Rectifier Circuit for Motorcycle Alternator - Page 1
Forum section: electronics / projects

author: spencer2004
author_rank: Contributor
timestamp: December 01, 2014, 09:15:15 pm
content: Hello.I own a crappy Chinese 125cc motorcycle which is having some issues with the output of the alternator. I think that the rectifier is at fault and rather than simply replacing it with a direct replacement I'd rather make things complicated by making my ownMy main goal is to make it as efficient as possible, I already have some low voltage drop schottky diodes to rectify the 3 phase output from the alternator but I would like some feedback on how I should regulate the voltage to roughly 14.5V. I was thinking either a buck converter topology or transformer with a DC to DC converter chip and external mosfet.

author: Zero999
author_rank: Super Contributor
timestamp: December 01, 2014, 10:58:04 pm
content: If the alternator has a wound rotor, as in a car, it's normally done by adjusting the field current.Quite often a motorcycle alternator has permanent magnets and gives a fairly constant current output. The usual rectifier circuit is just a large zener diode connected in parallel with it on a massive heatshink.

author: Paul Price
author_rank: Super Contributor
timestamp: December 01, 2014, 11:31:04 pm
content: Your problems are likely due to frayed wires or corroded connectors or other open/short circuiting wiring problems on the bike.Even you have a defective regulator circuit inside or outside the generator/magneto/alternator itself, you need to first find out how your charging circuit works and also especially to posit why it is not working before making anything to improve upon it.

author: johansen
author_rank: Super Contributor
timestamp: December 02, 2014, 04:35:15 am
content: this is a rather common problem.I would like to make a custom regulator for my brother's bike, but i bought a new one instead.usually they are PM alternators, which means the regulator has three diodes and three triacs to provide regulation. --poor pf, poor efficiency, they may burn out if you jumpstart the bike with a dead battery... etc.

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: December 02, 2014, 04:20:38 pm
content: Quote from: spencer2004 on December 01, 2014, 09:15:15 pmThese work excellent. High regulation, particularly at low idle speeds.http://www.digikey.com/product-detail/en/L9911K/497-11624-5-ND/2746835I conjunction with this schottky bridge, you're bulletproof...http://www.digikey.com/product-search/en?vendor=0&keywords=fus45

author: Zero999
author_rank: Super Contributor
timestamp: December 02, 2014, 05:22:37 pm
content: Quote from: SoundTech-LG on December 02, 2014, 04:20:38 pmIt depends on what sort of alternator it is. If it has a wound rotor, like a car it'll be perfect but it's not suitable for permanent magnet alternators which are often used in motorcycles.

author: spencer2004
author_rank: Contributor
timestamp: December 02, 2014, 09:00:22 pm
content: Many thanks for your replies.My alternator is the permanent magnet type.I was thinking of going for something like this below with a current limiting resistor so that the battery's maximum charge rate of 900mA isn't exceeded. Also I'll probably add a 16-ish volt zenner or some other over voltage prevention for safety reasons. I was thinking of a transformer with a 1:1 ratio. I don't know what voltage the alternator outputs as of yet (the AC part) so I will need to measure that at some point.Any problems with this?

author: Zero999
author_rank: Super Contributor
timestamp: December 09, 2014, 09:57:56 pm
content: Try measuring the alternator's open circuit voltage before testing it, as it may be very high.

author: johansen
author_rank: Super Contributor
timestamp: December 09, 2014, 11:54:34 pm
content: Quote from: Hero999 on December 09, 2014, 09:57:56 pmcould easily have 150vdc out of the rectifier at 10K rpm, if it is designed to charge the battery at idle.