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The purpose of these diodes?

  1. Jun 27, 2017 #1

    Hello not sure if i've posted this in the right section. But could someone tell me the purpose of the zener diode and the diode in parallel with the resistor? Never seen this configuration before. Thanks!
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  3. Jun 27, 2017 #2

    Charles Link

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    I would have expected the Zener diode to be in the other location directly adjacent to it so that it can be used to regulate the voltage. It's interesting, but I wonder if it may be in error. I would be interested in the comments from others with more electrical engineering experience.
  4. Jun 27, 2017 #3
    The diode parallel to the coupling is to protect the coupling from reverse voltages.
    The Zener and the resistor closest to it are to filter the input - specifically to keep transient spikes from tripping the coupler.
    That type of filtering is needed when you are using a switch with mechanical contacts.
  5. Jun 27, 2017 #4
    Ah right... I knew it was simple. Couldn't find anything even similar to this on the web though. cheers.
  6. Jun 27, 2017 #5
    It looks like the Zener diode simply subtracts some voltage from the rectified voltage (or, which is same, increases the threshold), in order to make the "zero" time longer so the signal be better formed.
  7. Jun 27, 2017 #6
    What is the coupling device?

    The obvious choice would be an opto-isolator. However, if it is a relay then the parallel diode has the correct polarity to dump the back-emf and protect the zener diode.
  8. Jun 28, 2017 #7
    Yeah the coupling I believe will be an opto-isolator. If this is the case the diode will be used as something else?
  9. Jun 28, 2017 #8


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    no, cuz it isn't going to supply voltage as the image states
  10. Jun 28, 2017 #9

    jim hardy

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    Good Observation !

    It'll pulse at twice line frequency.
    Perhaps zener is for intentional time delay after the zero crossing ? Who knows what that 'coupler' does?

    PLC for some reason is being told: "Button is pushed AND AC is present in excess of ??? volts as calculated by voltage divider R's and zener."

    old jim
  11. Jun 28, 2017 #10
    The image doesn't state the coupler supplies voltage. The arrows just show the coupler output goes off to be connected to a PLC input. The image implies that the PLC input provides a 5 V source via a pull-up resistor. The coupler just needs to either ground that input line or leave it open-circuit. Either an opto-isolator or a relay would be suitable for that.

    (Of course the OP shouldn't ass-U-me this. He should check the PLC data sheet.)

    The circuit is rather odd. If all that was required was to check the status of the push-button switch it could be wired directly to the PLC and not bother with the dangerous directly-connected mains circuitry. The circuit could check the frequency, phase or presence of the mains but if you needed that, why would you only do it under push-button control? Perhaps the push button is remote from the PLC hence the isolation is useful?

    Also, if I had a reason to work from the mains I'd look first to use a (mains class X, Y) dropper capacitor rather than the 3 resistors. Perhaps the presence of the anti-parallel diode implies that the designer started this way but changed their mind without removing the anti-parallel diode (which doesn't seem to be needed given the bridge rectifier and an opto-isolator).
  12. Jun 28, 2017 #11


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    You are all missing a key point. And that point is: Avalanche diode.
  13. Jun 28, 2017 #12
    Well, don't keep us in suspense. Do feel free to explain our omission to us.

    Note that we have already discussed the use of a zener diode which, despite the name, normally is an avalanche diode over 5.6 V rating.
  14. Jun 28, 2017 #13


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    The point was to look deeper since the schematic symbol (to me) is not a zener. Functions similar to a zener, yes.
  15. Jun 28, 2017 #14
    This is not an unusual electrical circuit for situations where PLC's are used. We are in a "process control" environment, most likely some sort of industrial setting - chemical process, manufacturing, machinery control. It is likely that only 230vac is ubiquitous outside of the electrically enclosed PLC. Not only may the button be tens or hundreds of meters from the PLC, but low voltage wiring may not be allowed in the machinery area. And ever if it was allowed, special shielded twisted pairs might be required to protect a low-voltage, relatively high-impedance line from picking up stray currents from light ballasts and other inductive sources.

    Also, there may be no push button at all. That push button may simple represent a power line that is either powered up on not. The circuit can be applied to any 230vac line - one controlled by a push button, a relay, or the PLC.

    As far as dropper capacitors are concerned, the objective is for something very hardy that can be used in many situations throughout the industrial setting. The objective is not electrical efficiency, it's something that won't fail and won't confuse anyone. These systems are maintained by electricians, not EE's. Think "Ladder Logic", not circuit analysis.
  16. Jun 28, 2017 #15
    True, hence my comment about isolation for a remote pushbutton. We don't actually have any context for this circuit, beyond the picture itself.

    Resistors dissipate heat, which isn't good for reliability.
  17. Jun 28, 2017 #16

    jim hardy

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    On The Other Hand - with capacitive current limiting, should switch close at instant of peak voltage there's a large transient current .
    Sinewaves spend half their time above 70% voltage.
    So statistically, the zener and coupler would get hammered by a current spike on most of the contact closures.
    Of course if it's a relay that's no big deal.

    old jim
  18. Jun 28, 2017 #17


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    No one (not even the OP) seems to know that the "coupling" is or what it does. Can we really carry on a discussion about this without knowing what the black box does?
    The values of the resistors is pretty relevant (Is R the same for all three? )to what actually goes on. That Zener could be any value, depending on what voltage drop there is across the Rs.
  19. Jun 28, 2017 #18
    It's been in the back of my mind since first reading this thread, but wouldn't a 230 VAC interposing relay be simpler?
  20. Jun 28, 2017 #19

    jim hardy

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    yes indeed.

    The rectified AC could be a timing signal, the full wave bridge sans filter gives 120 hz.
    My microwave oven uses something similar - half wave rectified(gives 60hz) from another diode in the power supply advances the clock.

    The zener could add the intelligence " there's enough AC present to start (something?) "

    "What Evil lurks in the minds of programmers ? The Shadow knows..."

    Yep - we're all guessing.

    old jim
  21. Jun 28, 2017 #20
    Went looking for examples of zener diodes used in PLC input circuits. So far, the only close analog is on this daenotes web page, and show an optoisolator. The accompanying text suggests this is intended to operate as a simple AC-to-DC logic level converter.

  22. Jun 29, 2017 #21


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    Looks to me like the circuit produces a train of trapezoidal pulses at twice the frequency of the 220VAC supply. R1 and R2 limit the LED current. R3 makes sure that the LED turns off by reducing capacitive coupling of dv/dt through the bridge and zener ZD. The zener ZD increases the dead time when no LED current flows which significantly lengthens the LED off-time.

    The input to the 5VDC processor is a rectangular digital clock signal that could be used to signal power β€œon”, or to synchronise AC phase angle switching.
  23. Jun 29, 2017 #22
    When relays are used, there is usually additional filtering placed between the relay and the PLC. And this is always the case when the relay is part of a PLC manufacturer supplied interface board.

    The further you get into the PLC circuitry, the more expensive it gets to process relay contact bounce and arcing - expensive in terms of digital processing circuitry and/or processor time. Once the signal reaches the PLC processor itself, it will either trigger processing at each transition (which could be tens of thousands for a single dry contact make or break) or it will be sampled (perhaps at an unrepresentative instant).

    So, if the coupler is part of the PLC or PLC-manufacturer supplied equipment, it is certainly an opto-isolator.
    Otherwise, it is probably an opto-isolator.

    The 120Hz signal does create 240 transitions per second - but this would not challenge the processor. It is probably filtered with the software version of a one-shot.
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