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triacs and triggering

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IamSmooth

Sun Feb 24 2013, 05:54AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

I hate to make a brand new thread, but I wanted to get an answer. I am researching triacs, which I have never used. I have read about them. I want to turn an AC signal on/off every minute to an inductive load. I plan on getting an alternistor (snubberless).

Do these work well, or should I just add an RC element?

All these triacs have low maximum gate voltages. They also list maximum gate currents. I plan on using a 5v digital output to control the gate. So, I'm looking at the data sheet of Q4008LH4. The maximum gate voltage is 1.3v; the maximum current gate is 35ma. Should I just make sure R is big enough that 5v/R < 35ma? In this case I would use R = 200 ohms.

Also, if my gate signal is always positive, will the triac conduct for both the negative and positive AC cycle?

BigBad

Sun Feb 24 2013, 09:31PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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Provided the gate is given current, then the Triac will turn on at the beginning of each half cycle.

So called 'snubberless' Triacs may or may not need snubbers, depending on the situation. If you get this wrong, all that will normally happen is that the Triac will stay on all the time, and you will need to modify the circuit to add RC components.

klugesmith

Mon Feb 25 2013, 12:54AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

@smoothie: Be careful not to misinterpret the minimum and maximum values on the datasheet.

For example, "maximum gate current" probably does not mean you need to stay below that value, to avoid damaging the device. Unless you are reading the Absolute Maximum Ratings section, where typical values are on the order of an ampere.
Generally it means the highest gate current required to guarantee triggering. The trigger current of a specimen with minimally sensitive gate = most demanding gate. Remember, unlike in a MOSFET, is it the current that activates a triac or SCR.

Triacs can be triggered to conduct in either direction, with gate current in either direction.
The four sign cases are generally unsymmetric with regard to gate current requirement. Inadequate drive strength might trigger the triac in only positive or only negative half-cycles, as I can personally attest. That would be bad if it is switching the power to a transformer or AC motor.

Are you intending to have any isolation between your 1-minute timer circuit and the mains? There are optoisolated triac drivers available, random phase or zero-crossing.

[edit] In your example, a resistor value of 200 or even 100 ohms would not guarantee triggering in a worst-case situation. If gate voltage were max (1.3 V) and current requirement were max (35 mA), a 100 ohm resistor would need 3.5 volts across it. Thus you would need at least 4.8 volts at the digital logic output, as it is sourcing 35 mA. Pretty unlikely for 5 volt logic, even if its supply voltage were nominal.

@bigbad: Congratulations. I bet many readers would like to know what triac you are using, and what kind of heatsink. Is the gate driven directly by an Arduino output port?

BigBad

Mon Feb 25 2013, 03:53AM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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Posts: 600

I got my arduino-controlled triac working just today with a kilowatt resistive load. I haven't tried an inductive load yet, I need to wind a coil.

BigBad

Thu Feb 28 2013, 01:37AM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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Wound a coil and potted it, seems pretty solid...but (of course) my circuit doesn't work properly with the inductive load, goes on and stays on.

So I added some snubbing... still doesn't work properly.

Put it back on the resistive load, perfect!

(grumble)

I need to hit the books and data sheets harder!!! (Except it turns out none of my dead-tree books cover triacs- doh!!!)

Useful resources I've found so far include:

Wikipedia:

Example circuit (from here) which is very similar to the one I'm using:

20schematic

I'll probably upload my circuit if/when I get it working, although it's a very, very eccentric maglev system because I have a subtle and definitively ambitious plan that needs it (rather than it being an end in itself).

IamSmooth

Thu Feb 28 2013, 03:08AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

put a resistor in your snubber. I just see a capacitor.

radiotech

Thu Feb 28 2013, 08:10AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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Triacs are commonly triggered with a Diac in series with the gate terminal. If someone is
clever enough to find GE application note 200.35 3/66 , on line you can get all 21
pages from 1966, when thse things were popular.

Steve Conner

Thu Feb 28 2013, 10:35AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

0.1uF and 100 ohms is something of a classic snubber combination.

The issue is: The triac turns off at zero current, but with an inductive load, that's not zero voltage. As soon as the triac turns off, the voltage across the inductor collapses to zero, and the voltage across the triac jumps up to whatever the line voltage is at that instant. If the rate of rise of voltage exceeds the triac's critical dV/dt, it turns straight back on again.

The snubber capacitance fixes this by reducing dV/dt. Driving the gate from a low impedance also helps by increasing the critical dV/dt. Seems like your circuit has no load on the gate at all when the optocoupler is off, so you might want to add a resistor between gate and MT1 to load it.

Patric

Thu Feb 28 2013, 11:01AM

Registered Member #2899 Joined: Wed Jun 02 2010, 06:31PM
Location: Deinze, Belgium
Posts: 255

This works perfectly for me. It regulates a MOT from 500 to 1500 Volts:

BigBad

Fri Mar 01 2013, 04:55PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

Thanks guys, I've snubbed it as best I can, added in 100 ohm resistor and it's half working, i.e. switching properly but only half wave, the other half wave is on all the time.

Dratted thing!

I'm not sure if I've undersnubbed it, and I'm hitting some of the asymmetry you often get with Triacs or whether the triac is partially blown.

But it worked OK with resistive loads, so I'm assuming it's still not enough snubbing.

It'll work, or it will die with me trying!

I've got a spare Triac anyway.

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