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What can cause the voltage of my circuit to fluctuate in the way it does?

  1. Jul 17, 2009 #1
    I have created a circuit. The batteries are in series, and the igniters for my rocket are in parallel. There are three igniters.

    When I check the voltage that will be going through the various igniters, the voltage appears to fluctuate. It goes from 0 volts to 2.5 volts to 6 volts to 18 volts. On other places where the igniters will be placed, the voltage will just stay at its correct voltage which is 18 volts. When I test fire this circuit and place the igniters into their places, the igniters ignite at different times. How can i get them to ignite at the same time? They have to ignite at the same time.

    I think the possible reason for this is that the batteries are in series. Do the batteries have to be in parallel?
     
  2. jcsd
  3. Jul 17, 2009 #2

    berkeman

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    What are you using to measure the voltages? What do you mean "fluctuate"? Do you mean that you hold the meter on the two wires, and the DC voltage changes like that? You have a connection problem, or a bad battery. Or does it change over longer time periods?
     
  4. Jul 17, 2009 #3

    negitron

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    Bad connections. Since your igniters are supposed to be wired in parallel, the open-circuit voltage where each is to be connected should be identical. Since it isn't, you've probably got one or more bad wire splices; for the best connections, you should solder them, instead just twisting the wires together and taping them.
     
  5. Jul 17, 2009 #4
    Ok. Without the circuit, the batteries work just fine. By fluctuate I meant they change in voltage. I will have to open up part of my rocket to redo the connections. I thought this might have been the problem. Thank you.
     
  6. Jul 17, 2009 #5

    berkeman

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    Sounds dangerous. Be sure to disconnect the batteries first, eh?
     
  7. Jul 17, 2009 #6

    negitron

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    And ground the wire ends. I don't know how sensitive electric match igniters are to static, but I wouldn't want to risk my face on it.
     
  8. Jul 18, 2009 #7
    Connecting the igniters in parallel across the same power supply is the worst thing you can do, and will certainly result in what you are seeing. The reason is that the igniters will each have a slightly different resistance, and the one with the lowest resistance will get the most power and also drop the voltage accordingly. Ideally each igniter should be driven by a separate current source, and it should be substantial (so ignition is rapid). That way a high-resistance igniter gets more voltage (so the power is more equal to what the other igniters are getting). You can make a poor man's current source for each igniter like this: Try a considerably higher-voltage supply and put a resistor in series with each igniter. For example, triple the voltage and put a resistor of about the resistance of the average igniter, in series with each igniter. Discalimer: voltages higher than 12V should be used only by a one-armed professional electrician wearing inch-thick rubber gloves and standing on a lucite plate.
     
  9. Jul 18, 2009 #8

    negitron

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    No, fleem. First of all, he's measuring the open circuit voltages; there is certainly one or more poor connections present. Secondly, even if one igniter does draw more current, in doing so it fuses open so the next lowest R one fires, and so on. This all happens within a very small timespace.
     
  10. Jul 18, 2009 #9
    According to the OP, the problem is precisely that it does not occur in a small "timespace".

    As I said, this is most likely because the igniters are not getting the same power, which is most likely because the igniters have differing resistance (whether intrinsic to the igniters or because of bad contacts). Certainly it goes without saying that bad contacts should be fixed. But whether the variation in resistance is because of bad contacts or because of variations in the manufacture of the igniters, causing the supply to be more of a current source will help considerably.
     
  11. Jul 18, 2009 #10
    Thanks for everyone's input. After rewiring the entire circuit, I found that the clips I was using to ignite the igniters were no good. Apparently, the clips I used cannot be used in this sort of project.

    I am still having a different problem with the circuit, however. It is at https://www.physicsforums.com/showthread.php?p=2276533#post2276533

    Thanks
     
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