Outputs: Relay, Triac & MOFSET

Of the three outputs mentioned above, what are the differences and why would one be selected over another?

Thanks.

Edit: That should be MOSFET.

Relay: a completely isolated output, it can be used as interface between two different voltage levels, or from AC to DC or DC to AC. disadvantages are that the coil uses more current than a transistor device, and it has mechanical moving parts that wear out eventually. Advantages are that relays are available to handle wide power, current, and voltage ranges. The original universal PLC output type.

Triac: An AC "electronic" switch, (two silicon controlled rectifiers joined together) it can be used to switch certain AC voltages, but the input voltage is the same (not isolated from) as the output voltage.

MOSFET: Metal Oxide Semiconductor, Field Effect Transistor, it is commonly used to switch DC voltages. As with any transistor, it has 3 legs. The output shares common voltage with the other two legs.

For all of three output types, you have to match the output device to the load being switched.

and beware the "leakage current" factor ...

and since you're asking about TRIACs, you might want to take a look at this old post – particularly the part about the "Scary Story" ...

http://www.plctalk.net/qanda/showthread.php?postid=12891#post12891

when I make this particular demonstration, my students ALWAYS take notice ...

Lancie,

Thank you for your definitions of the three outputs. Most of what I have dealt with are relay outputs. I did try to replace a relay card with a triac card once but it didn't work. I'm not sure why other than the leakage that comes from a triac. Never used or even seen a MOSFET.

So much to learn.

Ron,

Thanks for the link -- made for some good reading. I have never used a wiggy and from what I read I don't ever want to. I ran into a similar problem such as you described in the other thread yesterday. I was working on a light in a ceiling fan and had I turned the breaker to the fan off so there was no power going to it. I noticed a neutral wire had come loose and went to tie it back in with the others. When I touched the wire (my hand was also touching the metal frame) I got a little bite. Not much but enough to let me know something was wrong. I looked a little closer and saw a red wire touching the frame. The other end of the red wire was coming out of a small plastic box. I took the loose end and touched it to the neutral. I saw a small spark and then nothing. I touched both wires and everything was OK. I'm guessing the small plastic box was a capacitor of some type and I was getting the stored energy.

Again thanks.

PhilipD said:

Of the three outputs mentioned above, what are the differences and why would one be selected over another? Thanks. Edit: That should be MOSFET.

Click to expand...

The mechanical relays will have only so many on off operations in their life. So if you are switching them frequently like about once a second or maybe minute they may not be best choice. they will also always be either on or off unlike teh solid state devices which can have leakage.

The solid state in theory will never wear out since they have no mechanical moving parts - with frequent switching conditions they will probably give better life than mechanical but of course nothing lasts forever. However they can have leakage when off. In my days of being own instructor and student learning PLC I was going to use neon lamps as outputs - advice here told me not to do it they may not turn off due to leakage - for that reason and a couple others I fired myself as instructor and found me a much better qualified instructor.

If you have not I encourage you to read Ron Beaufort post above and read the post he refers to.

Dan Bentler

Dan,

Thank you for your input on this. I have indeed read Ron's post and the thread he refers to.

Funny you should mention neon lamps. I just bought a Micrologix 1000 (10 inputs, 6 relay outputs) off EBay and was looking at neon indicator lights to use as outputs. Now my understanding is that the relay outputs will not have leakage so neon lights will work OK. I too am both instructor and student so let me know if I need to fire myself. (Ron, though he doesn't know it, is providing me with a great education.)

Edit: Does the MOSFET have leakage like the TRIAC?

PhilipD said:

Dan,

Thank you for your input on this. I have indeed read Ron's post and the thread he refers to.

Funny you should mention neon lamps. I just bought a Micrologix 1000 (10 inputs, 6 relay outputs) off EBay and was looking at neon indicator lights to use as outputs. Now my understanding is that the relay outputs will not have leakage so neon lights will work OK. I too am both instructor and student so let me know if I need to fire myself. (Ron, though he doesn't know it, is providing me with a great education.)

Edit: Does the MOSFET have leakage like the TRIAC?

Click to expand...

IF relays are elecro mech neon will be fine. if solid state then a resistor could be used to load it down and eliminate leak or so I am told - have not tried

Not sure about leakage with mosfets.

Dan

Does the MOSFET have leakage like the TRIAC?

Click to expand...

Not sure about leakage with mosfets.

Click to expand...

All solid state switches have some leakage. The triac is a solid state switch for an AC circuit, the MOSFET is for a DC circuit. You can generally find the leakage current in the specs for the device. For example, the spec sheet for 1769-OA8 module (1769-SG002, page 11) lists the maximum leakage current 2.0 mA at 132 VAC and 2.5 mA at 265 VAC. The same guide, on page 20 lists the max leakage current for a 1769-OB16 module as 1.0 mA.

Don't forget, if you need reading material on just about anything... Wikipedia

http://en.wikipedia.org/wiki/Solid_state_relay

Thanks Steve,

You gave me all three: Good, Fast and Cheap.

http://www.plctalk.net/qanda/member.php?u=11382geniusintraining,

Thanks for the link. I actually did go to wikipedia and read up on MOSFETs. But, dang it, there ain't no one there to talk to. That plus the fact that most of it was over my head.

I will not use any of the outputs you mention except triac for speed in certain situations. Has to be done right.
I use transistors and relays of them - external.
lot easier to change an external relay in the middle of the night than a faulty (not what I am thinking) one on a PCB.