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Old 11th November 2007, 12:44 PM   #1
Dan2 is offline Dan2  South Africa
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Default 12 volts to 6

hey, just a quick question.

my friend's father is busy restoring an old car, and he got a radio with it that seems to be as old as the car.

he says the radio runs off 6 volts. i have not seen the radio or the car, but im sure the car will use a 12 volt battery - and if the radio does in fact run off 6 volts i will need to halve the voltage.

what would be the easiest (and cheapest) way to do this, cos it will probably need to deliver a few amps to the radio.
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Old 11th November 2007, 02:46 PM   #2
GDJ is offline GDJ  Canada
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Cool project!

I've done a few old trucks from the mid 40's for a time, and honestly the easiest way to get around this is to swap out all the electrical parts to 12V and convert everything to 12V operation. The wiring does not need to be swapped out since it tends to be much heavier (6V vehicles had to deal with alot more amperage than the 12V vehicles). So basically check everything, change every light bulb, the signal-stat (for the signal lights) headlights, etc. The blower motors might need changing, but the projects I participated in I did not need to change out (the motors back then are very robust). Besides, finding good 6V components can be difficult.

NOW, if the radio is a original unit, a big wire wound resistor would be in order to drop the voltage down. If the radio is a later model that uses 12V, then you're pretty much set.
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Old 11th November 2007, 03:22 PM   #3
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--- OR ---

You could do a SimpleSwitcher Step-down regulator to step down the 11-14VDC to 6-7VDC the radio will need. Some of the SimpleSwitcher chips are good for up to 5A!

Moderators: Bump to Power Supplies Forum??

Dan2,

If you're familiar with these Switching Regulator ICs, they are perhaps the easiest Switchers to implement. If you shield it and filter it well enough, noise in the AM broadcast band should not be an issue.

Depending on how much you want to put into it, If you're interested, look at National Semiconductor's LM2678 at www.national.com. To do a complete regulator, you would only need the chip, a coil (energy storage), a diode (to steer the energy correctly), two filter caps (input and output), and two resistors (to set the output voltage). Additional output cap(s) and a filter coil for a C-L-C output filter will quiet things down quite a bit.

I suggest the switching regulator only because at, say, 3A, dropping from 12-14V in to the 6-7V out you qill require, will result in over 20W of power being dissipated away as heat! Also, if the car has a DC generator instead of an alternator, you are most likely good for only 25-30A, so every watt of power saved is crucial.

Steve
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Old 11th November 2007, 05:49 PM   #4
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Easiest and cheapest would probably be a large resistor as was suggested earlier.

If you want to get a bit more complex, a simple linear regulator would work. You could use a 10v Zener diode and resistor to set a reference voltage. Connect the gate of an N-channel FET to the Zener. The drain of the FET would go to 12v. The source of the FET would go to the radio. This would give you ~6.5v.

A linear regulator will run warm/hot and WILL require a heatsink. A fan cooled heatsink used to cool a CPU (follow link below) should be able to keep it cool.

http://www.bcae1.com/images/fanreg/l...heatsink01.jpg
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Old 11th November 2007, 08:54 PM   #5
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"Easiest and cheapest would probably be a large resistor as was suggested earlier. "

Only if you intend to use 1 volume setting for ever..... Otherwise the voltage is going to swing all over the place depending on the power being consumed by the radio, not good....
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Old 11th November 2007, 10:56 PM   #6
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junglejuice:
If you tried this with a modern head unit, the resistor would not work.

For the older head units, the audio output power is minimal. If the outputs are operating in class A mode, there will be little change in the voltage if you use a resistor. The idle current will likely swamp out the current drawn to drive the speakers, so even if the outputs are operating in class B, the voltage shouldn't change much.

He didn't say how old the vehicle was but most of the 6v units probably use vacuum tubes.
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Old 11th November 2007, 11:40 PM   #7
Eva is offline Eva  Spain
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The resistor trick is only likely to work with head units employing a class A choke-loaded amplifier. The few units from 1950s and 1960s that I have had the chance to open were done that way, although they were all 12V.

The switching regulator is also a very good solution, nowadays those chips are becoming almost as easy to use as linear ones, but without the heat. You may also find ready-made adjustable-output switching regulators on ebay for quite cheap. Example: http://cgi.ebay.com/2-Way-3-Amps-Ste...QQcmdZViewItem
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Old 12th November 2007, 07:06 AM   #8
Dan2 is offline Dan2  South Africa
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i just did a switch mode dc - dc converter project (pwm driving a MOSFET) for one of my courses at collage. i wasn't very expensive but we had to limit the output current at 200mA (and the mosfet runs cool without a heatsink)

can anyone tell me what the maximum current output is of these converters?? and the mosfet switches at about 35khz - what will happen to the radio with a 35khz ripple (about 1%) on the supply??
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Old 12th November 2007, 09:58 AM   #9
quasi is offline quasi  Australia
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The simplest and probably the cheapest method would be to use 5 or 6 bridge rectifiers wired in series (+ve to -ve terminals). Each rectifier will provide a constant 1.1 to 1.2 volt drop irrespective of current flow and will provide a voltage that is about 6 volts less than the 12 volt car supply.

The actual bridges used and their forward voltage characteristic will determine if you use 5 or 6. Either way the whole thing is likely to be less than $10 US.

Whilst the car radio may have peak current draws of an amp or two the average draw will only be a few hundred milliamps. This means that the average dissipation for all of the bridges might be a couple of watts.

Cheers
Quasi
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Old 12th November 2007, 11:16 AM   #10
Dan2 is offline Dan2  South Africa
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OF COURSE!!!
but surely i can use diodes in series, instead of rectifiers?
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