Zero crossing comparitor

Thread Starter

Kimi555

Joined Oct 30, 2020
17
Hi everyone,

I’m using a comparitor (zero crossing) to generate a pulse train from a 3 phase motor (used as a ac generator). I’m using the pulse train to calculate motor rpm. I have the comparitor working however it only works when I contact the rotor or stator body with my finger. I guess this is some grounding issue which eliminates some noise messing with the comparitor but I’m unsure what to do about it! Any ideas would be appreciated. It’s a lm311 inputs to a phase and line with output going through transistor. Like I say it works but only with my finger in contact...
Thanks for reading!
 

Thread Starter

Kimi555

Joined Oct 30, 2020
17
Hi and thanks for the replies. As I was drawing the circuit I decided to try powering the Led directly from the transistor (originally it went through a Darlington transistor ic) and now it is fine. I do have another question regarding the lm311 - what is the best way to wire to prevent oscillation at the crossing when the motor is turning slowly? Just asking in case someone has experience with this.
Thanks again
 

Papabravo

Joined Feb 24, 2006
16,775
Hi and thanks for the replies. As I was drawing the circuit I decided to try powering the Led directly from the transistor (originally it went through a Darlington transistor ic) and now it is fine. I do have another question regarding the lm311 - what is the best way to wire to prevent oscillation at the crossing when the motor is turning slowly? Just asking in case someone has experience with this.
Thanks again
Use the slower LM339/LM393 and add hysteresis.
Also, detect peaks rather than zeros.
 

ericgibbs

Joined Jan 29, 2010
13,597
hi K555,
Look at this image for an example of the LM311 with added Hysteresis.
Choose a value about 10K for Rin, the Rhyst value will determine the level of hysteresis, try a 1meg as a test value, decrease the value to increase the level of hysteresis.
E
 

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