Resistor values with a PUT

May 26, 2010 1 Replies

From what I have read, the gate of a PUT is what determines its threshold voltage, so you would control that with two resistors, one between positive and the gate, and the other between the gate and ground. I put together a circuit using 1K for both resistors, and got a certain behavior (pulsing an LED). I then tried using 2 470K resistors instead, and got a different behavior (basically, nothing). My question is, shouldn't the voltage of the gate be solely determined by the ratio of the two resistors, and not depend on the actual values of the resistors? Or does the amount of *current* through the gate effect something as well?



thanks


The gate must see a low enough equivalent resistance for the internal SCR positive feedback mechanism to snap. So yes, you do need gate current, and you can't get enough if those resistors are too big.

John

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