PLC to control TRIAC

[ei_ei22]

Hi, all friends,

>>Can we use PLCs to control TRIAC?
>>What kind of output do we need to control a TRIAC?



Note : TRIAC is a very convenient switch for AC circuits, allowing the control of very large power flows with milliampere-scale control currents.

Pls reply as soon as you can. thanks.

 

[panic mode]

you can get plccard with triac outputs...

do you want to just turn something on and off or you want to do phase angle control?

 

[Stephen Luft]

A triac can most certainly be used with a PLC. In fact, many PLCs offer triac outputs modules. Our first design was with a triac, so they have been used for quite some time.

A triac is used as a solid state switch for an AC device. The DC version would be a transistor.

We did an article that compares a relay output to a triac output and the triac output of various PLC manufacturers.

www.entertron.com/press5.htm

Hope this helps with your question.

God Bless,

 

[Jiri Toman]

Below quotes are taken from Entertron's web site:

A triac output is an AC output. It has no moving parts to activate, therefore the potential for it to fail from life expectancy restrictions is nearly non existent

I for one disagree with this. Every PN junction wears down each time it operates. LED's eventually get dimmer and triacs will start leaking and eventually fail.
While triacs will offer longer life than relays there are restrictions on life expectancy.

As mentioned earlier, a relay requires approximately 8 - 10 milliseconds to turn on. For a triac to turn on, requires less than 1 millisecond

As far as I know a triac is turned on during the zero cross over.
This means that a maximum turn on time can be as much as 16.6 ms.
On top of it the turn on time is not deterministic it could be
anywhere from 0 to 16.6 ms!
If you need fast and deterministic turn on time you need to use DC not AC!

A relay is rated to operate for approximately 1,000,000 mechanical cycles.

You can get relays rated at 10 000 000 cycles!


On the other hand Stephen you forgot to mention disadvantages of triacs:
1. Triacs do not have 0 impedance in the OFF state, therefore there is a current leakage to consider. This OFF state current leakage in a large triac like yours could be substantial.
I didn't see a spec in that regard but I would assume that 5 to 10 mA leakage would not be out of the question. Relay will not have any OFF state leakage!
OFF state leakage matters in terms of safety and also
in terms of operation. I have had an instance not so long ago where Continental Industries solid state relays (Triacs) were leaking so much that I had to introduce a pull down resistor!

I also had instances in plating and injection molding applications where leakage caused
a serious safety issue. I had to provide relay contacts for
complete isolation.



2. Issue of flexibility. With triac you can only use AC devices.
Relay will allow you to mix and match voltages.

 

[Jiri Toman]

I do have one suggestion for Entertron. Look into using MOSFET switches on your outputs. These can be used either on AC or DC and have very large OFF state impedance i.e. the OFF state leakage could be as small as 0.1 micro Amps.

I don't know what all the implications are in as far as MOSFETs go.
It could be that there are problems I am not familiar with.

 

[panic mode]

triacs (and thyristors) can be turned ON any time (like a latch), they turn OFF during zero crossing (removing power acts as unlatch)!
by controlling turn on delay it is possible to control duty and vary load (that's what every AC dimmer does for example). to do this one needs to know when is zero crossed, and start counting delay.
no delay = full sinewave
delay that is just a bit shorter than halfperiod of the sine = no power (almost).

 

[panic mode]

MOSFETs have rather small operating voltage, specially when compared to SCRs and triacs. High spec devices would be more expencive too (check IGBTs). but how oft you need to use AC outputs? I didn't need them for some 10 years.

 

[Jiri Toman]

I stand corrected the turn on time is only 8.3 ms not 16.6ms.

MOSFETs have rather small operating voltage, specially when compared to SCRs and triacs

120 VAC or DC is no problem for a Mosfet. I have actually designed and used on many applications a custom Mosfet switch PC board.
Mosfets I am using have no problem with 120 VAC or DC.

The idea with a zero cross over is so that you deliver the full voltage. Yes you can switch it on at any time but you will create a chopper. We are talking about a discrete ON / OFF output (not a dimmer), therefore you need to be at full voltage when turned on otherwise you might not deliver enough power on the first half period of the sine wave to turn the output device on!

 

[Jiri Toman]

zero crossover

Actually the primary reason for zero cross over switching is as follows:
Electronic controllers (such as PLC's) usually switch the loads at the AC voltage zero cross over point which eliminates voltage surges. In PLC controlled systems, a voltage surge would be caused only when another input device (i.e. push button or limit switch) energizes the load.

If electronic controls (such as PLC's) are being used to control a load that is not being switched on at its zero cross over point, surge suppression should be use on the switching device by attaching a surge suppressor directly across the coil terminals.

 

[TConnolly]

Back to the original post:



The answer is yes. However how you are going to do it depends on what exactly you are trying to do.



1)Is the load current low, less than 1 amp? Use a PLC card that has triac outputs built in.



2)Are you switching heavy loads and using the triac as a solid state relay with simple on/off control? If so you simply wire the triac gate to a discrete PLC output with the proper voltage for the gate. But here is where it gets trickier. If the PLC discrete output is a solid state output then it will have a leakage current even when off and that leakage current is likely high enough to gate the triac. You will require a resistor network on the plc output to drain the leakage current to a level below the gate threshold. If you use a plc relay output then leakage current is not a concern. Solid state relays incorporating a triac in an industrial package configured for easy mounting are commercially available. The commercial units often have an internal device for draining the leakage current from solid state PLC outputs.



3)Are you using the triac as a phase angle power controller and not just a simple on/off control? To do this you will require external circuitry to measure the incoming AC sine wave and gate the triac at the proper angle. Commercial units to do this are readily available, allowing you to soft start the load and ramp up to a desired output, thereby varying the AC Vrms to the load. Most commercially available units of this type accept an analog command from the PLC of 0-10Volts or 4-20mA.



A little more information about the application will be helpful in providing you a more exact answer.





A note of warning about triacs: Any device is prone to failure and the triac is no exception. However, triacs almost always fail in the ON state, meaning that the device it is controlling will be left in an engergized state. You must account for this in your system design and you may not use a triac as the primary control element on any device that poses a safety hazard should it be left energized.

 

[Stephen Luft]

Jiri,

All information related to that article is specific to Entertron's solid state design and experience over 27 years. Varying results could also be affected by the circuit design.

There are two comparisons being done in the article. A PLC relay output compared to a PLC triac output and our design vs. other PLC triac designs.

Yes there are other issues that could have been added but that wasn't the emphasis of the article.

Although we didn't specifically emphasize leakage current, for which our triac output does emit a minimal amount, the emphasis of the article was reliability over time of a triac output compared to a mechanical relay output. Even with relays that can handle 10,000,000 mechanical cycles, the life expectancy of a solid state output is still far greater.

Yes, devices can fail earlier than their rating. The potential for a relay to wear out before a solid state output is far greater because of the armiture.

With regards to your suggestion of a mosfet, I have no experience with them so cannot say one way or another if they would be beneficial or not. Our design is reliable and proven over time. Our circuit design is set up whereby a triac or transistor (NPN or PNP) can be populated on the same board. Mixed I/O can also be accomplished in groups of four outputs tied to one common. (four commons / board)

 

[ei_ei22]

Hi, all friends who answer my question
Panic mode,Stephen Luft,Jiri Toman,Alaric

Thanks a lot for your time and consideration for my question. I really appreciat it.

Thanks again.