6 to 12 to 6
by Steve Delanty
Owners of old cars and trucks that came with
6 volt electrical systems
know what a drag 6 volts can be! Dim lights, slow cranking, weak spark,
replacing generator brushes as a maintenance item, etc .
The cure is obvious... convert to 12 volts! Usually it's
pretty easy to
convert to 12 volts. Install a nice 12 volt alternator and regulator from
the wrecking yard, change the battery, all the light bulbs, the windshield
wiper motor, and the ignition coil. The 6v starter motor usually works
real nicely on 12 volts.
The problem is what to do with the stock 6 volt gauges and radio?
Often it's desirable to retain these items and drop the 12 volts down
to 6 volts to run them. So how are You gonna do that?
Part 1: Resistors
One of the common ways to reduce the voltage is by using a resistor.
A resistor reduces voltage by an amount proportional to the value of
the resistor (in Ohms) times the current flow thru the resistor. The
formula (Ohms law) is: V = I x R, where V is the voltage dropped across
the resistor, I is the current thru the resistor in amps and R is the value
of the resistor in ohms. For example:
You have a 6 volt radio that draws 3 amps You wanna run it on 12volts...
Your 12 volt system actually is at about 13.8v with the motor running
and You want the radio to get about 6.8 volts, which is roughly what the
system voltage would be on a running 6 volt system.
So... You've got 13.8v, but You want 6.8v on a device that draws 3 amps.
13.8v - 6.8v = 7v, so You need to drop 7 volts across the resistor
at 3 amps..
Since V = I x R, it follows that R = V / I, and if we plug our numbers in
we get R = 7 / 3 , or R = 2.33 ohms to get 6.8 volts on a radio that
draws 3 amps. 2.33 ohms is kind of an odd value, and You will probably
have to use 2.5 ohms, which would give 6.3 volts instead..
Easy, yes? Ahh, but don't forget the resistor wattage rating!
The power drop across the resistor causes it to heat up, so we need to
make sure the resistor can handle the power load without burning out.
That's what the wattage rating is all about.. In our example, we dropped
7 volts across the resistor at 3 amps, and since W = V x I, our resistor
will convert 21 watts of power into heat. That means our resistor must be
rated for an *absolute minimum* of 21 watts. A larger wattage resistor
will run cooler, and it's good practice to use a resistor rated for at least
50% higher wattage than You expect to handle. For our radio example,
I would use a 2.5 ohm, 40 watt resistor to do the job.
21 watts is quite a bit of heat... think about how much heat a 25 watt
light bulb makes! Make sure that You mount voltage dropping resistors
where they can't be a fire hazard, or bake any nearby plastic or rubber
parts! Be safe, O.K?
One of the problems with using resistors is determining how much
current Your equipment draws so You can calculate the correct resistor
value. The easiest way is to connect it to a 6 volt battery and connect
an ammeter in series to measure the current it uses.
Unfortunately, often the equipment doesn't draw a
amount of curent. A radio draws more current when the volume is
turned way up than it does with the volume down. A gas gauge may draw
several times more current when the tank is full then it does when empty.
If we go back to our radio example, rather than drawing a fairly constant
3 amps it's much more likely that it will draw a current that varies
considerably with radio loudness, and may constantly be varying between
2 and 4 amps. Hmm, if the current varies from 2 to 4 amps that means
that with the 2.5 ohm resistor we used in the example, the voltage to the
radio actually varies from 3.8 to 8.8 volts! This is not a good thing...
What we really need is a resistor that varies it's value constantly and
instantly with load changes so as to always keep a constant output
Part 2: Voltage regulators
There's quite a few solid-state voltage regulators on the market that
can be applied to atomotive use. One of the simplest is the 7806,
a 6 volt, 1 amp regulator.
The part looks like this:
And on paper it looks like this:
These regulators are rugged, provide
over-current shut down, and
will give a constant 6 volts output for currents from 0 to 1 amp.
They are good for running low current 6 volt things like gauges.
If all Your gauges draw a total of over about 0.75amps, it's a good
idea to use more than one regulator with one or two gauges
connected on each regulator, or use an output transistor to boost
current. (Yeah, We'll get to that in a minute) When using the 7806,
it's a good idea to connect a small capacitor from the input pin to
ground and another from the output pin to ground. The value of the
capacitors is fairly non-critical and any value from 0.1uF to 10uF (that's
microFarads) @ 25volts or more will work just fine. The capacitors help
protect the regulator from electrical noise, and to stabilize the output
under certain load conditions. My favorite caps for this are 1uF, 35v.
tantalums that look like:
And on the schematic:
So the schematic for the complete regulator circuit looks
The 7806 will make a little heat, and needs to be mounted on a small
heatsink to keep it cool. A little dielectric grease smeared on the back
of the regulator will help it conduct it's heat to the heatsink.
The heatsink can be a 3" square of aluminum, or a commercially
available piece like this one:
These parts are all readily available and if You have a radio shack
nearby You can use these part numbers:
1ea. 7806, (radio shack # RSU 1392008) $1.49ea.
2ea. 1uF 35v tantalum capacitor (272-1434) $0.59ea.
1ea. Heatsink grease (276-1372) $1.99
1ea. Heatsink (176-1368) $1.49
The most obvious flaw with the 7806 regulator is it's rather limited
current output. Unless You are only using it to power a couple gauges,
1 amp might not be enough. The cure is to add a transistor to the
output of the regulator. This can increase the output current capability
to well over 10 amps using the right transistor and a large enough
heatsink. There are many high power NPN transistors that will work
fine, and I often use a 2N5881 which looks like this:
Top Bottom Schematic
The body of the transistor is the collector connection, so
is always "hot", directly connected to 12 volts. It is very important
to make sure that the transistor body can't contact any grounded
chassis parts! In order to insulate the transistor from the heatsink,
it's conveniant to spend another $1.50 and use an insulating washer
and transistor socket like these:
I usually add a resistor from the output to ground to
keep the output
from floating a little high under no-load conditions. It's not really
necessary most of the time, but the part and it's schematic look like
So, here's the full schematic:
Note that this circuit uses a 7808 8volt
regulator rather than the
7806 6volt regulator. This is because although the transistor
increases the output current of the regulator, it also introduces
a 0.7 volt drop caused by the transistors base-emittor junction.
This results in the actual output voltage being approximately 5.3 volts
if a 7806 is used. By using the 7808 instead You get a 7.3 volts output.
This is at the upper end of what a 6 volt auto electrical system should
have when the generator is running, so 7.3 volts is fine. If You would
rather have 6.7 volts instead, You can drop the voltage 0.6 volts by
adding a 1N4002 diode in series with the input to the base of the
transistor. The diode:
Put it in like this:
That's the circuit I just built for my girlfriends '51 C-word truck.
It puts out 6.8volts with no load and 6.1volts with a full load of
10 amps. The one I built doesn't have a large enough heatsink to
run 10 amps continuously, but can deliver over 6 amps continuous
with short bursts of well over 10 amps. I figure that's plenty of juice
to run all her 6volt accessories...
Here's what hers looks like from the back :
And the front:
So, that's about all
there is to the regulators. The key to making the
regulators work is keeping them cool, so be sure to use a good heatsink
and put a thin coat of silicone grease on the regulator and transistor and
any insulating hardware. Mount the heatsink where it can get some
air circulation, don't put it in an airtight box. Don't mount it where the
heat it genereates can cause trouble for any plastic or rubber parts
nearby. Remember that the body of the transistor and all the other
parts are electrically "hot" when the regulator is powered up, so make
sure nothing can come in contact with it and short anything out.
It's a very good idea to put an inline fuse on the input side of the
regulator in case "something bad" happens.
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