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'[EE] Relay pull-in power at a hold voltage'
2008\04\15@071213 by Jinx

face picon face
In order to reduce current consumption in a 12V system I'd
like to use 24V relays. Apart from taking some of the strain
off the mains-powered PSU it will also help with battery life if
power fails

In the past I've done this with 12V relays in a 5V system by
dumping the reverse charge from a capacitor in series with the
5V supply, effectively putting 10V across the coil. The relay
will stay closed at 5V after the capacitor is discharged

The current project is a little more serious than what I've done
before so I'd like some advice and hopefully practical experiences

Questions after this background

The situation with a prototype, which works fine, is -

A 13.6V 300mA rail is available and 12V relays are used on
pluggable module boards. These are a mix of Schrack 32A power
relays with a 360R coil and 1A LVDC relays with a 720R coil

There could be up to 24 relays on at once, and the PSU can't cope
with this at the moment. The 13.6V rail is needed for other things or
else I'd simply bump it up to 24V. Also out are latching relays

The 24V relays I'm looking at are respectively 1430R and 2820R
coils, which reduces current consumption considerably. They are
also suitable size-wise and price-wise

Some tests I ran this afternoon - 24V relays will pull in at 15V and
drop out at 7V. 13.6V seems high enough above 7V to make this
idea work. Hold current is around 4.5mA, which is just dandy

The proposal is to make a local voltage doubler on each board
that's enabled when a relay is needed to be energised, like this

PIC-enabled doubler - diode > cap < diode 13.6V

Cap+ is the hot end of the relay coil. Another PIC pin turns on
a low-side transistor to ground the cold end

A PIC output would drive a simple transistor doubler to charge
the cap to 25V-ish

The questions (finally) are

Does anyone do this already, and with what results/cautions ?

Can I calculate what holding power the coil has at the lower
voltage ? Or is this a 'manufacturer knows' parameter. I'm already
aware that the relay will be more susceptible to vibration, but don't
know by how much

Any other considerations ?

TIA

2008\04\15@073123 by Xiaofan Chen

face picon face
On Tue, Apr 15, 2008 at 7:10 PM, Jinx <spam_OUTjoecolquittTakeThisOuTspamclear.net.nz> wrote:

{Quote hidden}

I think the relay vendor should have the specification about
minimum holding voltage.

I have used similar tricks quite some time ago. We have
12V (to energize the relay) and 9V (to hold the relay).
But you need to check wheter the capacitor will be
discharged in certain circumstances (like user slowly
increase the voltage) and that circumstances are
important or not.

Xiaofan

2008\04\15@080124 by Jinx

face picon face
> I think the relay vendor should have the specification about
> minimum holding voltage.

I've already found 'a' minimum hold voltage. That is, the voltage
at which the relay drops out

I guess what I'm really getting at is whether the coil is saturated
enough, relative to the 15-24V pull-in range, at the lower current/
voltage to be considered a 'solid' hold in normal use

As you suggest, the manufacturer may recommend the lower
end of the pull-in range, ie > the 13.6V hold voltage I have

I'll have to ask, but for now I'll pencil in a voltage doubler and
maybe try some amateur vibration tests, for what they're worth

2008\04\15@081456 by Spehro Pefhany

picon face
Quoting Jinx <.....joecolquittKILLspamspam@spam@clear.net.nz>:

>> I think the relay vendor should have the specification about
>> minimum holding voltage.
>
> I've already found 'a' minimum hold voltage. That is, the voltage
> at which the relay drops out
>
> I guess what I'm really getting at is whether the coil is saturated
> enough, relative to the 15-24V pull-in range, at the lower current/
> voltage to be considered a 'solid' hold in normal use

The minimum hold *voltage* will increase with temperature. With most
relay designs the contacts are held with the same pressure whether or
not the relay is close to the drop-out current, absent shock/vibration,
but if the contacts open a bit with shock/vibration then you could have
big trouble, especially if the current is high enough to cause welding.

> As you suggest, the manufacturer may recommend the lower
> end of the pull-in range, ie > the 13.6V hold voltage I have
>
> I'll have to ask, but for now I'll pencil in a voltage doubler and
> maybe try some amateur vibration tests, for what they're worth

Maybe shock more than vibration can cause it to drop out, depending
on the axis of motion.


Best regards,
Spehro Pefhany
--
"it's the network..."                          "The Journey is the reward"
s...spamKILLspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com


2008\04\15@082400 by Apptech

face
flavicon
face
> In order to reduce current consumption in a 12V system I'd
> like to use 24V relays. Apart from taking some of the
> strain
> off the mains-powered PSU it will also help with battery
> life if
> power fails


> In order to reduce current consumption in a 12V system I'd
> like to use 24V relays. Apart from taking some of the
> strain

Rather than voltage multiplying to allow operation of 24V
relays  with 12V holding,  consider 12V relays with
electronically defined reduced hold currents. This allows
you to tailor the holding current electronically
retrospectively should you need to. This does transfer some
power dissipation from the relay coil to a now non
saturating driver but this is liable to be easily designed
for. If the system requires a professional design approach
then this way allows far more flexibility.

Reputable relay manufacturers will usually specify "must
operate" and "must release" voltages for relays of nominal
operating voltage. You can use coil resistances and
temperature information to design safe holding currents.

Using 24V relays on a 12V system assumes that the "must
operate" and must release voltages fall appropriately wrt
12V. Using 12V relays and electronic drive tailoring means
no assumptions need be made.



       Russell

2008\04\15@082711 by Xiaofan Chen

face picon face
On Tue, Apr 15, 2008 at 8:00 PM, Jinx <.....joecolquittKILLspamspam.....clear.net.nz> wrote:
> > I think the relay vendor should have the specification about
>  > minimum holding voltage.
>
>  I've already found 'a' minimum hold voltage. That is, the voltage
>  at which the relay drops out
>
>  I guess what I'm really getting at is whether the coil is saturated
>  enough, relative to the 15-24V pull-in range, at the lower current/
>  voltage to be considered a 'solid' hold in normal use
>
>  As you suggest, the manufacturer may recommend the lower
>  end of the pull-in range, ie > the 13.6V hold voltage I have
>
>  I'll have to ask, but for now I'll pencil in a voltage doubler and
>  maybe try some amateur vibration tests, for what they're worth

One more trick is to pulse the driving signal. You may
be able to drive the relay with full voltage for a shorter
of time and then apply the holding voltage for a longer
time.

Xiaofan

2008\04\15@082900 by Apptech

face
flavicon
face
> I've already found 'a' minimum hold voltage. That is, the
> voltage
> at which the relay drops out

Guaranteed the same for all relays in all batches?

Guaranteed over temperature?

Orientation affected?

Vibration you've mentioned.


       R


2008\04\15@085605 by Jinx

face picon face
> > I've already found 'a' minimum hold voltage. That is, the
> > voltage at which the relay drops out
>
> Guaranteed the same for all relays in all batches?
>
> Guaranteed over temperature?
>
> Orientation affected?

I'm surmising Schrack and Omron would be reasonably reputable
and that the drop-out voltage of 7V for a 24V relay could increase
even 50% and still be well below 13.6V

AIUI, relay open/close is designed to be a "snap" action. That
is, at a marginal current there's a mechanical hysteris to prevent
chattering and which also minimises arcing and noise

2008\04\15@085641 by Jinx

face picon face
> Rather than voltage multiplying to allow operation of 24V
> relays  with 12V holding,  consider 12V relays with
> electronically defined reduced hold currents

I've considered that, but what advantage does that have over
using a similar method with a coil with 4x the DC resistance ?

For example

12V supply, 360R 12V coil, hold at 9V (or whatever)
12V supply, 1430R 24V coil, hold at 12V

2008\04\15@094803 by Apptech

face
flavicon
face
>> Rather than voltage multiplying to allow operation of 24V
>> relays  with 12V holding,  consider 12V relays with
>> electronically defined reduced hold currents
>
> I've considered that, but what advantage does that have
> over
> using a similar method with a coil with 4x the DC
> resistance ?
>
> For example
>
> 12V supply, 360R 12V coil, hold at 9V (or whatever)
> 12V supply, 1430R 24V coil, hold at 12V


It allows operation at all times under manufacturer defined
conditions AND it allows you to EASILY tune the design
retrospectively.


   Russell


2008\04\15@095444 by Tony Smith

flavicon
face

> > > I've already found 'a' minimum hold voltage. That is, the
> voltage at
> > > which the relay drops out
> >
> > Guaranteed the same for all relays in all batches?
> >
> > Guaranteed over temperature?
> >
> > Orientation affected?
>
> I'm surmising Schrack and Omron would be reasonably reputable
> and that the drop-out voltage of 7V for a 24V relay could
> increase even 50% and still be well below 13.6V
>
> AIUI, relay open/close is designed to be a "snap" action.
> That is, at a marginal current there's a mechanical hysteris
> to prevent chattering and which also minimises arcing and noise


What's wrong with latching relays?

Apart from price, availability, unknown start-up states & extra pins and/or
circuitry needed to turn then off, that is.

Tony

2008\04\15@145256 by Rich

picon face
First you need to determine the picking voltage and the drop out voltage.
Here's an idea for the list to criticize. Use a zener just above the drop
out voltage and pulse it with a voltage above the picking voltage.

{Original Message removed}

2008\04\15@151122 by Martin

face
flavicon
face
I do believe another common way to do this is with an RC parallel
circuit that is in series with the relay coil. The resistor sets the
hold current and the (rather large) capacitor allows more current to
flow when the circuit is closed, but drops off when the capacitor is
charged. When the circuit is opened the resistor drains the capacitor
charge.
Also, a TVS in parallel with the relay coil allows the field to drop
quicker than a reverse biased diode.
-
Martin

Rich wrote:
> First you need to determine the picking voltage and the drop out voltage.
> Here's an idea for the list to criticize. Use a zener just above the drop
> out voltage and pulse it with a voltage above the picking voltage.
>
> {Original Message removed}

2008\04\15@152557 by Spehro Pefhany

picon face
Quoting Rich <EraseMErgrazia1spam_OUTspamTakeThisOuTrochester.rr.com>:

> First you need to determine the picking voltage and the drop out voltage.
> Here's an idea for the list to criticize. Use a zener just above the drop
> out voltage and pulse it with a voltage above the picking voltage.

A scheme suggested on USENET by the late Tony Williams back in '01
(view with monospaced font only). Variations on this method are in use
in production quantities. +V was 3.3 in the discussion below.

------

Schematically something like this;

   +V+-----+----------+
           |          /Sw2
           \          +
           /R         )
           \          )5V Relay coil
           |   + C    )
           +----||----+
           |         _|_
           +         \_/ D (Schottky)
       Sw1 /         ---
           +          |
           |          |
   0v------+----------+-----

 At rest Sw1 and Sw2 are open, C is charged up
 to about 3V (via D).  Close both switches.
 The +ve end of the relay coil is taken up to
 +3.3V, and C drives the -ve end to about -3V.
 There is now 6.3V across the relay.

 Hold both switches closed for about 2x the
 nominal flying time of the relay and, (as
 Robert noted), have a capacitor large enough
 to provide not less than 80% of 5V for the
 nominal flying time.  The relay will close.

 Now open Sw1.  The voltage across the relay
 coil will drop back to about 3V, and a 5V
 relay will hold in at that voltage.  With Sw1
 open, R starts to recharge C, ready for the
 next time.

 Sw1/2 are probably npn/pnp transistors.
----


Best regards,
Spehro Pefhany
--
"it's the network..."                          "The Journey is the reward"
s...spamspam_OUTinterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com


2008\04\15@182028 by Jinx

face picon face
part 1 635 bytes content-type:text/plain; (decoded 7bit)

> A scheme suggested on USENET by the late Tony Williams
> back  in '01 (view with monospaced font only). Variations on
> this method are in use in production quantities. +V was 3.3 in
> the discussion below

Herbert, I played about with this circuit last night. Works well.
If more than one relay is to be turned on with the same command
a short delay separates the individual turn-ons so the cap can re-
charge. Once all required relays are on, the voltage doubler is
turned off

I'll be going through Schrack's and Omron's sites to see if there's
any data available for hold voltages


part 2 2590 bytes content-type:image/gif; (decode)


part 3 35 bytes content-type:text/plain; charset="us-ascii"
(decoded 7bit)

2008\04\15@183709 by Jinx

face picon face
> > 12V supply, 360R 12V coil, hold at 9V (or whatever)
> > 12V supply, 1430R 24V coil, hold at 12V
>
> It allows operation at all times under manufacturer defined
> conditions AND it allows you to EASILY tune the design
> retrospectively.

If the second example is within manufacturer's spec (I'll be
checking ASAP) then it would be obviously much more
preferable because of the lower current

===========

> What's wrong with latching relays?

> Apart from price, availability, unknown start-up states & extra
> pins and/or circuitry needed to turn then off, that is

Yes, you're absolutely right. We're being too close-minded. I'll
get on to changing the expensive extruded housing, budget, PIC,
customer requirements and s/w right away

Thank you Tony, mine eyes have been opened !!!  ;-)

2008\04\15@190133 by Richard Prosser

picon face
part 1 1872 bytes content-type:text/plain; charset=ISO-8859-1 (decoded 7bit)

Jinx,
Just for interest, I've attached a simple circuit that applies twice
the input voltage to the relay coil when switched. It may be the
same/similar to that you originally used - or might be useful to
someone else. It's a little bit tricky as to how the switching is
performed so I'll add an explanation.

Basic operation:-
1. Initially C1 charges via R1 & R2. The voltage pulse across the
relay (L1,R5) is insufficient to energise it as it is only the supply
voltage at this stage.
2. The switch (U1) closes. (This can be a transistor/FET etc if
required). The voltage on the right side of C1 then drops to 0V. The
voltage on the right side of C1 then drops to -12V.
3. Q2 now has its emitter at considerably lower voltage than its base
so it turns on. there is now  24V (approx) across the relay and it
pulls in.
4. C1 discharges and the current path through the relay is maintained
via D1 and the switch until the switch opens.
5. When the switch opens, C1 is recharged by the current through the
relay. Once this current drops past the hold-in value the relay opens.

Richard P



On 16/04/2008, Jinx <@spam@joecolquittKILLspamspamclear.net.nz> wrote:
{Quote hidden}

> --

2008\04\15@190957 by Jinx

face picon face
> One more trick is to pulse the driving signal. You may
> be able to drive the relay with full voltage for a shorter
> of time and then apply the holding voltage for a longer
> time

>From Schrack/Tyco

http://relays.tycoelectronics.com/schrack/pdf/definitions.pdf

(via http://relays.tycoelectronics.com/schrack/techn/)

Reduction of coil power consumption

For monostable DC relays a reduction of coil power is possible if
vibration or shock requirements are low. After an energizing pulse
of 100 ms with rated coil voltage the coil, power consumption can
be reduced to 50% by using constant DC supply (71% of rated
coil voltage). After an energizing pulse of at least 100 ms
additional savings in coil power consumption of monostable DC
relays can be achieved if the coil current is controlled, e.g. by
pulse width modulation (PWM). For the minimum recommended
coil current please contact our technical support. The used
frequency should be chosen so that it is outside of the audible
range. The characteristics of the application regarding
electromagnetic compatibility (EMC) has to be checked when
PWM control is applied

In my situation (losses ignored), 13.6/0.71 = 19V. Operating
range for the relay I want to use is > 16.8V

2008\04\15@191557 by Jinx

face picon face
> Just for interest, I've attached a simple circuit that applies twice
> the input voltage to the relay coil when switched

Thanks Richard, I'll file that one away. I was actually thinking of

http://www.piclist.com/techref/io/relays.htm

1/2 way down page

Lowest cost for 5V only supply with higher voltage relay coil

2008\04\15@192540 by Apptech

face
flavicon
face
>> A scheme suggested on USENET by the late Tony Williams
>> back  in '01 (view with monospaced font only). Variations
>> on
>> this method are in use in production quantities. +V was
>> 3.3 in
>> the discussion below



With that circuit, IF you use a common doubler then all
relays are driven to 24V during energisation of any other.

The ability to turn on N at once, or not,  depends on the
doubler power capability.

Multiple (N or N/k) doublers could be used if desired.

Still results in a compromise 2:1 solution which has no
"design" capability retrospectively. Relays *MUST* be rated
to hold at V/2 under all conditions of supply voltage,
ambient temperature, coil temperature, product variability
etc. Will probably work well in most cases, but a
compromise.


       Russell



2008\04\15@195242 by Apptech

face
flavicon
face
>> One more trick is to pulse the driving signal. You may
>> be able to drive the relay with full voltage for a
>> shorter
>> of time and then apply the holding voltage for a longer
>> time

This would give him full designable control, unlike the
scheme he seems intent on pursuing.

Similarly, series resistors with switched full voltage for
turn on allow full rated voltage from a fixed supply for as
long or short as desired followed by power reduction to
whatever level is desired.


       Russell

2008\04\15@202117 by Richard Prosser

picon face
Jinx,
It's a similar circuit. But it doesn't let you turn the relay off
unless you switch the 5V_in line.
(And note the comment re the electro polarity & the 100 ohm resistor).

RP

On 16/04/2008, Jinx <KILLspamjoecolquittKILLspamspamclear.net.nz> wrote:
{Quote hidden}

2008\04\15@211230 by Jinx

face picon face
> > One more trick is to pulse the driving signal. You may
> > be able to drive the relay with full voltage for a shorter
> > off time and then apply the holding voltage for a longer
> > time
>
> This would give him full designable control, unlike the
> scheme he seems intent on pursuing.

"intent on pursuing" - an insinuation I don't particularly care
for. I haven't decided on anything

> Similarly, series resistors with switched full voltage for
> turn on allow full rated voltage from a fixed supply for as
> long or short as desired followed by power reduction to
> whatever level is desired.

I have 13.6V available, the PIC will be sleeping whenever
possible (power saving - remember ?), which makes dynamic
control unattractive. So what is so desperately wrong with a
static DC voltage that is within, AFAICT, manufacturer's
guidelines ?

2008\04\15@214044 by Apptech

face
flavicon
face
>> This would give him full designable control, unlike the
>> scheme he seems intent on pursuing.

> "intent on pursuing" - an insinuation I don't particularly
> care
> for. I haven't decided on anything

Got your attention, which was the aim :-).

> So what is so desperately wrong with a
> static DC voltage that is within, AFAICT, manufacturer's
> guidelines ?

Nothing, provided that the whole system is "designable" and
doesn't lock you into compromises which cannot be easily
repealed if needed. If you can always run the relays on
fully spec voltage and then reduce the power by a controlled
and retrospectively alterable amount when desired you are
master of the situation (as much as Murphy will ever let you
be). If you run the relays on below spec voltage and have to
boost to operate them, the hold voltage has a maximum value
which you can't increase (= Vsupply) and the boost voltage
may or may not be able to be tailored depending on how you
produce it. I'd be more worried about the hold voltage being
adequate in all circumstances than the boost issue.

The manufacturer commentary that you posted specifically
mentioned some caveats (eg low vibration environment) and
you previously mentioned amateur vibration testing so they
have identified as an official  concern something that you
already identified as a possible issue. If practice proves
more rigorous in practice than in theory (as happens) and
you have a fixed sub spec hold voltage then you may be left
vainly searching the creek for paddles.



       Russell

2008\04\15@215046 by Richard Prosser

picon face
Jinx,
I can see nothing wrong with what you are proposing. We use PWM to
turn down the drive to large contactors (100-1000Amps rating) but the
overall effect is the same. One of the contactor families we use has
two windings, one for pull-in and the other for hold-in with the
pull-in one switched out automatically on connect. And we still PWM it
to save power.

It's quite common for contactors to switch in a series resistance once
they connect (or, more accurately, bypass a series resistor when
disconnected) which is very close to your proposal. As long as you are
meeting the manufacturers ratings for voltage and temperature then
there should not be an issue.

One other thing we have started doing is, in the event of a failure to
detect a "connect" state, is to pulse the drive between the hold-in
and pull-in values at about a 1 second interval with a ~8:1  ratio.
That way if the contactor is actually pulled in it will neither
overheat nor disconnect. On the other hand, if a wire has fallen off,
it will connect shortly after the wire is fixed.

One thing to watch is that, if you are looking at getting UL on the
product, you have to be able to meet all temperature ratings etc. with
a single failure. Probably not an issue in you case as your max
voltages are still within the manufacturers ratings. Can be an issue
with a pwm arrangement however!

RP

On 16/04/2008, Jinx <RemoveMEjoecolquittTakeThisOuTspamclear.net.nz> wrote:
{Quote hidden}

>  --

2008\04\15@223417 by Jinx

face picon face
> One thing to watch is that, if you are looking at getting UL on
> the product, you have to be able to meet all temperature ratings
> etc

I'm making up a PCB now to test under 'battle' conditions. It's
quite conceivable that the finished product will be installed in a
remote location in a tin shed. Vibration and shock are not likely
to be a major problem but temperature very well could be in
the summer. I have guidelines for track clearances and so on.
Connectors are right next to the relay pins (5mm spacing) and
there's not a lot I can do about that except treat the board to
minimise the risk of arcing. There are a couple of applications
where the current through the relay would be a significant % of
rated and I'll be using the relay to control a heavy-duty solenoid
actuator for those. Otherwise, a 32A-capable relay is more
than sufficient

I'll report back with test results



2008\04\15@224926 by Apptech

face
flavicon
face
"Richard Prosser" said
> Jinx,
> I can see nothing wrong with what you are proposing.

That could mislead, obviously quite unintentionally.
Most of what he has been proposing is to run a relay at
below rated voltage and apply boost voltage to pull it in.

The paragraph

>> > > One more trick is to pulse the driving signal. You
>> > > may
>>  > > be able to drive the relay with full voltage for a
>> shorter
>>  > > off time and then apply the holding voltage for a
>> longer
>>  > > time

was from Xiaofan AFAIK and essentially parallels what you
then described.

I like your 8:1 "pull in on repair" mode.


       Russell

2008\04\15@232004 by Marcel Duchamp

picon face
Apptech wrote:
> "Richard Prosser" said
>> Jinx,
>> I can see nothing wrong with what you are proposing.
>
> That could mislead, obviously quite unintentionally.
> Most of what he has been proposing is to run a relay at
> below rated voltage and apply boost voltage to pull it in.

If Mr. Jinx has valid datasheets that specify guaranteed pull-in voltage
and guaranteed drop-out voltage and ensures that he operates
accordingly, then I see no reason not to.

If he was simply guessing that it works based on testing one relay and
extrapolating, then I see no reason not to consider him "edgy".

Obviously, he must consider temperature extremes etc. but as Spehro
Pefhany wrote, he knows of large numbers of units in the field working
similar to Jinx's idea.

ps: when I first clicked on send, Thunderbird email program suggested
that the string "Apptech" was mis-spelled and the first suggested
substitute was "pyrotechnic"!!!

2008\04\16@010227 by Apptech

face
flavicon
face
> If he was simply guessing that it works based on testing
> one relay and
> extrapolating, then I see no reason not to consider him
> "edgy".

I never consider him edgy - at least, not for electronic
reaosns :-) - but we can all get bees in our bonnets
occasionally (just look at me)(bess stings all over) and
need a few people to yell a bit louder about our ideas.
(I've had enough yelling for a while thanks :-) ),.

> ... "Apptech" was mis-spelled and the first suggested
> substitute was "pyrotechnic"!!!

I resemble that.
Our place was long a favourite with my children's friends at
Guy Fawkes.



       Russell.

2008\04\16@114630 by Tony Smith

flavicon
face
> > What's wrong with latching relays?
>
> > Apart from price, availability, unknown start-up states &
> extra pins
> > and/or circuitry needed to turn then off, that is
>
> Yes, you're absolutely right. We're being too close-minded.
> I'll get on to changing the expensive extruded housing,
> budget, PIC, customer requirements and s/w right away
>
> Thank you Tony, mine eyes have been opened !!!  ;-)


See, minor quibbles.  Your vibration problem is solved, and an added bonus
is that power consumption goes down.  Slap a 'we're green friendly' sticker
on it and you're done.

Tony

2008\04\17@000230 by Jinx

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I ran some tests this morning. Results in graphic

'moderate shock' means banging it pretty roughly on the bench
with a continuity meter on NO contacts

Qualified success I think, still to run timing tests for the s/w and
may try to get the 13.6V rail bumped up a little, maybe 14V




part 2 2913 bytes content-type:image/gif; (decode)


part 3 35 bytes content-type:text/plain; charset="us-ascii"
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2008\04\17@002859 by Richard Prosser

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Jinx,
Just looking at the circuit quickly - Isn't the "top" diode (from the
13.6V supply) redundant?

RP

On 17/04/2008, Jinx <spamBeGonejoecolquittspamBeGonespamclear.net.nz> wrote:
> I ran some tests this morning. Results in graphic
>
>  'moderate shock' means banging it pretty roughly on the bench
>  with a continuity meter on NO contacts
>
>  Qualified success I think, still to run timing tests for the s/w and
>  may try to get the 13.6V rail bumped up a little, maybe 14V
>
>
>
> --

2008\04\17@020938 by Jinx

face picon face
> Just looking at the circuit quickly - Isn't the "top" diode (from
> the 13.6V supply) redundant?

Yes, you're right, it is. Thanks

2008\04\17@031911 by Apptech

face
flavicon
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> 'moderate shock' means banging it pretty roughly on the
> bench
> with a continuity meter on NO contacts

> Qualified success I think, still to run timing tests for
> the s/w and
> may try to get the 13.6V rail bumped up a little, maybe
> 14V

I'm rather afraid this is heading towards where I was afraid
it was heading towards :-(.

I'll delete the following paragraph of details of what's
wrong with it, lest it may offend ....................
........................................................................................................................
........................................................................................................................
......................................... .

and just say

   There is *nothing* that can be done to this circuit to
make it hold the relay in better without changing the actual
circuit. If it is inadequate in practice you are in trouble.

An alternative that would be cheaper would be to use 12V
relays and to PWM the 5V drive signal during holding. The
PWM can be at extremely modest frequency and easily handled
as a low load interrupt task.

After N IRQs
   Turn off all "holding" drivers.
After M IRQs
   Turn on all "holding" drivers.

Current is M/(M+N) x 100%.

Cheap, simple, elegant. Choose any 3.
NO more hardware compared to on/off drivers.


       Russell




2008\04\17@055510 by Jinx

face picon face
> I'm rather afraid this is heading towards where I was afraid
> it was heading towards :-(.

Which is where ?

> I'll delete the following paragraph of details of what's
> wrong with it, lest it may offend ....................
>
............................................................................
>
............................................................................
> ......................................... .

"There's something wrong and you have to guess what it is"

> and just say

>     There is *nothing* that can be done to this circuit to
> make it hold the relay in better without changing the actual
> circuit. If it is inadequate in practice you are in trouble.

Well, what is "inadequate" ?

> An alternative that would be cheaper would be to use 12V
> relays and to PWM the 5V drive signal during holding. The
> PWM can be at extremely modest frequency and easily handled
> as a low load interrupt task.

I've already tried PWM. At 1kHz

Turn-on 54% duty cycle, 18.5mA
Turn-off 17% duty cycle, 6.3mA, absolute minimum hold

I'd have to measure the overall circuit current needed to do this
at various frequencies

2008\04\17@070519 by cdb

flavicon
face
Don't Maxim (waits for hisses and boos) have something that might do
what you want - MAX4624/4625

Colin
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2008\04\17@092114 by Apptech

face
flavicon
face
> "There's something wrong and you have to guess what it is"

I said it before.
But. The booster circuit uses parts outside manufacturers
specs and in the all important long time / normal situation
of relay holding it places you at the mercy of having got it
right when testing. If you use a 12V relay at 12V then you
are at the mercy of the manufacturer. If they are reputable
as eg Omron (who you mentioned) tend to be, then you have a
reasonable chance that over 'the range of normal' all will
be OK and quite a way outside normal it may be as well.

As a semi-random (blame Gargoyle) example
relays.tycoelectronics.com/datasheets/SDT-R.pdf
says that must operate for this family is 75% of nominal
voltage and must release is 10%. That's a VERY low value for
must-release and I suspect that they have set it very very
very clear of actual to be safe.
In your system running eg 24 volt relays at 13.6V you are
holding at about 55% which is well above the (should it
happen to apply here) MUST release 10% and probably well
above the WILL release xx% as well. It suggests that what
you are doing is PROBABLY OK. And your bench tests also
suggest this. If you are happy with the margins that you
measure for your sample relays you may be happy to proced
and you will quite possibly 'get away with it'. Coz alas
that's what it is - when we boldly go outside spec sheet
specs we are making engineering judgements (or guesses
deep-ending) and we cannot be sure, barring substantial
personal expertise, how it will work out. OTOH if you run a
12V relay at 12V for pulling in and PWM it (at zero extra
parts count) at a duty cycle of your choice to hold it in,
then you are total master of your destiny, in this small
part of reality anyway.

>>     There is *nothing* that can be done to this circuit
>> to
>> make it hold the relay in better without changing the
>> actual
>> circuit. If it is inadequate in practice you are in
>> trouble.

> Well, what is "inadequate" ?

Noting that that was an *IF*.
Customer is unhappy that it sometimes doesn't stay in when
he thinks it should.

Copper wire is drawn through dies. The dies wear with use.
The wire size changes with wear. When they get too too worn
I'm told that they swap them into the next size up and
continue to use them. Sounds a bit mickey-mouse to me (and I
don't mean the meaning that the US HP salesman meant decades
ago when he described his development system as pretty
mickey-mouse) but I'm told that that is what done. If so,
the real gauge of the wire you get varies across an
acceptable range.

Fat wire sounds good - lower R - can't be bad? BUT it
depends how it is used. If it is wound on to make N turns
the resistance will be low. If it is wound on by the
resistance the turns will be low. If it is wound on to fill
space the turns will be low. Resistance will alter current.
Turns will alter amp-turns. Do you know which result your
manufacturer is providing? Or, do they? I'm not going to
start to analyse how much effect the above has - and I don't
know. The point is that even things that you expect to be
well controlled may not be. If the manufacturer has a spec
sheet figure it should tell you something. If they do not
have a figure the variation MAY surprise you. And/or them.

If you cannot "design" the circuit with certainty life gets
more uncertain. If you can then ... . PWMing is cheaper and
fully controllable under software. That doesn't mean it is
neceesarily the best solution - just the cheapest and most
flexible :-).  There may be other issues that make PWM
unnacceptable.


{Quote hidden}

Looks like the PWM atio and current are nicely matched

> I'd have to measure the overall circuit current needed to
> do this
> at various frequencies

I don't understand that comment.
Isn't overall circuit current just the sum of the chosen
holding currents plus and operate-mode currents?



       Russell


2008\04\17@092115 by Apptech

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flavicon
face
Just happened upon this reasonably good and very
comprehensive relay operation tutorial.

       http://www.worldproducts.com/note1.htm

Good enough to be worth saving.
9 web pages

This page is especially apposite to the current discussion

       http://www.worldproducts.com/note3.htm

useful

       http://www.worldproducts.com/note4.htm



2008\04\17@115730 by William \Chops\ Westfield

face picon face

On Apr 17, 2008, at 4:59 AM, Apptech wrote:
> If you use a 12V relay at 12V then you
> are at the mercy of the manufacturer. If they are reputable
> as eg Omron (who you mentioned) tend to be, then you have a
> reasonable chance that over 'the range of normal' all will
> be OK and quite a way outside normal it may be as well.

One of the things that struck me recently is how common it is to use,  
say, a 5V relay in a 5V circuit, being driven by a darlington driver  
with 1+V Vce.  Oops?

So how come relays aren't rated by current, anyway?  Surely they're  
fundamentally current-driven?  I can't even figure out why a 120V  
relay has all that miles and miles of wire inside instead of being a  
12V relay with an internal dropping resistor...

BillW

2008\04\17@125534 by Apptech

face
flavicon
face
> One of the things that struck me recently is how common it
> is to use,
> say, a 5V relay in a 5V circuit, being driven by a
> darlington driver
> with 1+V Vce.  Oops?

Fair point - removes quite a large % or rated voltage with a
5 volt relay.

> So how come relays aren't rated by current, anyway?
> Surely they're
> fundamentally current-driven?

Traditionally used with mechanical switches off a given
supply. And makes it easier to show what order of voltage
they need.

> I can't even figure out why a 120V
> relay has all that miles and miles of wire inside instead
> of being a
> 12V relay with an internal dropping resistor...

You need a certain number of amp-turns to produce the
requisite magnetic field.
AND power = volts a amps.
If you increase voltage by 10 times and increase turns by 10
times and resistance by 10 times you get 10% of the current
and the same amp turns and the same power used.

BUT if you use a series resistor you need the same current
as before and so 10 times the power - 90% of which gets
dissipated in the resistor.

In practice they may play with turns and wire size to get an
acceptable amp turns figure. So a 120V relay may not have
the same power input as a 12V one. But they will be similar.



       Russell




2008\04\17@131638 by Spehro Pefhany

picon face
Quoting Apptech <RemoveMEapptechspamTakeThisOuTparadise.net.nz>:

>> One of the things that struck me recently is how common it
>> is to use,
>> say, a 5V relay in a 5V circuit, being driven by a
>> darlington driver
>> with 1+V Vce.  Oops?
>
> Fair point - removes quite a large % or rated voltage with a
> 5 volt relay.

Some of which is compensated by the fact that relays are
traditionally designed to operate on unregulated supplies of nominal
voltage, so there may be some budget to spend when you have a regulated
supply (but read the data sheet carefully, and be sure to allow adequate
margin for pull-in with a worst-case relay at minimum Vdd, maximum Vce
*and* maximum Ta effects). And be sure to include self-heating of the
coil, since in most cases it could happen that the relay drops out and
has to be re-energized hot.

Best regards,
Spehro Pefhany
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2008\04\17@160925 by Richard Prosser

picon face
>  Copper wire is drawn through dies. The dies wear with use.
>  The wire size changes with wear. When they get too too worn
>  I'm told that they swap them into the next size up and
>  continue to use them. Sounds a bit mickey-mouse to me (and I
>  don't mean the meaning that the US HP salesman meant decades
>  ago when he described his development system as pretty
>  mickey-mouse) but I'm told that that is what done. If so,
>  the real gauge of the wire you get varies across an
>  acceptable range.

As dies wear they develop a shoulder that increases the drag forces
required to pull the wire down. Eventually the wire breaks too often &
the die is changed. The die is then refinished to the next size up by
removing the shoulder and fine polishing. It is then stamped with the
new size and returned to service. I think we used to do this up to
about 3 or 4 times before the die was too worn to resize.

RP

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