At 01:25 PM 3/21/2004 -0300, you wrote:
>Hi all,
>
>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?

>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?

>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.

Best regards,

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

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