From: Jonathan Kirwan
Subject: Re: How to use a zener diode for voltage regulation?
References: <email@example.com> <firstname.lastname@example.org>
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NNTP-Posting-Date: Tue, 03 Sep 2002 16:23:09 GMT
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Date: Tue, 03 Sep 2002 16:23:09 GMT
On 3 Sep 2002 09:04:36 -0700, email@example.com (Sabertooth) wrote:
>Can you explain a few things to me?
>2. Why do you have diodes between the transistor's base and GND?
If you assume for a moment that the diodes are silicon and have a
little current flowing through them, there will be something
approximating one "diode drop" or about 0.65 volts across each one.
This provides enough of a voltage to turn on the NPN transistor and
also a little overhead left to supply some current through R1.
Another way to look at it is that D1's diode drop is in parallel with
the base-emitter diode drop of the NPN and that D2's diode drop is in
parallel with R1. Thus, there will be about one diode drop's worth of
voltage across R1.
This is very imprecise, but a useful simplification.
>3. Why 2 diodes (instead of 1)?
Because without D2, say, there will be no remaining useful voltage
drop to operate across R1.
>1. How did you arrive at the value for R1?
If you go with the simplifying assumptions, then you will have a diode
drop across R1, whatever it's value. Since you want about 20mA, this
means that R1=0.65/0.02 or about 32.5 ohms. Somewhere in that
neighborhood will be the truth.
>4. Why is a current source a better solution than a zener?
>5. When is a zener appropriate?
I'll leave these for others.