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Ground Power Unit (commercial or DIY)

When working on avionics (e.g. data update), the battery can discharge quite rapidly with all the electric and electronic gear.

I am therefore looking for the best way to build a ground power adapter. Airplane is a 24V system, therefore the GPU should supply 28V. Would a sufficiently strong (30-50A) regulated power supply be enough or is it advisable to also include a battery as buffer? How would the battery be integrated into the system?

There are ready made units available of course but quite expensive. $1500 for a 50A system was the cheapest I found.

I don't know if something like ours would help. It connects to a ground power plug we keep in the aircraft. About 60AH at 24V. Nice and simple. The box has wheels as it is HEAVY!!

Gloucester UK (EGBJ)

Two car batteries only give 24V whereas one needs 28V to power the avionics. With 24V, the low voltage light would be on.

I built one in 2002, using an off the shelf Lambda 24V 40A switching power supply module, which was adjustable to 28V. This was a quality power unit and wasn't cheap - £500 or so. I then put stuff around it, to indicate the voltage, current, to provide protection against current flowing back into it, and to implement an overvoltage protection circuit which disconnects the aircraft if the PSU's output rises above 31V.

It didn't take long to build. I put it all into a waterproof box, in case it has to be used outdoors. Note the mil-spec connectors. The output connector is the huge 3-pin type which came from some aviation supplier; quite cheap.

Due to the drop in the schottky diode in series with the output, I tweaked the PSU output voltage to something like 28.5V to get 28.0V out.

Shoreham EGKA, United Kingdom


Two car batteries only give 24V whereas one needs 28V to power the avionics. With 24V, the low voltage light would be on.

The avionics work perfectly well with 24V! Back in the days when we operated Cessna 421s, we had to truck batteries (12V 100 or 150 Ah) connected in series to power the aircraft on the ground and also to help the starter motors in cold weather. Just like the setup of Steve, but with bigger batteries.

EDDS - Stuttgart

The avionics work perfectly well with 24V!

Not all of them, the King KDM 705 DME is one example of a device that needs more than 24V to operate correctly. Also several modern avionics sense "engine out" when the voltage drops below 26V or so.

Why are 28V continuous power units so expensive?

This one, 100A, is GBP 2000…

One can buy a switcher module for 1/4 of that, 1-off, retail.

Some years ago I built one, using the most expensive parts possible, hermetically sealed so it could be outdoors in the rain, with milspec circular connectors, for a fraction of the price.

Last Edited by Peter at 05 Aug 11:14
Shoreham EGKA, United Kingdom

Engineering product price = £X
Same product for aviation price = £10X
If the aeroplane runs on JETA1 price £100X

Darley Moor, Gamston (UK)

I feel likewise, it’s not just ‘power units’ in this aviation world which are a bit £10x’s !!!

Last Edited by WarleyAir at 05 Aug 17:44
Regret no current medical
Sandtoft EGCF, North England, United Kingdom

28V= means limited edition means few units sold to make up for the cost of development (and certification, perhaps).
Plus, as others said, “the market will take it”.
Cheese us, Peter, you know all that better than I do.

EBZH Kiewit, Belgium
74 Posts
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