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Old 10-10-2017, 10:11 AM   #1
barry wilkins
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Default Switching Transitors

I was looking at making a fast pulse generator.its a very simple circuit using an avalanche transistor.It works like a flame detector where the diode conducts because of the UV light from the flame ,the HV on its anode collapses and it switches off,and it oscillates.

The circuit is very simple a 40 volt transistor supplied with 90 volts on its collector through a 1Meg resistor ,so it breaks into oscillation.

The question is "what exactly is a switching transistor? If you look a some data sheets like 2N4401 ,Philips say it is a switching transistor ,other manufacturers say it is a General Purpose transistor ?

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Old 10-10-2017, 10:14 AM   #2
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Default Re: Switching Transitors

Can we see the circuit?
Transistor can be used as a switch,as an amplifier, etc.
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Old 10-10-2017, 11:38 AM   #3
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Default Re: Switching Transitors

The 2N4401 is a TO-92 package device. It's not a power switch transistor like an MJE13007 is. It can be used for general purpose usage, and it switches really fast (for a bipolar transistor). It's not unlike a 2N3904 or (except for the package) a 2N2222A.
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Old 10-10-2017, 02:45 PM   #4
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Default Re: Switching Transitors

I would guess the circuit is a standard relaxation oscillator with the transistor running in avalanche mode, so the standard base-controlled switching characteristics probably don't matter that much. I haven't experimented with enough transistors but I think most of them will work fine and avalanche break down upon hitting a high enough voltage; whether or not that high voltage will destroy the transistor before the avalanche effect takes over is another question (and whether the current that flows during the avalanche will also destroy the transistor.)
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Old 10-10-2017, 11:04 PM   #5
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Default Re: Switching Transitors

It's a weird oscillator circuit:
Explanation here: http://www.learningaboutelectronics....transistor.php
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File Type: png Relaxation-oscillator-circuit-with-a-transistor.png (16.6 KB, 8 views)
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Old 10-12-2017, 02:07 AM   #6
barry wilkins
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Default Re: Switching Transitors

Here is the circuit attached ,just of the oscillator .The other part is a 90 volt step up converter.
Apparently the principle comes from a Tektronix plug in, the transistor is 2N 2369 which I dont have ,BC 108 do not work ,so it may have to be a much faster transistor with more power.

Does not work on Multisim ,not surprised by that lots of very strange things go on with that software where real circuits actually work ,but can never simulate them.

BarryFast Pulse cct.png
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Old 10-12-2017, 11:38 AM   #7
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Default Re: Switching Transitors

You may need to go past 90V, the 2N2369 was chosen because it has a relatively low avalanche breakdown. I had a boost converter go to around 120V+ volts (had it lying around for something else) for some random cheap 2N3904 to avalanche. No special transistor needed really, just be prepared to destroy it, 90V is well past specs for a 2N3904
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Old 10-12-2017, 12:07 PM   #8
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Default Re: Switching Transitors

look at the part in green . with correct condition the led flashes .
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Old 10-12-2017, 03:32 PM   #9
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Default Re: Switching Transitors

Except that's the astable multivibrator oscillator, not a relaxation oscillator (though I figure that posting might have been a misplaced post...)
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Old 10-12-2017, 04:15 PM   #10
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Default Re: Switching Transitors

Quote:
Originally Posted by eccerr0r View Post
Except that's the astable multivibrator oscillator, not a relaxation oscillator (though I figure that posting might have been a misplaced post...)
was me thinking aloud ..
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