Comments on: A Vintage Single Transistor LED Blinker

By: Néstor Oscar Copello

Néstor Oscar Copello — Thu, 31 Jan 2019 23:41:24 +0000

And here is a paper from 1964 about using transistors in avalanche mode to make use of breakdown to change from one stable condition to another, and a graph of negative resistance region.
https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=4858&context=rtd

By: Néstor Oscar Copello

Néstor Oscar Copello — Thu, 31 Jan 2019 23:33:45 +0000

More about breakdown voltage on transistors.
While most transistors have a breakdown voltage of about 5V for the base emitter junction, the actual collector junction breakdown voltage for a transistor is hard to predict. Manufacturer only guarantees BVCBO is ABOVE certain value. When selecting transistors to meet a requisition, they verify the transistor DOESN’T BREAK at a certain voltage. They don’t find out at what voltage will it actually break. So a transistor rated at, say, 35V might break above 300V, as I verified once with some Texas transistors TIP29A.

By: Néstor Oscar Copello

Néstor Oscar Copello — Thu, 31 Jan 2019 23:23:21 +0000

Thanks for your posting about relaxation oscillator. Had already forgotten it is possible to do it with negative impedance device.
I first saw it at university lab exercise using a tetrode back in 1971. The oscillator by negative plate dynamic resistance was called dynatron. It made use of a region of negative plate resistance on tetrodes. One could connect a pentode as a tetrode to make use of this feature.
The characteristic of negative resistance can be used to store one bit of information on a transistor. The transistor is taken to the breakdown and VCBE takes a lower value. It can be reset by reducing collector voltage under a certain minimum and allowing it go go back to a high value. The transistor is triggered into breakdown by opening the base or having a resistor from base to emitter and no bias voltage.

By: Néstor Oscar Copello

Néstor Oscar Copello — Thu, 31 Jan 2019 23:07:16 +0000

Continuing former comment:
1b – Negative resistance in reverse connection.
A transistor can work reversed, this is, using emitter as collector. HFE is very small, though. And breakdown voltage for the emitter-base junction is about 5V. So a transistor connected reversed has a VCBO of about 5V and should also have a negative resistance region.

1c- POWER TRANSISTORS usually do not have a negative collector resistance region. When collector breakdown is reached, voltage continues rising slowly as current is increased.

By: Néstor Oscar Copello

Néstor Oscar Copello — Thu, 31 Jan 2019 23:01:16 +0000

1a- Transistors have a negative region beyond collector breakdown voltage. When collector breakdown is reached, amplification effect on base makes collector voltage to drop.

2. SWB2433 is NPN silicon VCBO = 50V – VCEO= 20V according data at:
https://br.ebay.com/b/Small-Switching-Transistor-Transistors/4666/bn_119901163

By: Gordon Cooper

Gordon Cooper — Fri, 08 Jun 2018 09:53:56 +0000

Thanks for info. I built one with a BC337, 18 volt battery and started at 15K resistor. All worked well at first BUT the transistor seemed to slowly lose its properties. I had to keep dropping the resistor value to keep it flashing, else I just get a dim glow. Current from batteries is so low I doubt they were running out, almost as though I was slowly damaging the transistor. At 9 volts no oscillation at all. Happened over about 5 minutes. Are we wrecking transistors?

By: foxxpup

foxxpup — Thu, 27 Oct 2016 22:15:13 +0000

In reply to Michael Black.

I remember making a flasher with a LM3909 back in the late 1970s. It was very efficient. In regards to this circuit, I can think of one that is even simpler, although not so stable. I’ve seen situations with LED’s and an “almost-dead” battery where, unloaded the battery voltage is just a bit over the drop voltage of the LED and current flows until the voltage drops to the drop voltage and current stops. The battery recovers a bit and then the cycle repeats. I’ve seen that often with numerous 3×1.5 volt pocket LED flashlights. Thats a simpler circuit. :-)