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'[PIC]: 2.5KW Load'
2001\01\26@034641 by Francois Robbertze

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Sorry this is off-topic!

My pic is connected to a triac optocoupler(MOC3041) that drives a bigger triac (IT615) which is rated at 600V 15Amperes.
When I switch 2.5KW heater element on at 220Volt the triac(IT615) with heatsink becomes very-very hot. Must I get a higer rated triac or must I look for some other problems?

I am sure that the pic software don't switch the triac on/off the whole time..It switch on at a certain time and off after a 5minute delay.

Regards
Francois
             lllllllllll
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            ( @ @ )
o------oOOo-(_)-oOOo-----o
|                                   |
|   Francois Robbertze    |
|    spam_OUTfr10TakeThisOuTspammweb.co.za     |
|                                   |
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2001\01\26@035849 by Robert A. LaBudde

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At 10:44 AM 1/26/01 +0200, you wrote:
>Sorry this is off-topic!
>
>My pic is connected to a triac optocoupler(MOC3041) that drives a bigger
>triac (IT615) which is rated at 600V 15Amperes.
>
>When I switch 2.5KW heater element on at 220Volt the triac(IT615) with
>heatsink becomes very-very hot. Must I get a higer rated triac or must I
>look for some other problems?
>
>I am sure that the pic software don't switch the triac on/off the whole
>time..It switch on at a certain time and off after a 5minute delay.

1. You're operating at the capacity of the triac.
2. You can expect the triac to get hot when switching these currents.
3. What are you using as a heat sink? You'd need a pretty big slab of metal
for this job.
4. Do you have a snubber network across the triac? (E.g., 10-50 R + .033 C)
Otherwise the triac may be triggering itself.
5. You might consider upping the voltage spec on the triac. 600 V is not
much margin with respect to the switching transients.


{Quote hidden}

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2001\01\26@044720 by Alan B. Pearce

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>1. You're operating at the capacity of the triac.
>2. You can expect the triac to get hot when switching these currents.
>3. What are you using as a heat sink? You'd need a pretty big slab of metal
>for this job.
>4. Do you have a snubber network across the triac? (E.g., 10-50 R + .033 C)
>Otherwise the triac may be triggering itself.
>5. You might consider upping the voltage spec on the triac. 600 V is not
>much margin with respect to the switching transients.

600V is the normal triac to use for 230V mains. If switching resistive loads
then 400V is the norm.

Remember the rule of thumb is that a triac will dissipate about 1 watt per amp
of load current as the typical voltage drop across a triac is a good volt. You
have a 10 amp load, so your triac will be dissipating a good 10 watts. This will
require a large heat sink - as in a proper finned heatsink at least 4 inches
square depending on your ambient temperature.

You may also require a snubber network to stop the triac triggering on mains
carried interference and noise spikes. When trying to run any sort of motor you
will definitely need a snubber network.

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2001\01\26@064929 by mike

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On Fri, 26 Jan 2001 10:44:09 +0200, you wrote:

>Sorry this is off-topic!
>
>My pic is connected to a triac optocoupler(MOC3041) that drives a bigger triac (IT615) which is rated at 600V 15Amperes.
>
>When I switch 2.5KW heater element on at 220Volt the triac(IT615) with heatsink becomes very-very hot. Must I get a higer rated triac or must I look for some other problems?
>
>I am sure that the pic software don't switch the triac on/off the whole time..It switch on at a certain time and off after a 5minute delay.
If you need small size, low cost, and low heat, a good old-fashioned
relay is still hard to beat!

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2001\01\26@065347 by Alok Dubey

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if ur triac is hot.. i wont swicth off..making it hotter till kaboom
are u sure the triac is turning off ? mebe the coil impedence etc is
somethign u havent considered?
alok


> {Original Message removed}

2001\01\26@073113 by Roman Black

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Francois Robbertze wrote:
>
> Sorry this is off-topic!
>
> My pic is connected to a triac optocoupler(MOC3041) that drives a bigger triac (IT615) which is rated at 600V 15Amperes.
>
> When I switch 2.5KW heater element on at 220Volt the triac(IT615) with heatsink becomes very-very hot. Must I get a higer rated triac or must I look for some other problems?
>
> I am sure that the pic software don't switch the triac on/off the whole time..It switch on at a certain time and off after a 5minute delay.


That triac is not really suited to the load you are
driving. A 15amp triac is a fairly small one, and
you are driving a pretty hefty load at 2.5kW.

Most 15A triac have a fairly high sat (on) voltage
typically 3 volts or worse when near their max
current.

2500w/220v= 11.4A, could be dissipating as
much as 35w. Big heat!

I suggest at least a 20amp stud mount device
on a good heatsink (about 1 pound of aluminium
per 35 watts is a good guide) or maybe do what
most commercial systems do, go for a relay? No
heat loss! You don't need fast switching for 5
minute cycle times?

-Roman

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2001\01\26@093822 by Thomas McGahee

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Why not use a dual approach?: Have the MT1 and MT2 of your triac
connected ALSO to the contacts of a Normally Open relay.

When you want to turn on your 2.5kW load, FIRST turn on the
triac and wait about one cycle time. THEN turn on the relay.
Keep both activated until you want to turn the load off,
then FIRST turn off the relay, **wait long enough for the relay
contacts to fully open**, and then turn off the triac.
Easy to do with a PIC.

The triac can easily take the initial turn on load. Then
when the relay kicks in the voltage across the relay contacts
AS THEY CLOSE will be only a volt or two, since the triac
is on. But then the relay will conduct the load for the
several minutes the load is on. When turning off, the triac
will ensure that there is no arcing of the relay contacts.
Then the triac turns off.

I have been using this method for cutting down on the wear
and tear of a heavy duty relay I use to control an electric
room heater. The triac needs only a fairly moderate
heatsink, as the relay ends up handling the ON current
for all but the turn-on and turn-off transition times.

My unit (which is not PIC controlled, but rather controlled
by some logic gates and timer circuits) has been running since
1975 without a problem. Oh, yes, I use a snubber network
to ensure the triac does not get re-triggered at turn-off.

Fr. Tom McGahee

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2001\01\26@100242 by Roman Black

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Thomas McGahee wrote:
{Quote hidden}

That's brilliant! Nil contact arcing for the cost
of a cheap triac that is hardly ever on...
I love it! :o)
-Roman

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2001\01\26@113310 by M. Adam Davis

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Depending on the voltage drop and internal resistance, your triac may be
dissipating a significant amount of heat.  You didn't give any details
about your heatsink, but if it's a small radio-shack one then you'll want
to get a bigger one.

Look at the triac data sheet, it'll give you all sorts of information
about how big a heatsink you'll need based on your load.  Since you're
drawing nearly 12A with your heater (which is fine for this triac), you
can expect to need quite a large heatsink.

Another thing to think about is turning on the triac at zero crossing.
Perhaps your moc3041 has a zero crossing detect circuit in it already, but
if not replace it with an optocoupler which does.  If you turn on the
triac in the middle of the ac wavefrom you'll get a huge inrush current
draw which will provide a significant amount of heat for the triac -not a
good way to start the 5 minute on cycle.

-Adam

Francois Robbertze wrote:
{Quote hidden}

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2001\01\26@125947 by Bill Westfield

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   When I switch 2.5KW heater element on at 220Volt the triac(IT615) with
   heatsink becomes very-very hot. Must I get a higer rated triac or must I
   look for some other problems?

Note that once you get into "heatsink territory", using a higher-rated triac
will NOT necessarilly reduce the temperature.  The voltage drop across a
triac will be relative constant (someone said 1V - I thought it was
typically higher than that), so for a given load the power dissipation is
also constant.  Then you need a heatsink setup big enough to reach
equilibrium at an appropriate temperature with that much power going into
it.  A 2.5KW load at 220V is "about" 11 amps, or 11 watts - think about how
hot a 15W lightbulb gets...  Higher rated heatsinks are typically better at
transferring heat from the junction to the casing/heatsink, rather than
actually generating less heat in the first place...

BillW

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2001\01\26@130612 by Harold M Hallikainen

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On Fri, 26 Jan 2001 10:44:09 +0200 Francois Robbertze <RemoveMEfr10spam_OUTspamKILLspamPIXIE.CO.ZA>
writes:
> Sorry this is off-topic!
>
> My pic is connected to a triac optocoupler(MOC3041) that drives a
> bigger triac (IT615) which is rated at 600V 15Amperes.
>
> When I switch 2.5KW heater element on at 220Volt the triac(IT615)
> with heatsink becomes very-very hot. Must I get a higer rated triac
> or must I look for some other problems?
>
> I am sure that the pic software don't switch the triac on/off the
> whole time..It switch on at a certain time and off after a 5minute
> delay.
>

       You just need a larger heatsink. The heater is drawing 11.4 amps. The
triac has a voltage drop of 1.2 volts (typical of a triac at 15A, see
http://www.teccor.com/thyristor/triac.pdf) .  So, you're dissipating
13.68W.  You can calculate temperature rise by multiplying this 13.68W
times the degree C/ watt rating of your heat sink. If you're running
without a heat sink, the Teccor datasheet shows you can run only about 2A
with an ambient of 25C.

Harold



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2001\01\26@174544 by Bob Ammerman

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> Another thing to think about is turning on the triac at zero crossing.
> Perhaps your moc3041 has a zero crossing detect circuit in it already, but
> if not replace it with an optocoupler which does.  If you turn on the
> triac in the middle of the ac wavefrom you'll get a huge inrush current
> draw which will provide a significant amount of heat for the triac -not a
> good way to start the 5 minute on cycle.

Nah... you won't get appreciable heating for one non-zero-cross turn on
every five minutes or so.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

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2001\01\27@010338 by Alok Dubey

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actually he didnt mention the power rating of the triac.. that owuld make it
easier to figure out
alok


> {Original Message removed}

2001\01\27@040752 by Peter L. Peres

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Your triac probably develops about 5V x 12A ~= 60W and should have an
appropriate heatsink (large).

No, you cannot make it run cooler otherwise. The only way to make such a
thing run significantly cooler is to use something other than a triac for
switching (like a bidirectional power MOSFET module). This will be more
expensive than a triac. The minimum voltage drop across an open triac is
1.2Volts, but this is a theoretical value (if you drive the triac hard
enough using dc the voltage could be 0.6V at least on one half period -
but that does not count). Even so, the power dissipated would be ~15W.

Peter

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2001\01\27@041249 by Alok Dubey

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if ur drving a 2400W with a 60W triac ur gne there itself..
what is the power rating of the triac?
alok


> {Original Message removed}

2001\01\27@042954 by Peter L. Peres

picon face
If anyone ever tells you a cheap, small, and good 15A relay will not 'heat
up' when run at 12A after 30 minutes dont' believe him. Not to mention the
printed circuit board holding it and any connectors.

Peter

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2001\01\28@135155 by Peter L. Peres

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Triacs rated 16A are often rated for ~70-100W dissipation (TO220 case).
When they are not, then you have a problem. (this is usually disguised in
a duty cycle rating in the datasheet).

Peter

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2001\01\29@011730 by Thys Van Tonder

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Francois

I have seen a 3KW phase control dimmer using a CPU fan and heatsink. It
works well where size and space is a problem.

Thys

{Original Message removed}

2001\01\29@053701 by Francois Robbertze

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----- Original Message -----
From: Roman Black <RemoveMEfastvidKILLspamspamEZY.NET.AU>
To: <PICLISTSTOPspamspamspam_OUTMITVMA.MIT.EDU>
Sent: Friday, January 26, 2001 4:58 PM
Subject: Re: [PIC]: 2.5KW Load


{Quote hidden}

I also like this very much!!!

What is a save time delay to wait for the relay contacts to fully open or
close?

If I use a 250Volt 20Ampere relay together with a IT615(600V 15A) triac
(with a small heatsink). Will this be OK?

According to Peter L Peres this relay will also be hot after 30 minutes...Is
it a better option to look at bidirectional power MOSFET modules then?

I must also say that the ambient temperature is sometimes relatively hot
(30 - 50 deg C) and can affect the efficiency of the heatsink.

Space is not the main consern but the thing must be very reliable and it
must last for years...I also don't want the project to consist of 99%
heatsink if I can prevent it and the cost must be preferrably as low as
possible.

Francois

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2001\01\29@060330 by Vasile Surducan

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On Mon, 29 Jan 2001, Francois Robbertze wrote:

{Quote hidden}

 Time to fully open and to fully close contacts are not equal and depends
both on the realy mecanics. Better is to measure it using a timer for
relay switch. However from 8 to 20 ms you may found any value, smaller
with relay dimensions and distance between contacts.
>
> According to Peter L Peres this relay will also be hot after 30 minutes...Is
> it a better option to look at bidirectional power MOSFET modules then?
>
 I have doubts you can't find a good relay. However you may use a trick,
power the relay coil through one of his normal closed contacts, on the
normal open one put a resistor to V+. When the relay is on his coil will
be powered with just a fraction on V+ enough to keep relay closed but less
with about 30% thermal stress. Much cheaper than a MOSFET modules.

Vasile

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2001\01\29@062445 by Michael Rigby-Jones

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> -----Original Message-----
> From: Vasile Surducan [SMTP:EraseMEvasilespamEraseMEL30.ITIM-CJ.RO]
> Sent: Monday, January 29, 2001 10:43 AM
> To:   @spam@PICLIST@spam@spamspam_OUTMITVMA.MIT.EDU
> Subject:      Re: [PIC]: 2.5KW Load
>
> > According to Peter L Peres this relay will also be hot after 30
> minutes...Is
> > it a better option to look at bidirectional power MOSFET modules then?
> >
>   I have doubts you can't find a good relay. However you may use a trick,
> power the relay coil through one of his normal closed contacts, on the
> normal open one put a resistor to V+. When the relay is on his coil will
> be powered with just a fraction on V+ enough to keep relay closed but less
> with about 30% thermal stress. Much cheaper than a MOSFET modules.
>
> Vasile
>
This assumes the relay armature has enough momentum to actually reach the
normally open contact after the normally closed contact has broken.  If not
you will have a nice buzzer!  A better method may be to just use the
normally closed contacts with a resistor wired accross them.  At least the
coil will still have some current after the contacts have broken.

Mike

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2001\01\29@065348 by Vasile Surducan

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On Mon, 29 Jan 2001, Michael Rigby-Jones wrote:

{Quote hidden}

 Yapp! With too large value resistor you'll have the same buzzer !
 Same cloth, different colour...

 Vasile

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2001\01\29@071636 by mike

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On Mon, 29 Jan 2001 12:34:37 +0200, you wrote:

>----- Original Message -----
>From: Roman Black <TakeThisOuTfastvid.....spamTakeThisOuTEZY.NET.AU>
>To: <TakeThisOuTPICLISTKILLspamspamspamMITVMA.MIT.EDU>
>Sent: Friday, January 26, 2001 4:58 PM
>Subject: Re: [PIC]: 2.5KW Load
>
>
>> Thomas McGahee wrote:
>> >
>> > Why not use a dual approach?: Have the MT1 and MT2 of your triac
>> > connected ALSO to the contacts of a Normally Open relay.
>> >
>> > When you want to turn on your 2.5kW load, FIRST turn on the
>> > triac and wait about one cycle time. THEN turn on the relay.
>> > Keep both activated until you want to turn the load off,
I would be inclined to turn the triac off when the relay is on, to
avoid a triac overheating hazard if the relay fails, especially if the
triac heatsinking (or lack of it) is designed around the short times
for which it will be conducting.
>> > then FIRST turn off the relay, **wait long enough for the relay
>> > contacts to fully open**, and then turn off the triac.
>> > Easy to do with a PIC.

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2001\01\29@075419 by Roman Black

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Francois Robbertze wrote:

>
> If I use a 250Volt 20Ampere relay together with a IT615(600V 15A) triac
> (with a small heatsink). Will this be OK?


I think it will work very well. For switching once every
5 minutes you may need only a tiny heatsink for the triac.



> According to Peter L Peres this relay will also be hot after 30 minutes...Is
> it a better option to look at bidirectional power MOSFET modules then?

I think the relay will stay quite cool. Coil power
for that size relay will probably be about 0.5w to 1w,
very little heat compared to your 30w from just the triac.
The contacts will heat a couple of degrees over ambient,
but this won't matter as the contacts are rated for
20A and you are drawing 12A max, they are fine for
continuous use. Since you are using the triac to
do the initial switch on, you can parallel the relay
contacts safely. (which is normally bad!) I would
choose a 20A dual contact relay, so really it
is a 40A relay. Very rugged!


> I must also say that the ambient temperature is sometimes relatively hot
> (30 - 50 deg C) and can affect the efficiency of the heatsink.
>
> Space is not the main consern but the thing must be very reliable and it
> must last for years...I also don't want the project to consist of 99%
> heatsink if I can prevent it and the cost must be preferrably as low as
> possible.

Definitely go for the most rugged relay you can get
in your price range. The relay contacts are your
weak link. It should be very reliable if you allow
good surge and spike protection.

-Roman

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2001\01\29@080035 by Roman Black

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I do it in battery powered devices with a large capacitor
and resistor. The cap is BEFORE the relay's switching semi,
and connected to V+ by the resistor. On first turn on the
full voltage comes from the cap to pull-in the relay, and
as soon as the cap discharges the resistor provides enough
holding current. I normally aim for about 35% holding
current. Never had a problem with it. (also decouples
the supply from the relay back emf quite well) :o)
-Roman


Vasile Surducan wrote:
{Quote hidden}

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2001\01\29@080449 by Bob Ammerman

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> What is a save time delay to wait for the relay contacts to fully open or
> close?

Just a few milliseconds.

> If I use a 250Volt 20Ampere relay together with a IT615(600V 15A) triac
> (with a small heatsink). Will this be OK?

I'm guessing you will not need any heatsink at all.

> According to Peter L Peres this relay will also be hot after 30
minutes...Is
> it a better option to look at bidirectional power MOSFET modules then?

I think he's talking about routing the load power thru a PCB. Direct wiring
would solve that problem. The relay contact resistance should be low enough
that it won't overheat in itself.

> I must also say that the ambient temperature is sometimes relatively hot
> (30 - 50 deg C) and can affect the efficiency of the heatsink.

All the more reason to go with the relay.

> Space is not the main consern but the thing must be very reliable and it
> must last for years...I also don't want the project to consist of 99%
> heatsink if I can prevent it and the cost must be preferrably as low as
> possible.

This should be very reliable since it eliminates the major failure mode of
both components: heat killing the triac and contact arcing wrecking the
relay.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

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2001\01\29@105721 by mike

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On Mon, 29 Jan 2001 23:57:25 +1100, you wrote:

>I do it in battery powered devices with a large capacitor
>and resistor. The cap is BEFORE the relay's switching semi,
>and connected to V+ by the resistor. On first turn on the
>full voltage comes from the cap to pull-in the relay, and
>as soon as the cap discharges the resistor provides enough
>holding current. I normally aim for about 35% holding
>current. Never had a problem with it. (also decouples
>the supply from the relay back emf quite well) :o)
>-Roman
One thing to be aware of with doing this - if the product may be
subject to mechanical shock (e.g. a handheld unit that can be dropped
onto the bench) , this can cause the relay to drop out, and the low
holding current will not be enough to pull it back in.
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2001\01\29@111013 by Thomas McGahee

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>
> I also like this very much!!!
>
> What is a save time delay to wait for the relay contacts to fully open or
> close?

That depends on the relay used. It varies from a few milliseconds to several
tens of milliseconds. If you are not sure, try something like 20 ms as a
starting point.

Besides regular relays, I have also built units that used small
"contactors". These are more heavy duty than relays, and are
often used to control equipment drawing 20 amps or more.

>
> If I use a 250Volt 20Ampere relay together with a IT615(600V 15A) triac
> (with a small heatsink). Will this be OK?

Yes. One gentleman that I showed the method to decided to not use
any heat sink at all on his triac. His unit has been operating
without any problem for about 15 years now. Personally I don't
like to take too many unnecessary chances. I would rather pay
a buck or two more, throw on a decent heat sink, and KNOW that
my unit will not overheat.

>
> According to Peter L Peres this relay will also be hot after 30 minutes...Is
> it a better option to look at bidirectional power MOSFET modules then?
>

Well, there are two sources of heat involved. One is the heating of
the current-carrying contacts. You can reduce this somewhat by
wiring multiple contact sets in parallel. In other words, use a
DPST or DPDT relay and wire the poles in parallel, and the N.O. contacts
in parallel.

The other source of relay heat is the relay coil. You can alleviate
this source of heat by using PWM. BEGIN by applying 100% PWM for the
first 50-100 ms, and then back off to about 35% PWM for holding the
relay closed. To open the relay, apply 0% PWM. Use a PIC, of course!


> I must also say that the ambient temperature is sometimes relatively hot
> (30 - 50 deg C) and can affect the efficiency of the heatsink.

In that case your heatsink will be less effective, as the heat
transfer is based on the difference between the heatsink temperature
and the temperature of the air flowing over the surface of the
heatsink. A cooling fan helps.


>
> Space is not the main consern but the thing must be very reliable and it
> must last for years...I also don't want the project to consist of 99%
> heatsink if I can prevent it and the cost must be preferrably as low as
> possible.
>
> Francois
>

If you use a Triac rated at from 15 to 20 amps continuous duty, then
you can get by with minimal heatsinking if you use the Triac/Relay
method mentioned in my prior post. I often use triacs that are
insulated from their case, and use the metal enclosure as my
heatsink. All you really have to do is make sure the triac is able
to survive the relatively short turn-on and turn-off sequence
when it is operating alone (relay off). Even without any heatsink
at all, most triacs can handle this with ease. But I prefer
to be a bit conservative about such things. I want my stuff
to run for many many years with NO problems.

Fr. Tom McGahee

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2001\01\29@231958 by Roman Black

flavicon
face
Mike Harrison wrote:
>
> On Mon, 29 Jan 2001 23:57:25 +1100, you wrote:
>
> >I do it in battery powered devices with a large capacitor
> >and resistor. The cap is BEFORE the relay's switching semi,
> >and connected to V+ by the resistor. On first turn on the
> >full voltage comes from the cap to pull-in the relay, and
> >as soon as the cap discharges the resistor provides enough
> >holding current. I normally aim for about 35% holding
> >current. Never had a problem with it. (also decouples
> >the supply from the relay back emf quite well) :o)
> >-Roman
> One thing to be aware of with doing this - if the product may be
> subject to mechanical shock (e.g. a handheld unit that can be dropped
> onto the bench) , this can cause the relay to drop out, and the low
> holding current will not be enough to pull it back in.


Yep, that's definitely a good point. Generally I've found
that the pull in current is *much* higher than the hold in
current, because when holding, the armature is closed and
there is no magnetic air gap. Most hold in currents can
be reliably less than 10% of the pull-in current, but
I use about 35% as a general rule. At 35% I think it
would be practically impossible for a drop shock to
pull it open, based on how hard it is to pry open
with a screwdriver etc. But I would advise caution if
someone was to push that limit by using very low
holding currents. :o)
-Roman

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2001\01\30@055440 by mike

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On Tue, 30 Jan 2001 13:30:27 +1100, you wrote:

{Quote hidden}

Probably not a major problem with big relays. I only mentioned it as a
warning not to try squeezing the last drop of power out of the holding
current, as I have had this problem in a real application, with very
small signal-switching relays in a handheld instrument where we wanted
to minimise battery drain and thermal contact EMFs from coil heating.
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2001\01\30@142651 by Peter L. Peres

picon face
Look, the relay heats up because it and the surrounding wiring will be
passing 12A at about 0.5 ohms total resistance (including board, screw
terminals, and some wire). That's about 6W dissipated. Relays have no
heatsinks (not the relays you buy off the shelf at least), so they get hot
if small. Solution: large relay. == more money, more noise, and more drive
power. Also larger box. Powering the relay through one of its contacts is
a proven way to build vibrators (as in Tesla coil drivers) because all
sane off the shelf relays break before make, and the relay will never
switch over to the position with the resistor, it will keep falling back
to the NC contact. This will cause arcing on the other contact and maybe
melt the relay if left alone like this.

A goodish relay that does 12A and can be left alone unsupervised for 1
year in a box, without catching fire or melting is not cheaper than $6 in
ones, if you know where to buy it. An optofet should be available for up
to 3 times that much money ($20) if you know where to buy. This will solve
all your problems and comes with UL or CSA or other such compliance so
noone can sue you if it does catch fire after all. It still needs to be
screwed onto a small heatsink, but that is all.

FYI there are a LOT of el cheapo railmount industrial timers and other
items that claim 10 or 16A switching and melt right out of the outer
casing after a year or less of continuous operation. Especially in a hot
and/or wet climate.

It strikes me that one of the failure modes of the relay and/or switch is
to fail closed and if this heater will heat too much it could kill
something. It is up to you to decide whether the something shall be cooked
alive or freeze to death in such an event. If you do not want it cooked,
then you need to add a thermal breaker to the circuit, exposed to the
heat. This is an off the shelf part, that comes specced in degrees C and
Amps. You probably want 15A or 20A and 55 degrees C. This is a little bit
hot for your thermostat range but much better than explaining things to
the firemen imho. There are few if any 45C thermal breakers afaik.

Peter

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2001\01\30@142701 by Peter L. Peres

picon face
One more tiny small detail: Francois did mention that his appliance
switches on and off abt. every 5 minutes. In 1 year that's over 3 million
cycles for the relay (1.5 million full cycles at least). I don't know many
20A relays that can be relied upon for that many cycles. Even if it would
switch ten times less often (every hour or so) I would have doubts about
the relay life. The static module switch does not have this problem.

When considering the price of the module, you must think about the makers
of the modules knowing fairly well what relays can do, and competing with
relays. In other words, they probably win if you line up all the
advantages, and for very little more money.

Mr. McGahee is right, 12A is lower contactor territory, and small
contactors are often used for this range of current. They are slightly
more expensive than relays but far more rugged. Not 1.5million cycles
though. And I like Mr. McGahees solution too, it has its place, and it is
good. But I don't think that it's good for this thermostat.

Peter

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2001\01\30@172718 by Spehro Pefhany

picon face
At 08:35 PM 1/30/01 +0200, you wrote:
>Look, the relay heats up because it and the surrounding wiring will be
>passing 12A at about 0.5 ohms total resistance (including board, screw
>terminals, and some wire). That's about 6W dissipated.

Bad design if so, usually relays don't run hot. 0.5 ohm is extremely
high. Typically a T90 will have < 0.1 ohm contact resistance, which
decreases slightly during the operating life. But the operating
life is 100,000 operations at rated load. Life end is due to the
erosion of contact material from the arcing that occurs during switching
It's possible to get 1.5M operations, but difficult at 12A without mercury
relay technology (initial cost, disposal concerns), an industrial style
contactor (noisy and fairly costly) or solid state (reliability and
heat issues). Don't make assumptions about the effects of derating on
contact life on a mechanical relay or contactor, test.

{Quote hidden}

You think? The relays are UL/CSA and have fewer application issues.

>FYI there are a LOT of el cheapo railmount industrial timers and other
>items that claim 10 or 16A switching and melt right out of the outer
>casing after a year or less of continuous operation. Especially in a hot
>and/or wet climate.
>
>It strikes me that one of the failure modes of the relay and/or switch is
>to fail closed and if this heater will heat too much it could kill
>something.

No. This is one area where relays excel. Solid state switches almost
invariably fail "on" (and they can do so at any time, given some failure
mode or transient such as lightning). Relays that are properly rated from
good quality manufacturers (and proper contact materials specfied for the
load) virtually never fail on (weld). The eventually just fail to close.
*usually* this is much safer.

It is up to you to decide whether the something shall be cooked
>alive or freeze to death in such an event. If you do not want it cooked,
>then you need to add a thermal breaker to the circuit, exposed to the
>heat. This is an off the shelf part, that comes specced in degrees C and
>Amps. You probably want 15A or 20A and 55 degrees C. This is a little bit
>hot for your thermostat range but much better than explaining things to
>the firemen imho. There are few if any 45C thermal breakers afaik.

Putting an independent backup in there is very often a good idea. Sometimes
it doesn't much matter, for example in home thermostats where the temperature
won't get above 100'F or so even if the heat stays on continously (it's
self limiting at a relatively safe value, no fire can occur). Or sometimes
the oven or whatever will just burn itself out safely, no damage other than
the elements will occur. Which may be acceptable.

Best regards,

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2001\01\30@211433 by Roman Black

flavicon
face
Peter L. Peres wrote:
>
> Look, the relay heats up because it and the surrounding wiring will be
> passing 12A at about 0.5 ohms total resistance (including board, screw
> terminals, and some wire). That's about 6W dissipated.

Man you are so far off base! I have a good milliohm meter
and I am quite aware of the resistive losses in a
relay passing 12 amps! You figures are about 20 to 30 times
out of whack. Expect a few milliohms (<20) for any decent
quality 20A relay contacts. Get a copy of Ultracad's
"pcbtemp.exe" which will help you calculate PCB track
resistance for say a 1/4 inch track with solder stripe.
Again a few milliohms.

Also I would not use screwdown tewrminals I would solder
the wires into the board or ont PCB pins. But if you were
to include screw down terminals you would also have to
allow for them in your thyristor solution so it's not
fair to include them as part of the relay losses.


> Relays have no
> heatsinks

Of course not, a 20A relay runs quite cool at 12A and
doesn't need a heatsink. I work with relays every
week, and they have their place. I think it is bad
to turn someone off a relay solution based on false
evidence! :o)

-Roman

PS. Find a relay rated for more than 12A that
dissipates 6W and you have found your "relay with a
heatsink" ha ha!

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2001\01\30@214429 by Barry Gershenfeld

picon face
>One more tiny small detail: Francois did mention that his appliance
>switches on and off abt. every 5 minutes. In 1 year that's over 3 million
>cycles for the relay (1.5 million full cycles at least).

OK, I waited all day for someone to catch this...

Every 5 minutes = 12 times an hour.
x 24   = 288 times a day.
x 365  = 105,120 times a year.

Barry

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'[PIC]: 2.5KW Load'
2001\02\01@101726 by Francois Robbertze
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face
> If I use a 250Volt 20Ampere relay together with a IT615(600V 15A) triac
> (with a small heatsink). Will this be OK?

I find it difficult to locate a 20A 250V PCB mount relay...
The closest I get is a 16A 250V(single contact)

I suppose I can put two of the 16A relays in par. to give me a total of 32A
resistive load. Then I will be able to switch a 4kW (18A) elements together
with a 25A Triac in the "Dual Approuch"

Regards

Francois

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2001\02\01@161252 by Peter L. Peres

picon face
>>One more tiny small detail: Francois did mention that his appliance
>>switches on and off abt. every 5 minutes. In 1 year that's over 3 million
>>cycles for the relay (1.5 million full cycles at least).
>
>OK, I waited all day for someone to catch this...
>
>Every 5 minutes = 12 times an hour.
>x 24   = 288 times a day.
>x 365  = 105,120 times a year.
>
>Barry

Yes! I apologize. Sorry. I did the calculation twice and it seemed odd
both times, but it was late (2100 yesterday). Barry Gershenfeld is of
course right, and I was wrong. The number of cycles is still rather high
for a relay imho. According to Spehro Pefhany's numbers (150000 cycles) it
would not be likely to last two years.

Peter

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2001\02\01@161310 by Peter L. Peres

picon face
Roman, my numbers are based on reality, adjusted up for safety. The
original question was how to do this small, cheap, and good. The answer is
no.

Of course you can make wide tracks, use an overrated relay to be safe,
solder the wires to the board (but still there will be a connector
somewhere. The wires need to be 2.5mm^2 or more for 12A continuous in
enclosure. When all this is over you will have about 30cm of 2.5mm^2
wiring (including the relay contacts) in a box. That is still not 500
miliohms, but would you calculate it tightly ? Then give it to a clueless
person on good faith to burn his barn or egg hatchery down in 6-7 months
when the relay fails and 'cooks' itself by arcing between the bad contacts
?

Incidentally the static switches come with screwdown terminals on them at
that amperage.

As to 6W relays, fyi 4x50A contactors are pretty common and have 220V,
0.2A coils (sometimes 0.5A coils). ;-)

I have nothing against relays and I know that they are reliable etc etc
but 1.5 million cycles per year ?

Peter

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2001\02\01@171316 by Spehro Pefhany

picon face
At 05:16 PM 2/1/01 +0200, you wrote:
>> If I use a 250Volt 20Ampere relay together with a IT615(600V 15A) triac
>> (with a small heatsink). Will this be OK?
>
>I find it difficult to locate a 20A 250V PCB mount relay...
>The closest I get is a 16A 250V(single contact)
>
>I suppose I can put two of the 16A relays in par. to give me a total of 32A
>resistive load. Then I will be able to switch a 4kW (18A) elements together
>with a 25A Triac in the "Dual Approuch"

Potter & Brumfield (Siemens) has the T92 (DPST rated at 30A, IIRC) and
200,000 operations with a resistive load. IIRC (again) it not only works
at 250V but also at European 380V. It's a Faston relay though, not PCB.
It's around $5 in large quantities but you might have to pay a lot more
in small quantities.

Cheapest is two T90 form-A (SPST) relays (less than $2 in quantity,
depending on
supplier), but they may not meed Euro requirements for coil-contact breakdown
if that is a factor-they are PCB however. Rated at 100,000 operations
resistive
load at 30A/250V for the N.O. form A type.

Best regards,


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2001\02\01@181306 by Roman Black

flavicon
face
Peter L. Peres wrote:
>
> Roman, my numbers are based on reality, adjusted up for safety. The
> original question was how to do this small, cheap, and good. The answer is
> no.

Sorry if I was harsh Peter, but I still think the
relay/thyristor idea is the best. And I still have no
idea where you got the 0.5 ohms for the relay contacts
and wiring, although I do believe it's best to err on
the safe side.


> Of course you can make wide tracks, use an overrated relay to be safe,
> solder the wires to the board (but still there will be a connector
> somewhere. The wires need to be 2.5mm^2 or more for 12A continuous in
> enclosure. When all this is over you will have about 30cm of 2.5mm^2
> wiring (including the relay contacts) in a box. That is still not 500
> miliohms, but would you calculate it tightly ? Then give it to a clueless
> person on good faith to burn his barn or egg hatchery down in 6-7 months
> when the relay fails and 'cooks' itself by arcing between the bad contacts

I work on devices with relays all the time, including
2kW microwaves. Normally for mains power switching the
PCB has about a 1/4 wide track, with a simple solder
stripe in the middle. And they keep the tracks short
obviously. I test relay contacts in many of these mains
appliances with my milliohm meter, an excellent test of
the power connections and relay contacts. 20 to 50 milli
ohms is normal.


> Incidentally the static switches come with screwdown terminals on them at
> that amperage.
>
> As to 6W relays, fyi 4x50A contactors are pretty common and have 220V,
> 0.2A coils (sometimes 0.5A coils). ;-)

Smartie pants! ;o)


>
> I have nothing against relays and I know that they are reliable etc etc
> but 1.5 million cycles per year ?

But I remember the person's original post, he
said once the load was on it would be on for 5
minutes. I really didn't think it would be switched
EVERY 5 minutes.

Just on this topic, many microwave ovens use a
relay for switching the power to the main transformer
(that powers the magnetron). Whenever you cook, it
switches on and off every few seconds to give heat
control. Yes the relay does fail eventually, because
I'm paid to replace them. But I have also fixed
commercial microwaves from Cafes that get used all
day for years, and never had a relay fault. That is
switching the entire power into a nasty inductive
load like a 2kW transformer. Cook something in your
microwave on "medium" and you will hear this switching
every ten seconds or so. Many millions of cycles.
So I believe that if this person uses a thyristor
for the initial switching, and uses a decent relay
for the main period (with no contact arcing), the
thing should last pretty much forever!

PS. If he's reading this, try a microwave relay!
:o)
-Roman

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2001\02\02@015202 by Francois Robbertze

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> PS. If he's reading this, try a microwave relay!
> :o)
> -Roman

Thanks for the tip!

Regards

Francois

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2001\02\02@180919 by Peter L. Peres

picon face
Roman, microwave relays have three large advantages:

1. They are in a vigorously ventilated case

2. They never switch the entire power of the magnetron on make (as it has
a relatively cold filament when so, and that is only a small part of the
power of the unit).

3. The magnetron filament and the magnetron proper across the transformer
secondary damps the transformer so there is hardly any arcing on break.

Also many commercial microwave designs are properly designed for
continuous operation and have no maintenance problems as you have noticed.
Since they cost up to 10 times more than an equivalent home unit the
manufacturers can afford to use a good quality relay instead of the crappy
plastic relay in a $150 home unit. You may also have noticed that some
microwaves have a very limited timer range (30 minutes or so). Imho it
pays to read the leaflet if you operate such a unit, it may have a serious
duty cycle limitation (like 'use for 30 minutes out of an hour').

Peter

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2001\02\02@180924 by Peter L. Peres

picon face
>And I still have no idea where you got the 0.5 ohms for the relay
>contacts and wiring, although I do believe it's best to err on the safe
>side.

A normal plastic-cased piece of equipment that switches 10A via relay
develops less than 6W of heat on the power circuit (totals, including
short leads, relay, screw terminals, etc) . P=R*I^2 -> R = 6/144 ~= 0.042
ohms. This is 'rounded' up by x10 to account for various aging and safety
related problems to 0.5 ohms (~10 times). That will put about 60W of heat
into the same box. The idea is that at this rating the box shall not melt
but fail safely. For that all the power related parts in the box should be
able to handle 60W heat input (from themselves or otherwise) for a certain
amount of time (like 10 minutes or so). This is more or less how I ended
up with 0.5 ohms. Others who are more experienced than me are invited to
check my figures and compare with their results.

Peter

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