Motion Control and Motor Drives

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Measureing PWM

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Hi All,

I have a valve (see attached image) that I am trying to read PWM from. I am trying to see if I can read at least the average voltage values from NI9201. But I am confused about the wiring connections.


I have a DS212 oscilloscope that I am using to verify the PWM wave. Logically I hooked red probe to white wire (PWM output), and black probe to the black wire (-) of the valve (this black wire is connected to the (-) terminal of the 24V power supply. However, no reading. Then accidentally I connected the black probe to an independent wire coming from the (+) terminal of the power supply; and all the sudden I got reading (see wiring image below).


Does anyone have experience with this weird connection, and it throws me off as to how I would make the wiring connection to NI9201.



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Message 1 of 5
PWM OC seems to be an Open Collector output. Therefore, you have to connect a resistor between the power supply and this output.
All indications are likely to be in the datasheet of the valve.
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Message 2 of 5

Hi JB,


Thanks so much for the reply. This is very useful.


I have some questions:


The first source you gave mentioned "With open collector output, you simply cannot just connect the output device to the pin and then to ground. Open collector does not work that way. It must have positive voltage and then the load and the negative or ground side of the load connects to the output."


1. So from the way it is hooked up to the oscilloscope, my oscilloscope acts like the load right?

2. the current way that it is connected (can read PWM), its wrong? The black probe should be connected to the PWM OC, and red probe should be connected to the (+24V) power supply? (that may explain why my DS212 oscilloscope suddenly died)

3. Could you explain what is the purpose of the resistor? 



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Message 3 of 5
Accepted by topic author wew044

If that helps you, imagine a simple switch in place of the transistor.
Does this answer your questions?
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Message 4 of 5

Had to think for a bit, but i think i got it. Will put this to the test.


Thank you so much.

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Message 5 of 5