triac leaks

[libra]

tried using a triac relay to switch an ac outlet and found it still allowed 120v without being triggered by any dc.

Bad unit, tried another same thing.

went to the bench and placed one in series with and old ac radio line cord. radio volume was reduced to  trickle but radio still played.

What am I missing, I want to use them in a couple of applications but want complete shut off not just a reduction.

Libra

[wooferhound]

 I have worked with Triacs 30 years. My job is working with professional lighting systems where triacs are used in dimmers and remote control systems extensively. I learned early that when a triac is off, it's not really off. The triac can be controlling an incandescent light and the light will be off, but if you get your fingers across the output the shock will knock you on your butt. It's like there is no current but the voltage is still there. I have controlled strobe lights through a triac relay, when switched on they will strobe normally, but when turned off it would flash about once every minute.

So ,Yes, there is some leakage but normally it is so small as not to be a problem. I have an excellent circuit that uses about 6 parts to make an Electronic Relay out of a triac. I will post it sometime today. It uses optoisolators on the control side to isolate the control circuitry from the AC line.

If the stray voltage from an 'off' triac bothers you too much, you can place a 4 watt incandescent christmas light across the output along with your other loads. It will short out the extra power, like the power that is running your radio.

[libra]

Thanks for the info. I was using a triac to turn on a power supply to charge the batteries when they became too low. Thought that I had made a wiring error.

Also planned using a couple of triac to short out the wind generator in the event of over speeding due to freak high winds. Doubt that I would need it but once burnt feel better to have this option.

So the leakage as a shorting method is not significant when left across the three windings?

Sure was surprised but appreciate the info

Libra

[wooferhound]

I'm trying to reply, the board is working slower than ever today . . .

Yes, triacs would work perfectly fine to short the turbine as long as you don't exceed their ratings. Another thing to note is that a triac is never really fully On. There is some resistance and you will lose a couple of volts because it's a multijunction semiconductor device.

I'm still planning to post my triac relay schematic soon.

[Opera House]

The actual drivers for the triac consume current.  With self powered triac bricks, this can be considerable.  Using a 0.1 to 0.47uF capacitor across the load will provide a no heat load that will drop the load voltage to an acceptable level.  A 47 to 100 ohm resistor should be placed in series with the capacitor.

[rossw]

Triacs and SCRs switch very quickly. What you are seeing is almost certainly the snubber (filter) across the triac that is required in most countries in order for the device to comply with EMI regulations.

These are (in the simplest case) little more than a resistor and capacitor, which WILL pass a small current - which in the case of a very small load (like a careless finger, or a LED lamp) - be quite enough to notice.

The "standard" way to overcome the problem is to use something else in parallel with the load to swamp this leakage. Some use a 5W pilot (incandescent) lamp, some prefer a capacitor. It's not fixing the basic problem though.

[DamonHD]

I use an SCR after the rectifiers as an emergency stop for my (tiny) turbine.

Works fine.

See the circuit halfway down http://www.earth.org.uk/wind-power-pilot-autumn-2007-MotorWind.html

Rgds

Damon

[wooferhound]

OK here is the schematic of the Solid state relay that I have been using for years.

http://www.fieldlines.com/images/scimages/231/TriacRLY.GIF

good for 15 amps and 400 volts as listed and the triac is electrically isolated from the tab so you don't need insulated heatsinks.

[Ungrounded Lightning Rod]

Don't try that with a big turbine.  You'll blow your rectifier.

[rossw]

Hate to tell you this mate, but you have 100nF in series with 100R straight across the triac anodes - so at 50/60Hz and a couple of hundred volts, you'll get quite a tickle if you stick your finger from neutral to your "turned off, switched active".

[youmanskids]

thanks wooferhound, that looks like a useful curcuit. Roger

[wooferhound]

Yeah I did some R&D (Ripoff and Duplicate) when I copied this circuit from some popular disco lighting equipment I was using. I have built 15 of these solid state relays to control mainly lighting but I have switched may things with them. You are correct to say that you can still feel a strong shock even when the circuit is switched off. However a light switched with this circuit goes completely off.