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Battery Charging Circuit-Onan JC

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Old 10-19-2007, 05:24:25 PM
Geoff Morgan
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Default Battery Charging Circuit-Onan JC

I ran a search on the forum before posting this, but I will be the first to admit that I'm not the sharpest knife in the drawer on any computer formats, so thanks in advance and excuse me if I missed a previous post that addresses my question. I have an early 1980's vintage Onan 12.5 KW JC with a lot of borrowed and patched in parts, etc. The battery charging circuit is missing the ammeter (0-5-0) and the diode and heat sink. There is no capacater shown on any of the circuit diagrams and so I would think it safe to say that this is a very rudimentary charging system. I have an idea of how the charging rate is adjusted by use of the taps on the big resister on the ground side of the charging circuit. My question; Can anyone provide me with a rating or a diode number for the charging circuit. I would guess a rating something about 50 volts DC and 10 amps or 500 watts, but this is a wild guess, since that seems like a lot and the little heat sink that I have seen on other Onan JC units is just an aluminum sheet 1/16" thick and about 2 inches square. The Onan replacement ammeter was not horribly expensive from Cummins, ablut $20 (and it has not arrived on the "stock" order plan, so I may be in for an ugly surprise), but they wanted $28 for the diode and went cheap, since I'm sure that it is available at any of the electronics supply houses for much less. Geoff Morgan
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Old 10-19-2007, 08:25:13 PM
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Default Re: Battery Charging Circuit-Onan JC

Before spending money I'd check the output of the battery charging winding. The book says it should be above 19 volts AC.
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Old 10-20-2007, 03:58:43 AM
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Default Re: Battery Charging Circuit-Onan JC

The Onan parts lists show a single diode, 100V PIV so anything with a PIV of more than 100V and a current rating of 10A or so would be fine.

I would suggest getting hold of a device with faston tabs rather than solder tags, makes it easier to install.

For redundancy, you could buy a surplus dual-diode module, or even a 35A bridge off ebay and use that, if you ever lose the diode you'll have more there without changing anything!

35A bridges sell for a couple of $$'s, have a 5mm fixing hole and Faston tabs. The larger Semipack common cathode modules from Semikron, IR and Ixys are physically bigger and have dual-hole fixings with M5 tapped captive nuts.

For my money I'd go for the bridge, cheap and effective, won't need a heatsink if you bolt it down to the control box metalwork with a bit of heat conductive paste. If you try and get one of the metal-cased type, that would be better. Look for a KBPC35-02 or KBPC25-02, both of these are 200V PIV.


Bridges have two ~, one + and one -. In one of the ~ and out the +

Dual-Diode modules have connections according to their configuration. Same as above, go in on one of the ~ and out of the +.

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Old 10-20-2007, 09:50:39 PM
Geoff Morgan
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Default Re: Battery Charging Circuit-Onan JC

Thanks for the information, I'll post how things work out in the future. I think that I have an old bridge unit in my tool box; I'll check and make sure that the thing is not fried. I ran the JC unit today and checking the output of the AC charging circuit with my trusty(?) "Chicago Electric" digital volt meter, I read 8.2 volts (lead B2 to ground/case). This was measured with no control circuit in place which is to say that this was the output with only residual magnetism in the armature. Considering that fact, I will predict that the charging circuit will perform well after the armature flux is brought up to snuff with the control voltage.
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