Line voltage bucking, or: "Save that old radio!"

Roy Morgan morgan at SPECKLE.NCSL.NIST.GOV
Fri Aug 22 10:43:29 EDT 1997


or Don't Fry Your old Radio on modern line voltages!

Most old radios were designed for 115 to 117 volt operation:
your line voltage is likely to be higher, as it is in most h
ouses today.  122 volts is common, with 124 not unusual.

Here's the trick to use a filament transformer to reduce modern
too-high line voltage to lower (110 or 115) line voltage for old
radio equipment.

Most receivers don't take more than a couple of amps, and 2-amp
transformers are easy to find.  (If it runs hot, it's too small.)

                Filament Transformer

   black   | *   |
--------------   |
              )|||       black
 Line         )||(    -------
 Cord     PRI )||( 12v|
              )||(    |
              )|| -----          Output
--------------    *
  white    |             white

green                    green

    * Switch output leads if output is *higher* than the line!

Choose the filament transformer for the voltage needed to reduce
the line voltage to the desired amount, and current rating equal
to the needed load current or higher.

The transformer's primary is connected to the power line.  The
transformer's secondary is connected in series with the power being
sent to the outlet point.  If it's connected right, the voltage out
will be LESS than the voltage in.  (If it's backwards, the
voltage out will be MORE than the voltage in.  This is bad.)

If you're worried that the filament transformer is rated at 115 volts,
not 125,  you can connect the transformer primary to the output, not
the input.  (It'll work just fine, and this would increase the output
voltage slightly.)

You could mount the transformer inside the receiver as a permanent
part of the radio.  Attach the primary wires to the primary wires
from the main power transformer.  (If you do this, make
sure that you have the polarity thing sorted out before you run the
receiver on it.  125 PLUS 12 is TOO much!)

Keep em Glowing!
Roy, K1LKY since 1959

-- Roy Morgan/Building 820, Room 562/Gaithersburg  MD 20899
(National Institute of Standards and Technology,  formerly NBS)
301-975-3254 Fax: 301-948-6213  morgan at --

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