Stupid question or smart inquiry?

Discussion in 'General Electronics Chat' started by SmartWater951, Sep 4, 2008.

  1. SmartWater951

    Thread Starter New Member

    Sep 4, 2008

    I have a question, in which I think I know the answer, but I have to ask as I have never heard it asked or even seen it done before. From what I know ( which isnt much ) it doesnt seem plausible but I would like to hear different opinions.

    Question: Lets say that I have a unit that provides an open collector pulse output. I also have another piece of equipment, a PLC that accepts a 0-5v scaling. I have heard that if you use a 250Ω pull down resistor that you can actually turn it into a 0-5v input. I understand that this is the case for a analog ckt, but an open collector pulse? Can someone either confirm or deny, and if it is a confirmation, please elaborate, thank you for your time.

    *note* I am only interested in my question above, any input regarding the ability to change my output to what I would need my PLC to see has already been taken care of. Thank you for your concern. :D
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    An open collector output can only output the voltage it is tied to through a pull up resistor. So, with the 250 ohms (5% values are either 220 or 270, 1% value is 249 ohm) to a +5 volt supply, then the pulse output will be either 0 or +5 volts.

    Note - a pull down resistor is connected to either ground or a negative voltage.
  3. scubasteve_911

    Senior Member

    Dec 27, 2007
    beenthere is right, but you can try something else too.

    You can use a pullup resistor to 5V, and follow it with a low-pass filter. Making sure that the cutoff of the filter is at least 100 lower than your PWM frequency, for acceptable ripple rejection.

  4. SmartWater951

    Thread Starter New Member

    Sep 4, 2008
    Thanks guys!
  5. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008

    PWM frequency?