Headlight blowing

[oldman]

Think the reg side is working fine, it's the rectification that's failed, lighting coil produces a.c. voltage that should be converted to dc via rectification but it's not (as I understand it) , trying to get a reading at headlamp for dc voltage impossible, only a.c. there.

[Calum]

@oldman I would have thought it would be D.C. but on the flip side there is no reason why it wouldn't tun A.C.

I would have to check the wiring diagram to know what's gone wrong.

[oldman]

Sure I read that the lighting coil produces a.c., hence the need for rectification, no other way a.c. voltage could get to the headlight unless rectification failed I think, happy to be proved wrong though, different coils for the charging etc innit??

[Calum]

@oldman But even if that is the case, 16v over the lights seems too much to me for a 12v bulb.

[scrimsmustang]

@calum said in Headlight blowing:

@oldman But even if that is the case, 16v over the lights seems too much to me for a 12v bulb.

Yeah thats why the bulbs are blowing, mind you they will blow even if the battery is on its way out also. Even if reg/rec is good.

[oldman]

2nd battery fitted, new, so no issues there, should not be a.c voltage at headlight, blow within seconds as the revs rise high and low beam?

[oldman]

Replacement reg/rec arrived today, fitted, max voltage at headlight 14v, no blowing bulbs! Anyone know test procedure for old one? Would like to confirm failure because I am ****
Ta

[Calum]

@oldman IIRC Haynes manual says impedance test across the unit should be x ohms.

I would need to look at it to get the right value though

[oldman]

Will have a look, have old version of manual, thought it said refer to manufacturer as you are too dim

[oldman]

My old copy of Haynes manual has no details to test, is there anything in the newer edition of the manual?

[oldman]

On the old one the yellow cable has a mark that looks like carbonisation but no visible breaks in insulation (even with magnifying glass), could be insulation breaking down? Has not been pinched or damaged in any way and looks whole bar the markon the cable. Have seen before in commercial building applications, will check wiring diagram to identify cable and see if that could have caused failure, photo below.

Have cleaned off surface btw to show cable surface, black coating previously.

[finnerz89]

I doubt it's the cable, probably one of the internal components

[Calum]

As said, the rectifier IIRC is just a bunch of diodes. Diodes restrict current to one direction. So the rectifier will correct AC to DC. It's the regulator that should regulate the voltage.

If the bulb is designed to dissipate 50 watts on a 14V DC supply, and you suddenly start feeding it 16V AC, well there is your answer.

[finnerz89]

@calum yeah most likely a bridge rectifier, 4 diodes arranged to smooth out the sine wave.
Regulator will drop the voltage by also dissipating heat which causes issues of its own