Threshold Crossing Detector Help

[veryevil]

Hi, I've been trying for a while now to detect when a voltage crosses a threshold. I've been using comparators. First the LM311 and now the TL3016.

The threshold I'm after is 2.5v and I've been trying to use a hysteresis of around 100mv but nothing seems to work. Either its very noisy with HF spikes or the hysteresis seems not to be centred.

Ultimately I am try to detect zero current through a motor using a current shunt and current shunt monitor LT1999 or INA282.

These both give a voltage dependant on current and direction and is centred around 2.5v.

I used the LT1999 with its reference voltage pin into the TL3016 but it loaded it and affected the reference voltage. I then passed the reference though a MCP602 as a voltage follower and then into the TL3016 and this gave me a perfect output that triggered when the motor current was at zero (LT1999 output at 2.5v) however it only worked when my motor was in one direction. My motor drive uses PWM through a H-Bridge and runs in both directions.

Any ideas on getting the signal I need? Or another way of getting zero current through a motor?

[Mechatrommer]

show your circuit.

[Zero999]

Is the power supply voltage regulated?

What's the required tolerance?

If your power supply has adequate tolerance just use a potential divider across it and a suitable feedback resistor, for example if you're running it from 5V, use two 12k for the potential divider and 300k for the feedback. The output impedance of the potential divider is 6k which with 300k will give 100mV of hysteresis: 5*6/300 = 0.1V

[Rufus]

These both give a voltage dependant on current and direction and is centred around 2.5v.

If you are trying to sense zero (or actually less than a small value of) current with a bipolar voltage indication of current you need a window comparator.

That is two comparators, in your case with thresholds set at say 2.4 and 2.6v and the outputs ored together.

[veryevil]

A window comparator seems to be the answer. I've set up a MCP602 as one with two trim pots for now and it seems to be working a treat. I should be able to shrink the window when its not on breadboard as there will be less noise.

Thanks Steven