PI coil calculations

[Daren]

I need to build a Barracuda coil.
I already have some AWG 26 teflon stranded wire but was not sure if the using thicker wire would not be better.
I decided to calculate current numbers for different gauge wire. The key number I was looking for was instant coil current 100uS after switching it on. (Max current on the end of the sample)

Coil diameter 255mm; inductance 440uH ; 29 loops
wire length 78 feet
power 12V
coil ON time 100uS

max amps = 12 V / (resistance + 0.5ohm ) mosfet in line correction

[awg] [wire resistance ohms] [max amps] [100uS amps]
30 8.2 1.37 1.157
28 5.16 1.77 1.22
26 3.25 3.2 1.67
24 2.04 4.72 1.75
22 1.28 6.74 1.7
20 0.807 9.23 1.53

the numbers are for solid wire, stranded is probably 10% more resistive this was not taken into account.
it looks that wires 26 to 22 would be fine with 24 being the best.

here is the instant current calculator
http://www.bowdenshobbycircuits.info/rl.htm

Anyone have any empirical test results for Barracuda PI coils?
I have DD housing so it will be a regular "bunch" circular coil.

[bbsailor]

I need to build a Barracuda coil.
I already have some AWG 26 teflon stranded wire but was not sure if the using thicker wire would not be better.
I decided to calculate current numbers for different gauge wire. The key number I was looking for was instant coil current 100uS after switching it on. (Max current on the end of the sample)

Coil diameter 255mm; inductance 440uH ; 29 loops
wire length 78 feet
power 12V
coil ON time 100uS

max amps = 12 V / (resistance + 0.5ohm ) mosfet in line correction

[awg] [wire resistance ohms] [max amps] [100uS amps]
30 8.2 1.37 1.157
28 5.16 1.77 1.22
26 3.25 3.2 1.67
24 2.04 4.72 1.75
22 1.28 6.74 1.7
20 0.807 9.23 1.53

the numbers are for solid wire, stranded is probably 10% more resistive this was not taken into account.
it looks that wires 26 to 22 would be fine with 24 being the best.

here is the instant current calculator
http://www.bowdenshobbycircuits.info/rl.htm

Anyone have any empirical test results for Barracuda PI coils?
I have DD housing so it will be a regular "bunch" circular coil.

Daren,

See this web link. http://www.learnabout-electronics.or.../dc_ccts45.php

The short answer is that the current in you coil will rise to about 63 percent of max current in one time constant and in another time constant wil rise to about 86 percent of maximum current and the in another time constant rise to about 95 percent of maximum current, etc. Maximum current is the current calculated with the total coil resistance plus any series resistance (like MOSFET on-resistance) and coax wire resistance. Your AWG 26 coil wire is 3.2 ohms plus 0.5 ohms (MOSFET on-resistance) or 3.7 ohms plus approximately .05 ohms for coax lead resistance for a total of 3.75. Then you divide that total resistance of 3.75 into the coil inductance of 440 uH and you get 117.33 microseconds (uS) time constant for your coil current to raise to about 63 percent of max current of 3.2A (12V / 3.75 is 3.2A). In 100 uS, your coil current will be slightly less.

I hope this helps

Joseph J. Rogowski

[Daren]

Thank you Joseph.