Simple Semi Stuff

Interesting circuits based on a single semiconductor.

This page is for the exploration of electronic circuit with some kind of non-trivial behaviour.

SSS1: Relaxation LED blinker

The first circuit is based on a property of transistors called “negative resistance”. Normal resistors draw more current as more voltage is applied, in a proportional fashion. This very fundamental property is called Ohm’s Law.

With transistors, however, there is a region where a reverse voltage applied between its collector and emitter will break down in a runaway “avalanche” mode: once this happens, more current will start to flow, as shown in this voltage-versus-current trace on a Curve Tracer (V on X vs. I on Y):

The negative resistance leads to a left-leaning slope, starting at around 7.5V in this case.

This property can be used to create a very simple oscillator, and with suitable components the blinking will vary from a clearly visible 1 Hz @ 10V to > 15 Hz @ 24V. The circuit won’t oscillate with voltages under ≈ 9V:

The 1N4148 diode has been added to protect against reverse voltage. This circuit is easy to build and makes a nice little general-purpose voltage indicator:

The two test points at the top can be hooked up to anything up to around 30V - if it blinks, power > 9V is present. The blink rate gives an indication of the actual voltage.

For an example with a special-purpose avalanche transistor, see Eric Wasatonic’s 15-minute video on YouTube.