Fuze test rig

Discussion in 'The Projects Forum' started by emmy, Sep 11, 2006.

  1. emmy

    Thread Starter Member

    Aug 9, 2006
    22
    0
    Back Again,

    I need to build a test rig that emulates a fuze (well quite a few of them)

    Problem :
    Building a system to fire the fuzes uses a lot of fuzes.
    Resetting the Fuzes also take time and I'm getting rather bored spending 50% resetting my test rig.

    Solution :
    Build something that can emulate a full rig of fuzes.

    I can think of 3 different methods of doing this

    1, DC Circuit breakers (sounds expensive)
    2, LED indicators (but if you miss it you will never know ?)
    3, Some kind of circuit that has a dual LEDS that when dormant is green and when the correct current has been applied it then changes to red and stays in this position until reset. It would also need to give the indication back to the control box of open circuit.

    The behaviour that needs to be emulated would need to fit into the following specs,

    Bridge wire resistance 1.5 - 2.5 Ohms
    No Fire current 0.18 Amp / 5 min
    Fire current 0.8 Amp / 4 msec
    no firing impulse 0.8 mj ohm -1
    firing impulse 2.5 mj ohm -1

    Any other solutions ?
    I think that even though it may be extra work 3 is the best solution.

    Could anyone point me in the right direction ?

    Emmy
     
  2. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    1,437
    1
    hi

    have you considered using discrete parts like bjt or jfet to be an electronic fuse to emulate a fuse? this way you can control the trigger points you would want by varying the peripheral parts.

    moz
     
  3. emmy

    Thread Starter Member

    Aug 9, 2006
    22
    0
    Nope I have not.
    But sounds like a solution
    Got a Circuit ? I can but try.

    Emmy
     
  4. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    What will be the voltage source of your 0.8A fire current?
     
  5. emmy

    Thread Starter Member

    Aug 9, 2006
    22
    0
    24 Volts DC
    This voltage is obtained from 12 battery through a voltage doubler.
    It inturn will charge 2 X 4700 Uf caps in paraelle which will be the actcual source for the fire current.

    Due to cable lengths more current is better. 0.8A is just the minium required.
    Some cable lengths can be 200M along cat-5 cable which as you can imagine has some voltage drop. Especially when you have several fuses at the end.

    One of the additional reasons for the test rig is to get reliable limitations for the cable runs as well as for building new firing systems.

    Very brute force solution I know but worst case it only has to work once.
     
  6. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    I’ve assumed the most critical criterion is the requirement to fire the fuze in 4mS with 800mA current flowing. As your long leads may limit the current I have assumed the worse case will be when your fuze is 2.5Ω rather than 1.5Ω.

    The attached circuit may be what you want. The aim is to use the current to charge a capacitor, and when the voltage across it rises to 0.7V a latch is turned on to light a LED. When a capacitor is charging, the voltage across it rises according to V= I*t/C, where t is in secs, I is in Amperes and C is in Farads. Charging a capacitor in a time of 4ms to 0.7V with 800mA current would require around 4500µF, so this circuit uses the 2V developed the 2.5Ω resistor to charge a smaller capacitor. The accuracy will be dependent on the pulse shape of your discharging capacitive source, but it should be reasonably accurate.

    The two complementary transistors form a latch that will switch on when the cap gets charged to 0.7V. Once on, the LED will stay lit until the unit is switched off.

    Use a Tantalum for the 100µF, because the effective series resistance of an electrolytic is too high.

    I hope that’s what you want.
     
  7. emmy

    Thread Starter Member

    Aug 9, 2006
    22
    0
    Pebe,

    What will the effect be on the circuit if I am supplying 24V at less than 180mA for a period of time ?

    This is the current that is used to check the fuse.

    Being a test rig I will need to use this facility.
    I would assume over a period of time that the cap will charge and give a faulse indication of being fired ?


    Emmy
     
  8. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    If you have 24V giving less than 180mA then I assume you must have resistance in addition to the fuze in circuit? The test rig measures the current flowing through the 2.5Ω simulated fuze - not the source voltage.

    180mA through the 2.5Ω resistor will give only 0.45V across it. That is the maximum that the cap can charge to, and is not sufficient to turn on the latch.

    BTW, after uploading the circuit I realised it could be simplified. You can change the 56Ω resistor to 5K6, the 100µF to 1µF, and omit the 4K7 resistor.
     
  9. emmy

    Thread Starter Member

    Aug 9, 2006
    22
    0
    Pebe,

    Circuit works perfectly. I have enhanced it by adding additional resistors on a rotary switch which allow for testing and emulating bad connections and cable lengths.

    My only other question on this device is how can I get it to go open circuit after it has fired ?

    Emmy
     
  10. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    I thought you wanted the LED to stay on until reset. The latch will stay on until you remove the current flow. The switch was for that. Alternatively, you can fit a push button between base and emitter of either of the transistors.
     
  11. emmy

    Thread Starter Member

    Aug 9, 2006
    22
    0
    Pebe,

    Sorry I have not explained myself properly.

    The power source that fires the Fuze has a LED and resistor in series which limits the power to the fuze and also gives a indication that the fuze is operating.
    The firing current is obtained basically by bypassing these componets.

    Once fired the fuze goes open Circuit which the test rig emulates using a LED and I must say works perfectly.

    The firing box however still has a indicator light on as the 2.5 Ohm resistor is not open.
    It is not a huge problem but if I had thought about it I should try to make the test circuit go open when fired. That way the LED indication can be achieved from the firing box.

    When I have it working I will need to make a full rail of these things which means 32 seperate fuzes. I will have a look for a Latch in a TTL chip to reduce the amont of individual componets.

    Emmy
     
  12. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    1. Is the 4ms (specified) a consideration, or are you happy that your supply can provide a pulse of at least that duration?

    2. Once the 2.5Ω resistor has gone O/C, how would you want to restore it ready for the next test?

    3. Would you still need the LED at the test rig position?
     
  13. emmy

    Thread Starter Member

    Aug 9, 2006
    22
    0
    Pebe,

    1, I am happy that my supply can do it, and I can use the first test circuit as a seperate test for verification.

    2, Don't care. A reset button would be the best solution. Remebering there will be a number of these for 32 different fuses so a single reset switch for all.

    3, No the LED is not a requirement. I will use the LED indicator on the firing box as the indicator.

    Thanks again for all your help.

    Emmy
     
  14. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    I'm not sure I fully understand your application. Is it for rocket firing? Are they all fed by separate cables? Will they all be fired at the same time? If so, your supply will need to provide 25A for 4ms min. I don't think you will get that from a voltage doubler circuit feeding a couple of 4700µF caps.

    If you intend to fire them in sequence and the tester is just being used to verify that the lines are adequate, why not put a single tester at the control centre with the 2.5Ω resistor in series with the line and temporarily short the end of the line while you test it?

    Or have I missed something?
     
  15. emmy

    Thread Starter Member

    Aug 9, 2006
    22
    0
    Pebe,

    This is for firing fireworks.
    Usually a rail would consist of 24 or 32 fuzes then you may have up to 15 to 20 rails depending on the size of the show.
    Usually they are fired in a sequence not all at once.

    One of the things I will need to test later is multipul shots and chase sequences hence multipul test fuse circuits.

    The test circuit I am after will be used to test the firing boxes that I have been making. The first ones were really basic and I am gradually increasing the systems with timers and chasing circuits. What I am building is purely for the workshop to save time and fuses while I am testing.

    I will be back later for help improving those as they are another story.

    Right now all I need is a test circuit for the firing boxes.
    Also is there a simple circuit drawing tool you can suggest ? I would find it an advantage for explaining things as well.

    Emmy
     
  16. pebe

    AAC Fanatic!

    Oct 11, 2004
    628
    3
    Thanks for the info. I'll give it more thought.

    BTW, I use MS Paint for drawings. It's fairly quick and easy.
     
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