Burnt out regulator

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Burnt out regulator

Postby LarryV on Fri Oct 07, 2011 12:40 pm

I have a NS2 regulator installed to control my Lestek 135 amp alternator. The alternator is hooked directly to the Lifeline AGM batteries (410 AH capacity total), no switch. The starting battery is charged with the combiner. The AGM batteries are in good shape and are 3 years old.

This system worked well for 3 years then the regulator burnt out. I took the alternaor out and had it tested. No problems found. The circuit board is black in the area between terminals 1 and 3 which I think is a capacitor. The fuse also blew. It appears it shorted out first and then blew the fuse. I have the recommended 8 amp fuse in the power wire to the regulator.

I thought the regulator might just have gotten old so I bought a new one. The same thing happened after about 20 minutes.

I checked my battery cables which are new. All the connections seemed tight. I have gone through all my wiring and have found nothing obvious. The ground wire going to terminal #3 looked a little worn and may have had a bad connection.

Any ideas what may have caused this?
LarryV
 
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Re: Burnt out regulator

Postby Coulomb on Fri Oct 07, 2011 6:12 pm

Do you have a 1-2-both battery switch?
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Re: Burnt out regulator

Postby LarryV on Sat Oct 08, 2011 8:34 am

I do but it essentially does nothing. I wired it with a jumper to combine batteries 1 and 2, so it doesnt matter if its in 1, 2 or both position and the off position does not cut the batteries out of the system. The off position still works which shuts off all power to the boat.

So, the cable from the alt goes to a stand alone terminal lug which has a cable going to batt 1 and another to the switch. A jumper runs from this terminal to another stand alone terminal lug which has a cable going to batt 2 and another to the switch. Its impossible to cut the batteries out of the system but I can still use the switch to turn off all power to the boat.

I checked the connections and they were snug, but not absolute tight. I took them apart and there were no sign of arcing or anything else to indicate there was a problem connection. The only "potential" problem I found was that the ground wire from the alt to terminal 3 on the regulator may have broke loose from the terminal. The heat shrink was still intact but the strands inside were mostly broke. Doenst seem like even if this broke completely loose it should fry the reg. Wouldnt that just shut everything down?
LarryV
 
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Re: Burnt out regulator

Postby Coulomb on Sat Oct 08, 2011 12:04 pm

Good job on the battery switch wiring.

Pin 3 is the alternator ground wire that carries field current back to ground during the time the field driver is turned off. If that wire is missing, there is no place for that current to go. The result is a big transient that blows up the field driver MOSFETS.

It's the same phenomena as open circuiting a running alternator, just a smaller scale.
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Re: Burnt out regulator

Postby LarryV on Sat Oct 08, 2011 4:12 pm

Thanks Coulomb. Sounds like that is my problem. Old gnd wiring on the back of the alt is the culprit.
So, power from terminal 1 either flows to the field thru terminal 2 or to ground thru terminal 3 but just doesnt ever just "shut off", correct?

Do you think this regulator can be repaired and used as a spare? Got 2 of them laying around now.
LarryV
 
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Re: Burnt out regulator

Postby Coulomb on Sat Oct 08, 2011 4:40 pm

What really happens with field current is this: When the field driver MOSFET is on, current is dumped into the field winding. When the MOSFET turns off the field winding becomes a generator and dumps current through a diode back to ground. Like all generators, if the output is interrupted, the voltage will rise until something breaks down, I.I a MOSFET.

As long as traces on the PCB aren't burned, the unit can most likely be repaired.
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