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Ohm's Law
Boltar Offline
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Post: #1
Ohm's Law
Ohm's Law


To understand what Ohm's Law is and how it is relevant to us as vapers, it is first necessary to understand to what it relates. Ohm's Law states that any given electrical system will obey a strict equation. This equation is:
V = IR
This means that the voltage passing through a given system will be equal to the resistance of the load of that system (Ohms) multiplied by the current (Amps). Now we don't often use Amps in Vaping, we do occasionally when talking about batteries and their limits, but mainly we talk about power or watts. The unit of power, the Watt, is not actually part of Ohm's law, but it is usually bundled in with it because it is very much related. The power equation is very simple
P = IV
The power of an electrical system is equal to the voltage of that system multiplied by the current. So, for an example, let's say that we have just built a coil and have read it on our meter as being 1.6 ohms and you've gone ahead and bunged it on your Mod which has a nice fully charged battery in it. We can use Ohm's law to calculate the Amps this system is drawing from the battery. Remember algebra at School? Let's do some simple algebra. Since V = IR is no good for us in that form, we want the Amps not the voltage. We have to rearrange it slightly. So we divide both sides by R (what you do to one side, do to the other and the equation will remain balanced) and we get this.
V/R = I
So voltage divided by the resistance gives us the Amps, simple. Since a fully charged battery is usually 4.2V we have enough to solve it.
4.2 / 1.6 = 2.625 Amps.
Great, we now have the Amps. But that's not usually what we want, we want the Power or Watts. How do we get that? Well Remember the other equation for power P = IV? So that's simple yes? We know the Voltage is 4.2V and we just worked out the Amps as 2.625A; so:
2.625 * 4.2 = 11.025 Watts.
Now that was actually rather long winded way of going about it. There is a much more simple way, and that is:
P = V²/R
So let's work that out. 4.2 squared is 17.64 and if we divide that by 1.6 we get..... TADA!! 11.025. Now I must point out here that batteries are not an ideal power source, that means that their voltage will drop under load. The reason this happens is because batteries actually have resistance. It is called Internal Resistance and you can usually look this up online. Internal resistances can vary from battery to battery. Now you could go through a voltage divider calculation to work out what voltage you will drop to, then use that voltage in the V²/R equation with R being 1.6 Ohms. But the simplest way to compensate for battery resistance is simply to add it to your coil resistance. If the battery we are using here has an Internal Resistance of 0.1 ohms (pretty crap battery) then all we do is increase our 1.6 ohm coil to 1.7 to compensate. Then we can work out our revised power.
4.2² / 1.7 = 10.376 Watts
So what's left? We can work out the power we are getting from a coil now, but, what if we want to achieve a specific amount of Watts with this 1.6 ohm coil. Let's say we want 15W. How do we do that? Well we would need to change the Voltage. Modern VV mods usually have a VW mode as well which works this out for you, but it's good to know how to work it out manually too. So let's revise our P = V²/R. We want to know the voltage at a given power and resistance. So using algebra again we can rearrange it by multiplying both sides by R. This gives us:
PR = V²
But, there's a problem. It's not just V it's V squared. So to get rid of that, we square root both sides. And we end up with this:
V = √PR
Right! Now we can work out the voltage we need. We want 15W and we have a 1.6 Ohm coil so:
V = Sqrt(15 * 1.6)
V = Sqrt(24)
V = 4.8989 approx

On our VV mod we would thus select 4.9V to achieve 15W. Regulated mods shouldn't experience voltage drops like batteries do, even if they are battery powered, because a circuit inside the Mod makes sure it remains constant. So in this case we do not need to compensate for battery Voltage drop.



That pretty much covers 99% of what vapers need to use Ohm's Law for. to recap:

To work out the Watts you're vaping at:
Square the voltage you are vaping at and divide that by your coil resistance (plus battery internal resistance if using a mech).
P = V² / R [+BIR ]

To work out the Voltage required to achieve a certain amount of Watts:
Multiply the coil resistance by the Watts you want and square root the result.
V = √PR

If you know any two of the values, you can use this chart (thanks sej016) to work out the other two.
İmage

Also check out this web page.
http://www.csgnetwork.com/ohmslaw2.html
It has javascript calculators for every permutation of Ohm's Law imaginable.

İmage
(This post was last modified: 03/02/14 09:26 PM by Boltar.)
03/02/14 03:42 AM
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shandy27 Offline
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Post: #2
RE: Ohm's Law
Say I had amod made with an lm2596 and I set the voltage before attaching the atomizer, then measured the voltage under load, do I calculate power from the voltage under load?
03/02/14 10:29 AM
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Sh1no8i Offline
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Post: #3
RE: Ohm's Law
Great info Boltar! easy to follow nice read Smiling

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03/02/14 10:56 AM
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JustPoo Offline
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Post: #4
Re: RE: Ohm's Law
(03/02/14 10:29 AM)shandy27 Wrote:  Say I had amod made with an lm2596 and I set the voltage before attaching the atomizer, then measured the voltage under load, do I calculate power from the voltage under load?

Yes, if you want to know how much power the coil is getting.



Great stuff, Boltar, another sticky for the collection. Could you stick an ohms law calculator at the bottom for us lazy boys?
03/02/14 11:11 AM
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shandy27 Offline
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Post: #5
RE: Ohm's Law
Cool thanks, was just wanting to make sure. Been watching what your doing with your coils with great interest. Was think of building a mod with 4 18650 in series with a high resistance coil (lol I know it's not sub ohm) and a kick ass heatsink about 4 times the size of lm2596 attached to the chip just to see how much power it can handle.
03/02/14 11:39 AM
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sej016 Offline
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Post: #6
RE: Ohm's Law
This little chart is quite useful ?

İmage

Steve J
Vaporatum ergo sum

İmage

http://www.pipevapes.co.uk/
(This post was last modified: 26/10/15 05:28 PM by sej016.)
03/02/14 11:56 AM
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jSquared Offline
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Post: #7
RE: Ohm's Law
^^^^^ I like that, Steve!

Great job on the write-up Boltar.

JJ
03/02/14 12:02 PM
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Cheekiecharlie Offline
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Post: #8
RE: Ohm's Law
Excellent, great explanation boltar Wink

vaping is a passion ,not just a hobby,

http://forum.squonking.co.uk/index.php

İmage
(This post was last modified: 03/02/14 02:45 PM by Cheekiecharlie.)
03/02/14 12:11 PM
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Boltar Offline
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Post: #9
RE: Ohm's Law
(03/02/14 11:56 AM)sej016 Wrote:  This little chart is quite useful ?

İmage

sej, do you mind if I put that chart into the OP?

İmage
03/02/14 02:32 PM
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Daddy Cool Offline
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Post: #10
RE: Ohm's Law
Blimey maybe we should get out more !!!Biggrinsmiley
03/02/14 03:05 PM
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