the wiggy that would not die
Well, looking at post #14, I’d say that Steve D seems to come down squarely in the
I will trust my wiggy any day.
column. And I have no intention of trying to sway his loyalty - I know better than to come between a man and his wiggy. But for those among you who haven’t got your minds quite so firmly made up - you might consider a few ideas. First of all Keith was absolutely correct in post #15 - the reason that the output module can handle a solenoid load - is because the triac device in the AC output module always turns the output off at the “zero crossing point” in the AC power sine wave. So that means that at each and every turnoff event there is little or no built-up energy in the solenoid coil - and therefore no arky-sparky effect back into the module’s output circuit. On the other hand, when a technician uses a wiggy, then the precise instant at which he pulls the probes away from the circuit becomes a highly random event. Yes, he might be lucky for years - with never a problem. Then again, if he just happens to pull the probes away while the AC waveform is at, or near, the maximum - well, then there would be quite a lot of energy stored up in that wiggy’s coil - and we all know that energy is neither created nor destroyed. So where does all of that stored up energy go? Depending on just how the probes break contact, it certainly could go right into the output module’s triac. With that in mind let’s take a look at Allen-Bradley publication SGI-1.1 - April, 1990 which is titled “Safety Guidelines for the Application, Installation and Maintenance of Solid State Control”
http://www.ab.com/manuals/gi/sgi11.pdf
Paragraph “C.2.7 Transient Overvoltage” reads in part: “Solid state devices are especially sensitive to excessive voltage. When the peak voltage rating is exceeded, even for a fraction of a second, permanent damage can occur. The crystalline structure of the device may be irretrievably altered and the device may no longer be able to turn OFF.”
Now I don’t know about you, but that “permanent damage” - “irretrievably altered” - type language makes me a little skittish. Personally, I’d hate to have my boss come to the conclusion that me-and-my-wiggy were the culprits who had been “permanently damaging” and “irretrievably altering” his valuable PLC equipment. There are quite a few knowledgeable people out there who have decided that this wiggy-on-a-PLC issue is something to be avoided. What they say makes sense to me - and so after careful consideration - I believe it. But of course I’m not trying to change anybody else’s mind.
Now Roger brought up a another very good point in post #17.
a wiggie or any other test device I have ever heard of WILL NOT blow up the input card. We just discussed this in a different thread covering optical coupling.
Yes, the optical isolation is definitely a good thing - but still ...
Let’s take a look at Allen-Bradley publication 1746-2.35 - July, 1999 - which is titled “Discrete Input and Output Modules - Product Data” for the popular SLC systems.
http://www.ab.com/manuals/io/1746/1746-td006b-en-p.pdf
Page 50 lists the Environmental Specifications for these common I/O modules - and the specification for “Isolation” is given as “1500 Volts”. A footnote denotes that this specification refers to “Electro-optical isolation between I/O terminals and control logic.”
Now this points out something that we already know: The opto-isolation of a standard input module is NOT intended to protect the MODULE’S INPUTS against higher-than-normal input voltages. Rather, it is intended to protect the PLC’s internal logic circuits from these higher voltages.
And obviously Roger didn’t mean to imply that we could impudently connect a higher-than-specified input voltage to an input module - and then count on the opto-isolation feature to protect the module from “blowing up”. For example: Take an input module rated for 120 volts and connect a 240 volt signal to it. You can kiss that particular input circuit - and all of its fancy opto-isolation - goodbye. Of course Roger knows that - and he said so when he qualified his statement and referred to damage from “a wiggie or any other test device”.
But then what about that published “1500 Volts” rating for “Isolation” - what does it really mean? In simplest terms it means that “optical isolation” is good - but (like just about every other form of protection) it does have its limitations. The book is telling us that once we get up over 1500 volts - then all bets are off. In “extreme cases” - where levels higher than 1500 volts are concerned - then the opto-isolation system just might let a spike pass on through the module - and then right on into the PLC’s logic circuits. That doesn’t sound good does it? What would a high voltage spike DO to the PLC anyway? We don’t know - and we don’t WANT to know. But whatever happens, I can just about guarantee you that it won’t be something good.
Now the question remains - are we SURE that the wiggy is NOT going to give us any more than the specified 1500 volts? And - before you answer - remember that the ignition coil in our car gives us an output greater than 20,000 volts - while using just the 12 volt input of our battery. Now just how much of a spike do you think we might be able to generate with our wiggy - using an input of about 120 volts? Place your bets, gentlemen.
Well, I don’t know about you, but I’m tired of wiggies - at least for now. What follows is something that came to mind while reading about the “leakage current” issues in some of the earlier posts. It’s safety related and comes from the same publication I listed above. Allen-Bradley publication SGI-1.1 - April, 1990 “Safety Guidelines for the Application, Installation and Maintenance of Solid State Control”
http://www.ab.com/manuals/gi/sgi11.pdf
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 confirms that the coil drops out. Yes, indeed - the 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” 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.