Date:         Sun, 18 Oct 2009 16:25:06 -0400
Reply-To:     David Beierl <dbeierl@ATTGLOBAL.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Beierl <dbeierl@ATTGLOBAL.NET>
Subject:      Re: Jetta swap: Check Engine Light Install Electrical Questions
Comments: To: neil n <musomuso@gmail.com>
In-Reply-To:  <c4e7c5f90910181056w47375dfcq801b04699d2deb8e@mail.gmail.co m>
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At 01:56 PM 10/18/2009, neil n wrote:
>Right. Makes sense. My knowledge of transistors is sketchy, but I do
>recall that they are semi conductors. Upon Scotts direction, I figured
>there was something like a diode in the path I was trying to check.

A transistor is basically a big diode with a little diode stuck
sideways in the middle.  The big diode is reverse-biased, i.e. wired
into the circuit so that it blocks current flow.  If you inject
current into the little diode, the big one starts to leak and allow
current to flow proportionally to the amount of current going through
the little diode but bigger.  To use it as a switch, you bias the
little diode so the big one is either closed or full open.

If you hook it up backwards it will smoke.  There are two kinds of
regular transistors, NPN (most common) and PNP.  They are mirror
images.  In NPN you bring the base (the little diode) positive
compared to the emitter to allow positive current to flow from the
collector to the emitter (the two ends of the big diode).  With the
PNP you bring the base negative with respect to the emitter, to allow
negative current to flow from collector to emitter.

Of course there's a book or two that we haven't talked about.  But
that's the general idea.

Yrs,
d