wire - resistance - amps and ohms . . .

[Pyromaniac]

Hi guys - I need some advice on wiring, not for home or DIY though but regarding electrical ignition of pyrotechnics

( I am a fireworks display operator just taking up electrical firing )

I have a 12v system

The details on the ignitors are as follows -
( basically an electrically fired match head )

match head resistance = 1.6 +/- 0.3 ohms

Copper wire tails on the match head 1m long resistance = 0.17 ohms

No fire current ( max current to pass for non firing of the match head - used to test continuity ) = 0.2 amps

All fire current ( to actually fire the match head ) = 0.6 amp recommended fire current is min of 1 amp.

I am trying to figure out the best way to get maximum current from my firing system to the ignitors,

specifically I wish to fire 7 ignitors from one button push on my system.

It pushes out 12v DC

It needs to go through 50 feet of wire to get from my system to the match head - make that 100 feet as its 50 feet each way to make the circuit.

What would be the best size of wire to use ?

Am I right in thinking thicker gauge means less resistance so I can get more current using a thicker wire ?
Flexible wire is needed - not something like T&E !
Cat 5 type wire is too thin and fragile.

OR - if I found some form of multi-cored cable - would using 2 or 3 cores as one run of cable be the same as using one thicker wire ?

Electricity is not my forte !!

 

[electronicsuk]

OK, so let's get the basics first:

I have a 12v system

match head resistance = 1.6 +/- 0.3 ohms

Copper wire tails on the match head 1m long resistance = 0.17 ohms

All fire current ( to actually fire the match head ) = 0.6 amp recommended fire current is min of 1 amp.

specifically I wish to fire 7 ignitors from one button push on my system.

It pushes out 12v DC

It needs to go through 50 feet of wire to get from my system to the match head - make that 100 feet as its 50 feet each way to make the circuit.

What would be the best size of wire to use?

Ohms law says V = I * R, we know that the recommended firing current is 1 Amp, and the maximum resistance of the match head and 1m cable is 1.6 + 0.3 + 0.17 = 2.07 Ohms. These are pretty much static values, not variables, so if we plug these figures into the basic Ohms law calculation, we can work out the mimimum voltage you need on e-match to guarantee ignition:

V = 1 * 2.07 = 2.07v

So, 2.07 volts, in ideal conditions and not taking into account any internal resistance of the supply source and any other wiring, is enough to ignite one match. With a 12v supply, or say 10v worst case if your battery has pretty much had it, gives you around an 8v drop over your 50ft of wiring to play with.

A more sensible figure to work with might be a 4v maximum drop along the cable run, as you don't really want to be operating anywhere near limits. The conductor size you need will depend on how you arrange the electronic matches, if they will all be at the end of one 50ft/15m cable then said cable will need to be able to support 7 Amps with a maximum 4v drop along its length.

We normally calculate a cable's fault drop based on the amount of mV dropped, per amp, per metre. We can work the calculation in reverse and say that 4v divided by 7 Amps and divided by 15 metres = 4 / 7 / 15 = 0.038v, or 38mV.

So, what does that tell us? It means that to give a guaranteed firing of 7 ignitors @ 1A each all the the end of a 50ft/15m cable run, the cable needs to have a volt drop of 38mV/A/m or less. Two core 1.5mm flex has a V.D. lower than this, so would probably make an ideal candidate.

However...

The above calculations treat the ignitors as a static load of 1A, which of course they are not. At 8v each match would like to draw far more current than this, but the resistance of the match, wiring and the supply will be a limiting factor. This increased current draw means that the volt drop on the cable will now be far higher than initially calculated, which is going to completely skew the results.

A far better solution to this would be to treat the match and cabling as resistors, then you can work out the volt drop and current at every point in the circuit and work out exactly what will happen when you try this for real. Sadly I've just got back in from a night out (a little sad that the first thing I did was log on here!) and don't really have the willpower or figures in front of me for the cable to do that.

An aside, you may or may not know that larger displays tend to use capacitor firing systems with light gauge wiring to light their matches. The cap is charged to a relatively high voltage (48-100v or possibly more depending on the system) and bellwire or lamp flex is more than adequate for most interconnections, as the volt drop is more negligible with higher supply voltages.

You could also consider siting the battery next to where the matches will be fired, and then using a car headlamp relay and a light gauge wire to the coil and a switch at the operator firing position. This would be my preferred method if you didn't want to invest in a proper system, as it avoids high currents having to travel any large distances and is inexpensive and simple. I will assume that as this is your day job you will already be conversant with the codes that govern your work and the use of electrical control and firing systems.

 

[ban-all-sheds]

( I am a fireworks display operator just taking up electrical firing )

What are you - a man or a mouse?

Get in there with a portfire ya wimp...

 

[Pyromaniac]

Thanks for the explanantion BAS - makes understanding how electricity works a little easier, prob sounds a daft question to anyone electrically minded, but if you dont know you dont know ! And its not something I want to assume on when wiring up live gear in the middle of a field.

What are you - a man or a mouse?
Get in there with a portfire ya wimp

Ooh - Portfires are for wimps ( but the best way to go for Joe Public instead of matches and lighters )

My preferred weapon of choice for hand firing is this -

Bernzomatic Mk 3 - sent over from the states - much better than the Mk 2 version used by plumbers over here as it is unaffected by damp - ignites every time - and is reliable upside down.

And if THAT wont light my stuff then it stays unlit [/img]