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  1. #31
    Join Date
    7th February 2009
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    Arlington, TX
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    1,223
    Quote Originally Posted by Charles_McG View Post
    Does it take any special kind of meter to read those low resistances?
    Yes. At a minimum it requires a Kelvin connection. This uses four wires so that the excitation current doesn't pass through the wires that measure the voltage. The test current must also be lower, much lower, than the no fire current of the igniter, if connected.

    Purpose built igniter testers can be quite expensive. Valhalla 4314's seem to show up on EBay fairly often at not insane prices.


  2. #32
    Join Date
    18th January 2009
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    1,762
    Quote Originally Posted by jderimig View Post
    Jim, like I said it is tricky and you have to make the measurements like you are doing. Every case is different and what is safe for one instance may not be for another.

    A question that I have is are you running the shunt in parallel with your ematches with a series resistor between the power source and your parallel circuit?

    In otherwords do you include a dropping resistor that the shunt works against to drop the voltage across the match?
    Actually, I mispoke on the method I use. I don't assume the nominal voltage of the battery - that wouldn't work. Instead, I use published values for the maximum current capability of the battery. I also don't try to measure the resistance of the shunt wire itself - again, this is from published information.

    A few years ago, I posted the calculation I developed for the case of a 9 volt battery with a shunt. I've copied it below. I think a review of it will show the process isn't that complicated, although I have always recommended ground testing of the exact circuit just for peace of mind.

    More recently, I have gone to LiPo batteries (not having a choice with the EasyMega altimeters). There, I solve the current problem by using small LiPo's with limited current capability, again, combined with ground testing (per my previous post).

    Jim

    ...From February 2015 ...

    It's relatively easy to determine if your shunt is sufficient. A simple example.....

    I use a 9-volt alkaline battery for my altimeters (I specifically do not use LiPo's in this service). The maximum current a 9-volt battery can produce is around 5 amps. The statistics for the JTEK ematches I use are:

    - Maximum no fire - 300 ma
    - Minimum all fire - 750 ma
    - Maximum recommended test current - 40 ma
    - Recommended firing current - 1000 ma

    Let's say that the resistance for the ematch part of the circuit is 2 ohms (the match and some wire). If my shunt wiring is 1 foot long and I use 24 gauge wire, the resistance of the shunt could be as low as 0.026 ohms, depending on the switch used. If the altimeter fires and shorts the battery to the ematch, the maximum possible current through the ematch would be 0.026 / 2.026 x 5 = 64 ma. This is above the maximum recommended test current but well below the maximum no fire current. And, since the circuit has some actual resistance, the battery won't actually produce 5 amps.

    In the above scenario, the shunt would prevent the ematch from firing if the altimeter fires. However, if you used a battery or cap that could source more current, or a longer piece of wire for the shunt, or finer wire, you could approach the maximum no fire current. I ran this test case with 4 feet of wire (0.104 ohms) (edit - for the shunt wire) and a shorter ematch (1.4 ohms perhaps), which might have given 0.104 / 1.504 x 5 = 346 ma. That fired the match.

    Although my example case would be safe, I have gone to adding a resistor to the ematch pathway for sustainer ignitors. For a 9 volt battery, and with a recommended firing current of 1 amp, the ematch circuit could have a resistance of as much as 9 ohms. I use a 3 ohm resistor (plus or minus), giving 5 ohms total, to drop the maximum current to below the recommended maximum test current.

    Jim


  3. #33
    Join Date
    23rd January 2009
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    NE Ohio
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    2,431
    Quote Originally Posted by JimJarvis50 View Post
    Actually, I mispoke on the method I use. I don't assume the nominal voltage of the battery - that wouldn't work. Instead, I use published values for the maximum current capability of the battery.
    That approach is perfectly sound. A couple points and my opinion sprinkled in....

    Battery manufacturers do not rate or control the maximum current they can deliver, they control the minimum current the battery can deliver into a load. It is the series resistance of the battery that does the current limiting. For example a 9V battery has a limit of ~5A because it has a series resistance of about 2 ohms. Battery manufacturers try to minimize the series resistance not control it to a target value. So if a batch of batteries has a series resistance of 1.5ohms they will ship. As long as you build in a factor of safety (like 2x) you should be fine. Assume that a rogue battery can deliver 10A instead of 5A (2x) and still not fire the match.

    Same thinking applies to LiPo. Lipos that limit their current have high series resistances. Doesn't mean that a particular batch with have the same resistance as another. Apply a factor of safety here as well.

    Opinion inserted here: Choosing a LiPo with high series resistance....what's the point? Personally I prefer to use a very stiff battery that can deliver enough current to toast a bagel if necessary. If I was going to use a shunt I would choose a battery with very very low series resistance and THEN insert a resistor into the pyro leg prior to the shunt. This way I KNOW what the series resistance is of the system rather than guessing what the battery manufacturer gave me and the altimeter gets a solid stable voltage that is insensitive to varying current demands.
    John Derimiggio NAR/TRA L3
    MarsaSystems

  4. #34
    Join Date
    10th July 2007
    Location
    Melbourne, Australia
    Posts
    963
    Opinion inserted here: Choosing a LiPo with high series resistance....what's the point? Personally I prefer to use a very stiff battery that can deliver enough current to toast a bagel if necessary. If I was going to use a shunt I would choose a battery with very very low series resistance and THEN insert a resistor into the pyro leg prior to the shunt. This way I KNOW what the series resistance is of the system rather than guessing what the battery manufacturer gave me and the altimeter gets a solid stable voltage that is insensitive to varying current demands.
    I agree with this. If you are using the internal impedance of the battery to guarantee your shunt will work I think you are asking for trouble. Maybe not now, but in the future if you change your battery, or if something else unexpected changes. A deliberate resistance place in line (either wire resistance and/or extra resistor) takes it out of the guesswork category if you are using shunts.
    TRA 13430, Level 3

    "Everybody's simulation model is guilty until proven innocent" (Thomas H. Lawrence 1994)

  5. #35
    Join Date
    23rd July 2011
    Location
    Butte, MT
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    1,810
    Quote Originally Posted by OverTheTop View Post
    I agree with this. If you are using the internal impedance of the battery to guarantee your shunt will work I think you are asking for trouble. Maybe not now, but in the future if you change your battery, or if something else unexpected changes. A deliberate resistance place in line (either wire resistance and/or extra resistor) takes it out of the guesswork category if you are using shunts.
    Another option when using a shunt might be to place a high current capability diode in the pyro leg. A large enough shunt should be able to keep the voltage lower in the shunt leg than the forward voltage rating of the diode, preventing any current from passing through the pyro leg. In this instance you would want to carefully select a diode with a higher forward voltage rating.


    Steve Shannon
    Steve Shannon
    L3CC, TAP, Director, Tripoli Rocketry Association

  6. #36
    Join Date
    10th July 2007
    Location
    Melbourne, Australia
    Posts
    963
    Another option when using a shunt might be to place a high current capability diode in the pyro leg. A large enough shunt should be able to keep the voltage lower in the shunt leg than the forward voltage rating of the diode, preventing any current from passing through the pyro leg. In this instance you would want to carefully select a diode with a higher forward voltage rating.

    Steve Shannon
    I like your thinking Steve. Clever . This mitigates against contact resistance variation in the shunt well. Just have to make sure the diode is not going to go open circuit (act like a fuse) when passing the igniter current. Make sure you choose a diode with a suitably over-rated operating current. Be careful of using just the surge current, as it is normally specified for a very short time (micro to milliseconds IIRC). A better measure of the robustness of the diode is the I^2t (I squared t) rating of the diode. It is basically I^2t that rates a fuse. Some diodes provide this in the data sheet.
    TRA 13430, Level 3

    "Everybody's simulation model is guilty until proven innocent" (Thomas H. Lawrence 1994)

  7. #37
    Join Date
    23rd January 2009
    Location
    NE Ohio
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    2,431
    Quote Originally Posted by Steve Shannon View Post
    Another option when using a shunt might be to place a high current capability diode in the pyro leg. A large enough shunt should be able to keep the voltage lower in the shunt leg than the forward voltage rating of the diode, preventing any current from passing through the pyro leg. In this instance you would want to carefully select a diode with a higher forward voltage rating.


    Steve Shannon
    Thats a great idea. You still have to do the resistance calculation to make sure you drop the voltage below the Vf of the diode at the test current level of ematch. Shunts are an advanced idea, not exactly turnkey.
    John Derimiggio NAR/TRA L3
    MarsaSystems

  8. #38
    Join Date
    26th November 2009
    Posts
    4,363
    Thanks for this clever discussion to show the process can work but has to be carefully planned. My fear/point was a neophyte who blindly puts a parallel shunt in a circuit and thinks they're automatically "protected" is misinformed. I do remember
    a shunt circuit I copied for an external battery cable for a Yaesu FT-817ND. Had a diode and a fuse to "shunt" the current to the fuse if the polarity was connected wrong. Well, one time in a rush I did just that and it worked. The radio was saved.
    I replaced the diode along with the fuse to be on the safe side! Kurt


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