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  1. I've spent far too much time this evening thinking about this, and my head is now scrambled for something that should be pretty trivial!

    In the attached diagram, will the LED/resistor cause any current to flow in the line marked as 12-24v (that's the voltage to be switched to gnd, not an actual 12-24V source)
    Click image for larger version. 

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    I'm pretty sure it won't, due to the LED blocking any flow from the 12-24V, so it won't cause the 12-24V to be reduced as no current will be able to flow, but I've thought about this that much now, I'm doubting myself...
    Avoiding the rubbish customer service from AluminiumWarehouse since July '13.

  2. #2
    Kitwn's Avatar
    Lives in Exmouth, Australia. Last Activity: 10 Hours Ago Has been a member for 2-3 years. Has a total post count of 354. Received thanks 40 times, giving thanks to others 11 times.
    In theory with 5v on the anode and 24v on the cathode the LED is reverse biased and no current will flow. In practice you might well be exceeding the reverse breakdown voltage of the LED which will break it. LEDs tend to have quite low reverse breakdown voltages so check the spec of the device you're using.
    Engineering is the art of doing for ten shillings what any fool can do for a pound.
    Wellington.

  3. #3
    I can't see what it is supposed to do! I presume it is some sort of switchable state indicator for the 12-24v and you don't want it to interfere with the logic of the 12-24v line. Do you have any resistor values yet. I presume that you are intending to use a small signal NPN, in which case you will need a current limiting resistor to stop the magic smoke (or is the 12-24v line already limited).

  4. #4
    Kitwn's Avatar
    Lives in Exmouth, Australia. Last Activity: 10 Hours Ago Has been a member for 2-3 years. Has a total post count of 354. Received thanks 40 times, giving thanks to others 11 times.
    m_c,
    Another option which avoids the risk of reverse breakdown of the LED would be to connect the LED (with a suitable series resistor) to ground after the switch to 5v. This will not fully confirm that the 24v is pulled down but will indicate that the circuit has been operated.
    I assume that 'switch' isn't really a switch (otherwise you'd be switching the 24v directly) so you would have to confirm the actual device can deliver the LED current as well as the base current of the transistor.

    If you stick with the design as shown remember that the LED current must be added to the load current when rating the transistor and calculating the base current and hence resistor value.
    Engineering is the art of doing for ten shillings what any fool can do for a pound.
    Wellington.

  5. #5
    Neale's Avatar
    Lives in Plymouth, United Kingdom. Last Activity: 4 Hours Ago Has been a member for 6-7 years. Has a total post count of 1,375. Received thanks 262 times, giving thanks to others 8 times.
    Just put a small diode in series with LED, with adjustment to resistor value to allow for forward voltage drop when on? Should ensure that reverse voltage across LED is not excessive. Or put LED plus bigger resistor across the load directly?

  6. Thanks guys.

    I never thought about checking the reverse breakdown voltage of the LED, but as Neale has suggested, I can just add a normal diode in series to solve that problem.


    It's actually for a buffer for a touch probe, which is the last part I need so I can get the cabinet for my Triac shut and moved off the desk. The probe is designed for 5V at 20mA, and although I could connect it directly to the 24V 4K opto, I want an indicator on the machine. All the LEDs I've looked at are around 20mA, so some form of buffer will be better for long term reliability.
    The final circuit will have sockets for two probes (touchprobe + toolsetter), and detection for the touchprobe being plugged in (so I can disable the spindle). This is just the basic bit of the circuit, which I just hit a complete mental block with.
    Avoiding the rubbish customer service from AluminiumWarehouse since July '13.

  7. #7
    Neale's Avatar
    Lives in Plymouth, United Kingdom. Last Activity: 4 Hours Ago Has been a member for 6-7 years. Has a total post count of 1,375. Received thanks 262 times, giving thanks to others 8 times.
    Are you concerned about the long-term effect of switching 20mA on the switch contacts in the probe? This is the kind of thing that I might have worried about until I had an interesting conversation with a friend who had spent many years working with industrial control systems. His comment was that using 24V was not just useful for noise rejection but the slightly higher switching currents also helped with long-term contact life and reliability. Switch too low a current for too long and the eventual oxide build-up could lead to poor contact but the higher currents (but much too low for arcing to occur) helped break through any potential oxide. Not sure if it's relevant here but I thought I'd lob it in anyway!

    I spend many an hour musing on touch probes, generally based around the published designs for three-contact types, but wonder about mechanical switching reliability, where can I easily source gold wires/contacts, etc. In fact, anything to stop me actually getting on with the project...

  8. #8
    Quote Originally Posted by Neale View Post
    where can I easily source gold wires/contacts, etc. In fact, anything to stop me actually getting on with the project...
    https://www.proopsbrothers.com/gold-...1103-595-p.asp

  9. #9
    The transistor will attempt to short the 12-24V supply to ground and something will go bang.

  10. #10
    Neale's Avatar
    Lives in Plymouth, United Kingdom. Last Activity: 4 Hours Ago Has been a member for 6-7 years. Has a total post count of 1,375. Received thanks 262 times, giving thanks to others 8 times.
    Whose side are you on, Rob? Another good excuse up the swannee...

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