Triac Outputs

[machiv44]

Three of the outputs on my 1747-OA16 have voltage (113.8VAC - 116.6 VAC) present at the terminals even though the outputs are not energized. These outputs are wired to three pilot lights, all of which are faintly illuminated with the outputs off. When I energize the outputs, the measured voltage is approximately 117.5VAC and the lights are brightly illuminated.

Is it possible that the output card is leaking voltage at these three terminals? If so, why not at the other pilot light outputs (there are 5 total)? If the problem isn't leakage, is the card bad? Any suggestions? Thanks in advance.

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[Rube]

All is well! This is normal--do a little research on triac outputs and you'll understand them better. I would try to explain but I'd probably not do you any favors.

 

[keithkyll]

A Triac is turned on by putting a small current on the Gate. When switching inductive loads, the back EMF (as well as other electrical noise) can leak back through the Triac to the Gate. If this noise is strong enough, it will cause the Triac to self-trigger. In effect, it latches on.
In DC, you put a diode across the coil to cancel the back EMF. In AC, a "Snubber" is used. This is a resistor and cap in series. 100 ohms and .1 uF is typical. The Snubber causes about 4 ma leakage.

With Triacs, a Snubber is always needed. 40 Amp and larger SSR's use back-to-back SCR's instead of triacs. SSR's don't have a big problem with self-triggering like Triacs, so the Snubber is optional.

Connect another load to your outputs to bypass the leakage. A 33K, 2 Watt resistor is needed if your indicators are Neon. It will run a little hot when the output is active. Try 100K and higher if your lamps are incandecent. You just need a little more load to make the lamps invisible.

 

[Ron Beaufort]

and for any newcomers in the audience, don't forget the safety issues ...

The following was originally posted in an earlier thread ... since we're talking about triacs here, I thought I'd dredge up this "safety related" material for those who might be new to the forum ...

the following quote comes from Allen-Bradley publication SGI-1.1 - April, 1990 “Safety Guidelines for the Application, Installation and Maintenance of Solid State Control”

Paragraph “C.2.3 Off-State Current” says in part:

Off-state current is also referred to as leakage current in the literature. A solid state “contact” is a solid block of material which is switched from ON to OFF by a change internally from a conductor to an insulator. Since a perfect insulator does not exist, there is always some leakage current present as long as voltage is applied to the device. The presence of leakage current indicates that OFF does not mean OPEN. The reader is warned that simply turning a solid state device OFF does not remove the possibility of a shock hazard.

So here’s a scary story -

A technician needs to work on a motor starter - nothing fancy - just a plain-old everyday three-pole contactor with a 120 VAC coil. And incidentally, the coil is controlled by a solid-state triac-type AC output module on a PLC system. So the technician makes durn sure that the 480 VAC, 3-phase line power is disconnected - and locked-out and tagged-out - and checks with his meter to make sure that the juice is really off. Good - so much for the high voltage stuff.

Now the 120 volt AC coil voltage can be easily controlled by the PLC - so the technician just forces the proper output off - and visually confirms that the coil drops out. Yes, indeed the coil is de-energized - and the output module's LED for the coil’s output is off. And a meter test across the coil terminals indicates that there is less than 1 VAC showing up there. Well, that’s probably just a false "inductive pickup" kind of reading - maybe coupling in from nearby wiring. Anyway, less than 1 volt isn’t going to hurt anything. So now it’s OK to climb in amongst the wiring and fix the contactor - right?

What’s wrong with this picture?

The contactor coil is “loading” the circuit - and “draining off” the leakage current - and so the meter reading looks acceptably low. But a SERIOUS problem shows up once the wiring to the coil has been disconnected. Then there will be NO load on the output circuit - and the voltage will “float” up to its applied voltage level - 120 VAC. Now ... if the technician just happens to brush against that coil feed wire ... this could get very ugly. Especially if the technician happens to be wet with sweat and making good contact with something grounded.

The fact is - to the technician’s skin - that trickle of “leakage” current is going to feel just like a 120 VAC live wire - EVEN THOUGH THE PLC OUTPUT IS OFF! Now some people will say: “Well, that’s going to be a very low current signal - not enough to really hurt you.” Yeah - maybe so. The shock might not hurt you - but it can sure make you hurt yourself.