Forcing OFF

[allscott]

Maybe that's why this is a "Fluke". (I always loved the name as much as the products they make.)

Groan

 

[rsdoran]

Fluke tech support etc may not have thought of this for your application, the promo literature talks about stray or ghost voltages: http://www.coleparmer.com/techinfo/techinfo.asp?htmlfile=Fluke-volt-affects.htm&ID=292

 

[Ron Beaufort]

this is taken from the link that rsdoran provided in the post above:

The adapter presents a 3 k load to the circuit under test and thus will dissipate any stray voltage present if the circuit is not energized.

I just dialed in a spare potentiometer to 3K ohms and put that in parallel with my existing Fluke meter ... then I tested the voltage from a 1771-OAD output module (TRIAC type) while the output was “off” ... normally this would give an inconclusive reading of 120 volts - due to the leakage current we’ve been talking about ... but with the 3K ohm load across the meter leads, I read only about 6 volts ...



note: a few years ago I bought an Ideal brand tester for about $70 ... (I don’t have the model number handy right now) ... this one promised “low impedance eliminates ghost voltages” right on the package so I bought it ... but it still reads 120 volts whenever I use it on a PLC TRIAC-type output module - whether the output is “on” or “off” makes no difference ... obviously the “low impedance” it’s bragging about isn’t low enough to solve the TRIAC leakage current problem ...





question for Alaric: you said that you use this TL225 type of tester ... is the 6 volt reading that I’m seeing in my lab experiment consistent with the results you’ve been getting? ... 6 volts is low enough to work ok for my purposes ... the main thing is to demonstrate the difference between testing an output that is “on” and an output that is “off” ... so 120 volts “on” - and 6 volts “off” - is fine for me ... having the device UL approved is the main issue ... the lamp bulb and gator clip leads work fine - but they won’t fly in an industrial environment where someone cares about safety ...



thanks again, Ron ...

 

[rsdoran]

I do not want to look it up but as far as ON and OFF with plc outputs I would think is TURN ON RANGE i.e. if 24vdc then usually you have to exceed 5vdc for turn on, using that ratio (for simplicity) I would think you would need 20vac or higher to TURN ON an output. I think that link shows that a digital multimeter rarely if ever shows 0 (zero) voltage.

This may be something to help assist people with understanding about the devices they are using.

 

[Ron Beaufort]

all I'm really tackling right now is the 120 VAC type of output modules that use a TRIAC to switch the output on and off ... something like the 1771-OAD (PLC-5 platform); or the 1746-OA16 (SLC platform); or the 1756-OA16 (ControlLogix platform) ... I use all of these in my various "hands-on" classes ... I'm just trying to demonstrate a "SAFE" procedure for making a conclusive "good/bad" test of the individual outputs ... I use the 24 VDC modules also ... naturally there are some "safety" issues involved with testing those too - but they're not quite as critical as when working with the 120 volt modules ...

 

[allscott]

I don't remember ever seeing a Triac on an output card fail in the "on" state. Is this common?

I sure wish someone would have taught me in school about the whole leakage current issue. I figured it out the hard way after half dismantling a rack to try and figure out why a bunch of unused outputs were "on".

 

[Ron Beaufort]

I don't remember ever seeing a Triac on an output card fail in the "on" state. Is this common?

I’ve seen a couple do that ... failing “open” is much more common - at least in my personal experience ...

I sure wish someone would have taught me in school about the whole leakage current issue. I figured it out the hard way after half dismantling a rack to try and figure out why a bunch of unused outputs were "on".

I wish I had a dollar for every time that I've heard that one ...

and now just to nail down the ideas ... the “leakage” current that we’re dealing with in this thread isn’t really the same as the “ghost” or “stray” voltages discussed in the documentation that Ron kindly linked above ... the “ghost” or “stray” voltages are due to capacitive (non-contact) coupling between wires that are carrying current - and other (unconnected) wires that run along side them ...



on the other hand, the “leakage current” will appear even if there are NO current carrying wires located anywhere near the tested output ... specifically, the “leakage current” is actually conducted through the TRIAC device even when the TRIAC is turned “off” ... that’s completely normal operation - and has nothing to do with capacitive coupling ...



and regardless of where the “extra” voltage comes from, it sure makes things confusing when you’re just trying to find out whether the output is working ok or not ...



so even though on one hand we’re dealing with “ghost” or “stray” voltages from one source - I’m still hoping that the test instrument which eliminates those voltages will also be able to eliminate (or at least adequately limit) the “leakage current” that we get through a TRIAC-type PLC output module ...