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[FAQ] What the heck is Ohm's Law?
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Michael Offline
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[FAQ] What the heck is Ohm's Law?
Ohm's Law is a scientific formula used to describe the relationship between power (Watts=W), resistance (Ohms=Ω), energy (Volts=V) and current (Amps=A). All these things are important when vaping, as they correlate to the output of your device and safety of your cells.

So how does Ohm's Law work? And why does it matter?

It matters for people who are building your own coils, because it helps you know what is safe to vape or not. If you use a super sub-ohm build, it can draw too much power from your cells, and if you go over the amp rating of your cells, it can damage them and also damage your device or potentially your face. And we don't want you to damage your face.

DISCLAIMER: I need to say this up front, Ohm's Law does not apply perfectly in the real world, since it does not account for things like your mod's internal resistance, battery voltage sag, and tons of other factors. So use it as a general guide, but not as a perfect rule. If you use Ohm's Law and the math says it's safe to vape at .0003 ohms, DON'T. As a general rule, I consider it unsafe to vape with anything lower than .3 ohms, even if the theoretical math equation says differently. Better to be safe than sorry.

The Formula

There are a lot of variations of the formula used in Ohm's Law, so here are a few formulas that I find helpful:

A=V/Ω - this one is what I use to find the amps that are being drawn when I use a build with a certain resistance. 
Ω=V/A - this is what I use when I need to know the lowest resistance I can safely use on a given cell.
Ω=W/A2 - this is what I use to find the lowest resistance I can use safely at a given wattage.

How to Use It

So how does this work in real life?

Example 1: Let's say I want to know if it's safe to vape at .09 ohms on a Samsung 25R cell. Since all 18650 cells have a 4.2V maximum charge, I can plug 4.2 into any of the formulas that have a V in them. I need to solve the formula for A, so I can see if it's within the cell's rating limits. I'll use the first formula:

A=V/Ω
A=4.2/.09
A=46.67 amps

I know for a fact that the Samsung 25R is rated up to about 20 amps, so the .09 ohm build will be drawing far too much power and therefore not be safe to use. 

Example 2: So what resistance is safe to use? Since we know that the amps need to be 20 or less, we can plug 20 in for A and 4.2 in for V using the second formula:

Ω=V/A
Ω=4.2/20
Ω=.21 ohms

This means that the very lowest resistance I can safely use with a fully charged battery is .21 ohms. However, the cell's charge will drop significantly as I use it, and it's never a good idea to be "borderline safe" so I should build it higher than .21 ohms. 

Example 3: But what if I'm using a variable wattage device? That's when the third formula comes in handy. For example, I'm currently using my Sigelei-100W box mod, and I like to run it at 69W (don't judge me). So what's the lowest resistance I can use? The amps still need to stay under 20A, since I'm using Samsung 25R cells in my mod.

Ω=W/A2
Ω=69/202 
Ω=69/400
Ω=.17 ohms

As mentioned before, I should build at least slightly higher than the minimum, so round up to about .2 ohms should be fine. But you also need to keep in mind the voltage of the device. 

Example 4: In a Sigelei-100W, there are two 18650 cells in series, so the total max voltage is 8.4V. Let's use the second formula to check the lowest safe resistance for this setup:

Ω=V/A
Ω=8.4/69
Ω=.12 ohms

So the .2 ohm build should be fine, according to the math.


Unfortunately, the math doesn't work 100% perfectly. The reason for this is that Ohm's Law is only applicable to a theoretical "perfect circuit" which would mean there is no margin for error due to the internal resistance of the mod, voltage sagging in the cells, and tons of other factors. Because of this, Ohm's Law cannot be used as the sole guide of what's safe or not. So use your head. Like I said before, if the math says you can vape at .00004 ohms, please for the love of God DON'T. It's a good rule to never go below about .3 ohms at the very lowest, and you still need to use Ohm's Law to make sure your cells can handle that kind of resistance. 

If you have any questions about all this, please feel free to ask. I know there's a lot of scientific stuff in here that's hard to understand, and there are a few guys on here who know a lot more than me about this subject, and we're all happy to help. 


For more information and a simple, easy to understand guide to the terminology used here, check out the Ashtray Blog's guide to resistance, wattage and voltage.

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15-01-2015 05:52 AM
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RE: What the heck is Ohm's Law?
Great post Michael thanks!! that all makes perfect sense Good
15-01-2015 07:58 AM
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Michael (01-15-2015)
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Post: #3
RE: What the heck is Ohm's Law?
[Image: ohms-law-illustrated.gif]


Big Grin
17-01-2015 03:27 AM
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Michael Offline
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Post: #4
RE: What the heck is Ohm's Law?
That... actually is freakin' amazing. Hi

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17-01-2015 03:50 AM
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Almost Jaded Offline
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Post: #5
RE: What the heck is Ohm's Law?
A small quibble...

Michael Wrote:Ohm's Law is a scientific formula used to describe the relationship between power (Watts=W), resistance (Ohms=Ω), energy (Volts=V) and current (Amps=A). All these things are important when vaping, as they correlate to the output of your device and safety of your cells.


...you called everything else by the correct corollary term, lol.


You also state that...


Michael Wrote:Example 4: In a Sigelei-100W, there are two 18650 cells in parallel, so the total max voltage is 8.4V.



[font=Verdana, Arial, sans-serif]...and that would imply series wiring, not parallel.  Parallel wiring increases capacity whilst maintaining voltage; series wiring increases voltage whilst effectively retaining the original capacity (because they'll be drained at twice the voltage).[/font]


[font=Verdana, Arial, sans-serif]Just tryin' to help for posterity![/font]

Why are there weird font codes in my post that I didn't put there, lol

Remind one liberal per day, that laws affect only the law-abiding.
(This post was last modified: 03-05-2015 04:50 AM by Almost Jaded.)
03-05-2015 04:41 AM
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Michael Offline
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Post: #6
RE: What the heck is Ohm's Law?
(03-05-2015 04:41 AM)Almost Jaded Wrote:  A small quibble...

Michael Wrote:Ohm's Law is a scientific formula used to describe the relationship between power (Watts=W), resistance (Ohms=Ω), energy (Volts=V) and current (Amps=A). All these things are important when vaping, as they correlate to the output of your device and safety of your cells.


...you called everything else by the correct corollary term, lol.


You also state that...


Michael Wrote:Example 4: In a Sigelei-100W, there are two 18650 cells in parallel, so the total max voltage is 8.4V.



[font=Verdana, Arial, sans-serif]...and that would imply series wiring, not parallel.  Parallel wiring increases capacity whilst maintaining voltage; series wiring increases voltage whilst effectively retaining the original capacity (because they'll be drained at twice the voltage).[/font]


[font=Verdana, Arial, sans-serif]Just tryin' to help for posterity![/font]

Why are there weird font codes in my post that I didn't put there, lol

Thanks for pointing that out! Not sure how I missed that Tongue I'll  fix it asap. 

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03-05-2015 09:38 AM
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Michael Offline
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Post: #7
RE: [FAQ] What the heck is Ohm's Law?
Sticky stuck. 

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16-06-2016 04:08 PM
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