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Voltage regulator Design Problem? Answered

 Ah, voltage regulators. Easy Right! Not this one.

I am making a voltage regulator like for an automobile regulator for a DC generator.
(I'm replacing the old DC generator regulator on Pre 60's cars)
A battery is charged from a winding on a Generator  (20V @ 60 Amps)
A voltage sensor on the battery voltage turns on and off a switch driving the Field winding.
The Field winding basically "turns on and off" the generator output.
The battery is connected directly to the generator. (Why  and Why not is coming up)

But when the Genrator's output is lower than the Battery voltage, current will flow back from the battery to the generator winding and discharge the Battery. So I need some isolation between the generator output and the battery.
Alternators are easy since it has a bridge onthe output keeping the current from flowing backwards.
Old regulators used a relay which burned out sooner or later.

So easy, use a diode right? Just put a diode inseries with the generator to the batery.
By the way, the "open circuit" voltage on a generator is about 200VDC

The best diode I can find has a .7V drop at 60 Amps which is 42 Watts of heat to dissipate.
Ahhh, so use a FET, They can get down into the milliOhms right?
Except for the reverse polarity (and ESD) protecton diode always across the FETs defeat the purpose of a reverse polarity switch.

So who out there is smarter than me?
What goes inthe box marked ??
Any solutions accepted.
Ungefahrt (now neither young or fast)



6 years ago

I don't see where the objection here to a diode is, yes, it dissipates 42 W, that's why car diode packs are meaty things !


Answer 6 years ago

My objection to replacing a mechanical contact with a non glowing semiconductor seems obvious to me.
In the small space without sufficient heat sinking needed, the better answer (as in all semiconductor designs today) is more efficiency and longer life (Hot things don't last (see Marilyn M).
I want to get it down to a few watts. I build switching regulators that pump out 300 watts in a small space because I can get 98% efficiency with FETS. Just the dumb diodes aren't improving.


Answer 6 years ago

Use a dual 30 A Schottky diode then. Should give you 18 - 20W. You COULD I suppose you N channel and P channel Mosfets in series, and create a kind of synchonous blocking rectifier, but I can't see you beating 18W dissipation.



6 years ago

1. Old Bosch Regulators are no longer available.
2. If you find the correct one, it costs $200 for a used one.
3. The "relays" used inside usually have burned poinbts and only last a few months.
4. Being both a car guy and a analog electronics designer, I thought I'd make a solid state one.
Yes, we can spend a lot of bucks and buy used ones when I can find them, or maybe I would be smart and build better replacements.
THey are broken and I understand fully how they work, I wanted better.


6 years ago

I don't really understand what the problem is.

>Does the old DC generator regulator not work, and do you know why?
>Why not replace it with an alternator?

I get the idea that something is broken, but you don't fully understand how it used to work.