| Electronics > Repair |
| Help Identifying Vintage Component |
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| floobydust:
Note Prestolite is using the relay coil (resistance) as part of the voltage divider, giving this a tempco. Intentionally or not? Copper wire at 20°C vs 50°C around 19% shift up, increasing the trip point. System voltage at 16V for flooded lead-acid batteries is close but not quite a hard failure in very cold weather, you can find 15.5V below -30°C. Older alternator voltage-regs many did not have a "shelf" at temp extremes whereby they limit the charging voltage temp compensation, clip it at the low temps. The parts and circuit have a few discrepancies so it's not possible to extract the design secrets, but I would say there's enough for anyone to do a safe repair of the module. |
| Drjaymz:
--- Quote from: floobydust on May 25, 2023, 06:01:02 pm ---Note Prestolite is using the relay coil (resistance) as part of the voltage divider, giving this a tempco. Intentionally or not? Copper wire at 20°C vs 50°C around 19% shift up, increasing the trip point. System voltage at 16V for flooded lead-acid batteries is close but not quite a hard failure in very cold weather, you can find 15.5V below -30°C. Older alternator voltage-regs many did not have a "shelf" at temp extremes whereby they limit the charging voltage temp compensation, clip it at the low temps. The parts and circuit have a few discrepancies so it's not possible to extract the design secrets, but I would say there's enough for anyone to do a safe repair of the module. --- End quote --- Its odd that they tend to use remote regulators. That means they more often than not end up with oscillations due to resistance and interference, especially when you consider the average GA aircraft is over 40 years old. The over voltage relay is optional and unfortunately you dont know its failed until your radios explode one day which happened to a friend of mine in february as it fried the garmin glass g<something>. A good 12k of damage. You can't easily test it in place and its never tested on annual but worth doing. Theres nothing wrong with the relay and its not really a surprise that its a capacitor at fault. Later models will unlikely be repairable. The later models claim better accuracy but I think the important thing here is that the capacitor is really there to prevent it triggering spuriously so it does need to be there especially as its using a thyristor, once triggered it will stay on. I didnt know they latch on and can't find that information anywhere. But after reverse engineering that's the only way the circuit works. |
| fzabkar:
--- Quote from: Drjaymz on May 25, 2023, 09:20:10 pm ---Its odd that they tend to use remote regulators. That means they more often than not end up with oscillations due to resistance and interference, especially when you consider the average GA aircraft is over 40 years old. --- End quote --- It's odd that you should say that. :-) My parents had a 1967 Chrysler Valiant (car). The regulator was a remote type, but it was mechanical, so there were no stability issues. However, when I retrofitted a modern solid state regulator at the same position, the lights flickered with a 1 second period. IIRC I cured this oscillation by installing a relay so that the regulator was able to directly sense the alternator output (rather than via the ignition switch). |
| floobydust:
If you ever look at alternator field winding waveforms, from an old school linear solid-state regulator, they are actually low freq. PWM the field is bang-bang controlled. I expected a nice linear control but there is more gain than that, and the field winding just toggles on/off. That was a surprise to me. People running old aircraft retrofitted with electronics, ultralights as well, are finding you need bulk capacitance 10,000-150,000uF added in order to lower system ripple and noise, transients. I'd be adding some huge TVS they use for load dump as well. The 1Hz oscillations of car charging system voltage I have only seen when the battery is pooched and has a problem. A few cars with lights surging up and down in brightness at 0.5-1Hz. |
| Drjaymz:
--- Quote from: floobydust on May 26, 2023, 05:17:28 am ---If you ever look at alternator field winding waveforms, from an old school linear solid-state regulator, they are actually low freq. PWM the field is bang-bang controlled. I expected a nice linear control but there is more gain than that, and the field winding just toggles on/off. That was a surprise to me. People running old aircraft retrofitted with electronics, ultralights as well, are finding you need bulk capacitance 10,000-150,000uF added in order to lower system ripple and noise, transients. I'd be adding some huge TVS they use for load dump as well. The 1Hz oscillations of car charging system voltage I have only seen when the battery is pooched and has a problem. A few cars with lights surging up and down in brightness at 0.5-1Hz. --- End quote --- "Panel light flicker" is a very common complaint, if you look at the regulator part number you'll see other manufacturer equivalent parts and they all state "reduced panel light flicker" which tells you why people look to change it out. It is as you say either all the field current or none and there's a capacitor in the resistor chain which looks like they were trying to build an oscillator! I have a new revision M regulator which does use PWM which I never tried so might ground run with that and see if it helps. The older regulator uses a Zener reference a Darlington pair then a beefy transistor so yeah, lots of gain there. The transient suppression is taken care of with a diode and standard neon protects the main transistor. Prestolite's maintenance manual diagram attached if you are interested. This type of arrangement is probably common to 1960's automotive designs. Note that despite the diagram attached this is potted and so it really is impossible to fix. The thing about aircraft wiring and batteries for those not familiar is that they are lightweight, so both the wiring and the battery have huge resistance compared with a modern car. I feel a short alternator cable as thick as your thumb going to a good battery is infinitely superior but on the plane the alternator lead is both long at 5' and no thicker than your multimeter lead and its expected to carry 40A. Unlike a car they then have to go through a fuse or breaker, and a master switch. Which means that even when it was new it wasn't really ideal. For balance reasons some aircraft have the battery in the tail as far as possible from the engine and twins have two alternators to worry about. Then you could have 24 / 28 volt systems event on smaller singles like Cessna decided was a good idea. Ultralights use gel batteries and I think I've see lithium phosphate at aero expo but I'm not an ultralight aficionado. On certified aircraft you have to stick with the 1960's design even if they are massively deficient, again, the requirement for STC's and the impenetrable paperwork and cost requirement basically discourages improvement. |
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