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Random avionics internals

Let’s say it is a 35V zener; at 5A that is 175W

But who said anything about 5A? In the interval before the CB does its job, this zener is sinking the output voltage without any current limiting at all except as provided by the wiring and other components. The alternator is rated 120A so it will probably see 100A or more, i.e. 3500W. It’s unlikely it will survive long enough to trip the CB.

Putting a dead short across the bus doesn’t seem a great idea for the rest of the aircraft, either.

Last Edited by johnh at 27 Nov 11:52
LFMD, France

I am referring to a possible condition where you get overvoltage but the current drawn by the zener is not quite enough to trip the 5A CB.

Depends on the CB type. If it is the usual avionics thermal type, I agree that the zener will probably be smoking well before the CB opens. But it might be a magnetic one.

Zeners are bad for this anyway because they have a low mass junction and can’t handle large transient dissipation. It is like comparing metal film resistors with moulded carbon resistors; for snubbers you need to use the latter. They are thus not great at clamping unless the max current is well defined elsewhere.

I have sometimes used zeners for spike clamping in “vehicle” applications but used big ones like this:

Administrator
Shoreham EGKA, United Kingdom

johnh wrote:

But who said anything about 5A? In the interval before the CB does its job, this zener is sinking the output voltage without any current limiting at all except as provided by the wiring and other components. The alternator is rated 120A so it will probably see 100A or more, i.e. 3500W. It’s unlikely it will survive long enough to trip the CB.

Putting a dead short across the bus doesn’t seem a great idea for the rest of the aircraft, either.

But it won’t be a dead short as the Zener will probably be rated 15 V or thereabouts. As long as it survives, that is. So the power would be 15 V * the current the alternator can source when it is loaded enough to reduce its output voltage to 15 V.

ESKC (Uppsala/Sundbro), Sweden

True, but my guess is that the current limit under those circumstances will be the zener itself, given the rest of the circuit is rated 120A. I suppose voltage across a zener follows the same rule as for the forward direction, i.e. proportional to log(current), plus the resistive drop across the mass of the semiconductor. Either way, my money is on the zener exploding long before any mechanical circuit breaker can trip.

LFMD, France

Yes; this circuit is a total cockup.

Administrator
Shoreham EGKA, United Kingdom

The last few points are a reminder to me that maybe not everything one covers in the IR theory is BS.
Not being into electronics and such I remember almost banging my head against the wall trying to get resistors, diodes, ohms law etc into my thick head for the exams. For me it was the hardest part of IR theory by far.🤨🤪🙂

France

The level of teaching electrics/electronics in aviation exams is poor and is way below what one needs to understand how the above circuit works. There is really no point in teaching about components at the IR level, or at the EASA66 level.

Administrator
Shoreham EGKA, United Kingdom

For me it was the hardest part of IR theory by far.

I had no idea. why on earth would you need to learn about electronics to get an IR?

The more I look at this circuit, the less it makes sense. There’s an old caustic remark from the 17th century about English women’s shoes, that they “appear to be made by someone who has heard shoes described but never actually seen any”. Ditto this circuit.

The push-pull arrangement feeding the FET makes no sense at all. The load is in effect a ~1000nF capacitor, which the op-amp could perfectly well drive directly. Further, since there is no leakage path, as soon as either transistor turns on, the voltage will change rapidly (depending on the value of R28). I wouldn’t be at all surprised if the gate voltage on Q3 oscillates like mad., though it depends on the value on C1B.

What is VR2 for? Putting a zener on the input to a voltage regulator makes no sense at all.

And we won’t even talk about the rat’s-nest around U3/K1.

LFMD, France

Surely this is a PWM rather than linear regulator? There is positive feedback via R4/C4 which will flip it rapidly off/on. So it is a electronic version of the old vibrating contact regulator entirely in keeping with the pre-war technology it’s connected to!

Alan

EGST + Tibenham, United Kingdom

I don’t think this is PWM. U2 is a straight op-amp (the pin numbers are for sure) and R4/C4 form a single pole control loop with it. I see no circutry generating e.g. a sawtooth waveform and a comparator generating a PWM pulse train.

Administrator
Shoreham EGKA, United Kingdom
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