# Shorting Triac gate to ground doesn't turn it off

In the below circuit, 170 is a Q401E3 Triac: http://www.mouser.com/ds/2/240/E2Triac-18233.pdf

The capacitors are ceramic disc.

The Triac is rendered normally conducting when S1 is open.

When I close S1 the Triac is still conducting when I don't want it to be.

I'm not sure what is wrong or how to fix it; would appreciate any help. The only thing I can think of that is happening is that the path from Engine Magneto to ground through the S1 switch is of a lower impedance therefore making the Triac useless in the circuit below. The point of the circuit was that when S1 closes, it allows the spark plug to ignite.

Edit: circuit based on patent US5190019 https://www.google.com/patents/US5190019

• That's not shorted. There's still a 1k resistor in there, now in parallel with a 470 that's always effective. So the switch has far less effect than you think it does, based simply on that. – AaronD Jun 29 '17 at 4:27
• Also, that looks suspiciously like it was drawn for a patent application. Did I guess right? Patents are not required to work; only that no one else has applied for that idea yet. – AaronD Jun 29 '17 at 4:28
• Yes, It's from a patent but it was also produced in real life so I can't figure out why I can't get it to work. – klcjr89 Jun 29 '17 at 4:31
• The switch does not turn off the triac, rather it prevents it from turning on. – Spehro Pefhany Jun 29 '17 at 4:49
• @SpehroPefhany In testing it doesn't seem to based on the patent. – klcjr89 Jun 29 '17 at 7:32

The off-circuit magneto will be pulsed on and off by the engine points or equivalent. When switch S1 is closed, this will prevent the magneto from firing the next time the points open.

This works because the the magnetic field from the last points opening has collapsed, bringing the current through the triac to zero. Now the points open again, raising the voltage on the magneto primary. If switch S1 is open, the gate is fired from this rising voltage and the triac turns on. If S1 is closed, the rising magneto voltage is shorted to ground so the triac will not fire. As a result, there is insufficient current developed in the magneto primary to generate enough voltage on the magneto secondary to fire the spark plugs.

• There is no points in the system, and also the circuit isn't working. – klcjr89 Jun 29 '17 at 11:04
• For aircraft, I should have said "breaker" in the primary field. Same concept. If you have neither, perhaps you could explain your magneto circuit. – Glenn W9IQ Jun 29 '17 at 11:15
• If you are attempting to use this circuit with an electronic ignition, it will not work. – Glenn W9IQ Jun 29 '17 at 11:23

You have misunderstood the circuit description. S1 represents the "points" in a conventional ignition system, and it's the opening of the contacts that initiates a spark.

In any case, a triac cannot be turned off via its gate anyway. The only way to turn it off is to allow or force the current through the main terminals to fall below the device's holding current value.

• No, S1 is not the points, it represents the operator seat switch. – klcjr89 Jun 29 '17 at 4:43
• Whatever. I shouldn't have had to guess about the purpose of the circuit. The second paragraph still holds. If your triac is not turning off, it's because you're passing a DC current through it. Magnetos only generate AC, so you must be trying to do something that's outside the scope of the patent. – Dave Tweed Jun 29 '17 at 4:51
• You can look at the patent yourself: google.com/patents/US5190019 – klcjr89 Jun 29 '17 at 4:53
• I couldn't look at it before now. You should edit that link into the question itself. – Dave Tweed Jun 29 '17 at 4:54
• I didn't say that. Read everything above carefully. What exactly do you have connected to the "ENGINE MAGNETO" terminal of the circuit? – Dave Tweed Jun 29 '17 at 11:19