Imagine you have a 3-volt power supply and you want to connect a single red LED. How do you figure out the right resistor to use? Let’s break it down step by step.
First, we know that the power supply provides 3 volts. The LED, when lit, will have a voltage drop of about 2 volts. This means the resistor needs to handle the remaining voltage. So, 3 volts minus 2 volts equals 1 volt that the resistor must manage.
Next, we need to consider the current. The LED requires a current of about 20 milliamps (mA). To find the resistance needed, we use Ohm’s Law: Voltage (V) = Current (I) x Resistance (R). Rearranging gives us Resistance = Voltage / Current.
Convert 20 milliamps to amps by dividing by 1000, which gives us 0.02 amps. Now, divide 1 volt by 0.02 amps to get 50 ohms of resistance.
To make this easier, you can use an online calculator where you just input your values. This can save time and help ensure accuracy.
Let’s try another example. Suppose you have a 9-volt battery and want to connect a yellow LED, which also has a voltage drop of 2 volts and needs 20 milliamps of current. What resistor should you use?
Subtract the LED’s voltage drop from the battery’s voltage: 9 volts minus 2 volts equals 7 volts for the resistor. Using the same method as before, divide 7 volts by 0.02 amps to get 350 ohms of resistance.
What if you don’t have a 350-ohm resistor? You might have a 330-ohm or a 390-ohm resistor. Which one should you use?
To decide, calculate the current for each resistor. For the 330-ohm resistor, 7 volts divided by 330 ohms equals approximately 0.021 amps. For the 390-ohm resistor, it’s 7 volts divided by 390 ohms, which equals about 0.018 amps. Both are close, but to be safe and ensure the LED lasts longer, choose the 390-ohm resistor.
If you need an exact resistance value, you can combine resistors. This is a useful trick that can be explored further.
It’s also important to choose the right power rating for your resistor. Use the formula: Power = Current squared x Resistance. For the 390-ohm resistor, 0.018 amps squared times 390 ohms equals 0.126 watts. A quarter-watt resistor will work fine here.
How long will the battery last? If the battery is rated for 500 milliamp hours, divide this by the circuit’s current (18 milliamps) to get about 27 hours. This is the theoretical maximum, and actual performance may vary.
That’s it for this lesson! Keep exploring electronics and electrical engineering to learn more. Check out other resources and videos to continue your journey.
Gather the components needed to build a simple LED circuit: a 3-volt battery, a red LED, and a 50-ohm resistor. Assemble the circuit on a breadboard, ensuring the correct orientation of the LED. Observe how the LED lights up and discuss why the resistor is necessary.
Work in pairs to solve different scenarios using Ohm’s Law. Calculate the required resistor for various LED colors and battery voltages. Present your solutions to the class and explain your reasoning.
Use an online resistor calculator to verify your manual calculations. Enter different voltage and current values to see how the required resistance changes. Discuss how this tool can help in designing circuits.
Experiment with combining resistors to achieve a specific resistance value. Use different resistor combinations on a breadboard and measure the total resistance with a multimeter. Share your findings with the class.
Calculate the theoretical battery life for different circuit configurations. Use the formula provided in the article and compare your results with actual battery performance in a controlled experiment. Discuss factors that might affect battery life.
Here’s a sanitized version of the provided YouTube transcript:
—
Let’s say we have a 3-volt supply and we want to connect a single red LED. What resistor do we need? We know this wire is 3 volts and this one is our ground wire, which will be 0 volts. The LED has a voltage drop of around 2 volts, so our resistor needs to remove the remaining voltage.
3 volts minus 2 volts equals 1 volt. We know the LED needs a current of around 20 milliamps. Therefore, 1 volt divided by 0.02 amps equals 50 ohms of resistance. Make sure you convert your milliamps to amps for this calculation.
To make it easier, we do have a calculator on our website where you can just input your values. I’ll leave a link in the video description down below for that.
Now, you try to solve this one before I do. Let me know your answers in the comment section down below.
Let’s say we have a 9-volt battery and we want to connect a yellow LED, which has a voltage drop of 2 volts and requires 20 milliamps of current. So, what size resistor is required? We have a 9-volt supply, so subtracting 2 volts for the LED leaves us with a 7-volt drop for the resistor.
The current is 20 milliamps, so 7 divided by 0.02 amps equals 350 ohms of resistance. Now, the problem is that we don’t have a 350-ohm resistor; we only have a 330-ohm or a 390-ohm resistor. So, which one should we use?
We need to ensure the current doesn’t exceed 20 milliamps, so we will calculate which resistor suits us best. To do that, we divide the required voltage drop of 7 volts by the resistor value of 330 ohms to get 0.021 amps. Then, if we do the same for the 390-ohm resistor, we will get 0.018 amps. Both of these values are very close and will work, but to be safe, we will choose a 390-ohm resistor, as our LED will therefore last longer.
We can also combine resistors to get the exact value we need, and I’ll explain that a little later in this video.
We will also need to choose the resistor power rating. We can calculate this using the formula: power equals current squared multiplied by resistance. So, 0.018 amps squared multiplied by 390 ohms gives us 0.126 watts. A quarter-watt rated resistor will be fine for this circuit.
How long will this battery power our circuit? Let’s say this battery is rated for a typical 500 milliamp hours. We simply divide this by our total circuit current, which in this case is 18 milliamps. So, 500 milliamp hours divided by 18 milliamps will give us around 27 hours. Although this is the maximum, it probably will not achieve this in reality.
That’s it for this video! To continue learning about electronics and electrical engineering, check out one of the videos on screen now, and I’ll catch you there for the next lesson. Don’t forget to follow us on Facebook, Twitter, Instagram, LinkedIn, and of course, theengineeringmindset.com.
—
This version maintains the content while removing any informal language and ensuring clarity.
Voltage – The electric potential difference between two points in a circuit, measured in volts. – The voltage across the battery terminals was measured to be 9 volts.
Current – The flow of electric charge through a conductor, measured in amperes (amps). – The current flowing through the circuit was 2 amps.
Resistor – A component used in a circuit to resist the flow of current, thereby controlling the amount of current passing through. – The engineer added a resistor to the circuit to reduce the current flow.
Ohms – The unit of measurement for electrical resistance, symbolized by the Greek letter omega (Ω). – The resistor had a resistance of 100 ohms.
Power – The rate at which electrical energy is transferred by an electric circuit, measured in watts. – The power consumed by the light bulb was 60 watts.
Battery – A device consisting of one or more electrochemical cells that store and provide electrical energy. – The remote control needed a new battery to function properly.
LED – Light Emitting Diode, a semiconductor device that emits light when an electric current passes through it. – The LED on the circuit board lit up when the switch was turned on.
Resistance – The opposition to the flow of electric current, causing electrical energy to be converted to heat, measured in ohms. – The resistance in the wire caused it to heat up slightly.
Amps – The unit of measurement for electric current, short for amperes. – The circuit was designed to carry a maximum of 5 amps.
Circuit – A closed loop through which electric current can flow, consisting of various electrical components. – The students built a simple circuit using a battery, wires, and a light bulb.
