Resistor question

[basementcat]

So if I was making a 2 cell mod (2x3.7v bats) and put in a 1ohm resistor would I want to use hv cartos/attys to achieve the magic 9watts that's preferred? (ie 1ohm resistor, 5ohm carto to achieve 9 watts) I know it's voltage squared divided by resistor ohms to get wattage and from my calculations that would be right but maybe I'm wrong and the 1ohm resistor drops the wattage more when in line with an atty or carto? Any info is appreciated.

My math
7.4v x 7.4v = 54.76
1ohm resistor + 5ohm carto= 6ohms
54.76\6= 9.12 watts

Please correct me if in wrong. Thanx for any info.

 

[Java_Az]

Math looks correct. You would need a 10 watt resistor at least. Only problem i see with it is your using the 7.4 nominal voltage. Fully charged your batteries will be 8.4 volts they will drop to around 5.5 volts when protection kicks in. So right out of the charger you will have 10.4 watts and at the end of the charge 5.5 volts you will have 6.7 watts. If you want more of a steady wattage from start to finish pretty hard to beat a switching regulator.

 

[basementcat]

Here's another question. Do I account for the voltage drop before the atty connector or do I account for them together at the same time? Because if I account for it prior to the atty connections (say if the resistor is on the positive line on the atty connection) I would get about 5.9v going to the atty connector which would then mean I would then need a 3.8-3.9 ohm atty to achieve 9watts. Ugh I'm so confused. Any help Is appreciated.

 

[basementcat]

Anyone? Still stumped.

 

[Java_Az]

Good question i was just adding the two together and thinking they would be one. . Going to have to think about this one some more, but i have some steaks to cook up right now so i have to go.

 

[basementcat]

Good question i was just adding the two together and thinking they would be one. . Going to have to think about this one some more, but i have some steaks to cook up right now so i have to go.

Yeah I can't seem to figure out what would be correct. Enjoy your steaks!

 

[Java_Az]

The first one is not correct since i gave the atomizer the wattage the resistor would be using. 6 ohms resistance in the circuit @ 7.4 volts 9.12 watts is correct for the total circuit, But the resistor will be eating up some of that wattage so the atomizer wont be getting the full 9.12 watts.

 

[bigblue30]

You need to work backwards with this one..I will use your numbers, but you can figure this out for any combination you want…. Your numbers: 7.4 volts, 9 watts, 5 ohm atty.
To get 9 watts out of a 5 ohm atty you need to pass 1.34164 amps across it.
I = Square Root of (P / R).
To get 1.34 amps with 7.4 volts, you need 5.51827 total ohms.
. R = E / I
So you would need a .51827 ohm resistor with your 5 ohm atty with 7.4 volts to get 9 watts at the atty.
The atty would then see 1.34164 amps with 5 ohms, or the voltage across it would be 6.7082
E = I x R
And the power at the atty would be 8.9999 or 9 watts.
P = E 2 / R