Converting 3 Phase 70VAC to 12VDC

Thread Starter

Oliver Müller

Joined Dec 30, 2017
Hi guys,

I've got a alternator which provides 70Volts of 3 Phase AC and I want to convert this into 12V (or other voltage) DC! I know I have to use rectifier, but in which combination and which one have I to choose?

LG Oliver ~ Germany


Joined Oct 8, 2017
Is the alternator off of a motor vehicle? If so is the built in rectifier and regulator no good? We need more information. At least I need more info.


Joined Jan 15, 2015
Yes, you want a 3 phase rectifier which are very common and the circuit would look a little like this:
3 Phase Rectifier.png

The problem is while you mention 70 Volts you make no mention of the 12 Volt current demand. You are not providing enough information. Additionally as mentioned if the alternator was or is an automotive type there should be a way to control the field winding and control the voltage out. The above drawing does not show a field winding.



Joined Apr 10, 2015
You need to use a transformer, fed from the A.C. generator (aka "alternator") to get the correct voltage, then rectify with a diode bridge like Ron shows.

Power (measured by us as watts, but close to Volt-amps) is a constant. For example, if your generator is 12kw, at 12 volts it is capable of 1000A current! (I suspect your generator is smaller than that, so a 6000watt gen would be about 500A @12V, etc.)


Joined Oct 8, 2017
Don’t forget that alternators from vehicles will need some sort of excite voltage. I think that’s what it is called. It is used to start a electric voltage in the windings. After it starts to produce the voltage it becomes self sustaining.


Joined Jul 18, 2013
Don’t forget that alternators from vehicles will need some sort of excite voltage..
It may be possible to use residual field if it is present, if so, the output is fed directly into the field and soon as it reaches 12v it switches over to the regulator, same as a DC generator does.


Joined Feb 20, 2016
Really, as you have been asked, more info on the alternator is needed. What current do you need from the output and what is the power of your alternator and does it have any control of the output? And what are wou driving the alternator from?
I do wonder why it is so hard to get info from a lot of Thread Starters.....
But instead of using a transformer ( 3 phase transformer needed) to drop the volts, if you cannot control the alternator directly down to lower volts, rectify the output and use a switch mode supply to drop the volts to what you need.
Rectified and filtered 70V out will be getting pretty close to what is required to run an off the shelf universal input supply/charger so I'd investigate that.