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TRIACs and Inductive loads

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Munkey

Thu Jun 14 2007, 08:58PM

Registered Member #588 Joined: Sat Mar 17 2007, 11:06AM
Location: UK
Posts: 93

I know that you can't use the TRIAC light dimmers on inductive loads sutch as transformers, I don't know why but you just cant.
I won't be using a light dimmer but I would like to control a AC solenoid valve with a TRIAC, would putting a high wattage resistor say 30ohms at 10w in series with the valve to make it a resistive load work?

Thanks.

Hazmatt_(The Underdog)

Fri Jun 15 2007, 01:34AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

You can't?? Since when?

All the big 3000A and 5000A rectifiers used in Anodizing/Electroplating use primary side SCR back to back (as a Thyristor), and secondary side halfwave rectifiers.

If you're trying to control the flow through the valve, you're not going to get far doing it that way, its just going to buzz and chatter a lot.

You're going to have to elaborate on the purpose.

Munkey

Fri Jun 15 2007, 06:37AM

Registered Member #588 Joined: Sat Mar 17 2007, 11:06AM
Location: UK
Posts: 93

I thought you coulden't ever since I read this

Hazmatt_(The Underdog)

Fri Jun 15 2007, 07:38AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

If I put a conventional 100W dimmer on a 300W NST, yea ..I would expect a problem there.

Let's weigh the situation:

What industry does for ~30 years versus some internet article that a guy wrote while he was goofing around with a dimmer.

You still haven't elaborated on your purpose or what you expect to do.

Sulaiman

Fri Jun 15 2007, 08:53AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Using a TRIAC to phase-control the power into an inductive load is NOT straightforward.
Turning on an inductive load on for a few cycles then off for a few cycles is quite common.
Controlling an ac solenoid valve is quite straightforward;
During the time that you want the solenoid ON apply continuous/repeated gate trigger current
the most common method used to be to apply a squarewave at around 1 to 2 khz via a resistor and small transformer
(Like a GDT that we use for SSTC), something like 50 Ohms and +/- 5V should work for most triacs.
Check the data sheet for your triac for Gate trigger voltage and current requirements.
Put 100Ohms in series with 100nF across the triac to snub any switching transients.

Steve Conner

Fri Jun 15 2007, 09:11AM

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

The reason why lamp dimmers don't work with inductive loads has nothing to do with the triac itself, it's the cheap-ass circuit they use to drive the triac. With proper firing using a technique similar to what Sulaiman suggested, they will drive just about any load. I've "dimmed" things like vacuum cleaner motors and NSTs this way with no problems.

Munkey

Fri Jun 15 2007, 06:01PM

Registered Member #588 Joined: Sat Mar 17 2007, 11:06AM
Location: UK
Posts: 93

Ok, this is what I want it to do, turn on and off a solenoid about 1Hz, it is for a water feature thingy.

And woulden't putting a voltage on the gate for X amount of time then off, when the AC cycle reaches 0 it should tun off why pulsing at a high frequency.... I would be using a opto-isolator (I had my time with GDTs and hated it!)

Thats what I thought, why should it blow it, its the same as passing AC directly through it....

Hazmatt_(The Underdog)

Fri Jun 15 2007, 07:14PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

If that's what you want to do, then its very simple. Just use the Triac to the solenoid like a conventional switch, and trigger the triac using an optoisolator that has a Diac inside. Then whatever timer you want to interface only has to light the opto's led and the Triac will turn on.

Munkey

Fri Jun 15 2007, 09:48PM

Registered Member #588 Joined: Sat Mar 17 2007, 11:06AM
Location: UK
Posts: 93

Done like this......

Right?

Carbon_Rod

Sat Jun 16 2007, 06:57AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

I tend to use setups similar to bacons reference, except I tend to add a DIAC or SIDAC based symmetrical zero crossing driver with MOV protection and various other bits to meet CSA/UL harmonized standards. A 4n35 would be a bad choice in this case, that chip is only really good for DC stuff or H-bridge drivers. Try the MOC3041 series as they are much better for AC systems (the chip notes should explain why in detail with proper driver circuit examples.)

Steve, IIRC for high power switching the non-symmetric output of most simple TRIAC based circuits can become a real problem (as the above design would incur)... I am unsure if even keying symmetric cycle phase chopping would prevent these types of issues after the TRIAC has been fired several times.

Cheers,
=)

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