This tutorial is a guide to the Ohm's Law / Power cheat sheet. You may have arrived here because you received a cheat sheet with your Core Electronics order. If not, no worries! You can still follow along with the examples - we've provided a reproduction of the card for reference.
In this video we'll cover practical uses for Ohm's law in maker-electronics without dwelling too much in theory-land - the examples are also shown below for reference.
- How to Use the Card
- Ohm's Law Examples
- Find the Current in a Resistor
- Find the Voltage across a Resistor
- Find an unknown Resistance given Voltage and Current
- Everyday Example - Choose a Resistor for an LED
- Power Examples
- Find the Power dissipated across a voltage regulator
- Find the Power dissipated across a resistor (LED circuit)
Each side of the card shows a relationship. The blue side is Ohms law - the relationship between Voltage (V), Current (I) and Resistance (R) in a circuit. The red side has equations for calculating Power.
Let's take Ohm's law for example: V = I x R where:
- V is voltage in Volts
- I is current in Amps
- R is resistance in Ohms
V = I x R can be rearranged into the two other expressions shown on the card, but it can be cumbersome or confusing to remember these - that's why we provided an Ohm's law triangle on the card. To use it, pick the quantity you wish to solve for, cover it with your finger, and what remains of the triangle is the expression to be solved. For example, if we wish to solve for Voltage, cover V and what remains of the triangle is I x R. Likewise solving for Current - cover I and what remains is V divided-by R.
Let's proceed with a few examples.
We'll be referring to the blue side of the card which shows Ohm's law.
Consider the case where we know the voltage across a known resistor - eg. 5V applied across 470 Ohms.
The parameter we are solving for is current, so we will cover the I symbol - what remains is Voltage divided by Resistance
Consider the case where we know the current flowing through a known resistor, and we wish to find the voltage across the resistor - eg. 0.5 A flowing through 10 Ohm.
The parameter we are solving for is Voltage, so cover the V symbol - what remains is Current multiplied by Resistance
Consider the case where we know the voltage across, and current flowing through an unknown resistor - eg. 12 V and 5 A.
The parameter we are solving for is Resistance, so cover the R symbol - what remains is Voltage divided by Current
Here's a really common example, selecting the right resistor for an LED. The circuit below is of a diffuse red LED connected to 9V, with a current limiting resistor - to be chosen. The forward-voltage of the LED is 1.8V (common for this type of LED) - you can get this information from datasheets, or just measure it with a multimeter. We know the voltage across the resistor will be the difference; 9 - 1.8 = 7.2V.
We want to stay under a sensible current - say 10mA, let's use that to calculate the minimum required resistance. R = V/I = 7.2/0.01 = 720 Ohm.
We'll refer to the red side of the card which shows formulae for Power in an electrical circuit.
Components often have a power rating that we don't want to exceed - else they might fail, or shutdown to protect themselves. Take the case of a linear regulator, which converts a higher-voltage to a lower one. The regulator rejects the excess energy as heat. The rate heat is dissipated is power, measured in Watts.
The parameter we are solving for is Power, so cover the P symbol - what remains is Voltage multiplied by Current.
Here, Voltage is the voltage-difference across the device. For a linear voltage regulator, this is easy, it's the IN-voltage minus the OUT-voltage.
If we were using a 3.3V regulator (eg. AMS1117-3.3) to regulate 5V to 3.3V with a current draw of 500mA (0.5A) we can calculate the power dissipated as: P = V*I = 1.7*0.5 = 0.85 W
Linear regulators are easy because we know what the voltage drop will be across them. To use this formula for a resistor, we would first have to apply Ohms law to find the voltage across the resistor. Luckily, the Power formula comes with some shortcuts: we only need to know the Resistance and either the Voltage across the resistor or the current flowing through it.
Returning to our LED-resistor example from before, we can calculate the power dissipation in the current-limiting resistor to make sure it is below the component's rating. In the video I'm using a 1/4W resistor, so I want to make sure the result is below 0.25 Watts. Since we already know the current flowing through the resistor (7mA with a 1000 Ohm resistor) we can apply the formula P = I^2 * R. If we had not already calculated the current, we could instead use the voltage difference (7.2V, as shown earlier) and apply P = V^2 / R instead.
We've applied Ohms law, and not just for the sake of it - but to solve real problems and build real circuits. There are plenty of other applications for these formulae, so I hope you've found these examples intuitive enough to apply in your own designs. If you've found anything confusing or just have some questions, feel free to start the conversation below - we're full-time makers and here to help!
May your LEDs remain bright and your resistors unburnt!