View Full Version : Converting an Alternator from NEG to POS

23-07-05, 08:10

I got a Balmar smart charger, and I found out that I need to covert my alternator from NEG to POS and then drop a field wire out of the alternator. How hard is this to do? I am told it is not hard for a professional, but I cannot find one that knows what this is or how to do this in Croatia.

I have a french made NEG Valeo alternator and there is no documenation on the insides of the alternator anywhere on the web. From what I can see, I need to solder a wire inside the alternator, but I am not sure how to switch the NEG to POS.

thanks for your help

23-07-05, 09:59
It will need a different rectifier ie one with a positive output rather than a negative one and a different regulator ie one for a positive output. Neither should be a problem as alternators more commonly have a positive output. I've never heard of a negative one in fact.

You'll have to change the battery round and reverse the supply to every thing else that is polarity sensitive.

Are you sure about this?

23-07-05, 13:44
Check that they mean mean how the alternator is regulated, not the output.

To regulate the alternator output voltage you switch current in the rotor circuit, this may be between the field diodes and rotor, pos control, or between the rotor and ground, neg control.

Normally you will feed the field diodes to the brushes, through the rotor out the second brush, through the regulator to ground, neg system.

To convert to pos control, the diode feed is removed from the brushes, and feed to the new regulator, the output from the new reg is feed to the brushes, this goes through the rotor and out the second brush. You now need to bye-pass the regulator by connecting the brush to ground, giving you a pos system.

Does this make sense to you?

all the best


23-07-05, 16:02
If this is truly an alternator and not a DC-generator, the field polarity will have no effect on the output. The reason for this is that an alternator produces alternating-current (AC). The AC is then rectified by diodes to produce DC current. The polarity of the DC is dependent only on the polarity of the diodes. Most alternator-rectifiers I've seen use six diodes, three of one polarity and three of the opposite polarity. The case of the diode is one pole and the lead coming out is the other pole. I believe it would be possible to reverse the polarity by simply reversing the position of the three positive diodes with the three negitive ones. They are usually a press fit. You will first have to unsolder the leads, then press them out. Then reverse their position and re-solder the leads.

The field connection usually comes out to a terminal on the frame of the alternator for connection to the regulator.

23-07-05, 16:05
There are usually no diodes in the field circuit.

23-07-05, 16:23
It's the control circuit that is either NEG or POS control, not the alternator output. It controls the POS feed to the rotor, or the NEG side to earth.

There are three field diodes, they produce an independent pos to feed the rotor, that is only available when the alternator is turning.


23-07-05, 16:46
Gosh this thread is getting more and more confusing.

Re the field diodes, some atlternators have 3 diodes, one from each phase, to supply the field current (via the regulator) while in others the field current is supplied from the main rectifier bank but then an additional diode is incoporated in the main output to block the possible curent flow back from the battery to the regulator when at rest.

It would not be possible to reverse the polarity of the rectifiers I have seen as Q_E_Too suggests but I'm not suggesting he is wrong in saying that it may be possible with some.

23-07-05, 18:31
Vic if the regulator is before the rotor it's positve reg, if the regulator is after the rotor it's a neg reg. Depending on the design of the regulater it can be pos or neg, most are neg these days, but if you fit a smart reg it may be a pos one, thus you have to change from a neg to a pos alternator.


23-07-05, 19:01
Got that , I hope. This what you think Okolehao2 meant when he said he wanted to change from neg to pos. I assumed he meant he needed to change the output polarity. (wrong again) It certainly makes more sense. I can't picture though what he needs to do as I've never seen a circuit diagram for a neg regulated machine.

Okolehao, ignore my original reply that, it seems, was rubbish

23-07-05, 22:32
Yup, the Valeo alternator has a NEG internal regulation and the Smart Regulator from Balmar has POS regulation. I assume that I need to remove the old internal regulator, since you cannot have a POS and NEG regulator on the same circuit together. In reading your posts, I am still not sure what I need to do with the field wire. Do I need to tap the Field wire from a different location for the POS Smart Regulator to work?


24-07-05, 01:59
OK on many auto alternators the regulator is a plastic box attached by 2 screws to the back of the alternator. It likely has a wire connector to it which goes to the alternator lamp and then to the battery via a switch. If you remove the 2 screws the regulator comes out with 2 brushes attached to it. These when fitted go into the alternator to contact the slip rings on the rotor.
You would need to acquire a brush mount that doesn't have a regulator attached or destroy the regulator wiring so that the brushes are still mounted and attachemnet can be made to the brushes. you then need to find the diodes associated with the field supply and make a connection to them. Of course there are many different types of alternator and I am perhaps more familiar with Japanese or American alternators.
So this is an easy job if you know what you are doing but very tricky if you don't. In practice you may be better off getting the Valeo people to mod an alternator for you or get an alternator of a type where they provide clear instructions on modifying. Don't count on your average auto electrician being able to sort it out for you. Sorry this may all be inappropriate anyway good luck olewill

24-07-05, 02:00
Sorry meant get the Balmar people to mod an alternator. olewill

24-07-05, 10:38
Solder two wires capable of carying 5amps, one to each Brush connection and bring out of alt, apply the power with the alt stationary, measure the voltage to the case, the one with the the positive voltage is the one you need, insulate the discarded one, problem sovled.

24-07-05, 15:05
Yes, I think that is what I need. Remove the internal regulator. Connect two wires, one to each brush. When you say apply power, do you mean reinstall the alternator back in the engine, and run it? Then measure the voltage at each brush? One wire should read +5v to +14v and the other should read 0v? The one thats read positive voltage is the one that becomes my field wire for the smart regulator?

Also do I need to do something special to make my engine control panel function properly? I think I can get a tach out wire from my smart regulator.

25-07-05, 01:18
/forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif Get a smart regulator that can be setup for either POS or NEG. Saves the trouble of messing with the alternator. Sterling does those /forums/images/graemlins/cool.gif /forums/images/graemlins/cool.gif

25-07-05, 07:06
The Sterling Regulator won't change the polarity of the alternator. It's pos/neg option (doesn't exist on my older model) is for alternators which are already wired as either positive or negative.

To reiterate what has already been stated. The polarity of the output of the alternator is given by the diode rectifiers on the main windings. You have to change these (and the regulator) to alter the polarity!

25-07-05, 07:34
You need to locate the feed from the field diodes to the brushes, cut this wire in two, and connect the the two ends to your new external regulator. The existing regulator you need to bridge out, that is short the input to ground.
How easy or difficult depends on your alternator construction, with some newer ones, it may not be possible, ie combined brush box and regulator.

Best of luck


25-07-05, 13:33
I'm still thinking about this question.

With a NEG regulated set up presumably there is a negative feed from the field diodes to the regualator but a POS regulated set up (the kind I'm familiar with ) is going to need a positve feed to the regulator. This means that the field diodes will have to be reversed. Whether or not that is possible will depend on the design. It could mean replacing the whole rectifier/diode assembly with one intended for a POS regulated machine. If the Valeo alternator is of the type that doesn't have a three field diodes but an "isolation" diode instead (I'm looking at the diagram of an old SEV alternator) then that will have to be positioned in the positive ouput.

The wiring of the warning light must be different as well surely.

It all sounds like work for a specialist or a question of replacing the existing alternator with one compatible with the Balmar charger.

Or am I still totally wrong about all of this.

25-07-05, 13:44
No it's simpler than that.

A POS reg goes in the feed to the rotor, ie 12 volt.

A NEG reg goes in the ground side of the rotor, ie 0 volt.

That's it, all other items are common to both regs.


25-07-05, 13:48

you might find this document (http://www.xantrex.com/web/id/760/docserve.asp) useful for a conversion from N-type to P-type

25-07-05, 17:32
Really is simple then! Amazingly simple in fact. Presumably the field diode/isolation diode issue is all taken care of by the Balmar smart charger also the warning light.