Exceeded 6A firing current on SLCF

Discussion in 'Rocketry Electronics and Software' started by Buckeye, Dec 24, 2019.

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  1. Dec 24, 2019 #1

    Buckeye

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    I got warning beeps #3 and #4 on my first flight with Lipos. Exceeded 6 amps on drogue and main firings.

    I am surprised because I am using a common setup for the Perfectflight SLCF:

    Battery: Tunrnigy Nanotech 2S, 180mAh, 25-50C. Fully charged to 8.3V.
    Ematches: the cheap, orange wire matches from China. They measure 1.0 to 1.4 Ohms on my meter.

    By Ohm's Law, I guess that ~8A could be going through the firing circuits and raising the warnings? Is this gonna be a future problem? Should I consider adding more resistance (resistor or 2 matches in series)? Has anybody else seen this?
     
  2. Dec 24, 2019 #2

    rharshberger

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    As long as I did the C rating correctly the 25-50C 180mah can source between 4.5 and 9 amps. So that battery was about 33C.
     
  3. Dec 24, 2019 #3

    Johnly

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    The instructions for many PF products tell you to install a current limiting resistor when using LiPo batteries.
    That's what I do, and have had no issues.
     
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  4. Dec 24, 2019 #4

    mpitfield

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    Along with using a limiting resistor, I would also choose a lower C rated LiPo when you go to replace your current ones. No downside to it and although you could argue that it is redundant along with the resistor, it is still a good practice.
     
  5. Dec 24, 2019 #5

    jderimig

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    The current limiting resistor on the outputs is the best choice next to using 2 ematches in series (why waste energy heating a current limiting resistor when it could be doing useful work igniting another ematch?).

    I wouldn't choose a lower C rating LiPo as that defeats the purpose of using a Lipo. Might as well just stick to a 9V alkaline if you are going to do that. A higher internal resistance LiPo will drop voltage when the current load hits. Keep the higher C rated Lipo (I use 300ma) and keep your electronics running at constant voltage regardless of the intermittent firing loads.
     
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  6. Dec 24, 2019 #6

    mpitfield

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  7. Dec 25, 2019 #7

    ksaves2

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    I affirm it’s smart not to toast the outputs but I’d get a little nervous using a 180mah battery and have a 45 minute pad wait. Yeah, I know most devices don’t draw much on standby but I still have reservations. Club launch likely no problem. Mount on rail, clear the skies, countdown and launch with little or no wait. Big regional launch and “stuff” happens like getting a bunch of people L1 certified first or getting a drag race going before launching the average joes rockets. Since newer rules on drags, that won’t be as much of an issue anymore. Kurt
     
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  8. Dec 25, 2019 #8

    g.pitts

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    Hi Buckeye, I’d be a fan of the series resistor. Having over-current events could lead to long-term reliability issues with the output transistor used to drive the charge igniter or e-match. I’m guessing that the duration of your output firing is relatively brief, so any wasted power dissipation would be minimal.

    Do you have good characteristics for the e-match you are using? Specifically the max current with a guaranty of no firing, and the minimum required current to ensure that the e-match fires. I would want to see those specs from the vendor rather than trying to reverse-engineer them using a very small sample size. This should give you a ballpark resistor value to look at and ground test. You’ll also want to ensure that said resistor can meet the thermal requirements being imposed upon it. Some resistors will have a specification for transient power levels, but in the absence of that if it were my setup I’d want to see a resistor with a power rating at some percentage over the calculated expected resistor’s power dissipation level in this application.

    Just my $0.02, and remember that free advice is worth no more than what you’re paying for it!
     
    Last edited: Dec 25, 2019
  9. Dec 25, 2019 #9

    Reinhard

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    Another thing to consider, the "resistance" of the ematch might drop if you get an electric discharge through the plasma after the wire burned through. That's more of an issue with higher voltages. I sent more than a 1kA through an ematch with "only" 400V while messing with a capacitor bank, but I wouldn't rule that out completely with ematches where the combustion helps with generating the plasma. In applications like head end ignition it's probably even worse.

    Reinhard
     
  10. Dec 25, 2019 #10

    Buckeye

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    Thanks for all the good info.

    Do you guys have some examples/specs/pics of using a resistor and how to wire it up? Electronics are not my forte. I looked at DigiKey and got overwhelmed. Bear in mind this is a re-worked 38mm min diameter ebay, thus the reason I am using the SLCF and a small-sized battery. A "chassis-mount" resistor makes sense, I guess, but not sure I have room on my sled for two of them??

    https://www.rocketryforum.com/threads/the-avionics-bay-thread-post-your-photos.156649/#post-1946687

    I was hoping to avoid re-inventing the wheel for a fairly common altimeter/battery. The easiest thing would be two ematches in series, which I already do on occasion. I will also look at different battery options as mpitfield has discussed. Finding a 2S with low capacity and low C rating is nearly impossible, though.
     
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  11. Dec 25, 2019 #11

    jderimig

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    I use these series for altimeter output load testing. You would want around a 2-3W resistor. Even though the instantaneous power is closer to 25W dissipated in that resistor its only on for 1sec. A 2or3W resistor will not get too warm in that time.

    https://www.mouser.com/ProductDetail/Yageo/PNP300JR-73-1R2?qs=oypCK0zG3264Aq%2B5pHjErA
     
  12. Dec 25, 2019 #12

    Buckeye

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    Perfect. Thanks. I can simply solder the resistor to one leg of my ematch?
     
  13. Dec 25, 2019 #13

    jderimig

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    Sure.
     
  14. Dec 25, 2019 #14

    Johnly

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    Soldering a resistor to the e-match lead would be the hard way of addressing the issue.
    I keep the JST connector on the battery and picked up the mating connector assembly at the local hobby store.
    Trim one lead of the matching connector to compensate for the resistor length.
    Solder the resistor the shortened lead, install some heat shrink insulation over the resistor and solder joint.
    Secure the leads into the battery terminal of the altimeter. Done!
     
  15. Dec 25, 2019 #15

    woferry

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    If you're willing to put the resistor in series with the battery then you could also use the switch terminals on the SLCF and put the resistor there. Both mean that the electronics are more likely to see a brown-out during firing, though the cap on the SLCF is meant to help survive those events. Best would be only in the firing path so the electronics would always see the full battery voltage, though that would mean a resistor per channel if you wanted to protect both.
     
  16. Dec 25, 2019 #16

    jderimig

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    But the resistor on the battery does not prevent the overcurrent on the FETS if the capacitor is large enough to prevent a brownout. You want the voltage drop to occur after the Cap storage.
     
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  17. Dec 25, 2019 #17

    g.pitts

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    I personally would not do it this way, if I’m understanding your proposal correctly. Any voltage drop across the resistor due to high current events on the e-match output would cause a voltage drop as seen by the altimeter, and may create a brownout condition where the altimeter drops out due to low voltage and resets when the high current condition ceases. Of course this concern is dependent on the resistor value and the magnitude of the high current event.

    It would be safer to just solder the resistor to the charge outputs, although it means that you may need two resistors - one for each output.
     
  18. Dec 25, 2019 #18

    jderimig

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    ^ This. Putting a series resistor on the LiPo battery just makes it emulate a 9V alkaline battery (without the mah capacity). If you are going to do this just use a 9V alkaline battery.
     
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  19. Dec 25, 2019 #19

    Johnly

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    Best I can tell the brownout capacitor is isolated from the firing circuit and only supports the processor side of the device.
    If I'm correct, a voltage drop caused by a high firing current incident would not impact the processor function.
    I've used what I have suggested in over 50 ARLISS flights and over 100 others without issue.
    Best I can tell, it seems to work with the Perfectflite CF altimeters.
    That said, I'll hook-up a shorted e-match lead to the altimeter and force it to fire and measure the processor voltage on a scope.
     
  20. Dec 25, 2019 #20

    g.pitts

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    Ah, that explains it. There’s no doubt diode isolation on the hold-up capacitor so that any current going to the charges comes from the battery and not the cap. The cap is sized such that it will keep the altimeter alive through any disruptions to the voltage coming into the altimeter.

    Thanks for the further explanation!
     
  21. Dec 26, 2019 #21

    g.pitts

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    Trying to organize some thoughts on this topic...

    I've ranked some of the ideas presented here in the order of impact (this is my judgment, I don't know the degrees of freedom you have with your rocket):
    • Per your original idea, put two of your existing e-matches in series which doubles the resistance (and halves the current) seen by the SLCF output transistor.
    • Change your e-match selection (I'm using these with my Eggtimer Quantum: https://www.cobrafiringsystems.com/initiator_3ft).
    • Change your altimeter to one with "beefier" output transistors (the output transistors of my altimeters are electrically rated at 40A as one example - there are no doubt others).
    • Add a series resistor (2W or 5W wirewound non-inductive or carbon composition) between the e-match and one of the outputs from your SLCF.
    The double e-match idea on the surface seems to be the easiest. You'd need to solder a shortened e-match in series very close to the igniter portion of the primary e-match, and make sure the solder connection is rock solid. If it opens up on you in flight, you will have an open-circuit and your charge will not fire. This approach also exposes a "live" electrical connection where you've soldered the two together, so heatshrink or electrical tape is a must in order to avoid a visit from Mr. Murphy.

    The resistor idea also exposes conductive wiring in your electronics bay, so the same goes here for either heat shrink tubing or electrical tape. If you go this route, you might consider adding a terminal block into your electronics bay to tidy up the wiring, make it more reliable, and avoid having to solder the resistor each time you install a fresh e-match.

    I suppose there may be other things that could be done, but these are the top-of-mind ideas. Let us know what you decide to do.
     
  22. Dec 26, 2019 #22

    RCBrust

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  23. Dec 27, 2019 #23

    Buckeye

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    I ordered a couple dozen of the resistors that jderimig mentioned, not knowing if they will be one-time use. I also ordered some terminal blocks for a more permanent mounting of the resistors on the sled. I will do some testing.
     
    Last edited: Dec 27, 2019
  24. Dec 27, 2019 #24

    g.pitts

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    Outstanding! Photos or it didn't happen. ;) Back of the envelope calculation assuming the e-match resistance is stable at 1Ω (until it opens), the instantaneous power dissipated in the series resistor when the charge fires is I^2 * R = (8.3V/2.2Ω)^2 * 1.2Ω = 17 Watts. I noticed in the resistor datasheet the statement on transient power dissipation capability of "10 times rated power for 5 Sec." Based on the 3W power rating of this resistor, you should be golden.
     
  25. Dec 27, 2019 #25

    Voyager1

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    Many of us that use electronic deployment will use a 2-way terminal block on the outer face of the avbay bulkplates. Another option for installing a series resistor, if space permits, is to use a 3-way terminal block to connect the resistor and e-match to. It would be necessary to insulate the resistor with either heatshrink or narrow plastic tubing.

    upload_2019-12-27_15-39-58.png
     
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  26. Dec 27, 2019 #26

    g.pitts

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    A picture is worth 10^3 words!

    This is what I was proposing, but I was advocating it to be put into the electronics bay to protect the resistor from any moving hardware prior to or during deployment.
     
    Last edited: Dec 27, 2019
  27. Dec 27, 2019 #27

    Voyager1

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    I agree, inside the avbay would be preferable. However, I was thinking about those folks that already have terminal blocks outside the avbay. The resistor could be potted with epoxy onto the block, so it would be protected.

    As an aside, the over-current condition is not something I have experienced so far with my two SLCFs. I would generally use a 2S 350 mAh LiPo with each and a J-Tek e-match. If I recall, these e-matches have a DC resistance of about 1.0 -1.5 Ohms, resulting in an ignition current pulse of at least 7A - 5A, respectively. It's possible that the e-match goes open circuit before the SLCF current detection sampling detects the over-current situation. The current pulse is probably only tens of milliseconds in duration, but that would depend on the e-match being used.
     
    Last edited: Dec 27, 2019
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  28. Dec 27, 2019 #28

    g.pitts

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    Epoxying the resistor in place is a great idea! Epoxy is more a thermal insulator, but this is not an application that under normal conditions should result in any significant thermal energy anyway.
     
  29. Dec 28, 2019 #29

    manixFan

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    I have used the little 130mAh that Adrian sells for his Raven altimeters. As a test I left it on for 2 hours and then fired all 4 ematches I had connected to it. (It has 4 outputs.) They all fired, no problems. The new altimeters draw so little current on standby that they really can get by with the smaller batteries. I learned to fly using my RDAS compact, which draws probably more than 10x the standby current so I'm still stuck in that mode of thinking. But as I've tested both the Raven and the Stratalogger, I feel a whole lot more comfortable with the lower capacity batteries, especially since they can dump plenty of current to fire off an e-match.


    Tony
     
    Last edited: Dec 28, 2019
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  30. Dec 28, 2019 #30

    g.pitts

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    Wow, you aren’t kidding! I’m not familiar with this line of altimeters, but they spec the active current as 1.5 mA! With your 130 mAh battery, it would run your altimeter for over 86 hours (minus any time due to battery degradation and any current used to fire the e-match(es)).
     

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