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Summarized Ohms Law

Before reading this, we recommending reading the Ultimate Vapers Guide to Battery Safety

Electricity is an extremely difficult concept to understand, and we're going to cut out all the excess and leave you with just the simplified fundamentals for vaping.

We're going to cover just a brief intro into electricity and ohms law, we'll cover:

  1. What is electricity
  2. Voltage, Current, Resistance and Power
  3. The Triangle
  4. Calculating Current
  5. Calculating Power (wattage)
  6. Calculating Resistance
  7. Calculating Voltage
  8. What does it all mean?


what is electricity


Electricity is the movement of electrons. Simple enough right?

Without getting too deep, everything in the entire universe is made of atoms. At the core of the atom you have Protons and Neutrons, and in a cloud around them are Electrons!

Here's the thing, Electrons don't like each other. Electrons have a negative "-" charge, and when you put them together, they tend to push each other away!

BUT! Electrons love Protons! Protons have a positive "+" charge, but can't move around as easily as electrons can. So if you put an electron and proton together, the electron will move towards the proton!

Remember the old saying? Opposites attract?

So with that out of the way, now we can get to the building blocks of electricity (extremely simplified for our lesson)

  1. Voltage: An imbalance of of charge between two points
  2. Current: Amount of electrons flowing past a point
  3. Resistance: Resistance to the flow of electrons
  4. Power: Transfer of electric energy into another form

And how we measure them

  1. Voltage: Measured in Volts (V)
  2. Current: Measured in Amps (A)
  3. Resistance: Measured in Ohms (Ω)
  4. Power: Measured in Watts (W)

And their symbols

  1. Voltage: Given the symbol (E or V) for Electromotive Force or Volts
  2. Current: Given the symbol (I) for the Intensity of flow
  3. Resistance: Given the symbol (R) for Resistance
  4. Power: Given the symbol (P) for Power

Keep in mind this is just and intro and you can go much more in depth into your explanation and definition of these terms however, why not just keep it simple?

So how do they work together?

My favorite way to explain this is to imagine electricity as a tank of water going through a hose. The force pressing down on the water, or the pressure is Voltage. Because of this pressure, water is going to flow, the amount of flowing water is Amperage. But at one point in the hose, there is a kink! Less water can flow through here, causing Resistance.Now imagine you spray the water at a waterwheel, causing it to spin, Power!

Now that you know the terminology, we can get into the mathematical relationships



volts and resistance

Ohms law describes how Voltage, Current and Resistance relate algebraically, stating

Voltage (V) = Current (I) multiplied by Resistance (R)


or you can rewrite it many ways


So lets do an example, We have a RDA consisting of a 4.2v Battery and a coil reading at 0.5 Ohms. If we plug this into our equation it should look like this: 4.2v=I(0.5Ω). Divide 4.2v/0.5Ω and I = 8.4. 8.4 Amps will flow!

Now here's a fun little trick Draw a circle (or triangle) and draw a line horizontally in the middle. Leave the top half alone and draw a line vertically in between the bottom half. On the top half put an V, on the bottom left quarter put an I, and the right quarter put an R. Now, depending on the value you need, place your thumb over the symbol and you will get your answer! For example you need voltage? Cover the V and you are left with I multiplied by R! You need Resistance? Cover the R and you are left with an V/I!


The Triangle

volts current resistance 2

Inside the triangle you can see the three main elements in any electrical circuit, represented by the letters V, I, and R. I would vocalize the triangle as “V over I times R” with “times” being multiplication. The hardest part of this will be remembering what the letters represent, and that’s easy:

  • V = Voltage (your battery voltage)
  • I = Current (the amperage drawn by your coil)
  • R = Resistance (the resistance, in ohms, of your coil)

So, how do we use the Ohm’s Law triangle? Again, simple – the triangle visually depicts the relationship between voltage, current, and resistance. In the following examples we’ll explore how to use the triangle and formulas to help you build coils targeting the current and wattage you desire.


Calculating current

If you want to determine the current draw through a resistance (your coil) the formula is:

I = V ÷ R (or I = V/R)

How did we arrive at that? Look at the triangle and you will see that to solve for current (I)you must divide voltage (V) by resistance (R).

Let’s put the formula to work in a real life example. If you are using a mechanical mod, with a freshly charged battery you theoretically have 4.2 V available to power your coil. If your coil is 0.5Ω, you now have everything you need to determine current, in amps:

I = 4.2 V ÷ 0.5Ω (or 4.2/0.5)

I = 8.4 A

As you can see, with your 0.5-ohm coil and a freshly charged battery at 4.2 volts, the resulting max current draw will be 8.4 amps. If your battery has a 10-amp limit, you are well below the cap. Don’t forget that using a dual mechanical mod in series configuration will double your amp draw per battery, and you will have to build coils with twice as high resistance to be safe. Also note that as the battery depletes, the current will also tail off. For example, when the battery reaches 3.7 volts with the same load, current will drop to 7.4 amps (3.7 volts / 0.5 ohms)


Calculating power (wattage)

The next thing you will probably want to know is the power generated at the coil, or wattage. It’s not shown in the triangle, but the formula is simple. Just multiply the current in your circuit by the voltage applied:

P = V x I

In our original example, the formula would look like this:

P = 4.2 V x 8.4 A

P = 35.3 W

So that 0.5-ohm coil with a fully charged battery at 4.2 volts will pull a maximum of 8.4 amps and deliver 35.3 watts. You can see that as the resistance of your coil increases, current will drop and wattage will drop.


Calculating resistance

The second Ohm’s Law formula that can be of use to us is calculating resistance. Let’s say that you have a battery with a 10-amp current limit and you want to determine the lowest coil resistance that you can safely run without exceeding the CDR of the battery.

To calculate, you would use the following formula:

R = V ÷ I

Since you know that the battery CDR is 10 amps, you might want to target 9 amps in your calculation, to give yourself 1 amp of headroom. You also know that your max voltage will be 4.2 volts on a single battery mod. So the calculation goes like this:

R = 4.2 V ÷ 9 A

R = 0.47Ω

The result tells you that your safe lower limit with the 10-amp battery is 0.47 ohms – anything lower and you risk exceeding the current limit of the battery. Of course, if you have a 25-amp battery, your low resistance drops to 0.17 ohms:

R = 4.2 V ÷ 25 A

R = 0.17Ω


Calculating voltage

Finally, and probably not as useful to us, using the triangle you can solve for voltage in a circuit, as long as you know the values of the other two variables.

To solve for voltage when current and resistance is known, the formula looks like this:

V = I x R


What does it all mean?

Really, the most useful formulas for vapers, are the three that calculate current (I = V ÷ R) power (P = V x I) and resistance (R = V ÷ I). These will allow you to figure out the current your coil will draw and the wattage that will result. As you increase resistance, current and power will drop off. If you decrease resistance, current and power will increase. The resistance formula allows you to calculate a safe low resistance based on the CDR of your battery.

It’s all good information to help you stay within the safe limits of your batteries, and to tweak the amount of power at your coil to help you achieve your own vaping nirvana. There are other considerations like coil ramp time and the heat of your coil that are determined by wire gauge and mass. Ohm’s Law won’t figure any of that, and a site like Steam Engine can be helpful.

One final, and critical piece of advice: ALWAYS assume that your battery voltage is the equivalent of a fully charged battery: 4.2 volts for a single battery mod or parallel battery mod, or 8.4 volts for a dual series mod. People will argue that the coil will never see that actual battery voltage due to voltage drop within the mod, but to be safe ALWAYS use the full theoretical battery voltage (at full charge) in your calculations.


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