Looking at Tachometers – aka, ‘My Tach Stops Working When the Batteries are Full!’

It is common in Diesel engines to sample the Stator output of an alternator as a way to drive a tachometer.  After appropriate calibration it mostly works well – until it does not…

As a regulator senses the battery is fully charged it will cut back the field drive to reduce charging current being forced into the battery.  At some point the regulator might go into Float mode - maintaining just a basic charge – or it might even turn off the alternator all together.   When that happens the Stator output voltage drops rapidly from 14v+ while actively charging to well under a volt. 

During these times people will report their ‘tachometer’ stopped working.   Traditional approaches to correct this include always sending a small amount of field drive, even when no charging is desired.  This ‘Tach Mode’ can work, and is a capability of the this Alternator Regulator.  But it can be fiddly to dial in just right;  getting just the exact right level of field drive needed to produce a sufficient stator signal for the tachometer, while not starting to once again force energy into an already full battery.  It is one area I am looking to improve with the revision to the regulator. 

Here are some O’Scope captures of the two alternators mounted on our Cummins main engine.  Trace 1 is the OEM alternator used to recharge the starter battery, while trace #2 is the large Leece Neville 270A alternator attached to our house battery.  The O’Scope traces are showing Stator output after the engine has sat unused for 2 days.   The engine was started at idle and remained there.  All field drives were disconnected, so we are only seeing output as a result of residual magnetism. 

In this example there is a nice 500mVp-p signal.  Just from residual magnetism (No field drive).  Even after 2 days of sitting around.  The regulator currently uses an AC coupled  74S14 Schmitt Trigger to condition Stator input, requiring about a 1.8Vp-p or better signal to trigger.  Clearly over the 500mV we are seeing above, but well below the 12v-14v level needed to start charging.  There are two different approaches I am considering:

1. Creating a closed feedback look between the PWM drive and the stator Vp-p, managing to around a 5-6Vp-p when ‘no charging’ is required.  This will give a good stable tachometer lock, which not allowing any current to flow from the alternator into the battery.
2. Improve the sensitivity of the Stator input circuit.

Mostly I am leaning towards option #2, looking at a VRS chip such as the LM1815 or the NCS1124.  These have sensitivity in the range of 300mVp-p, and would clearly be triggered in my example above – even after 2 days of ‘sitting’.

Once the regulator is able to get a stable RPM reading it could ‘echo’ that out the Feature-out port to drive an external tachometer.  End results:  no more ‘Tachometer stops working when batteries are full’ situations – and also no risk of continuing to overcharge batteries by artificially providing field current to keep a strong Stator signal. 

Is this something you would find of value in?