Induced voltage problem

Discussion in 'General Electronics Chat' started by BPW, Jul 6, 2006.

  1. BPW

    Thread Starter New Member

    Jul 6, 2006
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    Whats the best solution?

    I have a multi-conductor cable that is used for various signaling applications. The problem I am having is that I am getting a small induced voltage (4.6 VAC) onto some of the lines which are connected to LEDs to provide status indication. This is enough voltage to cause the LED's to illuminate and give a false indication. Without too much elaboration there isn't an easy fix to the induced voltage.

    What can be done to keep the LED off at the 4.6 volt level but allow normal operation when fully powered to 120 VAC?
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    Greetings BPW,

    If I understand your problem correctly, it sounds like you are getting cross coupling between signal lines carrying large signal levels and signal lines carrying low level signals. All of the signal lines are bundled together in a common multiconductor cable.

    One thing to do is check your multiconductor cable and see if it is actually composed of several twisted pairs. If the cable is multiple twisted pairs then you will want to make sure that you have assigned your signals and their returns to the same twisted pair and not accidently assigned the signal to one conductor of one twisted pair and the return to the conductor of another twisted pair.

    If the multi-conductor cable is not made up of twisted pairs then two options come to mind.

    Option 1: Separate out the low level LED signal lines and cable then separately so that they do run in the same cable bundle. If you do this you may want to consider a shielded multiconductor cable for these low level signals.

    Option 2: Place a 1 microfarad capacitor in parallel with the LED. Make sure that you observe the correct polarity on the capacitor if you use a polarized type cap. If the LED needs to be pulsed at a rapid rate then you may need to reduce the cap to a 0.1 microfarad or maybe even smaller. You can experiment to see what size cap you can get away with.

    hgmjr
     
  3. Papabravo

    Expert

    Feb 24, 2006
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    I cannot get a picture in my mind of what you are talking about. I need a schematic or a picture of what you are doing.
     
  4. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    BPW,

    I agree with hgmjr's advice. I've had high powered [800 kW] RF at 100 kHz wreck havoc on signaling circuits where there were no shield. I've used capacitors to reduce level of that signal to something more acceptable.
     
  5. Papabravo

    Expert

    Feb 24, 2006
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    I'm trying to understand the geometry and the connections because noise and interference can be coupled in multiple ways. You can apply the suggested band aids but if they are ineffective you will be back where you started. It is exceptionally difficult to know if the solutions are effective without accurately identifying the source and magnitude of the interference. That's just my opinion and I could be wrong.
     
  6. Erin G.

    Senior Member

    Mar 3, 2005
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    You didn't mention whether you cable was shielded or not. If it is, check that the shield is grounded properly. If you're not using shielded or twisted pair cable, you may want to consider re-cableing with one of the two if it's practical.
     
  7. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Papabravo,

    Your correct in your opine. It's always best to eliminate the problem at the source and it may end up being they will have to replace the cabling with shielded pairs for signaling and a power cable for power.

    Sometimes you have to band aid things, it's certainly not my first choice, but it's certainly ... a choice.

    I'm sure the problem could be clarified if BPW told us the cable type and how much current was used by the device connected to the power. Alot of times, the troubleshooting or even suggesting without alot of the information only leads to speculating. And maybe that's all the questioner wanted to hear ... speculation.

    Any EMI should be eliminated at the source and like everything else, there are practical limits.
     
  8. BPW

    Thread Starter New Member

    Jul 6, 2006
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    Thanks for the input from all. This is a problem that we see often and I am just trying to get an idea of what others may have done or would do. As someone noted we know the idea situation is to stop the inductance but for to many reasons to cover here that isn't always practical.

    The cable is a 96 conductor with no overall sheild or individual shield. There is an outside armor braided jacket.

    If anyone knows where you can purchase a multiconductor cable this large with individual shields I would love to hear from you.

    The general setup is Pressure switches on one end switching the neutral leads and LED indicators on the other. We also have some momentary switches that are used in this multiconductor and it also distributes power (110 VAC) to multiple locations. As such I don't need pairs but rather single lines shielded.

    We have run into various different types of inductance but in this instance it is self induced by other lines in the multiconductor. We also have seen problems with inductance from outside lines. In particular with high voltage DC lines.

    So making the assumption that we aren't going to stop the inductance (yes that would be the best thing to do) then what other options are available?
     
  9. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    One option is to reassess your requirement for such a large cable bundled in one package ... it may be just convienence, but now that convienence is causing you heartburns right now.

    The best route is to seperate signaling and power. That way the signaling can be twisted shielded pairs and power could be an entirely other cable.

    I believe you would be best served using shielded pairs. I've seen signaling installations where they used one wire of a shielded pair for the "hot" side and had one wire servicing the ten "hot" sides as a return and none of the shields were connected. We would receive multiple false indicators until we cleaned up the installation using individual hot, returns, and grounding the shields.

    You might be able to get away with just running power cables to satisfy that need. Even as a temporary measure to ensure you won't need the shielded cabling on the signaling side.

    How long are your wire runs ... ?

    Is cost your prohibiting factor ...?
     
  10. SJG

    New Member

    Jul 7, 2006
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    The circuit is 120VAC.
    The LED operates fully bright at 12V to 250VAC at less than 1W.
    When we get the 4.6VAC EMI on this line the light dimmly lights.
    Getting rid of the EMI would be great but at this point not practical because the cable is already constructed and paid for. Shielding would probably be a good fix for future applications, but again, not practical now since the cable is already made.

    The 91 multi-conductor cable is not made up of twisted pairs. And is not shielded at all, just an armor braid. Shipboard cable.

    How will the capacitor not cause my light to blink, or is that the reason for testing out smaller sized caps in order to see when the light does not blink at 120VAC but does stop my EMI from dimming the light at 4.6VAC?

    Is there a way to use a resistor in parallel or series with the LED light to knock off the 4.6V. I am looking at parallel since I can shunt the EMI to ground across the LED (from L1 to neutral). Since the EMI is basically voltage with no VA (volts Amps). I have tried 40 ohm resistors 2W in parallel, and have gotten the LED to not Dim with EMI. But when I turn on the 120VAC the resistor smokes. I then tried a 10K ohm 5W in parallel and the light did Dim with EMI, and did not smoke at 120VAC. Maybe there is a happy medium I need to find. A range where I can shunt the EMI, but not smoke the resistor when I apply 120VAC. Since the LED will operate at 12VAC I just need to allow over 12VAC reach the LED during normal operation. Any advice here?
     
  11. SJG

    New Member

    Jul 7, 2006
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    Sorry for not introducing myself earlier, I am working with BPW on this problem right now.

    Since we know the LED will operate at full brightness at 12V to 250VAC, I am thinking I can decrease the voltage across the LED but still keep it operating when 120VAC is applied. By using a resistor in series maybe? But when just the EMI is present that same resistor will eat up most of the 4.6VAC leaving not enough to notice voltage to operate the LED.
    I did some fast calculations before I try putting a resistor in series with the LED. Here they are and what do you guys think about this?

    Resistance Value used: 10K ohm
    Voltage used: 125VAC
    Voltage Drop at LED: 75VAC
    Voltage Drop at Resistor: 50VAC

    Voltage used: 4.8VAC
    Voltage Drop at LED: 4.8VAC
    Voltage Drop at Resistor: 3.2VAC

    Resistance Value used: 20K ohm
    Voltage used: 125VAC
    Voltage Drop at LED: 53VAC
    Voltage Drop at Resistor: 71VAC

    Voltage used: 4.8VAC
    Voltage Drop at LED: 2.7VAC
    Voltage Drop at Resistor: 2VAC

    Resistance Value used: 40K ohm
    Voltage used: 125VAC
    Voltage Drop at LED: 34VAC
    Voltage Drop at Resistor: 90VAC

    Voltage used: 4.8VAC
    Voltage Drop at LED: 1.31VAC
    Voltage Drop at Resistor: 3.49VAC
     
  12. Papabravo

    Expert

    Feb 24, 2006
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    I'm still having a great deal of trouble figuring out what you guys are doing. One thing I do know is that LED's cannot drop large voltages, like 75VAC as your previous post indicates. It also seems to me that an LED will attempt to behave like a half wave rectifier by passing the positive half cycle and blocking the negative half cycle. I'm also beginning to suspect your measurement methods. That you are smoking large power resistors is a definite indication that you are trying to use LED's in a fashion they were never intended for. If the EMI is not affecting the operation except for the LEDs then I suggest that you find another way to do an indicator. "Grain of rice" neon bulbs might be a suitable alternative. I see no way to mitigate EMI without going to shielded twisted pairs for signal.
     
  13. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Can you give us the schematic drawing of one of the signaling circuits ... from source to LED. And one of the power circuits. It sure would clear things in my mind.

    120 Vac sure sounds out of wack. I know I certainly had something entirely different than 120 Vac travelling down the wire to an LED.
     
  14. awright

    Well-Known Member

    Jul 5, 2006
    84
    7
    Like all other responders, I am having a lot of difficulty understanding your application. It sounds like you smoked the resistors because you put them in parallel with the LED in the hope that you could drain off the low power induced voltage. But that puts the resistors directly in series with the 120 volt line which would fry them.

    If, in fact, your induced voltage is limited to 4.5 volts (PEAK?) can you put a zener in series with the LED so it will not conduct at less than the zener voltage? I certainly do not think this would be the optimum solution if you were starting from scratch, but given your constraints, it may be one option.

    Sure would help to see a schematic. Is your LED a composite device with diodes, resistors, active devices inside the package that allow operation over such a wide voltage range? Don't make us guess. Let us know what you are doing and what your components are.

    awright
     
  15. SJG

    New Member

    Jul 7, 2006
    3
    0
    We went ahead and switched the 120VAC lines going to the LED indication lights with 24VDC. This solved our problems by removing several lines of 120VAC which it looks like was the cause of our induction. Problem solved.

    Thanks, I hope this helps others.
     
  16. hgmjr

    Moderator

    Jan 28, 2005
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    Thanks for the feedback annoucing that your problem has been resolved.

    hgmjr
     
  17. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
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    I know this is a day late and a dollar short, but I've terminated dozens of high-count multi-pair cable aboard ships. Here's one example: M24643/42, Type LS1SU
     
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