Need info on SSR advantage for 220 vac contorl 5

[N3PSO]

Hello;

I need to control a 220 VAC Inductive Load from a TTL/CMOS control.

I always thought that SSRs were designed for this task. But, a co-worker says a mechanical relay driven by a ULN2003 or IRF511 and controlled by TTL/CMOS, would do just as well. I think the mechanical relay solution is very risky.

I need info on the SSR advantage.

Thanks.

 

[nbucska]

SSR: PLUS: don't wear out, fast, needs low power
No spart, less HF radiation.
MINUS: less forgiving about overload, momentary OL
can kill it.

MR: PLUS: can tolerate/survive momentary overload
MINUS: limited life-time, more power to drive it.
Sparking causes more HF.

Plesae read FAQ240-1032
My WEB: <

http://geocities.com/nbucska/>

 

[ScottyUK]

Some triac-based SSR's don't like highly inductive loads - the triac is unable to fully recover its blocking state before the device is forward biased again. Those using antiphase SCR's are much better in this respect. The following link has a fair list of the pros and cons of SSRs. I haven't read the whole article - there is some good and bad in the section I skimmed. Caveat emptor!

http://www.epanorama.net/circuits/semiconductor_relays.html

----------------------------------

Sometimes I only open my mouth to swap feet...

 

[danw2]

I always consider what happens when it fails.

The typical failure mode of an SSR is shorted output - it won't turn off.

What happens to your inductive load when you can't turn off the power?

Thanks, Scotty, for the link. Excellent summary.

Dan

 

[N3PSO]

Thanks, for your replies. All the info was good.
But, I need to know about driving large AC inductive loads from a TTL/CMOS logic control signal.

I thought that leakage currents were more likely in a mechanicl relay. And the leakage currents, even if buffered by a ULN2003, or IRF511, could work their way back to the TTL/CMOS circuit, and zap it.

Even if SSR and Mechanical Relays are equal in safty. I like the SSR for the simplicity of circuit connection.

Thanks again, in advance

 

[itsmoked]

Leakage currents??? You lost me. Where are these leakage currents you're thinking of leaking from and to?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Mike2468]

A TTL circuit can be easily zapped. A ssr should be a much easier load to work with than the inductive coil of a mechanical relay.

Either the mech relay or the ssr will offer isolation from the final 220vac inductive load and your control (ttl) circuit.

There is some leakage current between the ssr and the final load. But that should not impact the ttl part of your circuit.

 

[N3PSO]

When dealing with large AC inductive load, say 220vac 10a, I thought current flowing through a mechanical relay's contacts could generate a magnetic field around the contact.
the relay's coil becomes a secondary of a psudo-transformer.
The AC current could find a path from the relay coil, back to the TTL/CMOS source and zap it.

I thought that such a condition would be happen with an SSR.

So, for this large AC inductive load, am I safer with the TTL/CMOS driving an SSR?

 

[itsmoked]

I see.. Well the phenomenon you describe while possible is not at all likely. There are billions of relays controlling 10's of amps being run by embedded controllers that have no problem with that phenomenon. I would never even consider it. (Under about 80A) My electric range has multiple 10+ Amp mechanical relays run by the micro mounted on the same 8" x 5" circuit board.

Now improper layout and ignoring the inductive kick of the relay's coil will cause you problems. The standard solution is to put a diode across the DC coil (parallel) to it but in the non conducting direction (when energizing the relay).

Mechanical relay's are generally much less expensive then SSRs, and provide no heating issues. SSR's can have serious heating issues and are lossy (have a voltage drop) whereas mechanical relays can be considered to have no losses(in most cases).

Unless you have a rapidly switching application or need the zero current switching of an SSR they often don't make much sense. As previously mentioned SSR's don't even work with large inductive loads but are often destroyed by them.

We can give you more info if you describe your application in more detail. What load? What switching rate? What's controlling the relay?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[N3PSO]

Thanks Flamin, You gave me the answer I needed.

Paul

 

[jraef]

I thought current flowing through a mechanical relay's contacts could generate a magnetic field around the contact.
the relay's coil becomes a secondary of a psudo-transformer.
The AC current could find a path from the relay coil, back to the TTL/CMOS source and zap it.

That sounds to me lke an Electronics Engineers forensic deduction when trying to figure out why his CMOS device died. The phenomenon has nothing to do with it being a contactor and magnetic fields around the contacts, it's all about "inductive kick" of the coil itself, which can be (and is) successfully suppressed with RC snubbers, MOVs etc.

JRaef.com
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