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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 D_Hersey

    Hi,

     

    @Peter,

    Software: I use Proteus ISIS. It has enough power for what I need, and also it is interesting the interactive controls (switch, motor, LED,...) during simulation. On the other hand, some components models are missing.

    Hysteresis resistors: if I add a 1M resistor between +pin and output in the 1st comparator, then the voltage threshold (connected to +pin) is bouncing due to the effect of the capacitor, which produces abnormal functionallity of this comparator. How can I avoid that? In the 2nd comparator there is no problem to add it, I think.

    Spiky inputs: I added the caps as suggested.

     

    @Don,

    OC comparators: yes, precisely this effect has solved the problematic of making the consumption flag remaining active (low) even at low Duty Cycles, thanks to the quick discharge of the capacitor.

    PS fluctuation: if I understood your suggestion right, I can solve this by adding a parallel capacitor to the lower side of the voltage divider, isn't it?

    Hysteresis resistors: yes, we want saturation to provide a 'clean' binary flag. From that I understand that Peter's proposal is the right way, but still I got the problem with the 1st comparator as told above.

    Alternative circuitry: in the application there will not be a -5v supply. By the way, I need 'accuracy' (in terms of stability of the flag during PWM action) rather than speed of the signal, for my particular application.

     

    Thank you again!

  • 0 in reply to modelrailwaycat

    ah, I see, there are too many 1M resistors in that chain that includes the reference, the hysteresis and the pull up. Perhaps you could leave out the hysteresis.

     

    Alternatively, you might improve the circuit with an amplifier on the current sense resistor and you bring the operating current signal up to ~1volt. As Don suggests you could use a Vref for the comparator reference so that you are not dependant on the power rails, which will vary. The comparator reference could be derived with resistors around the 10k level. The 1M pull up could be down to <10k.

     

    Peter

  • 0 in reply to peterjcs23

    Rather than trying to deal with the tiny current sense signal by amplifiers you could increase the value of the sensor resistor to 10R. It would then drop far too many volts at high currents so you would need to shunt it with a schottky diode capable of handling the full load current.

    The loss in the diode would probably be not much worse than in the 0.5R resistor but you would have  a much bigger sense voltage to work with.

     

    Of course the linear range of current sensing would be reduced but since you seem only to want on/off detection this shouldn't matter.

     

    MK

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  • 0 in reply to peterjcs23

    Rather than trying to deal with the tiny current sense signal by amplifiers you could increase the value of the sensor resistor to 10R. It would then drop far too many volts at high currents so you would need to shunt it with a schottky diode capable of handling the full load current.

    The loss in the diode would probably be not much worse than in the 0.5R resistor but you would have  a much bigger sense voltage to work with.

     

    Of course the linear range of current sensing would be reduced but since you seem only to want on/off detection this shouldn't matter.

     

    MK

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