6 volt to 5 volt regulator or how to run a GPS unit off a 6 volt car system.

February 18th, 2013

This is a little project that came up due to a discussion on the Austin 7 friends site here.

The problem is running a car GPS unit in a car that has a 6 volt supply. Normally these units have a little power regulator that plugs into a 12 volt lighter socket and steps down the voltage to 5 volts the GPS units need. I don’t know if ALL of them use 5 volts but all the ones I have seen do. The problem is these regulators won’t work reliably on a 6 volt system because the voltage regulator component in them needs several volts more input voltage than the 5 volt output voltage.

They may just work then when extra load drops the input voltage (such as switching on headlights) they’ll stop regulating and your GPS unit will turn off. This is the case for any device operated in the car via a normal regulator such as cell phone chargers or radar detectors (radar detector in an Austin 7 – Ha!).

The way around this is to use a component called a LDO, or low drop-out, regulator. The following simple circuit will give you a 5 volt output from any input voltage down to about 5.5 volts. Below that it will give the input voltage minus 0.5 volts. A healthy 6 volt system in a car should be able to supply more than enough voltage so that never happens.

The device to use is a LM2940 regulator. It is more or less designed exactly for this application. It is protected from reverse polarity (connecting the wires backwards), short circuits on the output (getting your wires crossed) and thermal overload (getting too hot). It can supply up to 1 Amp of current which should be more than enough for modern GPS units. It will take an input voltage of 6 volts up to 26 volts so this regulator can be used on 6, 12 and even 24 volt car electrical systems.

The parts you needs are the LM2940 and two capacitors, a 0.47uF and an electrolytic of at least 22uF.

I used a 0.47uF polyester and a 330Uf 16 volt electrolytic.

I also used a little piece of strip board to mount it all on.

IMG_0687_1 Parts.

The regulator has three legs which when seen as above they are input, ground and output from left to right. The metal case of the device is also ground – this is important to note as we’ll see later.

The polyester cap on the far right can go in either way. The electrolytic cap is polarised and the negative lead will be marked, usually with a strip on the body next to it.

Wiring is simple. You can refer to the diagram in the linked data-sheets or my drawing below but basically the 0.47uF polyester capacitor goes between the input lead and ground. The 330uF electrolytic goes between the output lead and ground making sure the negative on the capacitor is the one attached to the ground. That’s basically it!

IMG_0689_1 IMG_0692_1 IMG_0695_1 Wiring.

The strip board is easy but you could just solder everything together without it dead bug style (all the wire just soldered to each other as necessary). I added four wires, red to the input lead, black to ground, a second black to ground then white to the output lead. A piece of heat-shrink covers and holds the wires in place.

The only other thing to do is attach the device to a heat-sink. Any piece of metal will do but aluminium is best. The size depends on the current you are pulling out of the device but I found a piece about 2 inches by 3 inches worked fine for me. You simply drill a hole in the heat-sink to bolt the device to with a 3mm machine screw, star washer and nut.

The device will work fine on a 6 volt or a 12 volt battery. On a 12 volt system it may need a larger heat-sink as it has to dissipate more power.

Now this is where you need to be careful. Since the device body is metal and is connected to ground if you bolt it direct to a heat-sink that will also therefore be ground. In a negative earth car you could then bolt the heat-sink direct to the car body and there would be no problem as the car body is also ground.

In a positive earth car though you can’t do this as you would be connecting the ground direct to positive – a short circuit! In this case you need to isolate the heat-sink or the device. Luckily they make little mounting kits to isolate the device. These are called TO220 mounting kits.

The TO220 is the style of package of the regulator. The kit is just a thermal insulating washer (clear mica or silicone), rectangular in shape with a hole through it, and a small plastic bush. The washer goes under the device to isolate it  from the heat-sink and the bush goes into the hole in the device through which you attach the mounting screw. That isolates the device from the heat-sink and the mounting screw.

To insulate the bottom of the circuit board I made a plastic shield from a piece of plastic drinks bottle. I scored and folded it to fit under the board and be held in place.

IMG_0697_1 Bolted direct to heat-sink for negative earth car.

IMG_3445_1 Device isolated from heat-sink for positive earth car.

In a negative earth car the red wire goes to the battery positive and the one black wire to the battery negative. On the output side the other black wire is negative and the white wire is 5 volts positive. These two wires would go into a plug to suit your GPS unit. It would also be wise to add a 2 Amp (or more) fuse to the red wire between the device and the battery.

In a positive earth car the red wire again goes to battery positive and the black to battery negative. And again the white and second black wire go to your GPS positive and negative. In this case you must make sure the two black wires cannot touch the body of the car! This would cause a short circuit. To protect against this I would put a 2 Amp (or more) fuse on the black negative wire leading to the car battery. You do have to be careful that the negative wire leading to the GPS unit never touches the positive car body or that fuse will blow.

In both cases put the fuse close to the car battery end of the wire. That way if there is a short the fuse isolates the power close to the source.

You’ll need whatever plug is necessary to plug it into your GPS unit of course. Make sure you get the polarity on that correct!

Also, if your GPS is in a metal housing make sure it doesn’t touch the metal car body either as this would cause a short if the body is positive earth. This is true if you use the GPS, or any electronic device, with it’s standard power supply in any positive earth car in fact.

Luckily most GPS units are in plastic cases these days. My test one however has a painted magnesium case!

IMG_0696_1 Testing.

I tested it on my ancient, first generation GPS unit I have had for years (made from production reject parts I got when I worked for Navman). This unit draws nearly 600mA when running. I doubt more modern units need this much current. This ran fine overnight with about 8 volts as the input voltage. The heat-sink got warm but not overly hot.

This diagram hopefully makes things clearer.

IMG_3446_1

Instead of a metal heat-sink  I would suggest mounting the whole thing in a die cast aluminium project box something like this one. Isolate the device from the box using the isolating method above and run the wires into it through a rubber grommet. The box will work as a good heat-sink and it will protect the electronics. It could be then be bolted directly to the body of the car which will act as one massive heat-sink!

 

5 Responses to “6 volt to 5 volt regulator or how to run a GPS unit off a 6 volt car system.”

  1. John Mason Says:

    Hi, Thanks for the circuit and how to put it together. My TomTom sat/nav had a plug to go into the cig lighter of a 12 volt car. Within the plug I assume it has a voltage converter to reduce to 5 volts. Into this plug is a usb socket for the conection up to the sat/nav. Does this mean to use your circuit I have to dispense with the plug in converter and conect direct to the usb lead. If so which output lead conect to which usb conections.
    Your John Mason in the UK.

    P.s. I am not an electrics person but believe with your well worded instructions I can make this little adapter.

  2. admin Says:

    Hi John, I’ll get in touch and see if we can’t work it out. Shouldn’t be too difficult.

    Simon

  3. salmaan Says:

    Thanks.
    I am not a electronics man but I think this is a good work
    again thanks

  4. nick coates Says:

    Hi

    Thanks for the instructions, really handy and clear! Just wondered- stupid question I know, but what size and type of wires did you use for the charger unit and how did you fix the wires to the battery(given their size)? I am intending to just slip the wires into the main battery terminal clamps- under the clamping screws.

    Nick

  5. admin Says:

    I just used normal, medium gauge hookup wire (multistranded wire). I never attached mine to an actual battery but if I were I would crimp on ring terminals and then attach the wires under the bolts that go through the battery post clamps.

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