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MG MGB Technical - Stepping down 12v supply

Hi All

What would I need to splice into the wiring to turn a 12v 8.3 amp DC supply into a 6v 1.2 amp DC supply?

The voltage I can do, but the current bit has me stumped.

Liam
Liam H

Liam

The 8.3 amp rating is the capability if the DC supply, not an inherent parameter like the voltage. Crudely, a 5 ohm resistor would drop 6 volts at 1.2 amps, however, the regulation would be terrible - i.e. 12v at no-load and 6v at 1.2 amp. There must be a simple electronic device that does the job.

Barry
BJ Quartermaine

Liam,

It appears that you have little experience in electrical matters. You do not "step down" amperage. Amperage is the current flow which results from a particular voltage with a certain resistance in the circuit.
If you have a 12 volt supply and you wish to use a 6 volt item in a circuit, you have to connect the item in series with a resistor that has the same resistance as the item itself.
If the item you intend to use is something that gets warm you need to know the resistance when it is warm (different).

Mick
Mick Anderson

Liam,

Maybe I should make it more clear.
Ohmn's Law states that resistance equals the voltage divided by the amperage, or R=V/I.
If the 6 volt item you wish to use has, as you say, a working amperage of 1.2 amps at 6 volts, the resistance must be 5 ohmns.
Now if you put a resistor of also 5 ohmns in series in the circuit the total resistance will be 10 ohmns.
Now the 12 volt supply will give a current flow of 1.2 amps, which is what you want.

An example of this is the Triumph 2000 or 2500 saloon which has a 12 volt system, but a 6 volt ignition coil as the standard factory fitting. The ignition coil has a primary coil resistance of 1.5 ohmns and so a resistor of 1.5 ohms was placed in series to have the coil work with the 12 volt system.
Why? So that a lead from the starter motor could supply a full 12 volts to the 6 volt coil only during starting.

Mick

Mick Anderson

Liam, you can buy, for a very reasonable price, a solid state regulator that will drop the 12V down to 6V. They are a three terminal device, one for input, one for output and one for earth. Go to any decent electronic component store and tell them what you want to do. They will sell you the part and if they are nice explain how to wire it up. You need to tell them the voltage in, the voltage out and the amps you want at the output.

Make it very clear to them whether your car is positive or negative earth as wiring these things up the wrong way stuffs them.

I have got a solid state regulator to supply 9V to the fuel gauge in place of the old regulator. It consists of one regulator chip and one capacitor which both fit into the original regulator box. The regulator will accept anything between 12 to 18 volts in and gives a rock steady 9 volts out.

Tony

Tony Oliver

Best bet would be something like an LM7806 voltage regulator (but you will need to check the current rating of the particular device as many only go up to 1 amp). You would also need to attach a decent heatsink and beware of polarity (as many of these devices have a live heatsink terminal). As others have noted, putting a resistor in series with the load to form a potential divider will only work if the load has a static, known resistance - otherwise the voltage will end up all over the place. You would also need a fairly hefty resistor to cope with the power you'd be asking it to dissipate.

However, as it doesn't sound like you're very comfortable with electrics (no offence meant) you might be best off with a pre-made regulator. They are very cheap and easy to find - often they're made to power portable CD players, laptops etc from the cigar lighter. Try the likes of Maplin or RS.

What's it for, by the way?

Cheers,
Tim
T Jenner

Liam,

Do you have the two 6 volt batteries setup?
Why not just run your 6 volt item from one battery? This will not affect correct battery charging.
If you do, you should use an inline fuse.
Just connect it to the terminal of the one battery that also has the earth cable to the chassis. Just connect to the other terminal of that battery. Is the item to be used polarity sensitive?

Mick
Mick Anderson

If Liam can already do the voltage bit as he says, all he needs to know is 'go ahead and do it' as the current will look after itself - at the levels he is talking about.

Similar confusion crops up when people want to uprate their alternator, thinking that because they install an 80 amp alternator they *have* to install 80 amp wiring. You do if you are going to be feeding 80 amp loads, but if you only have the factory loads the factory wiring will handle that just fine. And FWIW the factory rated alternator should handle factory loads just fine.
Paul Hunt2

Hi everyone,

Thanks for the replies, some useful food for thought.

Just to point out, I re-wired my V8 conversion from front to back from scratch with nowt but the manual to guide me. I do have some experience, and I'm comfortable with it. So please don't be worried I'll fry myself!

It has a heavy duty single 12v battery for the V8 and I ahve to adde extra earthing, uprated +ve feeds etc. I wanted to wire in my pocket DAB radio, which is 1.2 amps. But it's very sensitive to anything but a clean supply and I definitely dont want to fry that!

best,

Liam
Liam H

The voltages in an MGB are pretty dirty, you get about 250v spikes from the coil, the HT return path is through the wiring, and there is the alternator, instrument voltage stabiliser and fuel pump all adding their bit. Modern cars go to great lengths to have adequate regulation and filtering inside the boxes of electronics.
Paul Hunt2

Liam,

I would go for a proper regulator IC then (like an LM7806) with some smoothing capacitors for good measure. They're very easy to use with just 3 pins. Example data sheet here:

http://www.utc-ic.com/spec/LM78XX.pdf

But note this one only goes up to 1 amp.

Cheers,
Tim
T Jenner

Here's another option which will supply up to 2 amps:

http://www.maplin.co.uk/module.aspx?ModuleNo=8064&doy=7m5#overview

Cheers,
Tim
T Jenner

Final option! This is a really neat little unit which will supply up to 3 amps and doesn't generate much heat:

http://www.maplin.co.uk/module.aspx?ModuleNo=115314&doy=8m5#overview

Cheers,
Tim
T Jenner

>Why not just run your 6 volt item from one battery?
>This will not affect correct battery charging.

yes it will, the current to charge one battery has to flow thru the other, if you add a missmatched drain of any significant amount then the battry you use will run down slowly or the other will get cooked with over-current. Battery charging is self regulating, but only if the batterys are matched - break the matching and you'll end up with two dead battrys in short order.


Personally I'd go with Tims last option unless you are certain you know what you are doing. Linier regulators are cheap and nasty and will waste as much power in heat as you draw in output - this probebly doesn't matter on the road unless you chose to put it in a box (where does that heat go?), but if you plan on running the radio you are better with a switched mode power supply.
Will

It's not as bad as that. Batteries do need to be matched if they are operated in *parallel* or one can discharge into the other, it's not really an issue in normal series charging. Whether two 6v batteries will be matched on purchase is a moot point, they almost certainly won't be later during their life. Still better to drop the voltage from 12v to 6v though.
Paul Hunt2

This thread was discussed between 01/05/2008 and 12/05/2008

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