Stupid question or smart inquiry?

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SmartWater951

Joined Sep 4, 2008
2
Hello,

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
 

beenthere

Joined Apr 20, 2004
15,819
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.
 

scubasteve_911

Joined Dec 27, 2007
1,203
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.

Steve
 
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