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

modelrailwaycat

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!

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  • 0 in reply to peterjcs23

    Hi Peter, thank you for your reply.

     

    Yes, the motor turns OK according to the Duty Cycle defined.

     

    The voltage across Rsense (R3), Vsens, is according to the current on pin 1 (divided by 2). Attached is a capture of the waveforms for a simulation of 110 ms. PWM start is delayed 50ms. As you can see, the output of the op-amp (Vsat in blue, on the right axis) has some disturbance, but its value is in high state even at the start of PWM. Vsens is in green and Vthreshold in red. Duty Cycle is 10% in this case.

     

    image

  • 0 in reply to modelrailwaycat

    Hi John

     

    okay this is what I'd do.

    1. 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.
    2. An op amp is a poor comparator, the LM324 is just not working in this mode at a voltage near the rail.
    3. 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)
    4. The next stage could be your RC filter which needs to be around 4Hz for a 40Hz pwm say 39k with 1uF.
    5. 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.
    6. 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

  • 0 in reply to modelrailwaycat

    ...oh yes and one more thing...

     

    You show zener diodes on the motor drive for the fly back current. These should be rectifier diodes 2A fast recovery types. For example ES2A.

     

    Peter

  • 0 in reply to peterjcs23

    I'm pretty sure those are schottky diodes not zeners. The symbols used are very similar however usually with zeners the "flags" on the ends are at an angle, not straight up and down.