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Old 02-18-2010, 08:38 PM
kiai kiai is offline
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Default Fixing a motorcycle rectifier/regulator. Can the old rectifier be bypassed?

So, I've seen a couple posts on motorcycles, but no answers yet.

I've got a 1981 Honda CX500C with a 3-phase alternator. I'm not sure why yet, but this engine puts out a current of up to 24 amps or so, unlike other CX500 bikes that send out less than 20. (Maybe because of the CDI?) The voltage regulator seems to be working, but the rectifier, which is fitted into the same unit as the regulator has a burned out diode or more and the battery is not charging.

I was wondering, if a diode breaks, can I put a new diodes before and inline with the broken unit to get current to the regulator without having to remove the broken rectifier? The diodes are selenium and encased. Doesn't the current take the nearest/shortest path?

Thanks!
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Old 02-18-2010, 09:23 PM
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Do you have a factory wiring diagram of the system? If yes, please post it. The rectifiers can fail either open or shorted. I would have to see how they are deployed to figure out if they can be left in?
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Old 02-18-2010, 11:12 PM
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Default schematic of charging system

The attached is all they provide as far is wiring is concerned. Output must be 14-15V. There is a 30-amp fuse before the battery.

If I can't put 6 new diodes before the old rectifier, what should the voltage regulator have? A resistor in series with a zener diode, parallel to a transistor? I'm not sure how to spec that besides volts.

Can I use three Radio Shack diodes before the Zener diode and resistor? Would the rectifier have to convert AC to DC before the voltage sensor? I have a lot of questions.
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File Type: jpg charging system cx500c.jpg (112.8 KB, 105 views)

Last edited by kiai; 02-18-2010 at 11:28 PM. Reason: question
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Old 02-18-2010, 11:34 PM
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If a diode fails open, does that mean a phase is still AC?
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Old 02-18-2010, 11:51 PM
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I just found a spec calculator here: http://www.reuk.co.uk/Zener-Diode-Voltage-Regulator.htm

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Old 02-19-2010, 12:02 AM
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If a diode fails open, (or anything for that matter) just consider it not there.
If it fails closed, consider it a wire.
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Old 02-19-2010, 12:17 AM
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I had a CX500 Turbo from around the same vintage (more or less the same engine, different packaging). I never dug into the wiring, but from a quick surf around the web, it looks like Honda used the same setup they use on all the other Hondas I've ever owned (I think the Goldwing and maybe ST1100/1300 are different). Anyway, that setup is:

Permanent-magnet 3-phase generator;
Bridge rectifier; and
Zener-like voltage regulator

The rectifier and regulator are potted into the same package. I've often had diodes fail in the R/R, and when that happens, you either blow a fuse (if the diode fails short) or don't get enough charging current and run the battery down (if it fails open). Either way, you have to check the generator coils, because they often get damaged too.

You can replace Honda R/Rs with aftermarket ones (I've used Electrosport R/Rs with some success) and you can also use OEM units from other brands. I've heard that Yamaha makes some MOSFET-based units that are more rugged than the Hondas, you might be able to find a used one at a junkyard for a good price.

You can, theoretically, build a unit that's more robust than the OEM, but it's not easy or inexpensive. You just need a big 3-phase diode bridge and a REALLY BIG shunt -- the faster the engine runs, the higher the voltage. The R/R really just shunts enough current to ground to pull the generator voltage down to 14V or so, low enough to keep from boiling the battery.

I've wanted to build an inductive buck/boost controller for a while, but it's a lot harder than it looks to get everything working in such a harsh environment, plus you have to make it small enough to fit!

The best bet, if you prefer riding to tinkering, is to buy a replacement R/R, mount it to the frame with heat sink grease, and maybe add a 12V CPU fan that runs while the bike is running.
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Old 02-19-2010, 12:53 AM
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Quote:
Originally Posted by spacewrench View Post
You can, theoretically, build a unit that's more robust than the OEM, but it's not easy or inexpensive. You just need a big 3-phase diode bridge and a REALLY BIG shunt -- the faster the engine runs, the higher the voltage. The R/R really just shunts enough current to ground to pull the generator voltage down to 14V or so, low enough to keep from boiling the battery.

I've wanted to build an inductive buck/boost controller for a while, but it's a lot harder than it looks to get everything working in such a harsh environment, plus you have to make it small enough to fit!

The best bet, if you prefer riding to tinkering, is to buy a replacement R/R, mount it to the frame with heat sink grease, and maybe add a 12V CPU fan that runs while the bike is running.
A shunt! I was wondering if it was necessary. But doesn't the 30 amp fuse work to keep the battery from boiling?

I know how to build the rectifier; though according to that schematic attached, are those diodes leading to the voltage sensor also working as rectifiers?

I want to build a more robust unit. The bike runs. The battery simply isn't charging. (Isn't that failing closed? So, the current is AC still, not DC?) I had a rectifier problem on a CL360. I installed a new Radio Shack rectifier. Works fine now.

Last edited by kiai; 02-19-2010 at 01:02 AM.
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Old 02-19-2010, 01:18 AM
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The fuse ultimately would protect the battery, but then after the fuse blows, the alternator is still pumping out juice. And, you don't want the fuse to blow in standard operation. The shunt will dissipate the power that the battery doesn't want. And THEN if there is a dangerous amount, the fuse blows.
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Old 02-19-2010, 01:37 AM
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The six diodes used to rectify the three-phase stator windings are similar to what is used in automotive applications except that these are likely rated at about 25A at ~200PIV. What is different is the three Silicon Controlled Rectifiers (SCRs). They are triggered by the Voltage Regulator (VR), and effectively short the stator winding on a half-cycle by half-cycle basis, thereby reducing the average voltage output. These would also be required to stand off > 100V, and be capable of conducting ~25A.

Replacing six power rectifier diodes and three SCRs is not going to be cheap. Plus, you will have to devise a means for mounting them and heatsinking them. How much does Honda charge for the entire rectifier/VR module?
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