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Dominator rectifier

Submitted by robin_boulton on Thu, 09/08/2018
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Heavy Twins

I've recently replaced the solid state rectifier twice on my 6v Dominator which still has the original alternator, and both times they have stopped working after a few weeks.

The first solid state rectifier had lasted nearly 20 years but I noticed this year that one of the connection looked loose and there were signs of arcing around the terminal. Because of this I replaced the 35amp unit with a 50 amp rectifier; when this burnt out I complained to the supplier and he sent me another, but the replacement went the same way.

Last winter I dismantled the primary chaincase and on reassembly thought that the clearance between the stator and rotor was okay. Could this be the cause of my problem or is it just a bad batch of rectifiers?

Submitted by robert_tuck on Fri, 10/08/2018

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Hi Robin, I had a 12v one melt with the orriginal alternator, in my case I thought it because I fitted it as a direct replacement for the orriginal selenium one but stupidly did not make a heat sink for it. Use some special white heat conducting grease as used in electronics between the parts and mount in cool air if possible ,also the connections MUST be clean and tight to avoid resistance and heat generation. Problem not caused by clearance of rotor.

Submitted by robin_boulton on Fri, 10/08/2018

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Thanks Robert, I've got my rectifier mounted on the inside of the toolbox and although having never made any allowance for heat dissipation, the first solid state unit lasted nearly 20 years, while the next two ( despite upgrading them to 50 amp ones) have only lasted a matter of weeks.

The replacement units were installed in exactly the same place as the first one!

I have tested one of the faulty rectifiers and although it looks perfect on the outside, the meter shows continuity in both directions on each AC terminal to +DC terminal.

The diodes between the AC terminals and Neg DC terminals are okay

Submitted by gordon_johnston on Fri, 10/08/2018

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I've never encountered a similar problem though I abandoned the rather dismal original 6v system many many years ago. These days I run 12 volts and use a black box from Al Oz rather than a rectifier/dodgy light switch combination with excellent results.

Submitted by laurence_king on Fri, 10/08/2018

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Hi guys, just saw this thread and thought a possible solution If you are running a positive earth and the alternator (or its connecting wiring) has an earth fault, part of the generated current would be shorted by those two diodes

Regards Laurence

Submitted by alan_osborn on Fri, 10/08/2018

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Yes and No. The encapsulated rectifier I have been selling for the last 35 years (as a respectable/trustworthy dealer!) Has been and is virtually bomb proof. The trick here is to purchase such a device from a dealer who understands the product. At this point you will probably understand that 95% of said dealers do not understand the simple bridge rectifier (which it is) so if you buy an item from a supplier that proves to be unfit then avoid that supplier.

The Yes in above is true, ie if a Lucas type motorcycle permanent magnet alternator does fault to earth then in effect there is a circuit from the alternator though the diodes to what ever earth the rectifier is connected to. This is one reason the Boyer Power Box very often failed ie a short to earth on the alternator shorted out the switched rectifier to the point of failure.

I notice we still have a rubbish spell check system on here, The system knows a word is misspelt but it can't find a dictionary to give you a real word. Hmmmm

PS Encapsulated rectifiers from A O Services are not of dubious quality.

PPS What most of you don't realize is that the rectifier has to stand up to the ignition generated voltage on top of the battery voltage.......

Submitted by robin_boulton on Sat, 11/08/2018

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Thanks to all for your advise / suggestions.

I am still using the original 6v Lucas alternator and I have disturbed it this year by changing the crankshaft sprocket so perhaps I have damaged the alternator and caused an earth fault.

If I bought a quality rectifier as Al Oz suggests, would it be okay even with the earth fault, or do I need to replace the alternator as well?

Submitted by robin_boulton on Sun, 16/09/2018

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I have taken All Osborne's advise and bought a quality rectifier from him, which seems to have solved the problem (I have had three runs out since and all seems well).

Thanks for all the help and advise. Al's website has lots of good information about testing alternators as well.

 


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