Why do you need 100A ?
I don’t need 100A. I am actually building a 40A one for a colleague, but I looked and can’t see any 40A ones that are cheaper.
Yeah – that’s very cheap indeed. It’s based on a very cheap Chinese switching power supply which somebody screwed a handle on top of.
I wonder who makes/sells them.
The chap I am building this one for needs it switchable 14V/28V because he works on different aircraft, and realistically it needs to be waterproof because due to airfield politics he often has to work outside a hangar.
Also I think one needs to build in a completely separate overvoltage protection circuit, otherwise you could blow up a whole panel worth 100k if the power supply fails and goes up to some high output…
I’d really want the overvoltage protection especially with cheap switch mode power supplies. More than once I’ve seen an SMPS go with a loud bang and a significant puff of magic smoke, and briefly put full line voltage on the unfortunate circuit it was powering.
I agree. I have built countless PCs, all of which had a power supply way bigger than was needed, and perhaps 50% of those power supplies blew up within a few years (we run them 24/7). One notably top end and very expensive one, fanless, blew up within an hour.
I built a 28V 40A power unit for the plane back in 2002, using a high-end Lambda switching module (24V, adjustable to 28V) and that has an overvoltage protector implemented as a 200A MOSFET which the output current passes through and which is made to go open-circuit if the output voltage goes above 33V. So it isn’t the traditional “crowbar” which blows a fuse; that is always a bit drastic especially if the fuse is say 100A.
I gather that switching mode PSUs that do implement an “overvoltage protection” feature do it by inhibiting the offline switching stage, which is probably OK, but not 100%.
An overvoltage protection is absolutely wanted, yes – but shouldn’t it be part of the aircraft’s permanent wiring? Positioned just “downstream” of the avionics master switch, or the solenoid/relay that it controls?
Many years ago I had a regulator fail on a TB20, resulting in full alternator output on the aeroplane avionics bus. The overvoltage protection did not work. It blew up, along with most of the radios in the aircraft.
The regulator and the overvoltage protection were roughly the same technology as on my 1946 Ferguson tractor.
Cue … “yes but it can be overhauled forever” ^^
An overvoltage protection is absolutely wanted, yes – but shouldn’t it be part of the aircraft’s permanent wiring? Positioned just “downstream” of the avionics master switch, or the solenoid/relay that it controls?
I don’t think it is possible, in terms of the general “way of doing things” and “competence” in GA electrics/avionics.
Opening a relay would not be fast enough. Certainly not fast enough to block the most common thing which is the starter motor spike (a few hundred volts). Later TBs have a 4th relay on the avionics relay board which opens the other three as soon as the key is turned to the starting position. Same on all cars, I believe.
It really needs either a MOSFET which opens, or some kind of clamping device, which would then need to be capable of continuously dissipating a massive power if the failed power supply has a lot of capability, leading to a fire hazard.
Many years ago I had a regulator fail on a TB20, resulting in full alternator output on the aeroplane avionics bus. The overvoltage protection did not work. It blew up, along with most of the radios in the aircraft.
I don’t think the TB has any overvoltage protection. I would think that you will blow up any GA aircraft if you just put 24V right across the field winding and the engine is revved up a bit… The electrics are primitive, especially in not being sealed to moisture – a problem made worse by the older aircraft having the regulator on the front of the firewall rather than in the cockpit volume where the GT has its one.
I know somebody who blew up everything in a TB10 by starting the engine with the avionics master ON. Their insurance paid up. I know this because I nearby bought a share in it, in 2001.
Just almost finished building a 14/28V switchable 40A aircraft ground power unit
The voltage switch is under a screw-on cover (the little box on the side) which when removed shuts down the unit, so it isn’t easy to do it accidentally 
The switch is a £100 (ex Ebay actually) milspec switch.
The finished thing will be IP67 sealed (rain proof).
I built a fixed 28V one for my TB20 back in 2002 but this one is switchable.
There is a totally independent overvoltage trip set at 16/30V.
Parts cost about £700, of which £550 is a high grade switching power supply.
