A unconventional use for a NPN BJT

You're probably right. I remember learning to check trans with a VOM and was told that some - like horizontal outs - would zener from C-E, so I had to make sure I knew where the base lead was.

It is known that with a high value resistor, a capacitor and NPN transistor is possible to build a relaxation oscillator. The trick is to "turn upside down" the transistor, that is, the emitter is connected to the positive terminal of the capacitor (charged by the resistor) while the collector is connected to ground and the base is left floating. Above 12-15V the transistor displays a region of negative resistance which is the source of the oscillation. What is the explanation of this behavior from solid state physics viewpoint? Any ideas? Gene

Boy, this takes me back a few decades! I recall making these relaxation oscillators in a lab class in the late 1960s! I think this is an avalanche effect, where the conduction electrons knock loose others that increase the conduction further, so the resistance goes negative. A lower voltages there isn't enough energy to knock others loose, so resistance is positive. At higher voltages the current is limited by bulk resistance or something so the resistance is again positive.

Best regards,

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

One comment, there is also a mode (that worked with some 2n2369s) that uses the transistor in 'forward' mode (where the collector is positive and emitter grounded.) The circuit is like this:

V+ | | RC | --------- | | Collector | ---Base | Emitter Capacitor | | GND GND

Note that the base is 'open'. The transistor is partially damaged when operated in this mode. The V+ would be fairly high (like 40volts).

John