downside of using a Wiggins-type tester on PLC systems ...

yes, a Wiggins tester will indeed provide a load to a TRIAC output – and thus prevent a "high" reading which could (incorrectly) make it APPEAR that the output is constantly "stuck on" – even when the output circuit is perfectly ok ...

here are a couple of previous posts (both in the same thread) related to the downside of using a “wiggy” tester on PLC systems – particularly on 120VAC output modules which are based on TRIAC devices ... (suggestion: read the whole thread if you’ve got time) ...

http://www.plctalk.net/qanda/showthread.php?postid=12803#post12803

http://www.plctalk.net/qanda/showthread.php?p=12891&postcount=19

and there’s a link in the following post that shows the Fluke SV225 “Stray Voltage Adapter” ... this little device will pull an “off” TRIAC output down to about 6 volts ...

this is a lot safer (for the equipment) than using a modified "ignition coil" for testing – which is basically what a Wiggins-type tester amounts to ...

http://www.plctalk.net/qanda/showthread.php?p=224940&postcount=5

party on ...

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THanks for the info Ron. But the thing I dont understand about triac ouputs is that why are they on even you make the output off. Isnt that dangerous? Triac are just switches so they should go off when gate voltage is removed or current current falls below holding current. Also why triacs have a better response time then relays? Since triacs take some time to go off .Shouldnt be the opposite .Please explain

You will have to go back to the TRIAC semiconductor definition (http://en.wikipedia.org/wiki/TRIAC); a TRIAC is a current switching device.
When you turn a PLC TRIAC output off you will impede the capacity of driving a "load" (by restricting current flow), however, the (leak) voltage is still "high" from any Voltage Test Setting DMM's point of view.
A Relay type PLC output (dry contact) is an electromechanical device which will have to fight mechanical forces in order to change states; since semiconductor "switch lag" is measured in microseconds it is at least one order of power ahead of the electro-mechanical devices' "agility".

bornwild,

you must remember that a triac is an electronic switch not a mechanical switch like a disconect.

the electronic structure of a triac will allow the flow of energy until a large enough load is placed on the unit to cause the triac to turn off. this is due to the composition of the triac itself.

yes, they are dangerous. i got lit up when i used a wiggy and it said off and then tried to connect to that terminal.
that's how i learned about them the hard way.
that's why we try and put warning signs on the enclosure doors and train our electricians.

regards,
james