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Topic: make a transistor "snap on" (Read 2183 times) previous topic - next topic


without resorting to an IC or an MCU.. or even a unijunction..

how would you make a standard 3 pin turn on.. for example using an ldr or a voltage divider...

as the voltage rises on the base c/e start conducting... is there a way via discrete components to only trigger off digitally.. now it's on now it's off.. nothing inbetween


Nov 02, 2012, 04:45 pm Last Edit: Nov 02, 2012, 04:48 pm by Grumpy_Mike Reason: 1
With one transistor no. With two do a google image search for:-
schmitt trigger transistor circuit


even a unijunction

So what do you know about a unijunction then, I last used them back in 1968, where I used a PUT programmable unijunction transistor. Mainly to make relaxation oscillators.


years ago from dicksmiths here in aus... one of the circuits used a unijunction transistor to switch on > 3 volts.... i need to buy some sometime...

ah finally i understand a use for a schmitt trigger!

i shall check it out thanks :)


how would you make a standard 3 pin turn on.

It depends on the specifics of "turn on". For example, you can easily turn a bjt into a tunnel diode and they turn on / off in a matter of nano seconds, with minimum parts.


That's the BEST one YET an Esaki or Tunnel Diode from a transistor???   I DON'T THINK SO. the construction of a tunnel diode isn't even in the same league as a common bi-polar. A Zener diode yes, the emitter/base junction... somewhat noisy and not a real sharp zener knee but a 6 to 7.5 V zener can be made from a bipolar and an SCR can easily be made from a complimentary pair... an NPN and a PNP make a regenerative switch that works like an SCR and will switch in all 4 quadrants. Some make common base oscillators that can go quite high in frequency. But not a back or tunnel diode. I've never *that I remember* seen a bipolar device display negative resistance, or at least useful negative resistance...

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A little more information about tunnel diode:
Tunnel diode - semiconductor diode characterized by a small thickness of the pn junction, a very high concentration of dopants on both sides and a negative dynamic resistance for a certain range of polarizing voltages. It was invented in 1957 by the Japanese physicist Leo Esaki (hence sometimes it can be named Esaki diode). During research on semiconductor junctions he noticed their thus far unprecedented feature based on the tunnel phenomenon. This phenomenon causes charge carriers move through the narrow barrier layer at a very low voltage.

from: tunnel diode


The tunnel diode depletion zone is narrow enough for quantum tunneling to be important,
the depletion zone increases in width with voltage, tunneling has a steep exponential fall-off with
the width.  Thus increase in voltage causes rapid drop in tunneling.  Further increase in voltage
eventually causes breakdown.  The very high dopant concentrations make the depletion zone narrower
than in normal devices (which don't benefit from tunneling).
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Anyway back to the topic, the answer is positive feedback.
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For example, you can easily turn a bjt into a tunnel diode
Please tell us how.

Retired after 40 years as a chartered engineer working mostly with RF and analogue electronics.


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