tach input

[Krister Wikstrom]

>The range 12v to 400v is mildly discouraging.  I know national makes 
>devices that might fit this application.  I was hoping to use an 
>opto-isolator though, and the limited current range (1mA to 16mA) is 
>tough to match to 12v to 400v.   Hmmm.

I have used opto-isolator, and didn't have any problems. I found an old 4N25 opto, put two 
resistors in series, and a zener diode from between the resistors to the other side of the input
diode - the first resistor limits the current to the zener (I used 15V), and the next one sets the
max current to the opto input. The zener also removes any negative voltage, that the opto can't
handle (was it 3V max). This was then connected to the primary side of the coil, and works fine,
except I have some software problems (small 8031 design)...

4N25 can handle several amps short peaks, so this should be ok - I'm not an experienced designer,
but this has worked fine - any comments (I never looked at it with a scope)?

 Krister Wikstrom
 kwi at mamma.icl.fi