Safe DO/PWM Operation with Open Collector

  • Hi,

    I'm running a Mervis on Unipi Neuron s103 with an ebm-papst fan (W3G200 Series, 220V, can hurt fingers), setting RPMs with (low power) PWM. The papst PWM is either controlled by 0-10V or PWM using the same wire. So with an Open Collector I've a pullup resistor to 10V. The Transistor on the DO has to become conductive to pull the wire to ground. As long as the DO is low, the fan sees 10V/100% duty cycle and goes to full throttle. This is quite unsafe, the fan spins up as soon as I power up the unipi. I tried to set Inital Values for the DO and PWM but that does not seem to work as expected.

    Now I'm wondering what the best practise for this kind of situation is? Should I run the 10V through a relais, sacrificing another digital out? Or can I run DGROUND and AGROUND to a relais? That would have the benefit of providing a sort of "arm" circuit for the implement on the analogue out as well. Or is there any other way I couldn't think of?

    Thanks, Andi

  • administrators

    Hi @_andi_ ,
    I took a look at the datasheet of the fan and there are two types of control: analog 0-10V or PWM with the amplitude of 10V


    Since the PWM mode of regulation needs external components, I would suggest you to control the fan via analog 0-10V. You can connect the AGND of your Neuron S103 to the terminal #1 of the fan and the AO of your Neuron to the terminal #2 of your fan. Then you can apply any voltage between 0-10V thus regulating the speed of the fan. Which software do you use on the Neuron by the way?

    Best regards,

  • Hi Martin,

    thanks for the answer! I'm using the current mervis image on the unipi. Sadly I can't use the analogue out because it's already in use for a valve :-(

    Thanks, Andi

  • In case somebody else stumbles over this: Another Problem is that the Digital Output is not 0V on 0 PWM / LOW due to the transistor inside the unipi. The voltage (around 0.6V) is just enough to kick on the fan on occasionally / keep it running from time to time (starts around 0.4V).

    I mitigated all issues with a wago 859-791 optocouple for each PWM, a rather pricey solution.

  • administrators

    Hi @_andi_,
    thanks for the info. It is indeed as you describe. That minimal voltage of 600mV in OFF state comes from the nature of open collector output. You have mitigate it quite elegantly with the Wago optocoupler.


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