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Driving Relay Coils in parallel

I built a test box to switch loads during a thermal cycle test. I used relays (Leach KCL-D1A), 28 Volt coils, coil resistance 450 ohms.

I wanted the relays to switch together, so I wired the coils in parallel. The drive came from an existing test card we had on-hand that contained a UDN-2891 High side driver. Not knowing the 2891 had an internal clamp diode, I placed clamp diodes across each of the relays coils (regular diode). The voltage on the driver was 28V. The minimum pick-up voltage on the relays is stated as 19.8 volts at 125C. So I thought I was good. Tested the switch box several times, worked OK. Worked at low temp (0C).

At high temp (50C) only one relay switched. Came back to 20C, and the both switched again. OK, this could have been caused by bad solder connections or something like that, but is there any other failure mechanisms associated by driving the coils in parallel?

Mark

Reply to
Mark
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Driving the coils in parallel is not a problem. The built-in clamp diodes should also present no problem. I'm not familiar with the UDN-2891. Are you pushing its current drive capability? your load will be approximately 124 mA. It is possible that at high temperature, the driver went into current limiting, and the resultant output voltage was enough to pull in one of the relays, but not the other, due to slightly different pull-in characteristics.

Reply to
Jon

Driving parallel coils with a voltage should work fine. What voltage did you circuit apply to the coils at 125C? Was the driver circuit exposed to the varying temperature or only the relays?

Reply to
John Popelish

No, there should not be anything.

Those Sprague UDN parts are source drivers, I think old ones which I don't have a data sheet for, but IIRC, they will drop at least 2V, so you're down to 26V on the coil.

Being that those are fancy-schmancy mil-spec relays and not a high-volume COTS parts, maybe one of them is defective.

Suggest you compare the pull-in voltage at room temperature of the two relays. Copper increases in resistance by about +0.38%/K, and dropout voltage is probably proportional over the range in question.

Reply to
Spehro Pefhany

Not that I can think of.

You either have a fault in one of the relay circuits or insuficient drive. With marginally adequate drive to a pair of relays it is quite likely one will operate and the other not.

Reply to
nospam 

--
Suggest you check and see whether  your tech knows the difference
between "series" and "parallel".
Reply to
John Fields

The voltage was about 26 volts on the coils at room temp and worked fine. The driver is rated at 350mA

The tech who built the circuit learned that you should always place clamp diodes across the coils, so he put one on each relay, I just left them in.

Thanks

Reply to
Mark

Driver rated at 350mA and was outside the thermal chamber. Strongly suspect a bad connnection

Reply to
Mark

The relays only saw 50C, driver was outside the chamber.

Reply to
Mark

Here is a man who has done telephone support! He might just be right, too.

Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany 

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Good one!
Reply to
John Fields

Your UDN is a "P" type drive and there may be current limiting on that existing test card, your nominal load is ~ 28V/ 1/2K x2 or 120mA. Did you measure the voltage developed across each coil? A 25oC excursion off room temp is next to nothing so that the usual effects of increased coil resistance + reduced saturation flux density should not come into play. There is most likely something wrong with your construction. And that "minimum" voltage at max temp is not right- it should say maximum pick-up voltage. And why would you put a diode across each coil if they're in parallel?

Reply to
Fred Bloggs

Bad connection through that chamber connector or your connector then...

Reply to
Fred Bloggs

"Mark" ...

Maybe far fetched, but are the relays mounted close together and in reverse direction? Magnetic leakage may change sensitivity (or, as I found out, keep a nearby relay closed when it should open). The higher temperature may just change the sensitivity enough to have this effect.

Did you try connecting just one relay at a time? With equal voltage drive (measure it!) it should operate.

Arie de Muynck

Reply to
Arie de Muynck

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