RFguy Posted March 18 Share Posted March 18 From my rather critical POV, the UL twin problem indicates the these issues are ahead / beyond the UL staff and the buy is goign to need ot odo their own system design , and perhaps get advice .... My thoughts are , isnt this an obvious failure mode / system design problem that should be a well worn path by now for UL. Beware of twin of anything in electronics, same goes for everything else. Ultimately, the error though lies with the pilots not doing their checklist (check alternator contribution / battery voltage post start) 2 Link to comment Share on other sites More sharing options...
facthunter Posted March 18 Share Posted March 18 The more complex it is the more MR Murphy will have a say in the final out come. You need to fly the plane at the same time as you're playing with this stuff. Fancy expecting the Battery to raise the U/C with so short a CHARGE TIME. Nev 1 Link to comment Share on other sites More sharing options...
3rd harmonic Posted March 19 Share Posted March 19 (edited) On 17/03/2024 at 1:43 PM, RFguy said: take a good look at the fuel injection pump current requirements under various condix, they can pull a fair bit, and that you might have BOTH pumps on due to design (or error ?) Given that good quality, sealed non liquid electrolyte batteries generally do not have sudden failures, it's probably a good option as a power source. a strong schottky diode or MOSFET schottky implementation with some sort of current limit would be a simple and reliable option. A big schottky with say only 20V 30V reverse barrier will be pretty low voltage drop. maybe something like 12 or 14 gauge wire will serve as a current limit....maybe aim for wire resistance = diode drop under normal conditions, as a starting point. that will be fine as long as the charging system gets up to 14.4 , 14.6V region Good point regarding the fuel pumps - yes normal fuel pump powered off the normal bus and the reserve pump powered off the ECU bus behind a diode on the small standby battery. I've used these shottky diodes for a caravan 12V system where i wanted isolation from different charging sources, with both diodes in the package in parallel, for a very low forward voltage drop VISHAY DUAL 150V 80A DIODE PACKAGE In an aircraft i'd be looking at 2x packages, for a total of 4x diodes in parallel. The DA42 accident is really just a lesson in redundancy engineering that could have EASILY been avoided IMO. I'd say they had a partially sulphated, poorly charged battery that collapsed under load when the L/G was retracted, of course a backup ECU battery modification was suggested AFTER the accident. Whoever came up with the electrical system design obviously didn't have much imagination... Unfortunately battery voltage post start and alternator current output check ironically will look artificially GOOD with a sulphated battery. As the voltage will come up quickly and the charge current will be low with the inability of the battery to absorb any charge. A load test will quickly show a battery with high internal impedance. I don't think there's been any official findings on the UL power velocity twin accident ref reg: N106VT?? Edited March 19 by 3rd harmonic Extra comments Link to comment Share on other sites More sharing options...
RFguy Posted March 19 Share Posted March 19 (edited) too many engineers have never been technicians dont put diodes in parallel, they DO NOT current share. but by all means put two in parallel for redundancy, just rate each diode at the full requirement Diode- use a diode with a Vr of around 40V. that will have a far lower Vd than a 150V diode. If you like Vishay, suggest https://www.vishay.com/en/product/97219/ https://www.vishay.com/docs/97219/vs-175bgq045hn4.pdf At Tj=125 and If of 20A, it will have a Vforward of about 0.3V, IE about 7Watts to dissipate. bolt to a chunk of aluminium, at least 20 sq inches surface area, 3mm thick. it will need a silicon washer and bush under the tab against the heatsink, since the tabs are live. Edited March 19 by RFguy 1 1 Link to comment Share on other sites More sharing options...
3rd harmonic Posted March 19 Share Posted March 19 (edited) Yeah, i know they won't share the current evenly, although 2x devices on the same die at the same temp won't be too bad, yes it is really about redundancy. Parallel MOSFET's will because the negative temp Coefficient, BJT's with positive temp won't hence the emitter resistors that give some local current feedback. As generally seen in audio amplifier output stages (and i have repaired MANY!!)... The VISHAY bolt on devices would be a good option, abit of a heat sink would be needed. Shottky diodes don't like high reverse voltages, 60V PIV would be nice maybe. Edited March 19 by 3rd harmonic 1 Link to comment Share on other sites More sharing options...
RFguy Posted March 19 Share Posted March 19 I think 30V on a 40V is OK, that's plenty of headroom even under elevated temperature. Link to comment Share on other sites More sharing options...
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