Triac with DC gate control?

I was down at a local electronic components store when I saw a triac that claimed to be able to be triggered with DC voltage. Always the curious guy, I picked one up and tried it out with no success! lol. I'm figuring I'm doing something wrong here. In my niavity I thought that I would wire the positive lead to the gate and then there would be something internal that would allow the DC to go to ground regardless of which way the AC current was going. *CLEARLY* this was wrong, but could some explain the why or how to make it work? I'm guessing that ultimately my best solution is to swap to an opto-isolator which premits DC voltage as the trigger and AC voltage on the high side.

johnbeetem over 13 years ago

Have a manufacturer and part number? There are opto-triacs which use a DC voltage (and return) to control a triac. They're used in a recent Elektor article: "Thermometer using Giant Gottlieb Displays" ( http://www.elektor.com/magazines/2012/april/thermometer-using-giant-gottlieb-displays.2114766.lynkx ).
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Former Member over 13 years ago in reply to johnbeetem

I'm confess that I'm uncertain. It wasn't an opto-triac I don't think. It was a TO-202 form factor so just three pins.
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Former Member over 13 years ago in reply to Former Member

Basically you can't use that. TRIAC stands for "triode for alternating current". Trick is in the 'Alternating Current'.
The TRIAC is close to the Thyristor so I'll add that as well:
The basic problem with a TRIAC and a Thyristor is they will only switch off when there is not current (which normally means no voltage.)
So if you want to switch a DC signal 'ON' you can do that: ONCE! To switch off you need to turn off the power to the circuit.

You might think: "Simple, I connect an AC signal to the triac/thyristor". But then you have a totally different problem. Digitial logic (and your Raspberry-Pi output fails in that category) is defined as a positive voltage against a ground.
In AC there is not really a ground. A lot of AC signals take 0V as ground an then you define your AC as a sine wave of +/- XX Volts. NO GOOD! The negative Voltage (when you AC signal drops below 0 Volts) is a big NO NO! It will damage the output of your PI.
Also voltage is always relative. If your PI output is connected to the control pin and your triac voltage drops below 0V relative speaking your control pin becomse positive (As far as the triac/thyristor is concerned).
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Former Member over 13 years ago in reply to Former Member

hmm... I had a feeling that it was too good to be true, oh well. I'll just change my opto isolation from DC on both sides to DC on the low and AC on the high
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