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'[EE]: Another relay question'
2002\10\29@184741 by Jinx

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I probably have this figured out right, but just checking

There are two relays to control a motor. An SPST for power
and a DPDT for direction. I've found the relays I want, based
partly on physical size. The SPST is 8A continuous and the
DPDT is 2A continuous but are close in size and low profile
(important). The motor draws 0.4A when running. If the direction
(2A) relay is closed before the power (8A) relay, then the power
relay will take the brunt of the start-up surge ??

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2002\10\29@194606 by Dwayne Reid

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At 12:44 PM 10/30/02 +1300, Jinx wrote:

>There are two relays to control a motor. An SPST for power
>and a DPDT for direction. If the direction
>(2A) relay is closed before the power (8A) relay, then the power
>relay will take the brunt of the start-up surge ??

Yes.

dwayne

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2002\10\29@212351 by Jinx

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> >(2A) relay is closed before the power (8A) relay, then the power
> >relay will take the brunt of the start-up surge ??
>
> Yes.
>
> dwayne

Cheers. Wating for the "but......." &or other shoe to drop.......... ;-)

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2002\10\29@223641 by Russell McMahon

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> > >(2A) relay is closed before the power (8A) relay, then the power
> > >relay will take the brunt of the start-up surge ??
> >
> > Yes.

Agree

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2002\10\29@223956 by Dwayne Reid

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So long as the smaller relay can handle the full load current and the
larger relay can handle the start and stop surges, you should have NO
problems.  Watch out for contact arcing on the larger relay, though.

We used to build controls for variable pitch fan blades used on large
earth-moving equipment using almost exactly that technique.  While it
worked fine, we wound up going to a somewhat more complex arrangement with
2 relays switching the +28V supply and 2 sense FETs switching the ground
side in order to handle specific customer requirements.  But as far as I
know, there are still a bunch of the relay only versions still out there
working - we occasionally get units in for repair (mechanic: "I tried to
clean all the gunk off the box with the steam cleaner and I guess it got
wet inside.  Dunno how that happened.")

dwayne

At 03:20 PM 10/30/02 +1300, Jinx wrote:
{Quote hidden}

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Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
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Celebrating 18 years of Engineering Innovation (1984 - 2002)
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2002\10\29@234710 by Jinx

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> > >(2A) relay is closed before the power (8A) relay, then the
> > > power relay will take the brunt of the start-up surge ??
> > >
> > > Yes.
>
> Agree

Thanks Dwayne and Russell - and 1/2 point to me

I did purposefully choose an over-specced relay for the power
as it will do all of the active switching. By luck it just turns out
that the SPST matches the package size of the DPDT, which
keeps the PCB tidy

Is there any mileage in trying to use something like an LM317
(but bigger) to limit the initial motor current, or is it normally
unnecessary or not worth the effort if the power relay is over-
specced by a wide margin anyway ? This is for a 12VDC motor
running on a battery, and will run satisfactorily down to ~9VDC
so perhaps a bypass transistor as current limiter ? The person
who I'm doing this for has had experience with failing semis (FET
H-bridge) in the previous version made by someone else, the
main reason I chose to use relays instead

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2002\10\30@041745 by Roman Black

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Jinx wrote:
>
> > > >(2A) relay is closed before the power (8A) relay, then the

> I did purposefully choose an over-specced relay for the power
> as it will do all of the active switching.

> Is there any mileage in trying to use something like an LM317
> (but bigger) to limit the initial motor current, or is it normally
> unnecessary or not worth the effort if the power relay is over-
> specced by a wide margin anyway ? This is for a 12VDC motor
> running on a battery,


Have you measured the stall (or start) current
of the motor?? That is where you will get the
potential problems and maybe with accel/decel
current, ie it might be wise to actually monitor
motor current.

If energy efficiency is not critical you could use
a 5% or 2% drop resistor, same principle as the
light bulb resistor it wont do much when the motor
is running but will make a huge difference at startup
and in the case of a stall.

Likewise if the motor will be stopped/started a
lot you will always be switching the relay at the
full stall current of the motor when starting it,
and your 8A relay might not be up to the job. :o)
-Roman

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2002\10\30@052252 by Jinx

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> Have you measured the stall (or start) current
> of the motor?? That is where you will get the
> potential problems and maybe with accel/decel
> current, ie it might be wise to actually monitor
> motor current.

I've found out that the motor pulls 1A at the load limit. The
client is off to Sydney for a few days from tomorrow morning
and we'll do some measurements when he gets back. In the
meantime maybe I'll look through mine and see if I've got
something similar to twiddle with as per your suggestions

Inrush suppressors (as found in monitors) have been mentioned
before in connection with anti-blow protection for light bulbs and
I was having a look at some in the RS catalogue

Specifically 210-673 to 210-752 (p643), 50R-2A to 5R-6A

example info http://www.todmcpg.com/inrush/inrush.html

Now, these are said to be suitable for "small" electric motors. I'm
perfectly happy with the series resistor on the light bulb, so wonder,
without rehashing the light bulb debate, whether anything more than
a series resistor for the motor is needed. In this case, I'm trying to
make life easier for the relay contacts by reducing or eliminating arcing

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2002\10\30@105250 by Jim

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  "Inrush suppressors"

Can we call them "inrush limiters" - as it makes
more sense since we're *really* limiting the current
"rushing in" ...

RF Jim

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2002\10\30@123841 by Peter L. Peres

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On Wed, 30 Oct 2002, Jinx wrote:

*>I probably have this figured out right, but just checking
*>
*>There are two relays to control a motor. An SPST for power
*>and a DPDT for direction. I've found the relays I want, based
*>partly on physical size. The SPST is 8A continuous and the
*>DPDT is 2A continuous but are close in size and low profile
*>(important). The motor draws 0.4A when running. If the direction
*>(2A) relay is closed before the power (8A) relay, then the power
*>relay will take the brunt of the start-up surge ??

Yes if you allow enough time between them. Same thing applies when
breaking (open the big relay first). Using a hefty snubber should help,
and using a NTC or other current limiter even more.

Peter

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2002\10\30@144542 by Dwayne Reid

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At 05:45 PM 10/30/02 +1300, Jinx wrote:
> > > >(2A) relay is closed before the power (8A) relay, then the
> > > > power relay will take the brunt of the start-up surge ??
> > > >
> > > > Yes.
> >
> > Agree
>
>Thanks Dwayne and Russell - and 1/2 point to me
>
>I did purposefully choose an over-specced relay for the power
>as it will do all of the active switching. By luck it just turns out
>that the SPST matches the package size of the DPDT, which
>keeps the PCB tidy

One thing that I should mention: all relays that I destroyed as part of my
testing way back then failed because of arcing that occurred when trying to
STOP the motor.  We used dynamic braking (using a SPDT relay to switch a
load resistor across the motor coil after disconnecting from the supply)
and the contact arcing was ferocious.  Clamping diodes reduced the problem
to the point where we had relay failures only if a dead short at the load
happened.

dwayne

--
Dwayne Reid   <dwaynerspamKILLspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

Celebrating 18 years of Engineering Innovation (1984 - 2002)
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2002\10\30@162654 by Olin Lathrop

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> There are two relays to control a motor. An SPST for power
> and a DPDT for direction. I've found the relays I want, based
> partly on physical size. The SPST is 8A continuous and the
> DPDT is 2A continuous but are close in size and low profile
> (important). The motor draws 0.4A when running. If the direction
> (2A) relay is closed before the power (8A) relay, then the power
> relay will take the brunt of the start-up surge ??

Yup.  It would be a good idea if the power relay is open whenever the
direction relay is being switched.


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Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

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2002\10\30@164943 by Jinx

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> One thing that I should mention: all relays that I destroyed as part
> of my testing way back then failed because of arcing that occurred
> when trying to STOP the motor

Hmmm, good point. I'll check my code to make sure that the 2A direction
relay is off before the 8A power goes off. The motor has back-to-back
zeners and filter caps across it, but that was intended to reduce EMR,
not address back EMF

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