Triac Output Module

Hi all,
Great site and looking for some help.Doing my Electrician exams and PLC's are a topic on it.Heres an exam question from a past paper i cant find the answer to.Any help would be great!!

Q:
Explain the operation of a triac output module for a programmable controller and explain why the current drain must exceed its holding current.

If someone knows could they keep the answer somewhat condensed.
Many thanks,
Stephen

feno, Welcome to the forum.

A quick google search using these terms triac output holding current revealed (in the first10 results) this article

http://www.intusoft.com/articles/triac.pdf

We dislike doing students homework. Here is a post from a former student and now contributor to this forum.

http://www.plctalk.net/qanda/showthread.php?t=12271&highlight=traffic

In this thread he describes the right way to address this forum.

feno said:

Any help would be great!!

Click to expand...

I'm not being brusque but, why don't you Google the following terms and see what you come up with?

1. triac

2. holding current

3. solid state relay

Ok, first you have to get a grasp of a thyrister, which is a gate that allows very high levels of current. A thryrister only allows current in one direction (anode to cathode) and that current must be high enough to cause a phenomenon called "avalanch current" or as you termed it "holding current". You will have to talk to a physicist if you want to know the reason for this particular behavior of free electrons. But basicly as I understand it you need a particular amount of positive voltage, and thus current, from anode to cathode in order to brake the valence or free electrons free in the material. For reasons totally beyond me the thryister will continue to conduct even after the gate voltage is taken away. That is until it is reverse biased with positive on the cathode and negative at the anode and the gate is turned off.

A triac is two thyristers pointing in opposite directions, one's anode to the other's cathode and visa versa (in parallel but opposite directions), but controlled with their bases or gates tied together as a single on/off control. So when the gate is turned on, one triac conducts during the positive half cycle and the other during the negative half cycle. When it is off it acts as a perfect insulator.

So in short and in answer to...

"Q:
Explain the operation of a triac output module for a programmable controller and explain why the current drain must exceed its holding current."

A: A triac output module operates as a solid state switch capable of conducting AC current. The current drain must exceed the holding current or the thyristers from which the triac is made from will not switch into conducting or on mode.