15 Amp 13.5 VDC Regulator internal operation

[larrybl]

Hi all. for my other hobby I restore Garden Tractors. Most use a simple diode to rectify the A/C stator voltage (nominal 26VAC) to provide 13.5 DC charge for the battery. On my current restore, this GT uses a three terminal 15A regulator. The stator (26VAC) connects to two terminals on the regulator, and the center terminal is the 13.5 VDC output. I originally thought this might be a regular bridge rectifier, but the multi-meter tells a different story. I was wondering if anyone may have a theory as to what may be in the regulator incase I needed to build a replacement.

[SeanB]

I would guess it contains a bridge rectifier inside, followed with a simple high power zener diode clamp and then a blocking diode to keep the zener from discharging the battery. 15A bridge rectifier and a 15V 50W zener diode with a 15A blocking diode. In place of the zener they could also use a simple thyristor shorting the coil when the battery voltage rises to a preset voltage ( 14V4 normally) so it does not overcharge. Shorting the coil will not harm it provided it has enough resistance and heat dissipation capacity to not overheat the insulation. Providing it stays below 140C it will be fine when shorted.

[larrybl]

Thanks for the reply, the regulator for this machine is almost 40 years old. The GT was manufactured in 1978 and all original. I am familiar with how discrete regulators work, and with searches was coming up short on a 15 A unit. It make sense that there is either a bridge or dual diode setup for regulation. Where I am confused, there seems to be no internal component to case ground, I only get readings between the three terminals. I was searching for components available If I had to replace this.

[HighVoltage]

Try to remove the potting material step by step
Submerge the complete regulator in Acetone and wait a few days
Then you can peel out the potting material and the parts will become visible.

I have taken many potted systems like this apart.

If the Acetone will not work, try these:

- Methylenechloride, dichloromethane
- MEK, methyl-ethyl-ketone
- Toluene, NMP, n-methly-pyrollidone

The good thing is, afterwards you know the circuit and have the original housing to place new parts inside.

[larrybl]

I would guess it contains a bridge rectifier inside, followed with a simple high power zener diode clamp and then a blocking diode to keep the zener from discharging the battery. 15A bridge rectifier and a 15V 50W zener diode with a 15A blocking diode. In place of the zener they could also use a simple thyristor shorting the coil when the battery voltage rises to a preset voltage ( 14V4 normally) so it does not overcharge. Shorting the coil will not harm it provided it has enough resistance and heat dissipation capacity to not overheat the insulation. Providing it stays below 140C it will be fine when shorted.

I think this is most likely the internals except for the blocking diode, once the key is turned off the regulator is disconnected from the charging circuit. I have a couple high current bridges laying around, I'll see if I can find a 50W zener, shouldn't it be rated around 13.5 volts? I think the original regulator may still be good, won't know till I have the engine back together. Just looking for options to build a replacement incase it is bad. Oh, I also upgraded the lights to LED's so I shouldn't need as much current. The original lamps were 35W each and with two the current would be a little over 5A, the LEDS (although not as bright) pull 1/4 amp. 

[larrybl]

I quit searching for part numbers and just searched for (Onan Regulator) and found this for $44.99.
"This redesigned and improved Regulator Rectifier can be used on Onan P-series 16-20HP Engines.  It has a 20 AMP rating and 3 male terminals "AC-B+AC".  This rectifier is more efficient than the original and will fit right into the place of the original."
Guess I won't need to build one after all.
Thanks all!

[bills]

That would not be fun. Make one.

[larrybl]

I plan to. Just to see if I can.

[SeanB]

If there is no diode from the AC terminals to ground then you probably have a stator that has a pair of windings and a centre tap that is connected to ground. Then the regulator will only contain most likely the 2 diodes and then the big zener diode or amplified zener diode ( zener driving a transistor base so the transistor handles most of the current when the diode is regulating), or a thyristor with a charge controller to regulate the voltage with lower loss.

[larrybl]

I didn't have the multi meter set high enough, Regulator case to each A/C pin measures 196 / 86 M-Ohm when swapping the leads around. Stator winding is continuous end to end.

[johansen]

they usually contain three triacs and three diodes. and some kind of crappy voodoo circuit to drive them.


If you want to make your own, replace the three triacs with mosfets, and have an intelligent circuit drive them.. this would also give you the benefit of being able to charge the battery well below usual idle speeds.. by turning two or three of the mosfets on, you short the alternator, storing up energy in the leakage inductance.. this will require a robust capacitor nearby, but whatever.

[larrybl]

I went through a couple junk boxes and found my 28V 2A CT Transformer, and a heat slinked bridge and mounted on a plastic board, this gives me 29.4 VDC. I am limited to 2A, but this should allow me to experiment on the regulation side.

[SeanB]

Put a 12V 21W stop bulb in series with the transformer secondary, to limit the current somewhat, so you do not accidentally cook the regulator.