Help Identifying Vintage Component - Page 1
Forum section: electronics / repair

author: Drjaymz
author_rank: Contributor
timestamp: May 21, 2023, 06:03:09 pm
content: I was testing an over voltage relay and it was not working (its from 1970s home built aircraft). I reverse engineered it and its reasonably straight forward, potential divider, NPN transistor activating the relay when the supply > 16V. The potential divider is actually 3 resistors, with the middle one being a trimmer, the transistor base tap comes from the top of the first resistor that is grounded.This was a dead short to ground (hence why it wouldn't activate) with a component in parallel which I assumed is a capacitor to prevent it triggering on noise etc. When I remove that component the relay works correctly switching at just over 16V.Mystery component attached. What made me question my assumption was that I wondered if it was a zener or something else and I wondered if anyone else could identify it. It looks like a polystyrene cap and the positive lead is offset which is consistent with some capacitor designs. 15V75 sounds a bit like a zener voltage but we have to remember that its also from around 1975 and may just be a date code of something like week 15 1975 maybe.The 4070 possibly the value 40n *10^7 or something buy thats a weird value. 4.7 something would be more normal.Its worth checking over voltage relays because when they don't work the first you know if when your avionics explode or your battery literally boils.PS I already had an account on here but it says I didn't exist so just had to create account again.

author: floobydust
author_rank: Super Contributor
timestamp: May 21, 2023, 06:07:02 pm
content: It looks like a tantalum capacitor, probably wet 75uF 15V.

author: factory
author_rank: Super Contributor
timestamp: May 21, 2023, 06:20:51 pm
content: Looks like an axial wet tantalum capacitor to me as well, they do seem to use odd values a lot.David

author: fzabkar
author_rank: Super Contributor
timestamp: May 21, 2023, 06:25:37 pm
content: I used to see these tantalum capacitors in old Control Data disc drives. When they failed, they would burn a hole in the PCB. They are definitely not suitable for aerospace applications.

author: Drjaymz
author_rank: Contributor
timestamp: May 21, 2023, 06:47:20 pm
content: There are actually a couple of them in there. I know that tants like to short when they fail. The relay module is a Prestolite X17621 and they are certified but obviously do fail.Being certified doesn't necessarily mean it uses military or special grade equipment and in fact a lot of what I have seen is of a standard you wouldn't allow in a car. I'd expected there to be some form of freewheeling diode or snubber but there is nothing at all, to be fair it shouldn't usually activate and it has done 50 years. The reason I took it apart was that the contacts had a fair bit of resistance, which with the ropey design of aircraft alternator systems results in strong oscillation in the charge current.Are we saying its 75uF 15V? Sounds high for a cap that isn't much larger than a 1/2W resistor and from the 1970's. I imagine that the capacitance has probably escaped over the years. I'd like to put another cap in, we don't want it spuriously activating when the radio transmits.I cleaned the contacts carefully and got the 2 ohms down to the same reading as just the test leads so thats good and there's a rubber seal which used to seal it all up but that gave up sealing probably 25 years ago and let corrosion in, so I will deal with that too.

author: coromonadalix
author_rank: Super Contributor
timestamp: May 21, 2023, 07:55:35 pm
content: hum not sure at the time if it was 7.5 uf missing dot scheme or exponent valuecan you find one not shorted out and try to pop it out of the pcb and measure it

author: floobydust
author_rank: Super Contributor
timestamp: May 21, 2023, 08:32:13 pm
content: Quote from: Drjaymz on May 21, 2023, 06:47:20 pmNo - a 15V tant. cap in something running off >16V or even 12-14V makes no sense.Do you have more info or pics of the board?

author: factory
author_rank: Super Contributor
timestamp: May 21, 2023, 08:40:12 pm
content: Quote from: Drjaymz on May 21, 2023, 06:47:20 pmThey are often smaller in size than regular aluminium electrolytic caps of the same age.The rubber seal types have a high failure rate, the seal perishes and leaks out the corrosive contents. Those with a glass hermetic seal are much better and rarely fail, unless the voltage rating wasn't high enough.David

author: shakalnokturn
author_rank: Super Contributor
timestamp: May 21, 2023, 11:20:46 pm
content: Quote from: floobydust on May 21, 2023, 08:32:13 pmOTOH if it's between transistor base and ground as it must have been to not go up in smoke 15V is plenty. As for the value I too find that 75μF is a little optimistic considering voltage rating and age.

author: floobydust
author_rank: Super Contributor
timestamp: May 22, 2023, 01:34:50 am
content: It would make sense having the cap across the transistor input E-B or in the divider chain. I wondered if it was across power internally.I'm expecting a zener diode somewhere because a voltage divider/trimpot and transistor alone does not work well to trigger from a certain voltage, it's very finicky especially over temperature. We don't know if this module latches (SCR) or has hysteresis etc.The Prestolite over-voltage relay X17621 won't win any beauty awards. It would be neat to see the insides.If the cap is across the input then it's just filtering B+ and EIA standard value of 82uF to replace the 75uF would be reasonable.It isstandard practiceto derate these tantalum caps to 50%-60% applied voltage. So a 15V part max. seen should be 7.5-9VDC.Axial solid tants are expensive ~$20 each. If it is close to the engine I would make sure it can take heat, although the original had epoxy end seals it looks like.Mil-specVishay 150D seriesclosest is either:82uF 15V 150D826 and size S code is 0.786" L 20mm x 0.351" dia. 8.9mm68uF 15V 150D686 and size R code is 0.686" L 17.4mm x 0.289" dia. 7.3mmThese are good -55°C to +125°C but harder to find. Digi-Key has no stock, Mouser has the 150D686 for $8.Alternate is molded Kemet 322 series T322E826 size E code is 0.530" L 13.5mm x 0.280" dia. 7.1mm but still $21 each. Ouch, Mouser has stock 80-T322E826K015.Today, this is an oddball capacitor so it might take some hunting.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 22, 2023, 01:44:57 am
content: Nah not tant, looks more like M39018 series aluminum electrolytic to me. Example:https://www.tti.com/content/dam/ttiinc/manufacturers/barker-microfarads/Resources/bmf-min-axial-alum-caps.pdfTim

author: Swainster
author_rank: Frequent Contributor
timestamp: May 22, 2023, 02:42:03 am
content: If it was a mil spec cap (and it certainly looks like one) then it would make sense that '4070' was the dash number... which in turn suggests that the capacitance is 6.8uF. However, if it was part of the M39018 series aluminium eletrolytics then the voltage would be 250V, with an obsolete tolerance code. On the other hand, if it was a M39003 tant then a '4070' makes a lot more sense - it would be 6.8uF 50V ultra high reliability, however the epoxy end cap doesn't make sense. Size wise, hard to say from the pic, but the tant option looks more plausible.Conclusion, maybe 4070 is a red herring...Cut it open to see what it looks like inside?

author: fzabkar
author_rank: Super Contributor
timestamp: May 22, 2023, 04:10:38 am
content: Modern tants in a similar looking package:https://web.archive.org/web/20200925215647/www.mallory-sonalert.com/tantalum/articles/TS5%20Data%20Sheet.pdfhttp://www.newyorkerelectronics.com/Mallory_Sonalert_TS5_Tantalum_Capacitor

author: floobydust
author_rank: Super Contributor
timestamp: May 22, 2023, 04:38:32 am
content: Very early, tantalums were known for high reliability and used in military and aerospace. So I say it's not an electrolytic.Logo "H" OK I found the manufacturer is Hilton Capacitor but no joy on 4070 part number.

author: fzabkar
author_rank: Super Contributor
timestamp: May 22, 2023, 04:48:27 am
content: Quote from: floobydust on May 22, 2023, 04:38:32 amhttps://web.archive.org/web/20050210214206/http://www.tantalum-pellet.com/

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 22, 2023, 05:20:44 am
content: Quote from: fzabkar on May 22, 2023, 04:48:27 amWow, looks like a 50s era photo; and the other identical but for air-brushing out the identifier (presumably after being bought out), didn't even bother to photograph new parts, haha. Nicely spotted!Tim

author: Drjaymz
author_rank: Contributor
timestamp: May 22, 2023, 07:00:58 am
content: Wow, I would say that these are exactly what they are. So we know its a capacitor.Picture attached. The board is a fibre board, hand twisted blobs soldered and epoxied on - which looks a lot like 60's era stuff. I can't easily remove the fibre board because the tiny relay windings are bonded to it at the bottom and I don't want to break it, especially as it now works. I warmed up the solder and removed the cap with tweezers and any replacement will probably go on the outside.I have traced out the diagram and will post that a little later. The capacitor goes between the blue wire (ground) and the top most blob in parallel with a 6.2k resistor. That then goes down the 20 degree slopey bit to the trimmer. And that junction has an 11.1V zener that pops out below the trimmer and goes across to the transistor base (which you can see). So that capacitor does see considerably more than 15V. In fact we have to see > ~ 12V before the base lifts above zero at all.At the end of the day, its just a ripple filter for something that shouldn't ever activate so its value isn't that important. What's important is that it isn't open or short and has some capacitance probably in the 10 -> 100uF region.I'll draw up the schematic in a translatable form because my mistake was drawing it as a standard 12V top 0V bottom schematic which confused the hell out of me. It also latches once triggered and I couldn't work out how it did that last night but maybe will have better luck today. Now I know they are caps that makes tracing it a little easier because when you're reverse engineering you tend to draw what makes sense rather than what you actually see.BTW there is another tant filtering the RHS and I tried to measure that my disconnecting one end and it has no capacitance whatsoever, couldn't get it to read on my meter, so its either duff or something really low like a few pf. But if you think about it they probably should be in the uF range given the time constant, because the relay isn't going to respond at khz or Mhz anyway its going to trigger or hunt in the hz region - which is my reasoning.So quick summary of what this is meant to do, in an aircraft alternator excitation is switchable, and because of some incident probably in the 1930s they decided they don't trust the regulator to regulate and so for safety they measure the bus voltage and if it exceeds 16V then the excitation is cut off completely. Because in flight the engine is 75% full power if the excitation fails at full then the alternator wants to produce ~ 300V and quickly becomes an issue because the batteries are lightweight i.e. 35Ah or less. That and the satnav costs 15k and any instrument that fails is probably 10k. You'd think if you bought a modern cirrus for £1 Million that it would be more sophisticated, but thats not how aviation works. No, we still have mixture control and magnetos.

author: fzabkar
author_rank: Super Contributor
timestamp: May 22, 2023, 07:51:06 am
content: Something like this?

author: Drjaymz
author_rank: Contributor
timestamp: May 22, 2023, 08:22:02 am
content: Quote from: fzabkar on May 22, 2023, 07:51:06 amThat is just the regulator. I can see it uses the zener reference to control the field current (I say control I think its on or off). So if you look at my attachment this circuit is in the regulator part. Its more or less the same as that internally. It actually has a neon indicator used as a TVS device which is quite neat, and there is one capacitor between the zener and ground. In the actual aircraft that capacitor is also toast and thats the reason the ammeter bounces up and down. I do actually have a new (June 2022) certified part for the regulator. But I wanted to check the OV relay actually worked on the bench and I subsequently found out that it didn't!

author: Haenk
author_rank: Super Contributor
timestamp: May 22, 2023, 08:52:35 am
content: Some serious measures have been take to ensure the trimmer is not re-adjusting itself. Ever.

author: Drjaymz
author_rank: Contributor
timestamp: May 22, 2023, 08:53:59 am
content: Reverse Engineering Complete (Big Clive would be proud).But, the bit I don't understand is that if R3 really does take its supply from the -ve relay coil end then when the relay is energized doesn't that then turn itself off again. Because TR1 is pulling R3+ve to ground. I thought there might be a wire behind, but probing it suggests its as shown.When it actuates what you actually see is around Vbe ~ 0.8V T1 and about 0.7 Vce. And it latches on.If you then reduce the voltage to about 4V then the relay lets go but if you increase comes back on again. Until you turn the circuit off again.So could it be that ZD1 is actually not a zenner? How does it stay on? Its markings are 4545.None of this is really relevant as I just need to replace c1 and c2 and get on with life, but I still wanted to work it out.

author: David Hess
author_rank: Super Contributor
timestamp: May 22, 2023, 09:12:19 am
content: It looks like a hermetically sealed axial tantalum capacitor to me. It could be either wet or dry but this can be determined from what type of seal it has.Without the epoxy coating causing strain in the tantalum pellet, these axial capacitors support up to 75 volts without derating and the risk of failing short so they are common in high reliability applications. Based on the part number I suspect it really is a 75 volt part but 1.5 microfarads instead of 15, which is consistent with the size.

author: Drjaymz
author_rank: Contributor
timestamp: May 22, 2023, 09:35:27 am
content: That sounds about right to me, and 75V should be ok for the intended application. You can in a 12V system see really huge spikes of any polarity but its not directly across the supply. I would say its done ok for 50 years old. But then again, we don't know when it actually failed because the relay will fail closed which is the non-fault condition.So question comes to what to replace it with? I have 2.2uF 100V multilayer - but they still feel far too modern for me and 2.2uF electrolytic, I think you just have to reckon that these will gradually get larger ESR and lower capacitance over time. Capacitors - still the dilemma they always were.On films they start up a million year old alien space ships and everything works fine, but we can't get capacitors to work for a couple of decades. Good job those aliens didn't use rifa caps.

author: Swainster
author_rank: Frequent Contributor
timestamp: May 22, 2023, 09:43:43 am
content: Hilton SWT wet tantalums appear to be still in production, now made by QuanticTPC (formerly the tantalum pellet company). You can even "tour" the inside of their office/lab (factory?) via google maps. To be honest, it looks like each cap is being made by hand - you could probably get them signed by the operatorIt's probably worth checking with them about the specs of the cap, given the application.

author: Drjaymz
author_rank: Contributor
timestamp: May 22, 2023, 09:50:29 am
content: Quote from: Swainster on May 22, 2023, 09:43:43 amNice! but I can't see it being necessary to chase down the exact parts.I looked up the relay and they did make them by hand until about 1990 about £400 and now they are cylindrical construction and advertised as with "integrated circuit" for reliability. As if measuring a voltage and activating a relay was something difficult requiring pixie dust and that being able to trip within a millivolt is somehow superior when you consider that the system is already 2 full Volts over-voltage at this point.And by the way, that relay isn't anything fancy, the field current is up to 4A and in reality a couple of amps. But for anyone who doesn't know, DC is sometimes difficult to interrupt because the electric field never falls to 0 like it does for AC.

author: David Hess
author_rank: Super Contributor
timestamp: May 22, 2023, 12:42:31 pm
content: A 1 microfarad 75 volt Sprague 150D series solid tantalum is 6.79 dollars:https://www.mouser.com/c/passive-components/capacitors/tantalum-capacitors/tantalum-capacitors-solid-leaded/?termination%20style=Axial&voltage%20rating%20dc=75%20VDC~~125%20VDC&instock=y&sort=pricing&rp=passive-components%2Fcapacitors%2Ftantalum-capacitors%2Ftantalum-capacitors-solid-leaded%7C~Voltage%20Rating%20DCIf I wanted to save some money, I might use a plastic film capacitor in series with a 1 ohm metal oxide resistor:https://www.mouser.com/c/passive-components/capacitors/film-capacitors/?capacitance=1.5%20uF&termination%20style=Axial&voltage%20rating%20dc=100%20VDC~~250%20VDC&instock=y&rp=passive-components%2Fcapacitors%2Ffilm-capacitors%7C~Voltage%20Rating%20DC&sort=pricinghttps://www.mouser.com/c/passive-components/resistors/metal-oxide-resistors/?resistance=1%20Ohms&instock=y&sort=pricing

author: Drjaymz
author_rank: Contributor
timestamp: May 22, 2023, 03:15:05 pm
content: Quote from: David Hess on May 22, 2023, 12:42:31 pmMouser doesn't work so well for me from here, but I can see these on RS. I think 63V would be ok 50V probably not, having said that the transistor wouldn't survive above 50Vce - well it might but it would be annoyed.

author: David Hess
author_rank: Super Contributor
timestamp: May 22, 2023, 06:46:20 pm
content: Quote from: Drjaymz on May 22, 2023, 03:15:05 pmI think the circuit is using the capacitor as a snubber and not for its voltage breakdown. If you want to do the later, then a 75 volt TVS diode will be more suitable.

author: fzabkar
author_rank: Super Contributor
timestamp: May 22, 2023, 07:07:26 pm
content: Quote from: Drjaymz on May 22, 2023, 08:22:02 amDoh, yes, of course. I was confused because your description and photo reminded me of the regulator I built for a mining application where there was no battery. IIRC, the field was grounded via a relay until the alternator's output voltage rose to a level sufficient to energise the relay and open its contacts. The transistor and zener then took over. The need for a battery (to excite the field) was obviated by the residual magnetism in the field.BTW, your circuit makes no sense to me, either. :-?

author: floobydust
author_rank: Super Contributor
timestamp: May 22, 2023, 08:58:31 pm
content: It won't work as drawn unless it's using an SCR. C3 is not on the schematic, part values would help.

author: Drjaymz
author_rank: Contributor
timestamp: May 23, 2023, 08:46:54 am
content: Quote from: floobydust on May 22, 2023, 08:58:31 pmI agree, because it latches on. (I think I mentioned that in previous post).The mystery component which I thought was a zener has the markings 4545 in blue on a black case and no other markings except a ring for the anode. I identified it as a zener because it doesn't do anything until just over 11.1V where it appears to clamp and as you raise the supply up to 20V its breakdown voltage only rises slightly to 11.4V.Its common to have a zener reference that turns on a transistor in these type of systems, either over-voltage or the regulator (they are both the same). But as drawn it wouldn't work because once you turned on the transistor then the transistor biasing circuit would turn it off again because its supply is now Vce which is ~0.8V. And yet, here it is on my desk working. Hence why I'm perplexed. Quite often when you look at automotive electronics from the 60's and 70's the circuit isn't as obvious as it may seem until you figure it out and realise its very cleverly done. The regulator someone else posted is very similar but that would be non-latching.I noticed that I omitted C3 after I took the photo. Its actually between Relay -ve and top of C1. Its value isn't readable but its a high voltage small capacitance polyprop. Its not measurable with my meter out of circuit so its less than 1n at this age, I think its supposed to be 4.7nF.The other odd thing is that R2 measures 1K but its colours are Pink Black Red Gold - now I have seen Pink before and I looked up a weird series where pink is 7. But it could also just be age as its not uncommon for white to fade (although usually looks yellowish). This unit would not get warm as it should only activate under fault conditions.

author: Drjaymz
author_rank: Contributor
timestamp: May 23, 2023, 08:53:27 am
content: If we ignore the relay for a moment, then the junction between R1 and VR1 would scale with the supply voltage. Current does not flow through ZD1 until Vr1 > ~ 11.1V. Then Vbe raises to about 0.7V at which point the Relay activates.Then it stays activated. And thats where I am confused.If R3 took its supply from the input then that would work, but then the relay would de-activate when you reduced the supply voltage - and it doesn't, TR1 stays hard on until the relay lets go at 3v. If you then raise again it activates again at about 7V demonstrating that the transistor has never switched off. So its latched.Now, to get further into it I could dismatle it further, but since it now works and all I have to do is replace C1, I'm probably not going to risk breaking it. So it may end up keeping its secrets.I have thought about this a bit more, T1 Must be an SCR. I don't come across those much these days and all the more important that the capacitors have some capacitance because otherwise it could trigger on spikes and will by definition latch on. I'll see if I can find markings before I stick it all back together. I could also easily test this by desoldering the "base" and triggering it manually, if it stays on then thats what it is.So whilst I know what an SCR is and how its triggered, what I don't know is what voltage will be present on the gate once its triggered (if its not connected). I mean that we know you need a threshold to trigger it but we see 0.8V after the device latches on. Now, since its a stack of silicon that may well be correct. Hopefully someone understands what I meant there.I had a look at whats available today and T092 case style Thyristors with a gate voltage of 800mV do exist and perhaps the part number is irrelevant from the 60's anyway.

author: Drjaymz
author_rank: Contributor
timestamp: May 23, 2023, 01:27:04 pm
content: Quote from: fzabkar on May 22, 2023, 04:48:27 amThis is it. 4070 is the date code. Thats week 40 1970 which is correct because the aircraft was built in 1974. So 75 would not have been it.The rating capacitance and voltage rating then must be 15V75. And it cannot be 15 volts so it must be 75V and that leave 15 which is wither 1.5uF or 15uF. 15uF at 75V I think would be physically larger, this is 13mm long, 3.4mm diameter. So that makes it most probable that its 1.5uF. There is another cap next to it and I cannot see its markings but its slightly less diameter and length and its measures just under 1uF.So its a Tantalum Pellet 1.5uF 75V capacitor.And the weird circuitry - my diagram is correct, its not a transistor its a Thyristor, what threw me was that I can't access the markings and that in the on and off state the voltages were very close to an NPN. Its a T092 800mV gate voltage and would latch as it does.I may just change both tants for a 1.5uF, I mean the tolerance of -10 / + 30 means they could overlap! I can't understand why they are so expensive and why nobody has no stock unless as someone has said, they are indeed still hand made.

author: Drjaymz
author_rank: Contributor
timestamp: May 25, 2023, 10:40:16 am
content: This is fixed, tested and working nicely.Now, going back to the circuit diagram. Why does it use the zener at all? You could just arrange the potential divider such that it was biased around the Thyristor trigger.One reason could be that the zener is a stable voltage reference. I don't really see it being any better than a simple divider and transistor.

author: David Hess
author_rank: Super Contributor
timestamp: May 25, 2023, 05:06:09 pm
content: Quote from: Drjaymz on May 25, 2023, 10:40:16 amIt is much more stable than just the Vbe of the transistor. Placing a zener in series with the base or emitter is an excellent way to make an accurate reference and error amplifier for a regulator or reference. Pick the right zener voltage and the temperature coefficients of the zener diode and Vbe cancel. RCA used to sell transistors with the zener built in.

author: floobydust
author_rank: Super Contributor
timestamp: May 25, 2023, 06:01:02 pm
content: 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.

author: Drjaymz
author_rank: Contributor
timestamp: May 25, 2023, 09:20:10 pm
content: Quote from: floobydust on May 25, 2023, 06:01:02 pmIts 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.

author: fzabkar
author_rank: Super Contributor
timestamp: May 26, 2023, 02:31:09 am
content: Quote from: Drjaymz on May 25, 2023, 09:20:10 pmIt'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).

author: floobydust
author_rank: Super Contributor
timestamp: May 26, 2023, 05:17:28 am
content: 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.

author: Drjaymz
author_rank: Contributor
timestamp: May 26, 2023, 07:18:22 am
content: Quote from: floobydust on May 26, 2023, 05:17:28 am"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.

author: Drjaymz
author_rank: Contributor
timestamp: June 07, 2024, 01:50:20 pm
content: Future me here - I've been flying this thing for another year or so and occasionally the Over Voltage relay kicked in and no problem I just turned off the ALT switch, turn it back on again and all is well for the rest of the flight.When using the landing light at take off (thats 100W) and selecting flaps in, when the flaps hit the stop (there is no limit switch) it crow-bars the 12V bus due to the heavy load. This causes the charging system to work harder and the voltage occasioanlly overshoots resulting in the Over Voltage relay tripping in. Except now, it will NOT reset until the engine is shut down. So this means that current is still flowing through the over voltage relay thereby latching the SCR.I have supplied the entire charging circuit including the over voltage module we reverse engineered last year.In the cockpit the ALT warning light has now decided to work. It comes on when the ALT switch is turned OFF. I never saw this light before, it didn't used to work with the ALT switch in any position.Looking at the diagram the ALT warning light has one side connected to the 12V bus and the other connected to regulator. That means current is flowing from the main bus to ground via the warning light.I have checked and this is the way its been wired for 50 years but doesn't make a lot of sense to me. The warning light looks like it may prevent the OVR being reset by providing some current preventing the SCR resetting. Secondly how does the warning light work?If the switch is off, then the other side of the bulb must be seeing a low / 0V. Terminal I on the regulator can't pass current to ground because it has a diode (except for the resistor network) - it has to be passing it through the OVR coil. If thats the case - how you supposed to reset it in flight - or perhap you are not meant to/ The flight manual states that you shoudl try cycling the ALT switch and if that doesn't work then shed as much load as you can and land - which is thankfully obvious.Last time this happened I tried turning the master off and that didn't reset it either. I think thats because the alternator has its own ground and therefore if even the tinyest current flows through the fild coils, it will produce an output unless the engines stops. And sure enough when you turn the engine off, it resets.Now - this is Aviation and therefore this is a terrible design with long thin wires carrying large current, its also 1970s and on top of that its French.So what is my actual question? That ALT warning light that has just decided to start working - do you agree with me, that is preventing reset of the over voltage relay? What is that light supposed to indicate - you'd think it means the alternator fault but its a bit ambiguous, because it could be off and the generator isn't generating.Now, I have also attached the actual flight manual in case I have missed something. But seems to me that it would have been better if the over voltage relay output marked REG went to the input of the regulator which is I presume why its marked REG. And you can see by the tenuous OEM diagram that the OVR Doesn't go to the regulator.

author: floobydust
author_rank: Super Contributor
timestamp: June 10, 2024, 09:08:14 pm
content: Kinda quiet but I'll stick my neck out here. You got a few issues.The OVR SCR holding current is typically low ~5mA so the ALT lamp is enough I would say to keep it latched on. Did you replace the SCR or is it stock? Also note SCRs get much more sensitive when hot, if the module is located near the engine.It looks like a bit of a design flaw that you can't unlatch it unless the ALT breaker is open and the lamp pulled out. Or the regulator is also playing a part here and not loading down the I input as usual and stealing current from the OVR module when the lamp is on (so SCR can't latch). I think the regulator (I terminal) voltage drop might be 3-4V so the lamp would be normally a bit dim when on. But the SCR latching on would make it bright.Stopping the jammed flap motor is basically causing a load-dump around 400msec duration (I don't know the alternator size/p/n) but your battery should soak it up. It might be getting old or high resistance in some wiring and so the bus voltage kicks up past 16V. It would be hard on any avionics, have you considered adding bus protection for that? Ultra-lights (with PM generators) they add bulk-capacitance 20,000-100,000uF to smooth out bus power.The Bendix OVR module ideally would have a time-delay, be slow and take a second or two before tripping - but those cap values are too small for that.C3 is in a wrong place and mangles any delay but protects the SCR from +ve voltage spikes and some extra dV/dt coverage- there's no back-EMF diode/TVS on the relay coil. Ick. A bit of a kluge, they must have had a problem with the OVR, it's not the greatest design.

author: Drjaymz
author_rank: Contributor
timestamp: June 11, 2024, 07:17:22 am
content: Quote from: floobydust on June 10, 2024, 09:08:14 pmThanks for your reply. I can confirm that the light absolutely is why the OVR doesn't reset. The bulb didn't work for years and was recently damaged and replaced (there is a seperate LOW-V light which is mandatory). It now works and this interferes with resetting the OVR. If you look at the terminals one is marked BAT the other REG it would have made far more sense to connect the REG end to the REGulator Input terminal. This is I think the way its meant to be. But you know - French. The SCR is stock. I removed the bulb and now you can reset it no problem.The alternator is an ALY 8403 which is I think 40A, part of the prestolite maintenance manual claims its 60A but all the fuses are 40A and the wires are not much thicker than your multimeter lead, these would be swapped out for a different ALY alternator if overhauled but you're looking at £1500 at least. I checked and all the diodes are working, the windings are roughly even resistance but the slip ring contact was poor. With my meter on continuity across the field terminals when you hand move the prop the beep is scratchy. I removed the brushes, the looked fine about 3/4 inch on both and then I used emry on the dull slip rings. Which improved it a bit but I don't know how they can be improved further without renewing or machining. I also measured resistance from battery +ve to alternator output and to regulator input and wiggled all the cables and panels to see if I could find a poor connection. That all looked ok. The master switch is on the +ve side and not as depicted in the POH - which is no suprise.Looking at the OVR yes, you are correct that C3 will directly feed any supply AC noise directly into the top of the zener which isn't what you want - its really difficult to trace from the photo and I may pull that and have a look at it again. If you remember we guessed the value of C1 and C2 and the originals were more or less open circuit, I couldn't get a reading on any of them out of circuit, my meter just showed erronious values and then I tried a chinese component tested which thought it may be a 137pF cap before decided the others were just resistors of wandering values. I would say worth increasing the value of C1 and C2 with better quality caps. I would say noise immunity is mainly controlled by C1 which would have some time constant.To the battery itself, at annual they will note how well it turns the engine, check the plates are covered and may measure the specific gravity. I look it to an automtive place which measured its cranking current which was in spec - slightly better than spec. The battery is from 2016 and the internal resistance will have risen which will make all of the problems I am having worse with time - like they have done, one cell could easily be much worse than the others and will often still give a good current output but will sag a lot.https://sandalwoodservices.com/battery-dr400-certified---611817010-400605-401-p.asp<-- this is the battery, replacement isn't too expensive.So I think that I have a combination of issues.Aging Battery, sensitive OVR, alternator slip ring contact isn't perfect, french spindly wiring.

author: Drjaymz
author_rank: Contributor
timestamp: June 11, 2024, 07:29:16 am
content: I should just add - if it wasn't a certified aircraft I would swap out the battery for Lithium Phosphate Aviation Replacement, rip out all the wiring for modern better spec wire and replace the alternator / OVR and regulator (which would probably be in the alternator) and put limit switches on the flap position or have it pre-selectable. Replace the landing light with AeroLED. All of which could be done for less than the price of a (s)crap alternator overhaul. Then I guarantee it would be much safer, more reliable and operate without issue for another 20 years. - But because its a certified aircraft it will have to stay with an unsafe battery and charging system that isn't fit for purpose.Is certification for aircraft that are > 40 years old where most parts are not procurable actually counter-productive - discuss?

author: David Hess
author_rank: Super Contributor
timestamp: June 11, 2024, 11:07:13 am
content: Quote from: Drjaymz on June 11, 2024, 07:29:16 amNobody at the FAA every got fired for keeping things as they are. Notice how lawsuits killed all of the civil aviation companies.

author: Drjaymz
author_rank: Contributor
timestamp: June 11, 2024, 11:38:10 am
content: When you have a production of aircraft and you find that something specific failed leading to an accident - lets say cracks on nose wheel assembly (because that happens to every aircraft type) then the certification process allows them to find all aircraft, serial numbers that would be exactly the same - so in theory that traceability is a good thing. The problem with very old aircraft (and remember GA aircraft average close to 50 years old) then it becomes impossible to keep compliant because parts are not procurable and salvage spares aren't available either because they would all have similar wear. I don't think I've seen a 100% compliant Cessna 172 for at least 20 years. Indeed the FAA and CAA now have relaxed quite a few things for standard modifications, but its still painfully slow. Using the AeroLED landing light as an example - basically a par36 LED spot, only draws 4A instead of 10 and doesn't blow. Works 9V - 28V constant power, much less heat. Now for a 172 it was worth their bother getting that approved as a standard modification, but if you have an aircraft type where there are only a handful built - then no chance it will be on that list despite the engine, electrical system being the same and its a drop in replacement. As result the official paperwork and the practical reality diverge significantly as people get them bodged behind a shed somewhere - I am sure the situation got worse permanently following Covid because parts availability became either years or not at all.