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CO2 Laser Power Supply
● A Cycling Circuit
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LED Replacement of Bulbs
Q&A
■ WITH RUSSELL KINCAID
In this column, I answer questions about all
aspects of electronics, including computer
hardware, software, circuits, electronic theory,
troubleshooting, and anything else of interest
to the hobbyist. Feel free to participate with
your questions, comments, or suggestions.
Send all questions and comments to:
Q&A@nutsvolts.com
ADJUSTABLE POWER
SUPPLY, 0 TO 480 VDC
QI want to construct a
cheap and reliable simple
AC to DC power supply,
adjustable from 0 to 480
VDC at 100 mA. I have a circuit
(Figure 1) for one that goes to 330
VDC. What components and what
values should I use for this circuit to
bring the output up to 480 VDC?
I also want to connect a 500V digital
panel meter at the output.
— Theodore Karatzas
AI am sure you will want to
use the 1:1 isolation
transformer if possible;
transformers are expensive.
You can also use the bridge rectifier if
the rating is greater than 600V. The
circuit in Figure 2 is a voltage doubler.
The two lower diodes in the bridge
are doing nothing; if you are building
the rectifier from discrete parts, you
only need two diodes. The ripple is
still 120 Hz because we are using
both alternations of the input
waveform. I have increased the filter
cap from 150 μF to 1,000 μF because
the lower value will produce five volts
of ripple, which I think is excessive.
Q2 is a current limiter which is a
good thing to have because
otherwise, accidental shorts at the
output would cause Q1 to explode
(I have blown up a few in my day).
The switch at Q1 drain is to reduce
the power dissipation in Q1. The
power is 30 watts if you use the
switch; 60 watts if you don't. When
the output is below 300 volts,
connect the drain to the 300 volt
source. For outputs over 300 volts,
connect the drain to the 600
volt source.
You want a digital voltmeter. That
is quite easy, but you should be aware
that some meters cannot measure a
source that is common with the
power supply and have to be battery
powered. The meter I have chosen
runs on 5 volts and has an onboard
DC/DC inverter to produce the needed
■ FIGURE 1
■ FIGURE 2
-5V (Mouser PN 580-20LCD-0-5C;
( www.mouser.com). A five volt zener
(1N751) is soft and requires about 20
mA to be sure of getting five volts.
Better regulation at lower power is
obtained using LEDs. In this case, I
used a red and a blue in series for five
volts. The meter draws 400 μA so the
LEDs will be dim, but you can still use
them for power-on indication.
For very little added cost, you can
have some regulation (see Figure 3).
Move the 10 turn 500K pot to the
output and connect to Q3 as shown.
The lowest output voltage will be
Vgs(on) of Q3. Because Vgs is not
February 2009 27
