Star Tech Journal

June 1986 (8)
STAR*TECH Journal
Negative Voltages
From Positive Supplies
By Janes Beck
Greater Distributing
Atlanta, GA
co.
This nonth we will work on getting
negative voltages from a supply that
offers only IX)Sitive outputs. In this
colunn, we will concentrate on linear
supplies; that is IX)Wer supplies that use
a step down transformer and a series pass
transistor for regulation.
Fig. 1A and 1B show the two nost corrrron
rectifier circuits. To get a negative
voltage from these just tap into the low
voltage AC supply and make an identical
circuit with the diodes and capacitor
reversed. Fig. 2A and 2B shows this.
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The only real design consideration is
which AC supply to tap into. I have found
the +12 unregulated supply is usually
best. This will give you enough voltage to
use fixed regulators such as a 7905 or
7912 to get the output voltage you need.
It should be noted that for short circuit
protection a fuse can be added to one of
the AC input lines. A 1-1/2 MDL should
suffice.
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2.b
The filter capacitor should be 2000uf or
greater with a working voltage at least
20% higher than the continuous unregulated
voltage.
Well, that will wrap it up for this nonth.
I hope I have saved you a few dollars and
maybe a few headaches.
Next nonth, we will work on getting
negative voltages from the infanous
switching regulator when it has only
IX)sitive outputs.
16
2a..
Since negative supplies are usually used
to power op-anps in the audio section, the
current demand is very low. This gives us
a lot of design flexability. The
rectifiers can be just about any 1 anp
general pur:[X)se type. 1N4004s work great.
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