6 Volts -> 12 Volts -> 6 Volts
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by Steve Delanty
Owners of old cars and trucks
that came with 6 volt electrical systemsknow 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 toconvert to 12 volts. Install a nice 12 volt alternator and
regulator fromthe wrecking yard, change the battery, all the light bulbs, the
windshieldwiper motor, and the ignition coil. The 6v starter motor usually
worksreal 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 downto 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 ofthe resistor (in Ohms) times the current flow thru
the resistor. Theformula (Ohms law) is: V = I x R, where V is the voltage
dropped acrossthe resistor, I is the current thru the resistor in amps and R is
the valueof 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 runningand You want the radio to get about 6.8 volts, which
is roughly what thesystem 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 resistorat 3 amps..Since V = I x R,
it follows that R = V / I, and if we plug our numbers inwe get R = 7 / 3 , or R
= 2.33 ohms to get 6.8 volts on a radio thatdraws 3 amps. 2.33 ohms is kind of
an odd value, and You will probablyhave 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 tomake sure the
resistor can handle the power load without burning out.That's what the wattage
rating is all about.. In our example, we dropped7 volts across the resistor at 3
amps, and since W = V x I, our resistorwill convert 21 watts of power into heat.
That means our resistor must berated for an *absolute minimum* of 21 watts. A
larger wattage resistorwill run cooler, and it's good practice to use a resistor
rated for at least50% 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 wattlight bulb makes! Make sure
that You mount voltage dropping resistorswhere they can't be a fire hazard, or
bake any nearby plastic or rubberparts! Be safe, O.K?
One of the
problems with using resistors is determining how muchcurrent Your equipment
draws so You can calculate the correct resistorvalue. The easiest way is to
connect it to a 6 volt battery and connectan ammeter in series to measure the
current it uses.
Unfortunately, often the equipment doesn't draw a
constant, steadyamount of curent. A radio draws more current when the volume
isturned way up than it does with the volume down. A gas gauge may drawseveral
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 constant3 amps it's much more
likely that it will draw a current that variesconsiderably with radio loudness,
and may constantly be varying between2 and 4 amps. Hmm, if the current varies
from 2 to 4 amps that meansthat with the 2.5 ohm resistor we used in the
example, the voltage to theradio 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 andinstantly with load changes so as to always keep a constant
outputvoltage...
Part 2: Voltage regulators
There's quite a few solid-state voltage regulators on the market
thatcan be applied to atomotive use. One of the simplest is the 7806,a 6 volt, 1
amp regulator.
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 boostcurrent. (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 toground
and another from the output pin to ground. The value of thecapacitors is fairly
non-critical and any value from 0.1uF to 10uF (that'smicroFarads) @ 25volts or
more will work just fine. The capacitors helpprotect the regulator from
electrical noise, and to stabilize the outputunder certain load conditions. My
favorite caps for this are 1uF, 35v.
The 7806 will make a little heat,
and needs to be mounted on a smallheatsink to keep it cool. A little dielectric
grease smeared on the backof the regulator will help it conduct it's heat to the
heatsink.The heatsink can be a 3' square of aluminum, or a commerciallyavailable
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
limitedcurrent 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 theoutput of the
regulator. This can increase the output current capabilityto well over 10 amps
using the right transistor and a large enoughheatsink. There are many high power
NPN transistors that will workfine, and I often use a 2N5881 which looks like
this:
The body of the transistor is the collector connection, so the
caseis always 'hot', directly connected to 12 volts. It is very importantto make
sure that the transistor body can't contact any groundedchassis parts! In order
to insulate the transistor from the heatsink,it's conveniant to spend another
$1.50 and use an insulating washerand transistor socket.
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.
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.
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...
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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