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  1. #41
    Quote Originally Posted by Doddy View Post
    John - firstly, these are usually infra-red, so it may well have been illuminated even if you couldn't see it (hint - you can often get a mobile phone camera live-view to witness IR LEDs).

    Reverse voltage?, yup, a good way to fry an LED.

    The good thing is these are generally fairly standard devices, although pin-outs may vary. And drive voltages?, can be pretty much whatever you need. I'll assume you've not hit this before otherwise you'd not be asking.. Take the supply voltage, and subtract the forward voltage drop of the LED. Then divide that by your chosen forward current. That gives the value of a resistor to place in series with the LED for it to operate at that voltage. That Datasheet indicated a max forward current of 40mA - that's an absolute max rating, a rough rule of thumb is around 10mA.

    So, if you're trying to drive at 5V, and the Forward Voltage is 1.8V, with a Forward current of 10mA (0.01A), that's (5.0-1.8)/0.01 = 320 Ohms, nearest easy found value is 330 Ohms. If you want to drive at 12V, then (12.0-1.8)/0.01 = 1020 Ohms, nearest easy value = 1000 Ohms (1k).

    Whichever resister value you calculate, you place the resistor in series with the LED then drive that from the DC supply.

    Note, its generally considered unhealthy to reverse bias an LED - they go pop at quite a low reverse voltage. Even if they don't pop, then can be significantly degraded.

    Amazon sell reflective opto-couplers - there's generally not a huge difference in the 4-pin devices (2 pins for the LED, 2 pins generally for a photo transistor). They should be pretty much interchangeable.
    Hi again folks
    Have replaced the opto switches but i am struggling to follow the calcs above
    Taking figures from page 5 of the datasheet Forward voltage is 1.7 v max with a forward current of 20mA, so with two lamps in series we have a forward voltage of 3.4.
    Not sure how I choose the forward current but 20mA is the figure given in the conditions column so will go with that.

    So cautiously driving with a 5 v supply 5- (2x1.7) = 1.6V and at 20 mA I calculate a resistor of 80 ohms and I dont have one.
    And I still cant quite understand why Boxford were driving the same rig with a 10v output and no apparent resistor , so with tongue firmly in the cheek and having a couple of spare opto switches my inexperienced mind thinks why not give it a shot without resistor anyway.

    So I wired to plus 5 v and brought the single pulse (blue wire) back to pin 12 , set this up in Mach 3 I am not understanding the other settings I see in Mach.
    I get figures flashing up for true spindle speed as I switch on with say S500 but these figures die away to zero as spindle runs up.

    I am bewilderd by the range of settings I may need to address so if any one can guide me I would appreciate.
    I presume that spindle pulleys just selects a speed range so I think I am OK with that
    But Ports and pins , spindle setup , spindle motor , motor outputs , PID settings have lost me, even before I get into Encoder settings
    Last edited by John11668; 07-11-2020 at 02:01 PM.

  2. #42
    20mA is usually around the max rated forward current. It's a fairly conventional rule of thumb to select 10mA. That'd give a resistor of 160R, or thereabouts (selecting 10mA as the basis of the resistor calculation gives you a lot of wiggle - you can halve it and still be within the working limits, chances are you could double it and it'd still work). You get less flexibility as you chose a resistor for a larger forward current, of course.

    An idealised, simplified model of an LED would presented a voltage source of V-forward (1.7, above), and 2 in series would be 3.4V. That then would mean without a series resistor you're presenting 1.6V (as you say) into a dead short - theoretically infinite current and the blue smoke escapes. Of course, the real model of the LED would have some internal series resistance, and there's the series resistance of the cabling, and the PSU. It might work. It might work for a while. It might work for not very long. But you have spares!, and if you want any resistors thrown in the post I can do that. It's an interesting experiment in any case, and you can but learn from it.

    With Mach, can you confirm that with stationary spindle you get 0 RPM. Then at very low RPM you get some RPM (maybe not the right RPM, but some). The first thing you want to make sure is that the encoder input is working before getting too hooked into the inner workings. You should be able to monitor P12 input on diagnostics page and rotate the spindle by hand to find the index position - just confirm that is working with 1 PPR.

  3. #43
    Would appreciate a couple of resistors suitable for the task Doddy. You still have my address I presume.
    And yes spindle speed shows as zero

  4. #44
    Not seeing any illumination on pin 12 as I rotate the disc past the single slot

  5. #45
    Quote Originally Posted by John11668 View Post
    Not seeing any illumination on pin 12 as I rotate the disc past the single slot
    I suspect with no resistor you have blown the new optical sensors
    Take the pin 12 and touch it to ground. See if this triggers the pin in mach3
    If it does then you have likely popped the new sensor

    Paul


    Sent from my iPhone using Tapatalk

  6. #46
    Are there optical switches with a better voltage tolerance ?

  7. #47
    That's the purpose of the resistor.

    EDIT: I'll get a selection in the post on Monday morning.

  8. #48
    Am currently trying to set up a profile in Linux cnc .
    Would that be an easier to place to start .

  9. #49
    RS Components do free delivery on all online orders which makes it economical to order only a few LEDs, resistors etc. of various values costing next to nothing for experimenting. I used to feel guilty about this and try to wait until I had a larger order, but they end up sending your order in 5 envelopes from 3 different countries anyway so don't worry about it. RS are not always the cheapest supplier but for small orders the free postage is very useful and delivery (here in Australia anyway) is quick.

    If you're not sure what you need I'm sure there are several readers who could scan the online catalogue and suggest RS part numbers for you even if we don't have components we can send you directly.
    An optimist says the glass is half full, a pessimist says the glass is half empty, an engineer says you're using the wrong sized glass.

  10. #50
    (Deleted - retreats with egg on face due to not being able to count on fingers and get same answer twice)
    Last edited by Neale; 08-11-2020 at 09:05 AM.

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