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.