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July 10, 2024. Subthreshold Conduction

Subthreshold Conduction! What is it? In MOSFET devices there typically is a minimum threshold voltage necessary to have the device turn on. But there can be some "turn on" below that critical voltage. (Subthreshold Conduction) 

Even in standard transistors a small portion of current leaks (collector to emitter) even when they are technically switched off. When that leakage becomes significant things can happen!

[Simple Analogy: Mary Jo is "warmed up" without 1st having the Bob's Big Boy Hamburger]

Simple relay circuit activated from an Arduino. Depending on the actual relay used the 2N3904 may be underrated which could cause it to fail or have excessive leakage current. Use a TIP31C. If the worry may be stray RF activating the switching transistor slip several ferrite beads on the base lead.

Above is a back of an envelope sketch of a relay activated circuit triggered from an Arduino signal. A signal voltage from the Arduino pin turns on the 2N3904 and current flows enabling the relay to close and control various circuits.

A ham friend wrote me about some mysterious happenings where inexplicably the relay on a similar circuit was being engaged without the Arduino signal signaling such an action. The circuit had been working properly and then the issue and it is intermittently occurring. 

There are many causal possibilities for this condition, but certainly an overly leaky transistor has to be in the mix. Could this also be heat related -- my shack is 97F at noon so certainly that could make this an event in summer operations.

In a curious convergence of actions, I am seeing a similar mysterious "turn on" in one of my transceivers which uses solid state switching but no relays. We often assume that just because something is solid-state and low voltage that there never is component fatigue and wear and tear on the devices. Wrong!

So, one more item (leaky transistors) to cause hair greying and thinning!


Pete N6QW

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