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Ground Power Unit (commercial or DIY)

A piece of engineering art. My hat is off.

LKBU (near Prague), Czech Republic

With all my respect and admiration, I do not fancy the dual voltage facility. IMHO it is sure to happen one day: someone pushes 28V= into a plane expecting 14V= , frying most of the avionics ; especially as crowbars are apparently not in vogue. It is an accident waiting to happen.

Plus my existing reserves about the built-in crowbar: I think it is on the wrong side, it ought to be on the side of the equipment it must protect; and it should be a “parallel” shortcircuit, instead of a “series” enabler. Yes I know the MOSFETs are generously overdimensioned, but if one day one blows it becomes a shortcircuit without any indication, still allowing the load current to flow under normal conditions yet leaving you with a false sense of security.

I much admire the style of schematics drawing: this is exactly the quality required. Myself generally don’t get beyond some sketches, only rarely implemented literally. My only alternative is to create the schematics in some software but all I have found yet are tedious to use. Even if I know documentation is important, I am not going to spend more time at drawing than at building.

Of course I know it is easy to give comments from the sideline, I would much rather have this construction than the “consumer class” battery charger I got from the Lidl… At least the mechanical construction is exemplary, and that’s what I never was good at.

Last Edited by at 22 Feb 08:19
EBZH Kiewit, Belgium

I agree about dual voltage. I am building this (largely as a favour) for a friend who wanted dual voltage because he works on various planes. My own one (an almost identical design) is just 28V, 40A.

Also the usual players have been doing them for years and I reckon they use cheap and dirty chinese switching power supplies. They are certainly not going to be paying 500 quid for a good one, on a product retailing at 1500 (trade ~1000) and made in the USA.

The problem with a crowbar is that it works only if the supply can supply enough current, i.e. with a big margin.

The MOSFETs do get tested when the output on/off switch is operated, so if they were to become fused-ON you would notice, hopefully They are (together) rated at 400A. At 40A, the power dissipation in them is 1.6W total (Rds just measured at 1m-ohm) so they run cool as a cucumber.

What is somewhat amusing is the temperature of the load resistors

The dummy loads are drawing just 25A in this setup, so if your aircraft is drawing 25A at 28V (as many are) now you can see how hard the avionics fan has to be working! That heat (700W) is being dissipated somewhere… My plane draws about 10A, excluding lights and pitot heat.

Administrator
Shoreham EGKA, United Kingdom

Ah, you had told me before the FET’s get tested at power on, apologies.
The Rdson of 0,001R is really impressive! Something tells me these can’t be cheap – but the money will be well spent.

Regarding the test setup: those wires are fearsomely thin to draw that amount of power, at low voltage! But of course it is a short-lived test setup only, I do not suspect you of wiring up your plane like that. I reckon the blue wire carries the temperature probe?

EBZH Kiewit, Belgium

The FETs are about 5 quid each I think. That stuff is so cheap nowadays… But obviously their 200A rating is an illusion, when you look at the thickness of their legs

The market for big power bipolar transistors is almost finished. IGBTs have taken over at the top end.

The blue wire is the thermocouple, yes. That was just for a quick check; I don’t know if the insulation can take that temperature.

Administrator
Shoreham EGKA, United Kingdom

Finished… I found I had to thicken up some of the wiring

Some warning stickers too…

The box alone is about 100 quid so the retail price would have to be about 2000 and that assumes direct sales.

Administrator
Shoreham EGKA, United Kingdom

What is the green (unlit) LED for?

LFPT, LFPN

It is actually lit; just a crappy photo… mains power input.

I am done with this box now, just as SE UK is disappearing under the rain

A better pic of the front panel:

Administrator
Shoreham EGKA, United Kingdom

Very nice! Do you take orders? ;-)

Do you take orders?

Seems you would have to qualify as a friend, then…

Last Edited by at 22 Feb 19:54
EBZH Kiewit, Belgium
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