Dual 3.7s???

[Ridewithme38]

Well, first you have to measure your total resistance. From that, you divide the voltage (7v) by the resistance to get the amperage

I (amperage) = E (voltage) / R (resistance)

Then , you solve for the voltage drop that you will need - you want to drop 2 volts across the resistor you're going to put into the circuit.

I (amperage) * R (resistance) = E (voltage)


Let's do a hypothetical, using common values I've seen tossed about in the forum:

3 ohms seems to be a common value given for the resistance of an atomizer. 1.5a seems to be the max current they'll handle, long-term.

I = E / R
I = 7v / 3ohms
I= 2.333 amps (sorta high)

Now, let's figure out how much resistance we need to add to drop the voltage to 5v at the atomizer.

I * R = E
2.333 * R = 2
R= .86 ohm - that's gonna be hard to find, I think.

Let's look at it the other way around, and try to limit the amperage to no more than 1.5a

R = E / I
R = 7 / 1.5
R = 4.666 ohm total resistance (5 ohm would be closest) That means adding 2 ohms to that 3 ohm atomizer.

Let's work with that, and see what we get for voltage and amperage at the atomizer, shall we?

I = E / R
I = 7v / 5ohm
I = 1.4 amp - pretty close!

Now, let's figure the voltage drop:
I * R = E
1.4a * 2ohm = 2.8v dropped across the resistor, leaving 4.2v at the atomizer - a bit low.

Now you have to figure POWER, so we don't burn up the resistor:
P (power) = I * E
P = 1.4a * 7v
P = 9.8 watts - you're going to need at least a 10watt resistor, and 15-20w would be even better.

Damn this is an Amazing Post!!! This explains EVERYTHING! Thank you so much for this!

 

[Tugger]

Just for a little extra fun, the atomizer resistance is going to go up significantly as it heats up, so the voltage across the load (atty) will drop significantly as you drag.

Of course, as the resistance of the atty goes up, the amperage will go down, thereby decreasing the voltage drop across the internal resistance of the battery, which will allow the effective voltage across the battery to go up.

Fun stuff.

 

[Ridewithme38]

Just for a little extra fun, the atomizer resistance is going to go up significantly as it heats up, so the voltage across the load (atty) will drop significantly as you drag.

Of course, as the resistance of the atty goes up, the amperage will go down, thereby decreasing the voltage drop across the internal resistance of the battery, which will allow the effective voltage across the battery to go up.

Fun stuff.

Is there anyway to calculate or measure the voltage drop as atomizer resistance increases?

 

[Tugger]

You can try to calculate the resistance change:

Wire: Nichrome (tm) & Other Resistance Alloys - Tech Data

Of course, without measurements for diameter, length, chemical composition, and final temperature you will have to make some questionable assumptions.

 

[Tugger]

Actually, looking at that technical data page, a rise in temperature from 68F to 800F is only about a 5% increase in resistance, so you can safely neglect the change in wire resistance from your calculations, unless these things get hotter than 800F, which I doubt.

 

[Carlos49]

Just wanted to update this thread with the results.

batteries FINALLY showed up today from DX. After patiently waiting a few hours for them to charge up, I popped them into my box mod. Screwed on an atty and fired it up...

Yup...like a dumbass I didn't check the voltage BEFORE firing up the atty. POOF!!

Two things came of this. 1 ohm is NOT enough for this, and apparently 801 atomizers don't like 8.5v

After 1 month I'm still waiting for my batteries from DX I'm gonna have to find a better place, If possible. Sorry didn't mean to Bump!!

 

[Lucacri]

Carlos, try to buy them from here:
UltraFire LC 14500 3.6V 900mAh Protected Battery (2-Pack) $5.72 - Free Shipping

They are not as cheap as DX but i received my batteries in 4 days!

 

[MidnighToker]

Sorry to hear about the POOF. Might I suggest using a 2ohm 2watt resistor, which should bring it down to about 4.75 with fully charged batteries. Credit to Mars_mcc:

http://www.e-cigarette-forum.com/forum/battery-mods/26110-simple-box-mod-swappable-resistors.html

Thanks man. That's an awesome idea to use swappable resistors until you find your happy place.

(I knew something was off in my calculations when I kept having to look for 10 and 20 watt resistors...)

 

[mnealtx]

Toker - I think most of the high-power mods seem to be using 3v cells in series, not the 3.7v.

 

[SmokinScott]

Here's another suggestion:
"add a rectifier diode like IN4004 or IN4007 to drop the voltage a bit. it's less shocking to atomizers without sacrificing too much power".

Credit to youfillintheblank in this thread:
http://www.e-cigarette-forum.com/fo...7-high-voltage-fans-6v-box-made-simple-2.html

 

[jonnyb]

I used a 1ohm 2watt resistor with 2 3.6v spiderfire 16340,it dropped the voltage to 5.4v been using it for 2 weeks and no atty problems yet.awesome vape!