# Get RPM form alternator

Discussion in 'The Projects Forum' started by szotyi, Feb 15, 2016.

1. ### szotyi Thread Starter New Member

Jan 14, 2016
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3
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

2. ### gerty AAC Fanatic!

Aug 30, 2007
1,154
304
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...

3. ### Alec_t AAC Fanatic!

Sep 17, 2013
5,980
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That will divide by 2^6 = 64 in total. Not what the OP needs .

4. ### szotyi Thread Starter New Member

Jan 14, 2016
19
3
Oh yes, you must be right. I will measure the pulley sizes, and calculate the ratio. It should be than around 12.

Szotyi

Jul 18, 2013
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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.

6. ### cmartinez AAC Fanatic!

Jan 17, 2007
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There are easier ways to electronically measure an engine's RPM. Is your engine powered by gasoline or diesel?

7. ### szotyi Thread Starter New Member

Jan 14, 2016
19
3
Are there?
My engine is kubota v1505t, diesel
Thanks!

8. ### cmartinez AAC Fanatic!

Jan 17, 2007
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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.

9. ### crutschow Expert

Mar 14, 2008
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D2 is redundant.

10. ### szotyi Thread Starter New Member

Jan 14, 2016
19
3
Great, I will try that tomorrow.

Thanks for the tip!

Jan 14, 2016
19
3
12. ### cmartinez AAC Fanatic!

Jan 17, 2007
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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?

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

14. ### cmartinez AAC Fanatic!

Jan 17, 2007
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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.

Jul 18, 2013
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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.

16. ### cmartinez AAC Fanatic!

Jan 17, 2007
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Yeah... but even diesels have alternators, right? So maybe your suggestion makes more sense, since the circuit would be compatible with gasoline engines too.

17. ### crutschow Expert

Mar 14, 2008
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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 likes this.
18. ### cmartinez AAC Fanatic!

Jan 17, 2007
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I just learned something new... thanks!

19. ### debe Well-Known Member

Sep 21, 2010
955
185
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.

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Jan 14, 2016
19
3
thanks