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300 V Square wave voltage regulation to < 10 V ??

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#1 jhenise

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Posted 15 July 2013 - 11:08 PM

[font="calibri;"]I want to measure engine RPM using my Lab Jack U6.[/font]


[font="calibri;"]The engine has a magneto CDI ignition with a tachometer outout wire from the Cdi box.[/font]


[font="calibri;"]I probed it with my O scope this weekend and it’s a square wave of about 300 V peak to peak.[/font]

[font="calibri;"]How to I step this down to less then 10 V ??[/font]


[font="calibri;"]I am an electronics noovice, but have been reading about zener diodes?[/font]



#2 LabJack Support

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Posted 16 July 2013 - 10:58 AM

Do you need to measure this without affecting it?  I'm thinking if you used a series resistor with a zener to ground, you will clamp the voltage but the current you draw will decrease the voltage of the signal itself ... perhaps substantially.


I would consider something with an optocoupler or similar, to protect the U6 and your computer.

#3 jhenise

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Posted 16 July 2013 - 10:32 PM

I can effect it, no problem there, it serves no purpose other then a reference signal.

 But  I am wiorried though that if i pull too much current to groung through the zener it will kill the ignition system. basically I loose spark if i ground the signal...

#4 jhenise

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Posted 18 July 2013 - 12:19 PM

SO I ordered some 6N138 Optoisolators...




Any suggestions on how to hook one of these up?


Assume the input is a square wave with 150 to 300 V peak to peak amplitude, and teh supply voltage is teh USB +5 V.


Looks like i will need some sore of resisted in series with the input and one bridging supply voltage and output.

What size resistors should I use?


Circuit diagram:



6N138 Pinout:


#5 LabJack Support

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Posted 22 July 2013 - 03:46 AM

The wiring of an opto tends to depend on the load that needs to be driven, power considerations and frequency of the signal.


If I understand correctly the 300V pulse is the actuation pulse for the injector. It's hard to say how much power is behind that, but to be on the safe side it would be best to minimize loading.


I would use a large, 10 MΩ or more, divider to drop the voltage to something manageable. That signal should then be buffered by an op-amp before being sent to the opto-isolator input. The output of the isolator can then go to the LabJack. To ensure proper isolation across the opto the op-amp needs to be powered from a source other than the LabJack's. That's the simplest way I can think of to do this properly.


If you want to go an easier route you can put a 1MΩ resistor between an IO and the CDI signal. There are a couple risks here. The signal may be overloaded and impede the operation of the CDI. Or the ignition noise might interfere with the LabJack's operation or maybe even damage the LJ and connected equipment.


Also. If the engine has a crank or cam sensor it might be easier to monitor that signal.

#6 sirnak

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Posted 13 August 2013 - 11:08 AM

It's an engine RPM waveform which you'll probably be getting from the alternator so naturally with the increasing RPM, the Pk-Pk voltage will increase. You just need some zener diodes. Just put one (of range ~10V DC) in inverted position in the signals path with the ground and your signal will get limited to 10V. Feed this signal to any frequency counter circuit and that's about it.

#7 LabJack Support

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Posted 15 August 2013 - 10:39 AM

A zener is a nice, simple solution, but I would put a resister in line as well to prevent too much current from going through the zener. Keep in mind that this solution does not provide any isolation.

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