[productionlink]

I am an aircraft mechanic working on a light aircraft wherein I am installing
a diode and a resistor across the poles of the generator control switches.

This is a modification recommended by the manufacturer a number of years back but which was not done on this airplane. The issue is that when the engines are shut down with the generator field control switch in the on position, a back EMF ( resulting from a low resistance in the gen field circuit producing a large current back through and tripping the 5 amp circuit breaker connected to the switch).

The modification involves adding a 50 amp resistor and a diode in parallel across the poles of the switch. I have identified and sourced the exact 50 amp resistor called for but can not find the diode part number or specifications.

The diode was listed as PN "RS261-506". The DC generator produces 28 volts and 150 amps.

I would like help to find the diode as originally specified... or to learn the specifications of a substitute diode suited for this purpose.

Thanks,,

Russell

[Al_The_Man]

A bit more detail (circuit) as to where you are actually applying the resistor and diode, a generator field is not that large an energy source compared to the armature.
Is the diode in parallel with the resistor and what resistance value is it?
DigiKey has a wide variety of diode rectifiers.
Digi-Key - 1N1184A-ND (Manufacturer - 1N1184A)
Al.

[productionlink]

Oops! I wrote 50 amps..........but I was thinking ohms!

the resistor is 50 ohms......the diode in the sketch appears to be silicon tube type ...........the resistor and diode are connected in parallel on the circuit from the field control switch to the generator field ....the circuit breaker that has the tendency to trip is on the other side of the switch on the wire going to the bus bar. the bus bar would have battery power ....or generator power if the engine is running and the gen is switched on ...

the generator control switch connects the field circuit and also energizes the generator contactor to connect the gen out put to the bus.

The practice is to leave the generator switches in the on position at the pilots option when shutting down the engines........but this sometimes causes the gen control (5 amp) CB's to trip......the modification is to prevent tripping

What would be an appropriate diode for this application?

thanks again

Russell

[Al_The_Man]

Having a 50ohm res in parallel I would think this one or equiv should do the trick.
Digi-Key - 10A02-TDITR-ND (Manufacturer - 10A02-T)
Al.

[Al_The_Man]

The way I interpret it is a BEMF from the field winding, not the armature, if so it is not a armature generated voltage but a collapsing one from the field?
At least that is what I got from the explanation.
A drawing would have been ideal.
Remove the field and there is no generation.
Actually an excited DC motor is a generator 100% of the time it is revolving, the BEMF opposes the applied voltage, otherwise the current at max voltage would burn the motor out.
Al.

[Al_The_Man]

The current problem at hand appears to be a degree of energy requires to be suppressed when a DC generator is turned off , and the field is still energized, but it is not clear from the posters description whether the energy to be suppressed is from the field or the armature?.
It would seem the obvious way is to remove the field at switch off would cure the problem?
This is why I suggest a better explanation or a drawing at least.
Al.