Subject: GLOWBUGS digest 58

From: [email-masked]
Date: Wed, 27 Dec 1995 01:59:07 -0600 (CST)

GLOWBUGS Digest 58

Topics covered in this issue include:

  1) The Breadboard Lives
	by [email-masked]

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Date: Wed, 27 Dec 1995 02:56:34 -0500
From: [email-masked]
To: [email-masked]
Subject: The Breadboard Lives
Message-ID: <[email-masked]>

Greeting of the season to one and all.

Being one who has been recently been bitten by the glow bug so to speak, I
have found my various bench power supplies useful only for heaters or grid
bias supplies.  The string of 9 Volt transistor batteries I had been using
were getting weak and it was time to build power supply for plate and screen
voltage.  The only problem was I didn't have an appropriate box/chassis.
 Rather than waiting for the next swap meet to look for a chassis, I decided
to build a quick and dirty supply to get me by.  I had a piece of pine left
over from my last wood project and decided to build a breadboard.

The whole idea of using wood an an electrical insulator doesn't sit too well
with me and so I decided to make all electrical connections using terminal
strips.  Fortunately, terminal strips are available (and dirt cheap) surplus
and I happened to have a good supply on hand.  All the components were
mounted physically using wood screws but all electrial connections were
isolated from the wood.

An isolation transformer, 1N4005 rectifiers, 680 uF, 200 V cap and an
adjustable power resistor made up the circuit.  Thanks to the needs of the
computer switching power supply market, large capacity caps at voltages up to
350 Volts are readily available at reasonable prices.

After throwing everything together, I tried the supply out on the 3A4 regen
receiver I built.  I was extremely dissapointed by the loud hum that swamped
out all but the loudest signals.  Why would there be such a hum when I had
such a huge filter cap and my receiver used very little current at all (two
mA max)?  After trying many things unsuccessfully, I decided to bypass the
rectifier diodes and that did it.  The hum was gone and I had a quick and
dirty power supply for my experiments.

The question I have is WHY does this work.  This technique is a standard fix
for broad band noise generated by linear power supplies used in computer
equipment.  I have done this often as part of my work but never thought much
about why this works.  Any theories?

Eric, WD6DGX

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End of GLOWBUGS Digest 58
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