TO3 alternator regulator - Page 1
Forum section: electronics / repair

author: Nick9one1
author_rank: Newbie
timestamp: October 13, 2025, 07:20:23 pm
content: I have a very similar problem as this previous post;https://www.eevblog.com/forum/repair/to3-alternator-regulator/msg3238050/#msg3238050My alternator is generating 15v+. Unfortunately the OP's fix in the post above isn't the issue I have, so I think its the regulator.I came across this part, which looks like a simple three wire voltage regulator, mounted on a bit of copper.https://www.ebay.co.uk/itm/155842326818It's expensive for what it is, so can anyone suggest a regulator part number so I can build one myself?thank you!

author: Jan Steinman
author_rank: Contributor
timestamp: October 13, 2025, 08:38:18 pm
content: Alternators do not directly regulate their output through active devices, which would require huge heat sinks and a lot of wasted power.Typically, the only thing between the alternator and the battery is three rectifiers in a half-wave configuration for the three AC output phases of the alternator.Rather, an automotive regulator changes the strength of the field winding, which increases or decreases the magnetic field inside the alternator as output voltage goes up or down.If your alternator has two wires coming out of it, the thick one is the output stator and the thinner one is the input rotor field coil. In this case, you can go down to an auto-parts store and purchase a two-wire regulator to control the field winding.

author: floobydust
author_rank: Super Contributor
timestamp: October 14, 2025, 02:02:12 am
content: I think the original part is made by Sanken for Denso. Just a simple alternator regulator IC, a few transistors inside. I think the field winding is low-side controlled, switched to ground. Not a linear regulator IC.You can see in the pics the replacement is a TO-220 transistor bolted to a copper bar, then bolted to the alternator housing. So the tab is grounded, it's likely a PNP with a few extra parts in the heatshrink tubing.Another replacement the Carmo CARRTRG25 uses SMT parts. But it seems to be an NPN.For the dollars asked, both of these are a little hokey. Heat is a big problem and voltage surges need protection.OP if yours is at 15V, acting like it's just shorted and stuck giving full output, a multimeter resistance check between the field-ground pins (F-E) will show if that is the problem. Or else the internals are fungoli.The other thread it was actually working but the huge voltage drop on the ignition switch and wiring was causing the output to go too high because the regulator was not seeing the right system voltage.

author: voltsandjolts
author_rank: Unknown
timestamp: October 14, 2025, 08:11:14 am
content: I deleted my post above since it was mostly bollocks.So, I think this schematic is roughly the OP's alternator, with the 3 pin regulator that controls current through the rotor winding.The regulator is a NPN current sink, which ties in with the picture of the Carmo CARRTRG25 above.

author: Nick9one1
author_rank: Newbie
timestamp: October 14, 2025, 08:15:45 am
content: Adding this here as the manual is very detailed (I wish they still made them like this)

author: voltsandjolts
author_rank: Unknown
timestamp: October 14, 2025, 07:01:53 pm
content: Not an exact schematic, but for understanding the principal, this is roughly what's going on inside the regulator.Z1 and the resistor divider that feeds it, determine the current through the rotor field coil and thus alternator output voltage.The two aftermarket options above seem to have even less component count than this.

author: floobydust
author_rank: Super Contributor
timestamp: October 15, 2025, 12:43:23 am
content: It's a bit of work to make one, the design is basic but I think the challenge is the mechanical, to make something that fits and can take the vibration, water? and heat.I wonder where the alternator is located, that it gets so much water/corrosion on it, in OP's pic or maybe it's from sitting out in the junkyard?That heatshrink-special... USD $105 I don't think has enough parts to be half decent and the guy's price is gouging people.NOS one on eBay is USD $400.

author: voltsandjolts
author_rank: Unknown
timestamp: October 15, 2025, 07:19:28 am
content: The options available are:(1) Purchase original replacement regulator - very expensive.(2) Purchase drop-in replacement aftermarket regulator.The aftermarket device shown in the OP looks too feable and unsealed to be reliable. Expensive too. Not a candidate.The Carmo CARRTRG25 in reply #2 looks better made and sealed but fiddly to fit and expensive.(3) Purchase used alterator. Remove regulator and sell alternator again, without reg.(4) DIY but making something reliable would not be a quick job.(5) Find a cheap aftermarket regulator intended for something else and hack it to fit, if you're willing to experiment.Maybe the "TO-3" hacked out of this could work:https://www.amazon.co.uk/5DR-004-242-061-Alternator-Regulator/dp/B005OUOEE4/522-0959420-4055735...or...https://www.amazon.co.uk/5DR-004-243-051-Alternator-Regulator/dp/B005OUNNFU

author: fzabkar
author_rank: Super Contributor
timestamp: October 15, 2025, 08:29:19 am
content: Quote from: voltsandjolts on October 14, 2025, 07:01:53 pmI built something almost exactly like that 30 years ago.

author: Nick9one1
author_rank: Newbie
timestamp: October 15, 2025, 08:40:15 am
content: Someone on a bike forum has suggested the LM317K is a direct replacement and is working for them.Does this look like a suitable replacement?https://www.digikey.co.uk/en/products/detail/texas-instruments/LM317K-STEEL-NOPB/6176?gad_source=1&gad_campaignid=160636106&gbraid=0AAAAADrbLlgbUfrxSWa8eiYUw4Vw87PQY&gclid=Cj0KCQjwjL3HBhCgARIsAPUg7a65_fwsKrP-Va0upJ2h6nCYx06w_hAhZ5Mv87c_FkNY9ZGaz9TXIBkaAtkdEALw_wcB&gclsrc=aw.ds

author: fzabkar
author_rank: Super Contributor
timestamp: October 15, 2025, 08:45:52 am
content: Quote from: Nick9one1 on October 15, 2025, 08:40:15 amIt makes no sense at all to me. Totally inappropriate, AISI.

author: fzabkar
author_rank: Super Contributor
timestamp: October 15, 2025, 08:47:18 am
content: I wonder if this is electrically compatible:https://www.ebay.com.au/itm/388395803569

author: Nick9one1
author_rank: Newbie
timestamp: October 15, 2025, 11:49:06 am
content: thank you. I'm sure it is electrically compatible, but fitting it into the alternator would be a problem as it's form factor is quite different.

author: Nick9one1
author_rank: Newbie
timestamp: October 15, 2025, 12:03:17 pm
content: thank you for the very helpful suggestions. Some of those solutions would definitely work.I would much prefer to replace the regulator over fitting another of similar age.Forgive my ignorance, but is there something special about the regulator in an alternator that means you cant buy a direct replacement from mouser or digikey?

author: voltsandjolts
author_rank: Unknown
timestamp: October 15, 2025, 01:56:22 pm
content: Quote from: Nick9one1 on October 15, 2025, 12:03:17 pmIt's a good question. AFAIK these TO-3 alternator regulators are not available on Digi-Key or other similar electronics distributors. They're quite simple devices, so should cost about £2. But I assume they're made to order, kept in-house (not sold through distributors), so they can then marry them with plastics/brushes and sell them at huge mark-up. Another issue is the modern style of potted electronics and integrated brushes, the TO-3 style is 30 years and dying out. So, I agree, buying a new TO-3 device would be a great solution, but finding a vendor is problematic - maybe another member here has some ideas.I'm sure that a cheap alternator regulator will be available with a TO-3 device that could be cut out and then soldered into your plastic assembly. But you might have to buy a few before you find one that's suitable. If you were willing to chop the bad TO-3 regulator out of the assembly you have, and solder three long wires to exit the alternator to a regulator, my second link above might work.

author: voltsandjolts
author_rank: Unknown
timestamp: October 15, 2025, 02:44:03 pm
content: Here's a cheaper source for the option shown in reply #2, EUR24+shippinghttps://www.mvh-shop.de/Motorcycle-regulator-rectifier-voltage-regulator-YM-029It's a JMP KNVR-900"JMP is a brand of the German company Matthies wholesale company, and they manufacture their own motorcycle regulators and other parts. They produce high-quality, OEM-equivalent parts for various motorcycles. "How's your soldering skills?

author: floobydust
author_rank: Super Contributor
timestamp: October 15, 2025, 04:45:57 pm
content: OP, what do you want to do? There are replacements out there, a bit pricey. That "heatshrink special" as I call it is really basic and I wouldn't expect it to that great. WAI Global KNVR-900Make your own? It's a simple circuit but mechanically a PITA. The part dissipates a few watts of heat and seems to get direct fan cooling, but you need an mcore PCB, that's how I would do it.The Nippon-denso 25A alternator also in Triump, T1300016 Regulator Assy.

author: wraper
author_rank: Unknown
timestamp: October 15, 2025, 05:34:13 pm
content: Quote from: Nick9one1 on October 15, 2025, 08:40:15 amThat's bullshit. LM317 has 1.25V reference voltage. So without any external components, it's supposed to do what, regulate to 1.25V?

author: floobydust
author_rank: Super Contributor
timestamp: October 15, 2025, 05:47:18 pm
content: It's people that think a "voltage regulator" in a TO-3 package will work here, and no it's a different part.I'm jealous of the guy making a small fortune with his version of a replacement.It seems to be PNP, catch diode and a zener in the heatshrink as my wild guess.I would do an MCORE PCB and a few transistors as the usual fare, in these 70's style regulator circuit.Quote from: Nick9one1 on October 15, 2025, 12:03:17 pmYou can't buy the original part- it's obsolete, from Japan, and the whole automotive industry moved to using hybrid modules for their own custom regulators.There is a single-chip IC regulator butit's from outer space!InfineonTLE8881 in TO-220-5. Very complicated, needs LIN to communicate with an ECU, and has EEPROM but will not work alone. As well it's for grounded field windings, and needs the phase pulses. So it wouldn't work here but shows the industry got all super complex so idle stability and emissions would not get affected by alternator load.