Normally-On Solid State AC switching

[STIDave]

Hi all,

I'm trying to find a method for switching AC up to 400VRMS and a requirement is that if the controller unit dies, the AC will be on, i.e. I need a "normally-on" switching method. Electro-mechanical relays are out because of industry perception of reliability - is there a solid state way to do this?

 

[jraef]

I suppose it depends on your definition of "dies". SCRs when damaged will usually short and become conductors, so technically that would fulfill your requirement. But if the firing circuit "dies" and the SCR is not shorted, there is no way to make them conduct.

What could you possibly want that for?

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[itsmoked]

My question exactly... "When control dies go full on." Now there's an algorythm you don't see often!

 

[davidbeach]

A contactor with normally closed contacts. Control power turns off the circuit.

But why on earth would this be desired is a very good question.

 

[STIDave]

The reason for the requirement is it is a requirement of the customer that if our electronic controller fails (no power), the device we are controlling will default to an "on" state. I've looked at some opto coupled mosfet output relays but haven't been able to find any that meet my 565V pk voltage spec.

 

[jraef]

I would think that in this case, electro-mech is definately the way to go. If you use an Allen Bradley contactor, it is Vertical Lift, Gravity Dropout. You don't get any more reliable than gravity on this planet!

Oh wait, you said nothing about the current rating. Are you looking for a small PCB component or a large power device?

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[STIDave]

Yes, it needs to be a small, PCB mount component. Max current actually switched will be about 4 amps.

 

[itsmoked]

jraef I was lectured long and hard by my "Mech" buddies that "you can never depend on gravity". You can depend on it to be present but not to work your mechanical device.

If you want rock-solid dependability you do not want SSRs. There porpoise is to allow rapid contactless switching with isolation not dependability.


You will have a problem finding a PC-MOUNT 600V relay...

I didn't see any on a quick jaunt.

 

[jraef]

Well you can make a 600V rated PCB switch using an SCR, but the Normally Closed issue is a buzz-killer. Any SCR is going to stop conducting when the AC crosses zero without a gate signal. I can't see a way around that.



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[itsmoked]

Well you could just resistor pull-up the SCR/TRIAC/SSR and then have your circuit pull it down when ever it's happy.