L298 current sensor pin, convert pulsed current signal to consumption flag

Question

Hello,

I am designing a small, simple 12V motor controller fed by a L298 H-bridge driver. The H-bridge input pins are supplied by means of PWM at 40 Hz. The motor can be connected or disconnected from the output even with the input and enable on. For this reason, I am interested in adding additional circuitry in order to obtain a binary signal of consumption using the current sensor pins; that is obtain a permanent high level while some current is detected and a permanent low level when no current is detected (after at least 1 period, but this is flexible).

The idea is to use a 0.5 ohm resistor to connect the sense pin to ground, and then use the voltage across the resistor to obtain the signal. I consider that the voltage measured will be from 5 mV to 1V and the Duty Cycle can be down to 1%. Because the sensor signal is also pulsed, I need somehow to filter it and obtain an stable output while consumption is 'on'. The problem is that I had not success to set properly a low-pass filter plus op-amp buffer configuration from this signal working reliably with all the mentioned conditions. I also tried to pre-amplify the sensor signal with another op-amp, then output that into the low-pass filter plus buffer, but had no success too.

Any idea is appreciated. Thank you very much!

peterjcs23 · Accepted Answer

Hi John okay this is what I'd do. The LM324 is a very old fashioned opamp with poor offset and does not operate rail to rail. You might like to choose a better one like OPA4241. An op amp is a poor comparator, the LM324 is just not working in this mode at a voltage near the rail. I would make the first stage of the processing a positive gain amplifier to get a bigger signal (you need to know your max current to set the gain for full scale) The next stage could be your RC filter which needs to be around 4Hz for a 40Hz pwm say 39k with 1uF. The next stage should be your comparator, which is now working with a higher more manageable voltage. Use a LM339 for a cheap standard part. That will drive your led directly on the output which needs a pull up to make it work. If you are going to reverse the current, you will need to set up a different reference voltage as with everything referred to 0V you will not get a negative swing. And your comparator will have to work with + an - current as both represent the on state. Peter

D_Hersey · Answer

Can you draw it up? What op-amp are you using? Not all op amps have common mode input range all of the way to BoS.

D_Hersey · Answer

Not to be an arch-pedant but remember that the LM339 has an OC output. The output of the left-hand comparator will discharge the capacitor much more swiftly than the pull-up will charge it. Maybe that is the behavior you desire, IDK. As drawn, you are being kinda noodnik regarding PS fluctuation. You can shunt a cap around the lower legs of your reference strings for a cheap fix. Or maybe you don't care or maybe the current behavior is what you seek. Or use a reference instead of a ladder across the supply.

L298 current sensor pin, convert pulsed current signal to consumption flag

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