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'Circuit Challenge - was Re: [OT] A little help? or'
1999\07\02@221410 by

>If you can find some circuit I can not understand, I will be the
> first one to recognize and ask how it works. This is the only way
to

> learn, and nobody will mock me for asking, but they will mock
> me for faking knowledge.  Nobody can *guess* how to do a
> brain surgery, it must be part of a learning experience.

OK, I'll take Wagner up on his challenge.
I know it wasn't what he had in mind but this circuit is always fun.
It's been a while since it was mentioned here.
Those who know what's going on, no telling, OK?

>From memory, hope I've got this right.

Take any small signal NPN transistor.
Connect Collector to ground.
Connect base to about +12 volts DC through about a 1K resistor
(Yes, I know the base is not biased "normally")

1. Predict the voltage expected on the (open circuit) emitter.
Do this 1st before you measure.
No cheating y' hear.

2. Take an oscilloscope or standard electronic multimeter (typically
10M ohm plus input resistance)
Measure voltage between emitter (floating) and ground.
Clue -  you cannot break the laws of Physics, even though it appears
that's what's happening here :-)

have fun

Russell McMahon

Russel,
Tell me don't have time to do  your little test but love logic games  :o)
Tracey
just e-mail me off list

As I see it the base-collector junction is forward biased, so you would
expect to see the base sitting at about 0.7V positive wrt the collector
(ground).

There is also a diode equivalent between the base and emitter, which would
normally drop 0.7V if conducting.  The base is at 0.7V wrt ground, so I
would expect to see 0V wrt ground at the emitter.

I set up the circuit as described, and got something very similar: the base
sits at 781mV, and the emitter (which is dangling in the air) has 2.5mV on
it, as measured by my digital multimeter.  The transistor I used was a
2N2222A.

The only bit I'm puzzled about is why Russell said it appears to break the
laws of physics.  In my view, it is doing just what you'd expect.  Is my set
up behaving differently to his?

Regards,

Steve Thackery
Suffolk, England.
Web Site: http://www.btinternet.com/~stevethack/

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