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Open Collector Vs. Totem Pole Outputs

  1. May 7, 2007 #1

    cepheid

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    Hi,

    I have an electrically-actuated shutter that can be either in the "closed" or "open" positions. I want to use optoelectronic switches (slotted switches with an infrared LED source and photodiode detector) at either end that will detect whether the shutter is closed or open. The opto switches I am looking at come in either the open collector output or totem pole output variety (both common output stages used in the TTL logic family, from what I gather). I basically have to make a choice between one or the other. From what I can tell, applications of open collector outputs are the ability to connect the output through a pull up resistor to a supply that is *different* from the chip supply (if interfacing two different kinds of devices). Another application I read about on wikipedia was that several open collector outputs can be tied together to one wire, implementing a sort of logic AND or logic OR configuration (depending on whether you're using positive-true or negative-true logic). Neither of these applications is relevant to me. Since the input pin of the controller to which I'm sending the opto switch signals is rated for 5-24 V, and the Vcc supply to the opto switch is meant to be 5 V, it seems to me that a totem pole output would be fine, and consumes less power overall (in general).

    My question is, I've never had to deal with either of these types of outputs before. Are there relative advantages or disadvantages that I've missed. Which type would you choose for this application?
     
  2. jcsd
  3. May 7, 2007 #2

    berkeman

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    The main advantage to the logic output is generally that it is faster than you can acheive with the open collector output. So my suggestion would be to look at the speed requirements that you have, and partially base your decision on those. The TTL output is a little more expensive, as you pay for the extra speed. Is there a price difference in the units that you are considering?
     
  4. May 7, 2007 #3

    cepheid

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    Thanks for your reply berkeman!

    The OPTEK website doesn't give prices, nor do their datasheets. This is not uncommon, based on my past experience. My supervisor claims that our distributor would be Digikey, but I can't find OPTEK listed as a manufacturer on their site, nor can I find this product in the optoelectronics category of their catalogue. So I'm sort of stuck. As for the speed requirements, I'll have to look into that, but I should point out that all of the high speed inputs on the controller I'm using are taken up anyway (the controller's purpose is to do something else entirely, and I'm just taking advantage of leftover auxiliary I/O's for these sensors). So I'm left with the low speed inputs.
     
  5. May 7, 2007 #4

    berkeman

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    You might try running the part numbers through findchips.com -- they check a set of distributors for you, including Digikey.

    And on my speed comments, I just meant to check your error budget in speed. The slow rise times of O.C. output gates can introduce offsets and errors in your readings.
     
  6. May 7, 2007 #5
    play safe

    Hi cepheid
    me an old :approve: engineer used to transistors (remember them?)

    If the shutter is inductive (a coil) then I might run it anyway, from a transistor separate from the opto device, but run from its output in some way perhaps via a base resistor (etc...). Thats merely because I don't know the optocoupler capabilities, or the power the shutter demands.

    Yes open collector (which is what an external transistor would be) is more robust with inductive loads, you never really know what a totem pole pair on a chip gets up to when driving inductive load, the logic outputs are not necessarily designed to have a 'lively' load, and as you know, inductance creates voltage transients as it's switched off, the stored power in the inductor tries to get back into the driving device to escape. :uhh: (technical explanation!)

    Thats why often its open collector used, for slower inductive apps, with a diode to suppress switchoff tansients which could destroy the driving semiconductor. Whether you use the open collector optocoupler, or an external transistor, is your decision.

    Ideally you do need to investigate the optocoupler spec. if you're driving inductive load directly from its 5volt logic output. But for instance relays are inductive.. if its specified to run a small relay then it might directly run your shutter.

    Try it, see if it explodes..:wink:
     
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