Voltage Regulator help

Two wires feeding the "magneto" to generate a magnetic field? Sounds like you're describing an alternator. An alternator uses a field winding to generate a current-controlled magnetic field. A magneto uses permanent magnets for the magnetic field.

I'm not at all familiar with the electrical system on your bike, but to regulate the alternator output voltage by adjusting the field current is a very different task than what a linear voltage regulator lime the LM117 does. I would think you would need to compare the alternator output voltage to a reference, integrate the difference, and drive the field coil with a transconductance amplifier.

But if I remember my car's electrical system correctly, they actually use the battery to do most of the voltage regulation. The "voltage regulator" toggles between two field current levels, fast charge and trickle charge. This simply involves two resistors, a relay to switch between them, and a comparator to sense when to make the switchover from low to high. I don't know if your bike is made to work the same way.

If you wanted to build a regulator as a fun learning project, I would encourage you to go for it. But to save money? Unless your time's worth just about zero, you'll come out ahead paying the $100.

Any chance of opening up your failed unit and repairing it? If so, that would be a lot better than reinventing the wheel. After all, the original design was developed with the help of a number of lessons learned the hard way.

I wish i could just pop open the old one. its all sealed closed with epoxy

ok so i managed to get the old one apart. with a lot of patience and scraping away of a big rubber block.

I see a rectifier bridge and a regulator. the regulator has a component 44E3 Which i have yet to find any information on. But anyway that component was desoldered from the board. and clearly fried on the underside.

Ok so i found out the part

MJD44E3-1 : NPN DARLINGTON SILICON POWER TRANSISTOR 10 AMPERES 80 VOLTS

20 WATTS [MOTOROLA]

Does your bike's alternator use field coils or permanent magnets? Makes a big difference in how the voltage is regulated.

This last sentence I don't understand. The magneto and the charging system are completely independent. They aren't even connected. Are you sure you have a magneto? One generally sees them on small motors, not mainstream road vehicles. I've seen some Harleys with magnetos, but we're talking about older bikes and radical customs.

If you can come up with specifics like this, you ought to be able to describe your system in a way that makes more sense.

No you can't do it that way. I guess the only way to address your problem is to look up the wiring diagram of your particular bike to see what kind of charging system you have. Do that, and post a link if possible.

You could try soldering in a new one and hope for the best. If you can't get the exact part substitute something with similar specs. Digikey has a parametric part search that comes in handy for stuff like this.

You're doing great! The MJD44E3 is 49c at

Of course the s&h would be a killer on a single 49c part, but at least you know the part's available and inexpensive.

If the regulator's not too complicated, the best thing to do would be to reverse engineer the circuit (trace out the schematic). With the schematic in hand, often even if it's not 100% complete or accurate, you can begin to understand how the circuit is intended to work. That will help you whether you decide to repair the old one or build up a new one.

Of course, you could try replacing the transistor and hope for the best. But between the possibility that some other failed component blew out the transistor and the alternative possibility that the failing transistor took some other components with it, it would really be a shot in the dark. If you choose to go this route, at least take an ohmmeter and verify whatever components you can. You might pick up a shorted diode or something.

By the way, just because the transistor got hot enough to melt solder is no proof that's it's bad. Give it the ohmmeter test and see what happens. You never know. While a prudent repairman would certainly replace it even if it tests ok, . . . you do have this one in hand, right? If you're new to transistors, see

for the simple go/no-go test.

Field Coil, (thus the 2 wires running to the rotor.) They connect to carbon brushes that ride along the copper slip rings on the rotor.

read this for some ideas