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Triac flashing with no gate signal

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Jul 21, 2010
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Hi All,

I am trying to make an AC switch using a triac. The schematic is attached.

The issue is that with no gate signal applied, there is a small leak. The test load is a 10W lamp at 240V. The lamp flashes weakly on and off when no gate signal is applied and turns on brightly (as expected) when the gate signal is applied.

I cannot figure out the reason. Some application notes suggest that exceeding dv/dt values can cause random turn on but that doesnt seem to be the case.

Kindly help me out.
Thanks in advance


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Two problems:

1. when U1 is turned off, the triac gate is effectively left floating. Try adding a resistor between U1.4 and L22, suggest trying 1K.
2. depending on the type of lamp, the leakage current through the snubber network may be sufficient to operate it. At 50Hz it has an impedance of ~31K so around 8mA may flow through it. If you are using a low energy lamp that might be enough to start it but not continuously run it.

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What type of lamp are you using.
Many low cost CFLs will flash when connected to a dimmer. This is fairly common.
I've never seen it on a LED lamp but it could happen too.

The chances are you can remove the triac and it will still flash!

What happens is the leakage current through the resistor and capacitor of the snubber network slowly lets the power supply in the lamp charge up. When it has sufficient voltage to start the lamp it does so and the light comes on. It then tries to draw more current than the RC can provide so the voltage drops again and the light goes out. You really need a more stable load or you could try connecting a resistor across the lamp to leak some of the current away so the lamp never has enough voltage to start working.


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