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VARIABLE VOLTAGE LAB POWER SUPPLY USING PSU

  • I am an Automotive Electrician, so needed a power supply that could provide the range of voltages found on 12 volt and 24 volt vehicles during cranking and running. I have been caught before with equipment that worked fine at 12v or 24v, but would drop out at cranking voltages. The power supply also needed to have dual outputs.

    The internet has many examples of lab power supplies based on computer PSU's, but I couldn't find one that would suit my requirements. It was time to design my own using the PSU out of my old HP computer.

    Searching eBay I found a 150W 6A DC-DC Boost Converter for $US4 that looked promising, so ordered one. I also ordered a 0-10A digital ammeter, 0-30v voltmeter, and a pack of 1N4007 diodes.
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    The parts arrived a couple of weeks later and testing confirmed that they would all be suitable.
    Connecting the boost converter to 12v gave an output voltage that could be adjusted down to just under 12v. This is not low enough to simulate the voltage in a 12v vehicle during cranking. To get the required low voltage I used a "crude but effective" voltage dropping circuit using seven groups of six diodes in series. Don't laugh, it works well.

    I was able to remove the 10k output voltage adjustment potentiometer from the circuit board and use an external one.

    I drew this diagram with Express PCB.
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    I bought the banana plug sockets, panel mount fuse holders, and a 10 ohm 10W resistor from Jaycar, the local electronics supplier.
    The 10k 10 turn pot came from Trade Me, New Zealand's equivalent of eBay.
    I found a plastic box that was left over from a previous project to build the power supply in.

    Assembling the power supply filled in a relaxing Sunday afternoon while listening to a twenty20 cricket match on the radio.

    Testing confirmed that the power supply worked well.

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