A Beginner's Guide to Ohm's Law, Resistance, and Resistors
Concept
Math
Circuits
Understanding Ohm's Law, resistance, and how resistors function is essential for anyone learning electronics. Whether you're a hobbyist, student, or budding electrical engineer, this foundational knowledge will help you design safe and effective circuits, troubleshoot electrical problems, and avoid damaging components. In this article, we'll explore what resistance is, how to use Ohm's Law in practical circuits, how resistor power ratings work, and how to read resistor values using color bands. This is a great starting point for building your understanding of electricity.
What is resistance?
Resistance, measured in ohms (Ω), is a property of materials that resists the flow of current. Think of it like friction in a pipe: the more resistance, the harder it is for electricity to flow. Resistors are components specifically made to introduce resistance into a circuit.
Wires made from conductive materials like copper have very low resistance, similar to ice, providing little friction to skates. In contrast, resistors are like rough pavement, slowing the electrical flow intentionally. Every material, from metals to insulators, has some level of resistance, ranging from a fraction of an ohm to millions of ohms.
What is Ohm's Law?
Ohm's Law is a fundamental equation in electronics. It demonstrates the relationship between voltage (V), current (I), and resistance (R).
V = I • R
From this equation, we also find
I = V/R R = V/I
Ohm's Law example
If I have a 5V power supply and a 100Ω resistor, the current can be calculated:
I = V/R = 5/100 = .05 A or 50 mA
With Ohm's Law, you can calculate how much current a resistor will draw or what kind of resistor you need for a given voltage and current. This helps prevent ruining components or power supplies.
Real life application: avoiding overload with Ohm's Law
If I have a circuit with a resistance of 27Ω and a power supply that provides 6V and a maximum of 200mA, I would need to know if the power supply will be able to supply enough amperage for my circuit. Doing the math,
I = 6/27 = .2222 A = .222 A or 222 mA
This circuit would require more current than I can get with this power supply. This circuit can cause the power supply to overheat and potentially fail. Knowing this ahead of time can save your power supplies.
What happens in a short circuit?
A short circuit happens when electricity flows through a path with very little resistance, like touching a copper wire to both terminals on a 9-volt battery. Let's say that our piece of copper wire has a resistance of .0001Ω. Using a 9V battery,
I = 9/.0001 = 90000
This is an enormous amount of current. In reality, the battery cannot supply us with this much (thankfully!), but it will overheat, and even potentially explode. This is why it is necessary to always have a load (like an LED) or a resistor in your circuit.
Resistor power ratings and values
Resistors can overheat and start on fire if they have too much power running through them. This is why it is important to know what power your resistor is rated for. The equation for power (measured in watts (W)) is
W = V • A
Let's say that we have a 220Ω resistor that's rated for 1/4 watt, and we are trying to use a 5V power supply with a maximum of 2 amps. We can calculate the current used using Ohm's Law:
I = 5/220 = 22.7mA
Now, taking the amperage and voltage:
W = 5 • .0227 = .1135 W
.1135 W is less than 1/4 W, meaning this resistor would be safe to use in this circuit.
Unknown resistor power rating
Resistors have different sizes, depending on their power rating. Here's a chart that demonstrates these differences:
| Physical Size (typical body length) | Power Rating |
|---|---|
| ~1/4 inch (6.3 mm) | 1/8 or 1/4 watt |
| ~1/2 inch (9 mm) | 1/2 watt |
| ~3/4 inch (11 mm) | 1 watt |
| ~1 inch (16 mm) | 2 watts |
| Larger | 5+ watts |
Reading resistor values
You may have noticed that resistors have little colored bands on them. The colors on the bands hold different values based on where they are located:
| Color | First 2 or 3 bands | Multiplier | Tolerance |
|---|---|---|---|
| Black | 0 | • 1 | |
| Brown | 1 | • 10 | ±1% |
| Red | 2 | • 100 | ±2% |
| Orange | 3 | • 1K | ±.05% |
| Yellow | 4 | • 10K | ±.02% |
| Green | 5 | • 100K | ±.5% |
| Blue | 6 | • 1M | ±.25% |
| Purple | 7 | • 10M | ±.1% |
| Gray | 8 | • 100M | ±.01% |
| White | 9 | • 1G | |
| Silver | • .1 | ±5% | |
| Gold | • .01 | ±10% |
4 Band Resistor
Each band on this resistor has a different meaning:
- 1st band: 1st digit
- 2nd band: 2nd digit
- 3rd band: multiplier
- 4th band: tolerance
Let's say that we don't know the resistance of this resistor:
By taking a look, we see that the first two bands are red, the third is brown, and the last is black. Looking at the table above for the value of each color, we can walk through the steps of calculating the resistance.
- 1st and 2nd bands: red and red; 22
- Times this value by the third band: brown; 10. We are at 220 now. This is the ohmic value.
- The last band, black, means that the resistor can be within ±5% (within 209 and 231 ohms)
You can double-check with this resistor calculator.
5 Band Resistor
The resistor we are trying to figure out looks like this:
Similarly to the 4-band resistor, the first three bands are the digits. The fourth is the multiplier, and the 5th is the tolerance. Calculating the value for this resistor:
- First three bands: red, red, black; 220
- Multiplier: black; 1
- Tolerance: brown, ±1% (between 217.8 and 222.2 ohms)
We get a final value of 220Ω.
Conclusion
Understanding Ohm's Law, resistance, and resistor ratings is essential for designing safe and effective circuits. Burning out resistors or power supplies can be very dangerous, which is why it is crucial to understand Ohm's Law and how to use it. If you're a beginner in electronics, check out the article on how to get started with breadboard circuits. Whether you're building breadboard prototypes or more advanced electronics, mastering Ohm's Law will save time, money, and possibly even your equipment.
