In sci.electronics.repair on Mon, 17 Jan 2005 06:42:51 GMT "James
Sweet" posted:


"meirman" wrote in message
.. .
I almost can't believe it, but after 35 years, I still get confused
about positive and negative. Sorry to bother you, but I just can't
remember.

I'm replacing a bridge rectifier with 4 separate diodes, and I know
where the negative should be. If I put one diode on one AC wire, and
another diode in the opposite direction on the other AC wire, and if I
set the VOM on 200V DC, and I put the common/negative/black lead from
the voltmeter on the unconnected end of one diode, and the
positive/red lead on the other diode, and the needle moves to the
right (I get a positive reading) does that mean the diode with the
black wire on it is putting out negative?

That's what I think, but I'm not sure.


If I went by the silver/grey band on one end of the diode, would that
end be the positive or negative end?

These are all laid out parallel to each other and taped together, with
narrow white paper tape on one end, and red paper tape on the other.
This means that the end with the red tape puts out positive voltages,
right?

Thanks.

Meirman

Connect two diodes together at the striped end, that'll be your positive
output, connect the other two together at the non-striped end, that'll be
your negative output, now connect the two free pairs together in the middle
and apply AC across those.

Second reply. Posted and mailed.

Well, you were right of course, and the tiny air compressor is fixed.
I bought it and a lawn mower at a junk yard for 20 dollars. Fixed the
lawn mower somehow without spending a dime (although it may need a
helicoil when next spring comes and I try to mow a whole lawn with it.
The mower looks brand new and the closest model to it sells for 175
dollars.

The bridge rectifier had shorted, I guess, and burned out a trace on
the circuit board. Then it blew the fuse. I replaced the fuse, ran a
wire in place of the circuit board connection, and put in 4 diodes.
But I had all the stuff so it didn't cost anything. The compressor
sells for 75 dollars, although that would include a hose and other
attachments. A set of those sells for 10 to 15 dollars.

So I'm doing well.

I alswo bought at the junk yard two gas powered Homelite weed wackers
for 7 dollars, figruing I could combine them to make one good one.
Failed to notice that neither had a carburetor. I'm not sure what I'm
going to do but I'm still ahead of the game.

Thanks for the help.

Meirman

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