# Alternator 'W' Phase Terminal AC to Arduino

This is my first post so please bear with me.

I'm trying to drive a tachometer (three prong, three phase from an old airplane) from the engine's alternator. It being a mechanical diesel I have to run it off of the alternator's W terminal, which is one of the three phases.

I believe it puts out a peak 14V AC sine wave (which means ±7VAC?), 1190Hz @ 5000rpm and ~200Hz at 850rpm.

I'm trying to figure out a feasible way to get this as a stable input to the Arduino, in this case an Attiny13a, to basically calculate the proper PWM signal to drive the electronic speed controller to drive the tachometer.

Edit: Found a solution using a 5v zener, resistor, and capacitor shown below.

I believe it puts out a peak 14V AC sine wave (which means ±7VAC?), 1190Hz @ 5000rpm and ~200Hz at 850rpm.

Check the actual voltage, it might or might not be 14VAC.

The relationship between peak and RMS is the square root of 2, I generally use 1.414 as being close enough, so 14VAC peak is 14/1.414 = 9.9VRMS

To get the frequency you need to know the number of poles the alternator has. A 2 pole machine outputs 1 cycle per revolution, a 4 pole machine 2 cycles per revolution.
At 5000RPM you get 5000/60 = 83.333 revolutions per second, so 83.333Hz for a 2 pole machine, 166.67 for a 4 pole machine (how on earth you arrived at 1190Hz will be interesting to learn).

At 850RPM you get 14.167Hz for a 2 pole machine, I'll leave the rest to you.

A tachometer converts mechanical power into a small electrical signal for measuring rotation speed - yet you
talk of “driving the tachometer” from an alternators W terminal?

Do you mean you are trying to drive a tachometer’s meter unit? IE replace the tachometer with a different signal source?

raschemmel:
That being said, all you need to do is add a 1k resistor from pin 5 of the 4N35 to the arduino +5V
and then the analog pin will read voltage.
But do you really want to measure voltage ?
Is there any doubt that the voltage you measure is going to be 5V (correct answer: no)

My apologies, I was going based off of a circuit I had found here, I mustve not copied it over right into my notes:

I’m not too familiar with datasheets, and as far as understanding the diagrams explaining the chip goes is not good as I’m new to making circuits. From what you’ve told me I went and simulated a circuit that I believe would be okay to use. So that square wave output of high and low would be something I could use to measure the frequency?

PerryBebbington:
how on earth you arrived at 1190Hz will be interesting to learn

I was going based off of this post’s frequencies, I figured the frequency was what came from the alternator’s W terminal.

I also found the same frequency posted here:

MarkT:
A tachometer converts mechanical power into a small electrical signal for measuring rotation speed - yet you
talk of “driving the tachometer” from an alternators W terminal?

Do you mean you are trying to drive a tachometer’s meter unit? IE replace the tachometer with a different signal source?

The tachometer I’m trying to drive is an old three phase gauge from an airplane.

What I’m trying to do is take the W signal from the alternator (one AC phase wave) to measure the frequency of the pulses, divide that by the difference in terms of alternator pulses to crank rpms, generate a pwm based off of this new frequency which will go into the ESC and ‘drive’ the three phase tachometer.

So essentially, Alternator AC → Arduino -(PWM)-> ESC → Tachometer

I’m just looking for a good circuit that takes the Alternator’s frequency and makes it safe for the Arduino to read.

I was going based off of this post's frequencies, I figured the frequency was what came from the alternator's W terminal.
Alternator W terminal | VW T4 Forum - VW T5 Forum

Thank you, I learned something ++Karma

I took it that you were measuring the AC from the alternator before the rectifier. I didn't know there was another output as described there. My comments about the frequency of AC from an alternator stand for the generating bit, I do not know if they apply in this case, to the W terminal, about which I no nothing more than you have told me.

After some more research I’ve decided to go a different route, ditching the opto-coupler.

I had to pull apart my alternator and solder in a output for the W terminal as there was no terminal there, but there was a cutout for a terminal and a casted W right where it should be. Few long screws and some solder later and it was done.

I used a 5v zener, 4.7k resistor, and .47uf capacitor to get the input signal to the arduino using the circuit attached. Uploaded the attached code and it works!

Now all that’s left is to get a laser rpm meter and make sure it’s reading correctly, and then figure out a driver for the gauge as the ESC doesn’t want to power it.

What guage ?

raschemmel:
What guage ?

It's an old plane tachometer, P/N AN5530-2. Has two dials and two three pin inputs on the back. Like this.

I've changed to using a atmega328p instead of the attiny13a. I figure something like three digital pins outputting 120 degrees apart, to a transistor then +12v dc mosfet bridge to drive it, like this:

Except I'd be connecting the mosfet outputs to the gauge inputs.

Only thing I'm unsure of is how I'd go from the hz reading inputted to the correct output frequency for the gauge. Maybe a map calculation in the arduino?

Where did you get that diagram from.
It won't work.
Voltage swing on the emitters of the NPN/PNP pairs is 0.65 to 4.35volt (emitter followers).
Impossible to drive mosfets with that.

Are you trying to do something like this?
Leo..

Wawa:
Where did you get that diagram from.
It won't work.
Voltage swing on the emitters of the NPN/PNP pairs is 0.65 to 4.35volt (emitter followers).
Impossible to drive mosfets with that.

Are you trying to do something like this?
Leo..

I don't think it's the same thing as a stepper motor. On Page 11, this says it's a synchronous motor.

I got that diagram here.

This is like what I'm trying to do: working with real RPM Gauge Arduino and Flight Simulator - YouTube

I'm not sure how he has this hooked up, but appears to be 12v in from the pc's power supply and the 3 digital outputs from the Arduino controlling it.

Right, there must be a three-phase motor inside that instrument that needs to be powered with some sort of VFD (variable frequency drive).

Should be able to do that with H-bridge chips, as in the link I gave you.
Could be that the youtube project used the L298.

Don't try to build that "electronics-project-hub" circuit.
It seemst to be designed/drawn by someone who knew very little about electronics.
Leo..

I think the two hands on the tach are for
left engine and right engine.