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Optocoupler If question... — Parallax Forums

Optocoupler If question...

MicksterMickster Posts: 2,693
edited 2017-09-04 10:46 in General Discussion
I need to couple an external 24v to a Propeller input (tied to 3.3v, via 4.7k). I have a TLP620-4 that I liberated from an old industrial I/O card. The datasheet states a maximum If of 50mA and a recommended 16mA. There is also an indicator LED in series with the optocoupler input. I was looking at using a 1k limiting resistor for ~20mA If but then I saw that the other design uses 2.2k. This means a If of <10mA. The datasheet doesn't specify a minimum If. Furthermore, these fixed 24v power supplies that I work with are unregulated and rarely do I get the full 24v. Which way should I go regarding the limiting resistance?

Edit: Ah, I just found "+/- 8mA" for this 4-channel device. This feels a bit borderline, considering that the nominal 24v can be closer to 20v.

Comments

  • Peter JakackiPeter Jakacki Posts: 10,193
    edited 2017-09-04 11:43
    Figures and graphs indicate that there isn't any problem with driving them at lower currents. The lower current may mean higher Vce(sat) and perhaps slower response but unless you need high speed there shouldn't be a problem plus the Prop thresholds aren't CMOS or TTL as it doesn't really have a noise margin since the inputs need to switch with the Sigma Delta counter modes etc and high/low trips around 1/2 Vdd at approximately 1.65V.

    A resistor of 2K2 driven from 24V is a bit low even taking into account the forward drop of the IR and indicator LEDs and perhaps less than 24V supply.. I would probably use a value of around 3k3 or so myself. Datasheets also specify test conditions which shouldn't be confused with operating conditions. They like to achieve a low Vce(sat) etc under all conditions etc. The absolute max of 50mA is not something that you should ever be striving for :)
  • MicksterMickster Posts: 2,693
    Cheers Peter.

    I'll try the 3k3 option although I kinda like the indicator brightness that the 2k2 provides. Response is not an issue.
  • kwinnkwinn Posts: 8,697
    Mickster
    Mickster wrote: »
    Cheers Peter.

    I'll try the 3k3 option although I kinda like the indicator brightness that the 2k2 provides. Response is not an issue.
    I suspect that Peter means the resistor on the output transistor of the opto while you are referring to the current limiting resistor on the input. If that is the case I would suggest a 2K resistor on the input and a 3.3K as Peter suggested on the output. Could even go as high as 4.7K on the output.
  • Peter JakackiPeter Jakacki Posts: 10,193
    I do mean input resistor, there is no need for this to be so low, especially at 24V.
  • MicksterMickster Posts: 2,693
    kwinn wrote: »
    Mickster
    Mickster wrote: »
    Cheers Peter.

    I'll try the 3k3 option although I kinda like the indicator brightness that the 2k2 provides. Response is not an issue.
    I suspect that Peter means the resistor on the output transistor of the opto while you are referring to the current limiting resistor on the input. If that is the case I would suggest a 2K resistor on the input and a 3.3K as Peter suggested on the output. Could even go as high as 4.7K on the output.

    Peter's on the right track; I have the 24v feeding the input via the resistor-in-question and a series connected indicator LED. The output is to pull-low the prop pin that is tied to 3v3 via a 4k7 resistor. I'm all for the least-possible component stress, so the lowest-but-reliable If, the better.

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