Triac outputs blowing up, please explain

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Dec 2010
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Burlington, NC
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I have an application using AB 1771-OAD output cards to drive pneumatic solenoids for diverters on a sorter system. Production wanted a way for the operator to fire the solenoids locally to clear a jam. Running new wiring for inputs was impracticable, so we wired a switch at each valve to energize it (120 Volts). Started blowing fuses (each output fused externally) and eventually blew up two of the outputs on different cards.

What causes that? I checked that the output card voltage source and the switches voltage source were the same. I am perplexed.

I did come up with an alternative solution of using a spdt toggle switch with the solenoid as the common, the card NC, and the 120v line NO.

Thank you for your time.
 
if I'm reading your question correctly, then what some people call the "flyback" effect might be causing your problem ...

it's the same effect that allows you to get at least 20,000 volts for the spark plugs in your car's ignition system - from a 12 volt battery ... (think: "ignition coil") ...

basic idea: when the solenoid is energized, a magnetic flux field is produced ... when the solenoid is quickly de-energized (by opening the circuit with the manual switch) the flux field quickly collapses through the conductors of the coil (think: "generator") - and an incredibly high voltage can be produced ...

the reason that the solenoid coils don't normally blow out the module is because the TRIAC device (the "switching" part of the module) is a "zero turn off" type of device ... in other words, when the PLC tells the module to turn off the output, the TRIAC automatically waits until the AC waveform crosses the zero point ... therefore there is no flux field at that particular point in time - and therefore no "flyback" effect - when you're operating under normal (automatically controlled) conditions ...

does that help? ...

and welcome to the forum ...
 
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Is the new switch circuit powered from the exact same power source, exact same phase of the same transformer, or is the new switch wired to a different 120V source?

If the latter then you are blowing fuses and triacs because you are creating a line to line short between two different phases. If so then you need to rip it out and do it right.
 
Back-Emf = L dI/dt

As ron was supposing.

Small dt lange L and large di will cause a large current spike.
 
I have an application using AB 1771-OAD output cards to drive pneumatic solenoids for diverters on a sorter system. Production wanted a way for the operator to fire the solenoids locally to clear a jam. Running new wiring for inputs was impracticable, so we wired a switch at each valve to energize it (120 Volts). Started blowing fuses (each output fused externally) and eventually blew up two of the outputs on different cards.

What causes that? I checked that the output card voltage source and the switches voltage source were the same. I am perplexed.

I did come up with an alternative solution of using a spdt toggle switch with the solenoid as the common, the card NC, and the 120v line NO.

Thank you for your time.

How about having a "Manual" function, either by selector switch or use a HMI to switch to triac outputs WITH the plc = no more damaged cards?
 
I see that my distinguished colleague Alaric has a different point of view about your problem - and you should definitely start with his idea before you jump on board with mine ...

the part about "blowing the fuses" tends to make me think that the "flyback" effect I mentioned earlier might not be the root cause of your problems after all ... specifically, you can get a WHOLE LOT of voltage from the "flyback" effect (far more than enough to blow a TRIAC output) - but usually not enough current to blow a fuse ...

please let us know how this finally turns out ...
 
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I did come up with an alternative solution of using a spdt toggle switch with the solenoid as the common, the card NC, and the 120v line NO.
Assuming it's not the scenario that Alaric described, I think this is probably the most appropriate solution in your particular case, since you want local control without additional input wiring.

I'm sure you realize this, but the type of switch you use is important. Your 'toggle' switch is probably a 'break before make' style, which is exactly the kind you want in this application. IOW, the NO contact 'breaks' (opens) BEFORE the NC contact 'makes' (closes), and vice-versa. This isolates the NO and NC connections from each other.

If you use a typical pushbutton or selector switch with NO and NC contact blocks on the back, it's possible that the contacts will overlap during the switchover. Some manufacturers offer special 'slow make and break' contact blocks for applications like this.

🍻

-Eric
 
I think the Flyback effect is the one I am dealing with. The 120v came from the exact same source as what supplies the cards.

I am using regular toggle switches which normally break before make, but they have not come in yet (damn UPS). I will let you know how they turn out.
 
I think the Flyback effect is the one I am dealing with.
well, if so, remember that the problems it causes can be EXTREMELY intermittent in nature ... specifically, you could test your system out with perfect results 1000 times – and then fry a PLC output on test number 1001 ... the reason is that the voltage of the AC waveform (sine wave) is constantly reversing its polarity (think: 60 Hz) ... this means that the magnetic flux field is constantly building and then collapsing ... this is normal operation ... consider that as the field builds and collapses, it becomes very weak when the voltage is near the zero point of the wave ...

so ...

suppose that each time you "test" your switching arrangement, the flux field just happens to be in its "near zero" condition ... in that case you'd have very little "arkey-sparkey" effect – and the TRIAC outputs might work just fine ... no problems ... so you put away your tools and sign off on the project ...

time marches on ...

sooner or later (usually around 3:00 am) the switch gets thrown while the flux field just happens to be in a "fully built" condition (it's a random thing) ... oops! ... your TRIAC output could suddenly be subjected to 20,000 volts or more ... that's not going to be a pretty picture ...

incidentally, this "flyback" effect is precisely the same reason that you're not supposed to use "solenoid" type voltage testers (think: Wiggy) on your solid-state I/O modules ...

you might want to take a look at page 8-8 in the Classic PLC-5 Hardware Installation Manual ...

suppressors.jpg
 
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