Alternator Control 5V PWM

My alternator is a P-D type. The P terminal requires a 0-5V PWM at 250Hz to initiate charging and control the voltage output.

As far as I’m aware there isn’t a suitable output from the ECU (maybe a spare IGN output - I’m not sure if they’re 12V or 5V).

Couple of ways I’ve thought of controlling the alternator:

  1. Switched 12V to a DC-DC converter, 5V from the converter to the alternator then use an AUX output to pulldown PWM. I’m not sure if this will work as the DC-DC converter outputs a regulated 5V and would probably freak out being pulled to ground 250 times a second - I’m not sure if this is correct, it might be fine…
  2. PWM from the Hbridge (12V) into a voltage divider circuit to achieve 5V. The problem I see with this method is that the Hbridge 12V will be more like 14V when the alternator is charging which means it’ll be difficult to select the right resistor pair to end up at 5V.

Has anybody PWM controlled a 0-5V accessory (alternator or otherwise) with the EMU Black? What is the best way to do it?

Auxiliary outputs have internal pullups so can be used for this control

Oh right! That’s much simpler.

Will the internal pull-ups pull to 12V or can they be set to pull to 5V?

Just having a look through the software and see user selectable pull-up resistors for analogue inputs, but nothing for auxiliary outputs…

Am I missing something in the software?

I have done this with a pull up resistor on the aux channel PWM’ing the alt.

I assume you have a GM alt of some kind?

I am not sure what Greg means by the aux outputs have internal pullups as I don’t believe they do. The H bridges can be flipped to use 12V out (high side), but the aux, inj and ign are low side control when used like this. Perhaps he can elaborate

It’s a Mitsubishi alternator that will go onto my 2.5 Duratec. I was mistaken about the 0-5V requirement, its actually 0-2V.

What did you pull up to?

My current plan after talking to an elec engineering mate is to use a voltage divider to get down to 2V from 12V and include a zenner diode to cap the output at 2V to account for changes in Hbridge output between charging and not charging.

I pulled up to the ECU’s 5V in that instance. 2V sounds very strange lol, your solution or one like may be the way to tackle that one.
Never heard of a 2V controlled alt, what make/model is it from?

The alternator is from a BK1 series Mazda 3 2.3L.

It is a weird one, I got the control voltage numbers from consulting a workshop manual.
page 921 of this (generator control signal):


Other diagnostics say that a working alternator should have voltage 0.7V to 2V at that pin so that checks out too.

I’d have thought pulling up to the ECU’s 5V might cause havoc with sensors?

Aux 4 has pullup to 12v.
If you pullup other aux to +5v nothing wrong happens.
The resistor will limit the current and nothing will broken.