Murr's Electric Match Reference Guide
The following was compiled by Murr Rhame from a variety of industry sources as a quick reference guide for pyrotechnic operators. He's posted it to the Show-fire mailing list and a few other pyrotechnic newsgroups and mailing lists, so we asked him if I could put it into a web page ….. and here it is.
It is very helpful and useful information when planning and setting up a show or production, especially in this short, simple format with the examples he provided.
Thanks to Murr for sharing this with us all!
Copyright © 1996 J. Murr Rhame, Jr. Distribute freely for non-profit use with complete article including copyright notice and signature intact.
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Amps = Volts / Ohms |
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Amps = Watts / Volts |
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Amps = Square Root of (Watts / Ohms) |
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Ohms = Volts / Amps |
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Ohms = (Volts2) / Watts |
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Ohms = Watts x (Amps2) |
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Volts = Amps x Ohms |
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Volts = Watts / Amps |
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Volts = √(Watts x Ohms) |
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Watts = (Amps 2) x Ohms |
Watts = Amps x Volts
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Watts = (Volts 2) / Ohms |
Net Ohms = R1 + R2 + R3 + …
Net Ohms = (Resistance of One Resistor) / (Number of Resistors)
Net Ohms = 1 / ( 1/R1 + 1/R2 + 1/R3 + ... )
Net Ohms = (R1 x R2) / (R1 + R2)
In firing circuits, the resistance of the wire must be accounted for. Some firing circuits will have more resistance from the wiring than from the electric matches themselves.
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AWG |
Feet/Ohm |
Ohms/100ft |
Ampacity* |
mm |
Meters/Ohm |
Ohms/100M |
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10 |
490.2 |
.204 |
30 |
2.588 |
149.5 |
0.67 |
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12 |
308.7 |
.324 |
20 |
2.053 |
94.1 |
1.06 |
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14 |
193.8 |
.516 |
15 |
1.628 |
59.1 |
1.69 |
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16 |
122.3 |
.818 |
10 |
1.291 |
37.3 |
2.68 |
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18 |
76.8 |
1.30 |
5 |
1.024 |
23.4 |
4.27 |
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20 |
48.1 |
2.08 |
3.3 |
0.812 |
14.7 |
6.82 |
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22 |
30.3 |
3.30 |
2.1 |
0.644 |
9.24 |
10.8 |
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24 |
19.1 |
5.24 |
1.3 |
0.511 |
5.82 |
17.2 |
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26 |
12.0 |
8.32 |
0.8 |
0.405 |
3.66 |
27.3 |
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28 |
7.55 |
13.2 |
0.5 |
0.321 |
2.30 |
43.4 |
These Ohms / Distance figures are for a round trip circuit. Specifications are for copper wire at 77º Fahrenheit or 25º Celsius.
* Ampacity figures are approximate continuous current capacity. The number of conductors, ambient temperature and or insulation type will effect the maximum safe continuous current capacity of wire. Wire can safely carry much higher currents for very short periods of time. With typical electric matches, the bridge wire will break in few milliseconds when fired by a current of 1 amp or more.
For these examples the electric matches have an all-fire current of 0.5 amps, a series-fire current of 1 amp, and their total resistance is 1.5 ohms. Actual specifications vary from brand to brand of electric match. All examples assume firing systems that operate directly from a high current batteries. Capacitor discharge systems, such as blasting boxes, require slightly different calculations.
The current requirement for reliable series firing is greater than the current per match needed for reliable parallel firing. Additional current is required for series firing to insure that all matches receive enough energy to ignite the primer before the circuit is broken. All electric matches in a series circuit must be identical for reliable operation. Some people go so far as to use only electric matches which are from the same lot number in series firing circuits.
The following examples show the absolute maximum allowable wire runs under the conditions specified. Many people prefer to use at least double minimum amperage in electric match firing circuit as a safety factor. For maximum reliability, typical electric matches should be limited to no more than 10 amps in series circuits. Matches need not be limited to 10 amps per match in parallel circuits.
Resistance for 0.5 Amp at 12 Volts = 24 ohms (12 Volts / 0.5 Amp = 24 Ohms)
Max wire resistance = 23.5 Ohms (24 Ohms total - 1.5 Ohms Match)
24 AWG is 19.1 Feet per Ohm.
Max 24 AWG wire run = 449 Feet (23.5 Ohms x 19.1 Feet per Ohm)
5 matches in parallel = 0.3 Ohms (1.5 Ohms / 5 Matches)
Total current needed for 5 Matches = 2.5 Amps (0.5 Amps x 5 Matches)
Resistance for 2.5 Amps at 12 Volts = 4.8 Ohms (12 Volts / 2.5 Amps)
Max wire resistance = 4.5 Ohms (4.8 Ohms total - 0.3 Ohms Matches)
20 AWG wire is 48.1 Feet per Ohm.
Max 20 AWG wire run = 219 Feet (4.5 Ohms x 48.1 Feet per Ohm)
10 Matches is series = 15 Ohms (1.5 Ohms x 10 Matches)
Current need for series fire = 1 Amp. For series firing the minimum current required is the same no matter how many Matches are used.
Resistance needed for 1 Amp at 24 Volts = 24 Ohms (24 Volts / 1 Amp)
Max wire resistance = 9 Ohms (24 Ohms total - 15 Ohms Matches)
22 AWG wire is 30.3 Feet per Ohm.
Max 22 AWG wire run = 272 Feet. (9 Ohms x 30.3 Feet per Ohm)
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™ , La Crosse, WI USA. All rights reserved worldwide.Revised July 17, 1998