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  1. #1
    In the middle of my router control box I have a Pilz safety relay, bought cheap from eBay. New but obsolete model, box suffering from long-term storage but the device was fine and has been running perfectly for a few years. However, yesterday it refused to trigger when I hit the "reset" button. I have opened up the case but it's not easy to get at all the bits inside without dismantling with a soldering iron. I need to do a bit more testing before I go that far, but I wondered if anyone else has ever had a problem with one of these things, or has any other comments.

    At least it failed safe!

  2. #2
    Just to draw a line under this one, in case anyone in the future stumbles across this post...

    After looking at how the thing was put together, it just did not look practical to take it apart at all easily - at least, not in a way that was likely to work again afterwards. A PC board that is fixed to its supports via around 4 dozen soldered pins (of which about half were bent over so that there was some kind of mechanical support) isn't the most maintainable item in the world and the chances of breaking a track or two were high. However, by looking at the simplified internal wiring diagram in the data sheet and a lot of inspired prodding with a test meter, it looked as if there was a problem with the internal connection between the 24V supply and the terminal feeding the e-stop switch chain. There is an internal (current-limiting?) resistor and diode shown between these two. I can only guess that these are protective measures in case of external wiring problems. However, my 24V supply is current-limited anyway, and as the e-stop supply terminal and 24V power terminal are literally next to each other on the relay, I moved the e-stop switch chain connection to run directly off the 24V feed rather than via the internal diode/resistor. Problem solved - all works as it should now.

  3. #3
    Out of interest is the diode in series with the supply - I.e. prevent reverse connection? Or is it across the coil -I.e. fly back protection when switching. If across the coil maybe it will stress the power supply if bypassed ?
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

  4. #4
    The only info I have is from the sketchy diagram in the data sheet. I think the idea is that you can see how it works (5 relays, apparently, with all sorts of interconnections) but without much detail - there's a lot more in there than they show! However, the diode is shown as being in series with the supply, together with a resistor (no value given). I'm guessing protection against a short to another supply in the external e-stop circuit (diode) or short to earth (current-limiting resistor). I would guess that in my case the diode has gone pop (never trust a semiconductor) but it might just be a broken track. No back-emf diodes shown, although they might be there somewhere. I have another 24V relay in the box, driven from the 24V SMPS, with no diode involved. I don't know what kind of back emf could be generated but these aren't high-current relays anyway.

    If this hadn't worked, I was considering making my own poor-man's version. Take the usual latching relay type of setup, with a momentary n/o switch to enable. Put a second relay in parallel, and wire NC contacts in parallel and NO contacts in series. That should cover you against either relay failing to operate or sticking on.

    I run e-stop switches plus fault outputs from the stepper drivers through the e-stop input on the safety relay although limit switches just go to the motion controller. The safety relay will enable/disable the motion controller, the stepper drivers via the enable inputs, and the stepper PSU via another relay.
    Last edited by Neale; 19-08-2019 at 06:42 PM.

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