×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Normally-On Solid State AC switching

Normally-On Solid State AC switching

Normally-On Solid State AC switching

(OP)
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?

RE: Normally-On Solid State AC switching

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?

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more." Nikola Tesla
Read the Eng-Tips Site Policies at FAQ731-376  
Member, P3

RE: Normally-On Solid State AC switching

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

RE: Normally-On Solid State AC switching

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.

RE: Normally-On Solid State AC switching

(OP)
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.

RE: Normally-On Solid State AC switching

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?

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more." Nikola Tesla
Read the Eng-Tips Site Policies at FAQ731-376  
Member, P3

RE: Normally-On Solid State AC switching

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

RE: Normally-On Solid State AC switching

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.
 

RE: Normally-On Solid State AC switching

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.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more." Nikola Tesla
Read the Eng-Tips Site Policies at FAQ731-376  
Member, P3

RE: Normally-On Solid State AC switching

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources