>>I have a question about the contact current specs of relays.
>>
>>I need to switch a PWM signal for a solenoid between two PWM sources. Let's
>>assume that I switch the relay when both PWM solid state drivers are off.
>
> So it is switching zero current, right?

In this case, yes.

>>Would I have to use the peak current of the PWM signal to match the contact
>>current, or is the resulting average current ok? (The second case would
>>result in a much handier relay...) I'm talking here about average currents
>>up to 3 A, with peak currents maybe up to 15 A or so.
>
> Neither. ;-) It's the RMS current that results in contact heating.
> With an inductive load and a suitably HIGH PWM frequency, there is little
> ripple and average ~= RMS ~= DC. Why are your peak currents so high?

I don't control the PWM frequency and I don't have specs for one of the PWM
controllers (the other one is my own controller, so I can control it here).
I'm not sure they always drive the solenoids with a high enough frequency
to get an averaging through the inductive effect. They might rely on the
mechanical averaging. I've seen PWM frequencies as low as 30 Hz, which is
not high enough for the effect you are describing.


>>If I can't guarantee that both PWM drivers are off when I switch the relay,
>>I guess I have no choice anyway and need contacts that are able to handle
>>the full peak current. Right?
>
> Might be worse than that. If the voltage doesn't reverse you may not be
> able to use the AC rating of the contacts. Hopefully your voltage is low
> and you can use something like the 24VDC or 28VDC rating in that case,
> because DC-rated relays for higher voltages tend to be rare and expensive.

This is an automotive application, so the voltages are generally in the 12
V to 16 V range.

Thanks a lot for your thoughts.

Gerhard

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