Variable Voltage Lab Power Supply Using PSU

[Kiwi]

Kiwi submitted a new Showcase Item:

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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Read more about this showcase item here...

 

[rich-38]

You use this for your startmotor, ive seen a lot, since my father had a lease/repair company, but smps can'tstand the powerdraw. Just google and you'll see that they just use supercaps, which can withstand the currentdraw .... You need a big buffer
which can deliver that current. Psu's from computer suck, that's chinese rubbish and certainly not made to the purpose you want it for. 20A for a longer time Isn't a problem for thyristors or fets, not constantly, your pcu has also 10 powerfets switching fast, just like a grapic card, but whatever you use supercaps are the safest. Watch an poweramp class d. and the row fets and capacitors to deliver the powerdraw

 

[davenn]

I don't think he is really worried about the starter motor, rich-38

for starter motor turning on the avg 12V powered car, you require a minimum of 600 -700 amps
peak current draws often hit 1000 amps for brief moments

Dave

 

[Kiwi]

What I was referring to when I mentioned "cranking" was the effect that the low voltage during engine starting would have on the equipment I was working on.

Electronic circuits designed for automotive applications need to have a fairly wide voltage supply range.
The vehicle may have a nominal voltage of 12volts, but the circuit must be able to operate between about 9 - 15volts without failing.
You would get pretty annoyed if your car stereo lost all its radio station pre-sets every time the engine was started, just because the voltage dropped to 10volts.

 

[Maddhung73]

Cool project man, keep that imagination a CRANKIN!!! (no pun intended)

 

[peterlonz]

To be honest I can't see how you intend using your DIY device.
This does not help anyone wishing to suggest improvements.
One thing I'd suggest is you allow for case ventilation if you intend operation for more than 10 mins or so at a time.

 

[Kiwi]

Hi Peter,
Sorry I haven't replied earlier to your semi-negative comments regarding my project.

It has been getting quite a bit of use at home and at work. Beats hunting around for the correct voltage wall wart to run something.

As I said at the start, the unit was made to help me with the automotive equipment I work on. This is probably a different situation to most of the people on this forum, so may not be totally suitable for everyone's usage. Being able to simulate the voltages present during vehicle cranking and charging has been a great help.
Anyone got any suggestions for improvements?

Overheating has not been an issue, even when used for extended periods. If you look closely at photo #5 you will see eight holes through the bottom of the case. The fan in the power supply sucks air up through these holes, past the heat sinks on the voltage booster and then out the back.

 

[Irv]

Very neat job! I like the "glass top" case

 

[dhe]

good job bro.....I apreciated what you've done.....

 

[Kiwi]

Crikey, 11K views!
Didn't think this would be so popular.
Hope it has been an inspiration to a few people to build something similar for themselves.

Power supply is still working fine, never had a problem with it.