Splitting PWM (Open Collector)

Discussion in 'General Electronics Chat' started by Tobias, Mar 9, 2010.

  1. Tobias

    Thread Starter Active Member

    May 19, 2008
    I need to split a PWM signal 24 ways. Do I need to run the signal through an Op-Amp for each split? Or will I be ok with just splitting the signal? I am just reading the signal, not using it to drive anything. The PWM does have the capability to drive 6 amps.
  2. ericgibbs

    Senior Member

    Jan 29, 2010
    If I understand your post correctly, what is the point of splitting a PWM signal 24 ways just to measure it.?
    Surely all 24 paths will be the same.?
  3. Tobias

    Thread Starter Active Member

    May 19, 2008
    The PWM is going to send out a signal to 24 individual speed controllers for 24 1/4 DC motors that are operating on 24volts.. Before I commit to getting the boards built, I just want to make sure I am not going to run into any problems. I don't see any problems, yet I am not an electrical engineer by no means.
  4. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
    All the motors will be the same speed more or less as you're feeding them from a single PWM source. What are you trying to build?
  5. Papabravo


    Feb 24, 2006
    That is fine but what do the 24 loads require. Are they all the same? With 24 loads I would be very worried about rise time. What value of pullup resistor will be pulling up the open collector output? BTW I don't think an op-amp is appropriate in the neighborhood of an open collector signal.
  6. Tobias

    Thread Starter Active Member

    May 19, 2008
    I am putting the electric motors on my dads planter. Currently everything is hydraulically driven. We are converting it over to the electric motors. 1/4hp 24 volt motors.

    The GPS system on the tractor outputs a PWM signal that I want to use to control the speed of the motors. So I am taking the PWM and using it to trigger N type MOSFETS. The MOSFETS will drive the motors. The GPS requires a feedback signal so I have an 18 tooth sprocket on one motor with a hall-effect sensor for feedback. Why I want to 'split' the signals is because the GPS system can do individual row shut-off. So I am thinking of just using an Opto-style relay triggered by the PWM and individual row signal. The output will drive the N type MOSFET.

    I am attaching a schematic. The motor will be getting 24v+ from the harness and the board will be supplying ground through the MOSFET.

  7. SgtWookie


    Jul 17, 2007
    Trouble ahead... those CPC1035 OptoMOS relays you're using to control the MOSFET gates are slow as molasses; 1mS to 2mS switching time, and using a 10k resistor to discharge the MOSFET gate is fine for a "safety" turn-off, but not for PWM.

    Do you have an idea of the PRT or frequency of the PWM signal?

    What is the motor's current requirement; stall and no-load?
  8. Tobias

    Thread Starter Active Member

    May 19, 2008
    Tomorrow we will have the GPS controller in hand. Talking to a few technicians, they believe they PWM frequency can only go as high at 120 hz. The tech thought it was adjustable. Tomorrow I will know more.

    Measuring the resistance between the two legs on the motor I get 0.7 ohms. Can I calculate stall load using 24v/.7=34.3 amps? On the test bench at 24 volts with no load measure with my Fluke meter I get 1.5 amps.

    I made a little board using the schematic from above but only 12v..I'm waiting on another battery to get 24v. Then I am using my function generator to simulate the GPS PWM signal. I am watching RPM in real time with a data acq system I have on the bench. I measure RPM using the 18 tooth wheel on the output shaft of the motor. The rpm seems pretty constant. I think the battery charger has a hard time keeping up with draw from the motor. Attached is a screen shot from the data system.

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