Alternator tacho sense

Captain Haddock

Senior Member
I am considering taking a tacho signal from an alternator phase tap (thats what the tacho uses) to check to see if an engine is running before carrying out a start procedure, I was going to use an optocoupler/resistor before the picaxe but not yet sure of the unregulated ac output range, would a zener diode be good to control the voltage before the optocoupler or would that mess up the tacho reading?
No code or circuit diagram yet as it's early stage in the project (actually a full re-vamp of an existing one), I won't be able to check the tacho signal voltage range for a few weeks either.
I know it's a 12 pole alternator and fairly sure it runs at 2x or 2.5x crank speed so should be able to use pulsin to get an idea if it's running or not in a very quick time so should add an element of safety to an existing starting system, just need to condition the pulse to a safe level.
I could also use the same signal as an engine sync indicator.
 

SAborn

Senior Member
A simple little circuit we use for rpm input of a wind turbine alternator is like the schematic below, the opamp is not entirely required but it adds some filtering and conditioning to square the pulse signal to the picaxe, as without it there would be some odd high false readings recorded in the data logging files.

There is no reason this same circuit would not work for your alternator application too.

Opto-Opamp.JPG
 

Captain Haddock

Senior Member
I'm sure I will need more on the alternator side of the optocoupler just to keep the optocoupler safe, I know motorbike generators can get up to 80 volts before the regulator/rectifier unit (and drop to less than 12v)but never checked on an automotive alternator so will be interested to check, as said it will be at least 2 weeks before I can do that though, also need to check the exact drive ratio but thats no problem, while I'm at the boat will have to cruise to the nearest pub for the night, life is so hard and cruel....
 

SAborn

Senior Member
I'm sure I will need more on the alternator side of the optocoupler just to keep the optocoupler safe,
I dont, but its up to you with what you feel happy with, its a circuit i have used many times on some higher voltage output mills.

Your alternator will be clamped to battery voltage and the voltage controlled by the regulator, the AC will only be a fraction higher than the DC side, and not 80v as you think.
The most you will see is about 15 volts on the 3 phase AC.
 

Hemi345

Senior Member
How new are these engines? If fuel injected, chances are good that there is a crank position sensor that you could tap into that would do exactly what your looking for.
 

Captain Haddock

Senior Member
They are old peugeot xud9's Hemi so fuel injection is all mechanical distributor pump, the project will be mounted under the helm so tacho signals are ideal as only 2' away from mounting position.
I think SAborn is right about the voltage not going over 15v, will check when next there.
 

TheChief

Senior Member
You could look for pulses from a CT that is looped around the ignition ht lead. Not sure if this would be safe with the high voltages induced in the ignition and also the magnetic field collapsing within the CT
 

SAborn

Senior Member
One problem that comes to mind with using the alternator AC for a rpm signal, is the way the voltage reg controls the alternator by switching on/off the field windings, this would cause some very eradic AC pulses and perhaps periods with no AC.

As for using a CT, every diesel engine i have ever seen dont have ignition ht leads. :confused:

A common method with a lot of heavy diesel engines is to use a proximity switch and monitor the crankshaft or a tailshaft, often something like the connecting bolts of a shaft coupling is used for proximity targets.

A basic NpN sensor is used as they allow 12 volt DC operation and direct connection to a micro controller input, as the micro input pin is pulled high to 5 volt via a resistor and the proximity sensor takes the input pin low via the NpN transistor in the sensor.
Most proximity sensors have a led on the back to show operation as the led lights in the presence of a target, this also makes trouble shooting easy, as well as the sensor is a sealed device making it useful for hostile environments.

The way i would tackle this project would be with a proximity sensor fitted to each motor, simple, tidy, cheap, and reliable.
Also very easy to calibrate, as the number of pulses per revolution is equal the number of targets per revolution.
 

hippy

Senior Member
Not sure if RPM is wanted but the original plan was to detect the engine was running. A piezo sounder used as an input sensor bolted to the engine may enable both.
 
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