Yearly breakdown of alternator field diodes

[zeeotter]

About every year the field diodes of my alternator (placed on a MD2030 in a Westerly Oceanranger) break down. I have replaced all wiring and checkt, double checkt and triple checkt on loose connections. If the possibility a loose connections is discarded what could be another sources that could course this almost yearly breakdown. Any thoughts?

Thanks Gerard

 

[pampas]

ac leakage into the dc on the battery side is the likely cause.

 

[Billjratt]

Is it JUST the field diodes and not the six generator diodes that blow? The field diodes being "spiked" could be the regulator, Ign lamp circuit or brushes.
If it's the main diodes it sounds like a dud connection in the charge circuit AND the protection diode on the back of the alternator has been sacrificed.
If there's a splitter relay could it be chattering. It's good to have a proper set of splitter diodes feeding the banks for this very reason - permanent connection, no switches or clickey things in the charging circuit regardless of the 1-B-2-O switch.

 

[VicS]

About every year the field diodes of my alternator (placed on a MD2030 in a Westerly Oceanranger) break down. I have replaced all wiring and checkt, double checkt and triple checkt on loose connections. If the possibility a loose connections is discarded what could be another sources that could course this almost yearly breakdown. Any thoughts?

Thanks Gerard

Only the field diodes? Main rectifier diodes unaffected?

Dodgy regulator?
Slip rings and brushes OK?

I think I might pop the alternator in an auto electrician for a thorough check if you have not already done so.

 

[rustybarge]

Is it JUST the field diodes and not the six generator diodes that blow? The field diodes being "spiked" could be the regulator, Ign lamp circuit or brushes.
If it's the main diodes it sounds like a dud connection in the charge circuit AND the protection diode on the back of the alternator has been sacrificed.
If there's a splitter relay could it be chattering. It's good to have a proper set of splitter diodes feeding the banks for this very reason - permanent connection, no switches or clickey things in the charging circuit regardless of the 1-B-2-O switch.

Diodes are very voltage sensitive, a surge caused by disconnection of the battery load will cause diode failure.

Did you select the battery switch to 'off' when the engine was running?
Is there a loose connections to the battery?
Are the diodes overheating, too low a rating for the alt output?

 

[William_H]

I find that surprising that the field diodes die. Silicon diodes available now can be rated at a high reverse voltage 600v being normal at a current rating appropriate to the job typically to rectify one amp or so for field current. I suspect that the person who replaced the diodes each time fitted exactly the same diode when much higher voltage diodes are availbale very cheaply. i imagine someting like this should do the job. http://www.altronics.com.au/p/z0118-1n5408-1000v-3a-silicon-diode/ Just a thought olewill