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'[EE] thyristors vs triacs'
2011\12\30@182257 by KPL

picon face
Hi,

I'm building some simple pic-based thermostats, which will have to
keep some technical rooms above freezing.
Controller part is fine, and one version of power part is fine, too.
It's based on MOC3062 optocoupler and BT138 triac. Connections are
directly as per MOC's datasheet, with resistors of 360 ohms.
I used ordinary 1/4W resistors, without any problem, that device is
working fine in test mode, switching a 500w heater.
(230 VAC 50Hz here)

For another application, halogen lamp is used as a heater, mainly
because they can be obtained really really cheap, and will direct the
heat, so I can mount it out of way. But those burn out very quickly
when switching on-off too often.
So, I decided to use one-way thyristors instead of triac, so i could
turn on half power initially, then power another one in opposite
direction after a delay. Looking thru available info, it seemed like
same circuit should be fine, in this case, two of them. But the gate
resistor is letting out magic smoke, it receives much more power!
Thyristors used are BT151, from datasheet their gate currents are even
smaller than for BT138.

I have forgotten many things about these, so probably I'm missing
something. Probably MOC3062 can not detect zero crossings, and is
feeding gate current differently? Probably thyristor requires
different circuit? Any ideas?

I really really do not want to just use bigger wattage resistors,
since whole device will be mounted in not-so-big plastic case and
unnecessary heat is unwelcome.

-- KP

2011\12\30@183124 by John Gardner

picon face
Don't know what's wrong with your circuit, but ISTR running halogens
at reduced power also shortens their life.

Jac

2011\12\30@184318 by KPL

picon face
Thanks, had no idea about that.
But that reduced power mode would be quite short, kind of soft-start.
One second, or probably even less.

Halogens I'm talking about are those with tubular bulbs, like this one:
http://www.sparksdirect.co.uk/500w-enclosed-halogen-floodlight-p-13363.html


On Sat, Dec 31, 2011 at 01:31, John Gardner <spam_OUTgoflo3TakeThisOuTspamgmail.com> wrote:
> Don't know what's wrong with your circuit, but ISTR running halogens
> at reduced power also shortens their life.
>
>  Jack
>

2011\12\30@185615 by M.L.

flavicon
face
On 30 December 2011 18:43, KPL <.....kpl.listesKILLspamspam@spam@gmail.com> wrote:
> Thanks, had no idea about that.
> But that reduced power mode would be quite short, kind of soft-start.
> One second, or probably even less.
>
> Halogens I'm talking about are those with tubular bulbs, like this one:
> http://www.sparksdirect.co.uk/500w-enclosed-halogen-floodlight-p-13363.html

A side note to your original question - but I'd use quartz tube
heating elements like these:
http://www.quartzinfrared.com/

They put out infrared heat so they will also be able to direct the
heat to some extent. They should last a long time.

-- Martin K

2011\12\30@185734 by alan.b.pearce

face picon face
> Don't know what's wrong with your circuit, but ISTR running halogens at
> reduced power also shortens their life.

I believe that they need to get up to a proper working temperature for the tungsten-halogen cycle to work properly. If they are not hot enough the halogen doesn't stop the tungsten evaporating off the filament, or something like that. I do remember it involves an evaporation redeposition of the tungsten.


-- Scanned by iCritical.

2011\12\30@190644 by KPL

picon face
That could be true, but at half power it just glows bright orange,
probably tungsten does not get to evaporate yet.

Not many options are available here locally. I know about proper
halogen infrared heaters, but they are basically 10 times more
expensive.

>
> I believe that they need to get up to a proper working temperature for the tungsten-halogen cycle to work properly. If they are not hot enough the halogen doesn't stop the tungsten evaporating off the filament, or something like that. I do remember it involves an evaporation redeposition of the tungsten.
>
>
> --
> Scanned by iCritical.
>
>

2011\12\31@034750 by Electron

flavicon
face

Check breakdown voltage of SCR gate when going negative.

I hope this fixes it.

SCR's are nice, TRIACs are just two SCRs after all, when working
with DC some people use TRIACs when it would make sense to use SCRs.

Cheers,
Mario


At 00.22 2011.12.31, you wrote:
{Quote hidden}

>

2011\12\31@092503 by Spehro Pefhany

picon face
At 06:22 PM 12/30/2011, you wrote:
{Quote hidden}

The SCR will block in the reverse direction, but the triac-output
optocoupler will turn on. The SCR will not trigger, but the gate
will break down with reverse voltage (well in excess of the abs max,
which is usually just a few volts), so gate current will flow-
peak of maybe 0.5A (on the negative half-cycle- think of the load
in series with the 360 ohm resistor in series with the optocoupler
output in series with a zener diode representing the SCR gate--
all across the 230V for the negative half cycles).

The result is too much heat in the SCR gate (maybe > 1W)
and WAY too much heat in the series resistor (>50 watts).

To fix it, just put something like a 1A 1kV diode in series with
the opto output- that should work. In fact, you can retain the
triac and add the diode to get half cycles- short the diode to
get full power.

(I suppose you know that drawing much current with just half
cycles is frowned upon- it creates harmonics and can overheat
transformers-- since you've only got a few A it shouldn't be a
problem). Also, thyristors (including triacs) tend to fail
'on' so please take that into account in your safety
considerations.


--sp







>I really really do not want to just use bigger wattage resistors,
>since whole device will be mounted in not-so-big plastic case and
>unnecessary heat is unwelcome.
>
>--
>KPL


'[EE] thyristors vs triacs'
2012\01\02@150306 by Dwayne Reid
flavicon
face
At 04:22 PM 12/30/2011, KPL wrote:
>Hi,
>
>It's based on MOC3062 optocoupler and BT138 triac. Connections are
>directly as per MOC's datasheet, with resistors of 360 ohms.
>I used ordinary 1/4W resistors, without any problem, that device is
>working fine in test mode, switching a 500w heater.
>(230 VAC 50Hz here)
>
>So, I decided to use one-way thyristors instead of triac, so i could
>turn on half power initially, then power another one in opposite
>direction after a delay. Looking thru available info, it seemed like
>same circuit should be fine, in this case, two of them. But the gate
>resistor is letting out magic smoke, it receives much more power!
>Thyristors used are BT151, from datasheet their gate currents are even
>smaller than for BT138.

I'm assuming that you are using the same opto-isolator for the SCR.  If that is the case, place a series diode in-line with the gate lead (K towards the SCR).  Also replace the SCR because you have probably damaged it.

What is happening is that the opto turns ON for both half-cycles but the SCR turns on for only the positive half-cycle.  When the opto turns on for the negative half-cycle, the gate breaks over and conducts.  This allows a large voltage across the gate resistor (360R) and smokes it.  This large, reverse current also damages the SCR's gate.

I have a couple of suggestions, if you wish.

1) Note, as others have already done, that the lamp has to run at full temperature for the halogen cycle to work.  However, if you are only ramping up to full current, the ramp won't do any damage at all.

2) I would monitor the AC power's incoming zero-crossing point, then do a fade UP rather than a hard switch.  Note that a hard switch-on for half cycles is about as destructive as a hard switch-on for full AC power - the destructive inrush current happens within the first half-cycle.

3) If you *DO* decide to do a fade-up, consider changing to a MOSFET and doing a "reverse" fade-up.  That is: each cycle of your fade starts at the beginning of the half-cycle rather than the end.  Note that a thyristor latches ON when triggered, which means that the fade can happen only from the end of the half-cycle.

The reason for doing the reverse fade is that you eliminate the vertical current spike that occurs when the thyristor turns ON.  Instead, the MOSFET is ON at the beginning of the half-cycle, then turns OFF at the appropriate time.  This lessens the shock to the filament.

dwayne

-- Dwayne Reid   <dwaynerspamKILLspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
http://www.trinity-electronics.com
Custom Electronics Design and Manufacturing

2012\01\02@162603 by KPL

picon face
Thanks Dwayne and Spehro, (and others)
Currently I replaced a triac into the same pcb, and added a 5 ohm NTC
for overcurrent protection, since found many reports on a web that
it's enough for bulb startup current limiting, will see if it works.
Switched to this approach currently mostly just because I want it to
be installed by tomorrow :)
I just feel I will have to find a proper heater at some point, so
probably it's not quite worth the trouble, except for learning
experience.
Those cheap bulbs tend to fail quite violently, I am not quite sure
protective glass will always help.

I earlier tried to implement soft-start circuit, based on U2008B,
which I was playing with a few times before. That is not very
straight-forward circuit, as can be seen here:
http://frantisek.rysanek.sweb.cz/U2008B.htm
Optocoupler would be used to switch on a slow start circuit at pin 3,
as i have done before. But this time it worked not so good, there was
a short bright pulse at the very startup, then it would turn off, and
slowly increase brightness the way I want. Do not know what caused
that pulse.

Will play with other options too, as probably some more similar setups
will be needed.

{Quote hidden}

>

2012\01\02@173741 by IVP

face picon face
> Those cheap bulbs tend to fail quite violently

One of the halogens in our kitchen did just that. A very loud bang
and the glass front shattered. Shards went everywhere, even some
splinters embedded in the lino

Do you really need part-cycle dimming -  on:off of whole cycles
would do the same on average. I made a 2-bar heater controller
using 4017s, MOC3041 + triac and a rotary switch to select the
ratio. Simple enough, a temperature sensor could be added

Jo

2012\01\03@035213 by KPL

picon face
> Do you really need part-cycle dimming -  on:off of whole cycles
> would do the same on average. I made a 2-bar heater controller
> using 4017s, MOC3041 + triac and a rotary switch to select the
> ratio. Simple enough, a temperature sensor could be added

But then you need to detect zero crossings, otherwise pic will not
know when to switch on/off? That seems like a lot of overdesigning for
just a soft start.

-- KPL

2012\01\03@052338 by IVP

face picon face
> But then you need to detect zero crossings

The MOC3041 is a ZC opto-triac, so if you're using an opto
anyway there's no added complication to the design

Cheap 4-pin opto-transistors can be used as ZC detectors too,
to detect one or both of the cycle peaks

http://home.clear.net.nz/pages/joecolquitt/txless.html

with a little s/w to get the actual timing spot on

Jo

2012\01\03@133719 by KPL

picon face
> The MOC3041 is a ZC opto-triac, so if you're using an opto
> anyway there's no added complication to the design

Yes, I'm using MOC3062 which kas ZC detecting, but controller has to
know when to instruct a optocoupler to open. Otherwise it will not be
in sync. Am I wrong?

> Cheap 4-pin opto-transistors can be used as ZC detectors too,
> to detect one or both of the cycle peaks
>
> http://home.clear.net.nz/pages/joecolquitt/txless.html
>
> with a little s/w to get the actual timing spot on
>
> Joe


I do not like these circuits, since you get a serious heat source, in
that 100K resistor. There was a work-around circuit, described here:
http://www.edn-europe.com/improvedoptocouplercircuitsreducecurrentdrawresistledaging+article+2021+Europe.html

-- KP

2012\01\03@135427 by Isaac Marino Bavaresco

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face
Em 3/1/2012 16:37, KPL escreveu:
>> The MOC3041 is a ZC opto-triac, so if you're using an opto
>> anyway there's no added complication to the design
> Yes, I'm using MOC3062 which kas ZC detecting, but controller has to
> know when to instruct a optocoupler to open. Otherwise it will not be
> in sync. Am I wrong?


The MOC only turns the TRIAC on at zero crossing. The TRIAC only turns
off at the next zero crossing by itself.
Simply turn the optocoupler on for slightly over one half-cycle and it
is guaranteed that the TRIAC will be on for exactly one half-cycle.

Isaac

2012\01\03@135736 by Isaac Marino Bavaresco

flavicon
face
Em 3/1/2012 16:37, KPL escreveu:
>> The MOC3041 is a ZC opto-triac, so if you're using an opto
>> anyway there's no added complication to the design
> Yes, I'm using MOC3062 which kas ZC detecting, but controller has to
> know when to instruct a optocoupler to open. Otherwise it will not be
> in sync. Am I wrong?


If you monitor the zero-crossing, then you don't need a zero-crossing
MOC, you could even control the conduction angle to have greater control
over the power.

Isaac

2012\01\03@150508 by IVP

face picon face
> I do not like these circuits, since you get a serious heat source, in
> that 100K resistor. There was a work-around circuit, described here:
> http://www.edn-europe.com/improvedoptocouplercircuitsreducecurrentdrawresistledaging+article+2021+Europe.html

Thanks, I'll add that link

2012\01\03@170107 by Electron

flavicon
face

On ( another :p ) side note on the subject of thyristors vs triacs, I'd like to
point out an experience I made.. triacs are much harder to use than SCR's, because
the latter can switch off much more easily, as for half a cycle they won't conduct
no matter how you drive the gate. Triacs instead will be able to unlatch only when
zero crossing the AC waveform, and since current and voltage aren't in phase in
some cases, you will do it when there's a non-zero voltage accross the triac (this
for example has EMI implications).

The OP had a good intuition using two SCR's, as a way to gain more control on the
heaters (e.g. also allowing one semi-wave (alternating please!) to limit power).

On a final note, SCR's, unlike triacs, CAN be switched off even when current is
passing through them, by sending a powerful enough negative pulse on the gate.
There are special SCR's (gate turning off SCR's are called, if I recall well) that
are specifically designed to switch off faster than a SCR driven with a - gate.

Just sharing the very little that I know, as what I learnt on this list is still
much more than what I could ever give. (unfortunately)

Cheers,
Mario

2012\01\03@171045 by Electron

flavicon
face

PS:

On ( another :p ) side note on the subject of thyristors vs triacs, I'd like to
point out an experience I made.. triacs are much harder to use than SCR's, because
the latter can switch off much more easily, as for half a cycle they won't conduct
no matter how you drive the gate. Triacs instead will be able to unlatch only when
zero crossing the AC waveform,

I mean, it's a very short moment and this has some implications. For example I don't
think you can use triacs easily at more than line (50-60Hz) frequences.

Also, both triacs and SCR's if turned on at zero crossing, the gate must be kept on
for a relatively long time, because at zero crossing it's hard to keep the device
held in latched state, and if you turn off the gate too soon, the device will unlatch.

and since current and voltage aren't in phase in
some cases, you will do it when there's a non-zero voltage accross the triac (this
for example has EMI implications).

The OP had a good intuition using two SCR's, as a way to gain more control on the
heaters (e.g. also allowing one semi-wave (alternating please!) to limit power).

On a final note, SCR's, unlike triacs, CAN be switched off even when current is
passing through them, by sending a powerful enough negative pulse on the gate.
There are special SCR's (gate turning off SCR's are called, if I recall well) that
are specifically designed to switch off faster than a SCR driven with a - gate.

Just sharing the very little that I know, as what I learnt on this list is still
much more than what I could ever give. (unfortunately)

Cheers,
Mari

2012\01\03@172026 by Electron

flavicon
face

this was sent in color but the color was missing when I received it back. The PS: was in
the 2nd and 3rd paragraph, i.e. from "I mean, it's a very.." to "the device will unlatch".

---

PS:

On ( another :p ) side note on the subject of thyristors vs triacs, I'd like to
point out an experience I made.. triacs are much harder to use than SCR's, because
the latter can switch off much more easily, as for half a cycle they won't conduct
no matter how you drive the gate. Triacs instead will be able to unlatch only when
zero crossing the AC waveform,



I mean, it's a very short moment and this has some implications. For example I don't
think you can use triacs easily at more than line (50-60Hz) frequences.

Also, both triacs and SCR's if turned on at zero crossing, the gate must be kept on
for a relatively long time, because at zero crossing it's hard to keep the device
held in latched state, and if you turn off the gate too soon, the device will unlatch.



and since current and voltage aren't in phase in
some cases, you will do it when there's a non-zero voltage accross the triac (this
for example has EMI implications).

The OP had a good intuition using two SCR's, as a way to gain more control on the
heaters (e.g. also allowing one semi-wave (alternating please!) to limit power).

On a final note, SCR's, unlike triacs, CAN be switched off even when current is
passing through them, by sending a powerful enough negative pulse on the gate.
There are special SCR's (gate turning off SCR's are called, if I recall well) that
are specifically designed to switch off faster than a SCR driven with a - gate.

Just sharing the very little that I know, as what I learnt on this list is still
much more than what I could ever give. (unfortunately)

Cheers,
Mario

2012\01\03@214656 by c h

picon face
IVP wrote:
>> Those cheap bulbs tend to fail quite violently
>
> One of the halogens in our kitchen did just that. A very loud bang
> and the glass front shattered. Shards went everywhere, even some
> splinters embedded in the lino
>

The book "Lighting control-technology and applications" says in "8.3
Thyristor And Triac Dimmers, 8.3.1 Introduction" that thyristor pair
better withstand high currents caused by tungsten lamp failure.
That can be the reason to choose thyristors because a halogen lamp is
used as a heater in one of the OP apps

2012\01\03@220021 by c h

picon face
>
> The book "Lighting control-technology and applications" says in "8.3
> Thyristor And Triac Dimmers, 8.3.1 Introduction" that thyristor pair
> better withstand high currents caused by tungsten lamp failure.

The book is by Robert Simpson "Lighting Control: Technology and
Applications", Focal Press, 2003
http://www.amazon.com/Lighting-Control-Applications-Robert-Simpson/dp/024051566

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