Open collector output terminology

Discussion in 'General Electronics Chat' started by strantor, Aug 17, 2013.

  1. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
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    I'm a little confused by the data sheet of this sensor. See excerpts below:

    My confusion is in the terms "high level" and "low level" with regards to the term "open collector." The functional diagram shows no pull-up resistor, so when they say "high level" and "low level," is it inferred "after you add a pull-up resistor"? or is there an internal pull up resistor inside tied to Vcc that has been omitted from the functional diagram?

    If there is no pull-up resistor internally then would it be OK to use the sensor to low-side-switch a relay like this one (coil resistance = 360Ω, @12V = 33mA)? If yes, then I am further confused by the terms "high level" and "low level"; does "high level" mean "transistor switched" or "transistor not switched?"

    Thanks
     
  2. crutschow

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    Mar 14, 2008
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    Yes, the terms are somewhat ambiguous. The terms "high level" and "low level" are indeed with an added external pull-up resistor. Generally there is no internal pull-up resistor. That means that the output transistor is ON for a "low level" and OFF for a "high level". Thus a relay connected from the output to 12V (which is ok to do if you add a diode across the relay coil, cathode to positive for transient suppression) would be energized when the output is "low level".

    Make sense now?
     
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  3. strantor

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    yes, thank you. That is what I suspected, but I kept talking myself out of it. Thank you for the sanity check.
     
  4. #12

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    Nov 30, 2010
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    I thought I saw the bat signal...and then I typed until the conversation was over:(

    The way I interpret this, considering the 10k resistor mentioned on the bottom of page 5, is that High level means the 10K resistor is supplying some current from Vcc and the transistor is off. And...visa-versa. "Low" means the transistor is on.

    It seems wrong to me too, that an open collector device is described as having a "high" output when in fact, the device itself can not perform a high output, but you see the same sort of thing on a comparator data sheet.

    The current limit is 50 ma, so it should pull down a 33ma relay (with diode protection, of course). And...you must be using the OPB720A because it has a 30 volt limit and the other one has a 7 volt limit. Notice that the graphs do not say, "10 volts". They say, "normalized output level".

    The graphs do not show the saturation voltage curve from 0 to 50 ma, the data sheet only shows V Out Low is .8V at 1 ma, so expect some sag there.
     
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  5. strantor

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    So for future reference, when I see this again, I'll assume that "high" means "off."
     
  6. #12

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    Works for me:p
     
  7. WBahn

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    Mar 31, 2012
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    The way I interpret the terminology is that is the output has two states, "high" and "low". These are just the names of the two states, so you can think of the "high" state as being the alternative to being in the "low" state. Another way to think of it is that the "high" IS a "high" in the sense that it is "not low", but it just isn't "actively" asserting a "high"; in contrast, the "low" level IS actively asserting a low level.

    But that just seems to be the way that I was able to put my mind around it when I first was introduced to open-collector outputs and it's been good enough to not require me to revisit it. I think at first I felt, somehow, that there was some importance to making a distinction between an "open-collector" output and the situation you would have with a "tri-state" output if you confined the latter to either driving a "low" or being tri-stated. But other than some timing fine points, I think that is another way to view it.
     
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