Indicator Lamps - Voltage Drop

Discussion in 'The Projects Forum' started by msnead, Jan 17, 2013.

  1. msnead

    Thread Starter New Member

    Jan 17, 2013
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    I'm trying to design a "tester" for checking continuity of "through wires" in an electric hinge we make. The hinge will be mounted to a swing test stand and cycled repeatedly to test the integrity of the wires. The wires in the hinge are about 24 or 26 AWG. I am using a 2191U5-24V LED indicator lamp (Chicago Miniautre Lighting - Newark PN# 88M9621) to see that the wire is "good". On one side of the hinge (wire) I am connecting to +24VDC through the use of a power supply. The other side of the hinge (wire) connects to the RED lead on the LED, and finally the WHITE lead is connected to -24VDC. No problem.

    However, I need to connect as many as 12 wires for a total of 12 LED's and I need to power up a 24VDC Ice Cube relay (for breaking the "enable" on the motor for the hinge swinger). It's been a long time since I was in electronics school, but it didn't take me long to remember VOLTAGE DROP. With only 1 (one) LED connected in series to the positve for the coil on the relay, I don't have enough to pull the relay in. I have only 6VDC. I've tried connecting the lights in series and in parallel, but still don't end up with enough voltage to pull in the relay. A PLC would solve the problem easily enough, but I was hoping to save cost. Does anyone have any suggestions?
     
  2. wayneh

    Expert

    Sep 9, 2010
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    So you're saying that your power supply can no longer trip your relay if you add LEDs in parallel with the relay? I'd say you need a bigger capacity power supply or a different relay.
     
  3. msnead

    Thread Starter New Member

    Jan 17, 2013
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    Yes, I admit I probably didn't do the "homework" related to the circuit before I dove right in. You are probably right about the power supply. I looked at it closer (it's an AB 1606-XLP30E and only has 1.3 amps output).

    While waiting for replies, I've continued "tinkering" with it and did find, however, that if I connect the positive (from the power supply) to every one of the RED leads on the LED's and then take every one of the WHITE leads and connect them to one side of the coil (ex: terminal 2), then come out of the coil (terminal 7) and take it to the negative of the power supply, that all the LED's energize and so does the coil. The actual voltage across the coil is NOT 24V, but it is enough to pull it in. Also, the more LED's I connect this way, the higher the voltage is across the coil. I'm so "rusty" in my electronic circuits. What would you call this connection method? Wouldn't that be truly parallel?

    The drawback to this, however, is that it would not allow for the coil to be de-energized if any of the wires were removed (unless less than 4 remain connected). Therefore, although you would have an idea that a wire on the hinge is bad, but you wouldn't be able to stop the motor so you would know what count the wire failed.
     
  4. wayneh

    Expert

    Sep 9, 2010
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    I get confused without a picture (schematic). Sounds like your power supply is plenty beefy, that much I get. How much current are your LEDs taking? Maybe 10-20mA?

    And your connection test is serial and parallel; the LEDs are in parallel but all of them are in series with the coil. That's not a good arrangement, IMHO.

    As I understand it, you want an LED indicator for each conductor and you want the relay to trip if any one of the LEDs goes out, indicating a conductor failure. Is this right? If so, I'd look at using several LM393 quad comparators. (There are other options but this IC is everywhere and easy to find.) Each comparator can watch for continuity on a conductor. The outputs can all be "OR"ed together so that the relay will only stay energized if there is continuity on all conductors. Failure of any one would trip it.
     
  5. msnead

    Thread Starter New Member

    Jan 17, 2013
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    I'm working on a schematic now. I have one, but it's an Autocad file, so I'm converting it to something I can post. Not sure of the amp draw on the LED's.

    IMHO? Not a good arrangement? Works, but not a "standard" practice?

    Yes, that's exactly what I need to do. The reason for the relay is that there's an "enable" circuit on the motor controller (KBMM-125). Closed is "run" and open is "off".

    Wow ... never hard of a "comparator" before. My line of work usually involves programming, wiring, and working with things that are "built" elsewhere. Sounds like what you are talking about is an "Integrated Circuit" or "chip"?
     
  6. msnead

    Thread Starter New Member

    Jan 17, 2013
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    I don't have a website so I am uncertain as to how I can post a picture.
     
  7. wayneh

    Expert

    Sep 9, 2010
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    I just mean it introduces a few problems. For one, you need enough LEDs in parallel to get enough current to trip the relay. Maybe you need 10 LEDs to trip it, and 12 is better, but even 9 will hold it once it's pulled in. All of this is dependent on power supply voltage, maybe the temperature of the LEDs and/or coil, and so. It's just not a reliable, solid arrangement.

    And yes, I was referring to an IC. I don't see how you can monitor continuity of 12 conductors, and then control a single relay, without using an IC. Maybe someone else has a clever idea.
     
  8. msnead

    Thread Starter New Member

    Jan 17, 2013
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    Thanks, wayneh. I've heard mention of "comparators" from other sources, and I agree that this will be the best approach. It's just been a long time since I worked with a "detailed" circuit such as this. I really appreciate the help.
     
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