AC out Solid State Relay for DC

and have passed a no load 12V thru it so I see it works.

That's a bit of a contadiction in terms...if it's 'no load' then there is nothing going 'thu'.
Anyhow asside from that pedantry, the easiest way is to try it on a nice safe 1A load, I'd expect it to be fine, but there is a slim possibility that the output stage requires the zero crossing of AC to do it's on/off switching.
Conclusion=try it and see, or study the manufacturers spec sheet.

Del

Thanks for the quick reply. Let me assume it will pass the DC with no problem, is there a current difference between AC 25 amp and DC? I know these are very simplistic questions for the learned, but treat me as if I were a smart 5 year old.

I agree with Del. Normally, an AC device counts on the zero-crossings for a shut-off so my guess is, you will be able to turn it on but not turn it off unless you disconnect the DC source for the load.

Yes. Take to full load current of the AC device and multiply by 0.707 to derive the equivalent DC component.

DC equivalent rather than DC component - sorry. I must have been deluded!

Thank you for the answer to my question.

trophyspeed; they use back to back scr's or triac's ,your right, they won't turn off. but they make ssr using power mosfets. perry

Hi Perry, So, you are saying that with the advent of power MOSFETs used in SSRs, that an AC device no longer relies on the zero-crossings for a turn-off?

No, SSRs for AC loads use back-to-back SCRs or Traics, SSRs for DC loads use MOSFETs. There are no SSRs that do both AC and DC load switching that I know of. But you can get the control voltage either way. That control voltage is often referred to as the "input", but people sometimes get confused by that term in thinking that it will convert DC to AC or vice versa, which of course it does not.

There are essentially 4 types of SSRs, because the control of the SSR can be AC or DC:

AC load, AC control = back-to-back SCRs or Triac

AC load, DC control = still back-to-back SCRs or Triac

DC load, DC control = power MOSFET or IGBT (although IGBTs are becoming rare)

DC load, AC control = still power MOSFET, but not very commonly used since if you have DC to switch, you have it available to control as well.

That's where I was confused: after believing that MOSFETs were used in AC SSRs. If there is no means of holding an AC SSR in the on state after a trigger pulse as applied to the control input, then how would the device operate in PWM mode. Thanks for clarifying that for me.

When switching AC, the current is passing through zero at a rate twice the frequency right? So what you can do is turn the SCR(s) on at any point in a sine wave, then when you want o turn it off, remove the control signal and the next time the line source current crosses zero, it will cease to conduct. So yes, you can do PWM with SSRs on AC systems, but your carrier frequency cannot be higher than the line frequency.

When switching DC, your PWM carrier frequency is limited only be the switching speed of the device.

tropicalspeed; they also make random turn on ssr, control data used them to replace the the starting contacts in single phase motors, theirs had a reed switch inside the case, used 20 volts dc. my daughter lives in rockport, likes it there, but misses the tall trees in minnesota. perry

You cannot use a solid state relay using thyristors to switch DC. Once the thyristor is conducting with more than 2v across the junction it will stay in the 'on' state until you remove the voltage across it, the gate has no control after switching on. You will therefore need a means of switching it 'off' as well as 'on'.

I think, like most questions asked on this forum, the answers get very technical very fast and totally confuses the poor guy trying to solve his problem. I would think the answers could be super-simplified if we knew exactly what this fellow is trying to control. A simple schematic (or wiring diagram) would solve his problem if the desired results were known. -- Just a thought.

The relay will be used to supply 12V to a HHO (Hydrogen Oxigen) generator To suplement auto fuel. The 12V is controlled from the negitive side with a current limiting PMW. My turning on and off the positive Voltage will be with a 3V input. The only question was would my found AC relay with DC input work and what the conversion was to get DC current from AC. The relay is rated 25A 240 V. If it don't work I'll get the real deal online. I have been able to follow most of the chatter and thank you all for your help. If your interested in the outcome let me know.

Bye

Tom

You're welcome. Sure, post something and let us know how you made out.

OK, so in case you didn't get it, no you will not be able to use your AC SSR for that purpose. You need a DC SSR, and they are different.

Now as a separate subject, has anyone explained to you that you will consume more energy in separating Hydrogen and Oxygen that you will gain in using it?

Something I use is the International Rectifier IR 3310. It has current limiting determined by a resistor value, and an active low control line. It is available through Digikey (their part # IR3310-ND). In one quantity it is 5.38 each. I believe you can get info on this through www.irf.com . I think it is a cute little critter, and it should handle your 20 amps easily.

Sincerely

Bill