# Get RPM form alternator

#### szotyi

Joined Jan 14, 2016
19
Hello!

I have a Denso 19630-64013 alternator. I would like to get the RPM from it. So far I have got:
There is a connector: 90980-11349, one pin is the P terminal, according to some datasheet this is the stator. I have connected this terminal to the base of a bc548 transistor through a 10 kohm resistor. And counting the falling edge with Arduino. It works okay, but my problem is that I have counting around 180 pulses per sec (10800 rpm) with no gas, with full gas around 570 pulses per sec (34200 rpm). If I divide those numbers with 12, then I would get 900 rpm to 2850 rpm, which sounds ok. But how do I get the 12? Or is it correct?

Thanks,
Szotyi

#### gerty

Joined Aug 30, 2007
1,305
If this is on a gas/diesel engine and you're looking for engine rpm the pulley sizes on engine and alternator must be identical. If you just want alternator rpm...disregard this post...

#### Alec_t

Joined Sep 17, 2013
14,237
To divide by 12, rotate right the counting file 6 times, each rotate right divides by 2,
That will divide by 2^6 = 64 in total. Not what the OP needs .

#### szotyi

Joined Jan 14, 2016
19
If this is on a gas/diesel engine and you're looking for engine rpm the pulley sizes on engine and alternator must be identical. If you just want alternator rpm...disregard this post...
Oh yes, you must be right. I will measure the pulley sizes, and calculate the ratio. It should be than around 12.

Szotyi

Joined Jul 18, 2013
28,533
If the terminal is the DC output, this will have 6 ripple pulses per Electrical revolution, for 4 pole (two electrical revolutions) this would be 12.
Not sure if this is what is referred to?
Max.

#### cmartinez

Joined Jan 17, 2007
8,181
Oh yes, you must be right. I will measure the pulley sizes, and calculate the ratio. It should be than around 12.

Szotyi
There are easier ways to electronically measure an engine's RPM. Is your engine powered by gasoline or diesel?

#### szotyi

Joined Jan 14, 2016
19
There are easier ways to electronically measure an engine's RPM. Is your engine powered by gasoline or diesel?
Are there?
My engine is kubota v1505t, diesel
Thanks!

#### cmartinez

Joined Jan 17, 2007
8,181
A diesel engine normally has electric actuators in charge of governing the amount of fuel reaching the injectors. These actuators are triggered electronically, and have electric pulses that can reach up to 120 Volts for about a millisecond.

If you were to tap into the actuator's wires, and connect them to an optoisolator with current-limiting resistors and a couple of diodes for reverse voltage protection. You can then use the opto's output to trigger an MCUs input pin.

The value for RPM is then obtained by doing some simple algebra.

#### crutschow

Joined Mar 14, 2008
34,078
........................
If you were to tap into the actuator's wires, and connect them to an optoisolator with current-limiting resistors and a couple of diodes for reverse voltage protection. You can then use the opto's output to trigger an MCUs input pin.
.................................

D2 is redundant.

#### szotyi

Joined Jan 14, 2016
19
Great, I will try that tomorrow.

Thanks for the tip!

#### cmartinez

Joined Jan 17, 2007
8,181
D2 is redundant.
Maybe... 1N4148's datasheet states its reverse voltage is 75V with a peak of 100V. And the opto's diode reverse voltage is only 6V. Also, I failed to mention that the actuator's coil experiences a kickback of up to -90V, that's why I placed the second diode.

You're far more knowledgeable than I am, @crutschow, why do you say it's redundant?

Joined Jul 18, 2013
28,533
Just by a quick Google it would seem the method of choice for a diesel tach is the alternator output?
Max.

#### cmartinez

Joined Jan 17, 2007
8,181
Just by a quick Google it would seem the method of choice for a diesel tach is the alternator output?
Max.
I just googled it as "diesel alternator tachometer circuit", and I couldn't find a suitable circuit. Do you really think that would be an easier circuit to build?

On the other hand, I'll grant you that my circuit is more difficult to connect, since tampering with the wires reaching the actuators is not recommended, and finding the adequate connectors for the actuators so as not to have to cut into their wires is not an easy task.

Joined Jul 18, 2013
28,533
Many show a particular pin on the alternator depending on model, I suspect it is a full wave ripple pulse from a small bridge or the main?
I have never been involved with a diesel tach.
Max.

#### cmartinez

Joined Jan 17, 2007
8,181
Many show a particular pin on the alternator depending on model, I suspect it is a full wave ripple pulse from a small bridge or the main?
I have never been involved with a diesel tach.
Max.
Yeah... but even diesels have alternators, right? So maybe your suggestion makes more sense, since the circuit would be compatible with gasoline engines too.

#### crutschow

Joined Mar 14, 2008
34,078
Maybe... 1N4148's datasheet states its reverse voltage is 75V with a peak of 100V. And the opto's diode reverse voltage is only 6V. Also, I failed to mention that the actuator's coil experiences a kickback of up to -90V, that's why I placed the second diode.

You're far more knowledgeable than I am, @crutschow, why do you say it's redundant?
I didn't know you assumption about the voltage kickback.
But it would be preferable to use one diode of a higher voltage rating, such as a 1N4004, then to have two lower voltage units in series.
The problem with two in series is if one has a reverse leakage current higher than the other, then excess voltage can appear across the one with the lower leakage.

#### cmartinez

Joined Jan 17, 2007
8,181
The problem with two in series is if one has a reverse leakage current higher than the other, then excess voltage can appear across the one with the lower leakage.
I just learned something new... thanks!

#### debe

Joined Sep 21, 2010
1,389
Find the center of the Star winding & run a wire out of the altenator. Theres about 8V AC & this is used as a Tacho feed on Diesels. This is how VDO tachos are wired to altenators & they have a set of program switches to get the right RPM to match the engine.