test circuit schematic

[zotdoc]

I have set up a fireworks control panel that will electronically fire the fireworks devices. It is simple, with a pushbutton switch that closes the circuit from a 12 volt battery and melts a 2" piece of nichrome wire. There are a lot of these individual "circuits", and the nichrome wire is wrapped by hand around the fuse of each firework device, and is held in place by twisting the nichrome into the stranded copper wire from the control box. Is there a way to set up a test circuit that would light an led to show continuity of the firing circuit without actually heating the nichrome wire? Any help would be greatly appreciated.

[toejam]

if you wire a led in series with a 1k resistor across each switch, (minding the polarity), the led will light if there is contunity.

[zotdoc]

Thanks toejam, but will this make the nichrome wire heat up?, I don't want anything going off prematurely. I'll try it with a dummy fuse and see, and I'll post later on with the result. Thanks again.

[sofaspud]

Obtain the specs on the nichrome wire and get the "melting current". Replace the 12 volt battery with one of smaller voltage, with a current-limiting resistor in series with an LED, that will allow the LED to light w/o providing enough voltage & current to melt the wire.

[toejam]

e=ir
12=1000i
12/1000=.012amps or 12 ma. thats eniugh to light a led, if it changes the temperature of the nichrome wire a measurable amount i would be very surprised, but you cant go wrong by testing the circut.
good luck
tj

[haklesup]

The LED will detect open circuits but not shorted circuits.<p>You will need to pass a current through the ignitor to determine its continuity. 12mA may be small enough. Aside from experimentally detrmining it, you can measure the resistance of the NiCr ignitors. You probably need 25W to 50W minimum to ignite the fuse and you're using 12V so the resistance is probably in the 2 to 4 ohms range (requiring 2 or more Amps). At that R, 12mA would heat the ignitor a paltry 288uW.<p>I assume the final connection and continuity check is made when everyone is well away from the rockets not as you go.<p>As a final improvement, you can put a master switch on the battery which will put an additional 1K resistor in series with the battery and all circuits. In this configuration, if a switch fails (as a short) then you do not get premature ignition (max current is limited). You can also test the switches because the LED will go off when the switch is closed and the LED is shorted out of the curcuit.<p>Another way to do this is to configure a rotary switch with the input to a Multimeter and measure loop resistance as a test. This would be more informative about marginal resistive contacts and not give a false pass when the ignitor is shorted out.