Rewinding Coax Relays for 12 Volt Operation
Paul Wade N1BWT 1992

Introduction
Microwave operation from high places requires portable operation for most
of us. The most convenient power source is usually the twelve volt
battery in the vehicle that gets us there, and modern solid state devices
work fine at 12 volts or less. However, most surplus coax relays are
designed for operation at 28 volts or higher and don't switch reliably at
12 volts. When available, 12 volt coax relays are exorbitantly priced, so
it would be nice if the highervoltage ones could be converted. Since
relays are ancient technology, digging through some ancient issues of
QST yielded an article which detailed the calculations necessary
to rewind relays for different voltages, which I will summarize here
since the back issue is probably no longer available. By now, Kent
(WA5VJB) is probably saying "It's simple  you just take some turns off
the coil until it works at 12 volts." Usually he's right, and I'm told it
often works for 28 volt relays, but let's go through the numbers and see
how well.
Calculations
The relative force generated by the coil to close a relay is conveniently
measured in ampereturns, simply the current through the coil
times the number of turns. If we are rewinding a relay, rather than count
thousands of turns we can simply fill the bobbin with wire and assume
that the volume of wire is constant. Using this assumption and using
standard US wire gauge (a.w.g.) sizes simplifies the equations. The awg
wire diameters decrease geomentrically with increasing awg number, so
that each size is approximately 1.12 times (or 10^{0.05}) smaller
than the preceding size. Using this relationship, we can
calculate^{1} the number of turns per square inch (N) of bobbin
cross section changing by a factor of 10^{0.1}, or 1.26, per wire
size, and the resistance (R) per cubic inch of winding changing by a
factor of 10^{0.2}, or 1.59, per wire size.
Since we are rewinding the same bobbin, area and volume are constant
(k), so
N^{2}/R = k
Multiplying by I^{2} / I^{2} , this becomes:
(NI)^{2} / I^{2}R = k
we can recognize NI as ampereturns and I^{2}R as watts,
or power.
What this means is that for any wire size that fills the bobbin, the
same amount of power applied provides the same number of ampereturns,
so what we need to calculate is the wire size that will draw the same
power at the desired voltage:
V_{1}^{2} / R =
V_{2}^{2} / R
If V_{1} is the original higher voltage and
V_{2}
is 12 volts, then we must increase the wire diameter by:
number of wire sizes = 10log(
V_{1} /
V_{2})
remembering that a larger diameter wire has a smaller awg number. The
original article^{1} had a graph, but this is easily
solved on a calculator, which they didn't have in 1956. The most
common voltages are also summarized in the following table:
Common Relay Voltages
Original  Desired  # of Wire Sizes 
28 volts  12 volts  4 
48 volts  12 volts  6 
115 volts  12 volts  10 
Procedure
The rewinding procedure is straightforward: peel the old wire off the
bobbin, measure the wire size, and rewind it with larger magnet wire
as calculated above. Your local Radio Shack® carries several sizes
of magnet wire, which may include the one you need. Mechanical details
should be pretty much like the original relay; take notes during
disassembly. The most difficult part is often prying the bobbin off the
metal pole. Relays in sealed cans are a larger problem, and I welcome
suggestions.
The fastest way to put the winding on is to wrap masking tape around a
dowel or pencil until the bobbin fits snugly over it, chuck the pencil in
a variable speed drill or lathe, and run it slowly to wind the wire on.
At low speeds, the wire can be guided with your fingers.
Example
I found several excellent coax relays with Nconnectors at a hamfest,
quite cheap because they required 48 volts. They were wound with #38 awg
wire. From the equation above, converting from 48 to 12 volts requires
wire roughly six sizes larger in diameter, so I rewound one with #32 awg
wire. The original winding required 48 volts at 54 ma., pulled in at 35
volts and released at 15 volts. After rewinding, it draws 265 ma. at 12
volts, pulls in at 8.5 volts, and releases around 2 volts. The power is
slightly higher now, because six wire sizes is an approximation, but I
can be sure it will still operate on a low battery. My 903 MHz station
now runs entirely on 12 volts.
I also rewound a relay with a 26 volt coil for 12 volt operation; results
were similar to the one above.
Alternative
As mentioned previously, we could have just removed turns to increase the
current until the relay draws the same power at the lower voltage. If we
take off half the turns, the resistance drops in half. The original
resistance is 48 volts / 54 ma = 888 ohms. At 12 volts, we need
216 ma for the same power, or a new resistance of 55 ohms, so we need
onesixteenth as many turns. We increased the current four times, so we
end up with onequarter as many ampereturns as the original, or only one
quarter as much force pulling in the relay. If we weaken the spring
enough, it may work, but will it be reliable?
A few more trials will convince us that no matter how many turns of
the original wire are removed from a 48 volt relay, there will be only
onequarter as much force pulling it in at 12 volts. A 28 volt relay
isn't as bad  the force is only reduced by 12/28, to a bit less
than half the original force. There is probably a combination of turns
and springbending that will work pretty well, but, if you've done
enough disassembly to remove some turns, why not take the rest off and
rewind it for 12 volts?
International
I haven't tried any relays from other countries, but I wouldn't be
surprised if they don't use awg wire sizes. In the U.K., they may still
use s.w.g. sizes, which are different, but the relative sizes are close
enough so that increasing the diameter by the number of sizes calculated
above should work. So measure the wire, convert to the nearest awg or swg
size, and go from there. I don't know what metric standard wire is
available.
Conclusion
Rewinding a surplus coax relay for 12 volt operation requires only one
simple calculation and perhaps an hours work; why not try it rather than
paying exorbitant prices or using inefficient DCDC voltage converters?
Notes
1. L.B. Stein, Jr., W1BIY, "Some Hints on Relay Operation,"
QST, June 1956, pp. 2125.