How to prevent triac short energizing loads?

unsaint32

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I know it's a simple question, but I did my thread search on this and googled it but came up with nothing, so here I am posting this seemingly rudimentary question.

If an output module's triac is shorted, (let's say the LED for the output terminal went off) the output load will remain energized creating a dangerous situation. What kind of safety measure can be made to prevent this?

Thanks.
 
The basic step is that the output power for that PLC module has to run through the Emergency Stop circuits.

E-Stops are an enormous kettle of fish, but the basic idea hasn't changed: the output power gets interrupted when the button (or pull cable, or light curtain, etc) is pushed.

It's hard to diagnose a shorted triac without using a separate input module, which is why diagnostic DC modules have shown up on the market in recent years.
 
just to be sure we're talking about the same thing here ...

are you talking about a TRIAC which is actually DEFECTIVE/SHORTED - or maybe just that the load on the TRIAC is too low - and so enough "leakage current" is flowing through the TRIAC to keep the output energized - even when the PLC is turning the output OFF? ...

reason for asking: the "leakage current" problem is a VERY common situation which quite often causes people to assume (usually incorrectly) that the TRIAC is faulty ...

yes, sometimes TRIAC outputs DO indeed go bad - but if the load device is a very small one, then you should investigate the possibility that "leakage current" might be causing your problem before you condemn the output module ...
 
Ron Beaufort said:
just to be sure we're talking about the same thing here ...

are you talking about a TRIAC which is actually DEFECTIVE/SHORTED - or maybe just that the load on the TRIAC is too low - and so enough "leakage current" is flowing through the TRIAC to keep the output energized - even when the PLC is turning the output OFF? ...

reason for asking: the "leakage current" problem is a VERY common situation which quite often causes people to assume (usually incorrectly) that the TRIAC is faulty ...

yes, sometimes TRIAC outputs DO indeed go bad - but if the load device is a very small one, then you should investigate the possibility that "leakage current" might be causing your problem before you condemn the output module ...
Click to expand...

I meant the faulty TRIAC, but my question actually applies to either case since both cause unintended load energizing.

But I hadn't thought of the leakage current. Does the leakage current level depend on the load device size? The device's resistor size?

Or can I adjust the module's leakage current level?
 
unsaint32 said:
I meant the faulty TRIAC, but my question actually applies to either case since both cause unintended load energizing.

But I hadn't thought of the leakage current. Does the leakage current level depend on the load device size? The device's resistor size?

Or can I adjust the module's leakage current level?
Click to expand...

No and no you can't adjust it. The leakage depends on the solid state device itself, the manual will tell you how much leakage there is.

Most inductive loads will not energize, lights, resistive loads can be a problem.

If you don't have a diagnostic module as Ken pointed out you may need to have feedback from whatever the loads controls, pressure, movement etc..
Then feed that back to an input, the PLC logic should know when the load is supposed to be on/off then create logic to monitor the loads control function (pressure,movement) by way of the extra input and take action when it is not supposed to happen.
 
Last edited:
Take temperature controllers, for example. The temperature controller has an alarm output. Typically, a relay. Use the relay or a contactor (driven by the relay) in series with the Triac output. During an alarm condition (overtemp or open sensor), the relay will drop out, disconnecting the AC supply to the triac.
 

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