Learn how to design a simple LED circuit.
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Let’s say we have a 3 Volt supply and we want to connect a single red Led. What resistor do we need? Well, we know this wire is a three Volts, and this one is a ground wire which will be zero V. The Led has the voltage drop of around 2 volts, and so our resistor needs to remove the remaining voltage, so 3 volts – 2 volts = 1 volt.
We know the Led needs a current of around 20 milliamps, so 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. Check it out HERE. Okay, now you try to solve this one before I do. Let me know your answers in the comments section down below. Let’s say we have a nine 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? Well, we have a nine volt supply. So subtract 2 volts for the Led, and that leaves us with a seven volt drop for the resistor. The current is 20 milliamps, so seven 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. So which one should we use? As we saw earlier, we need to ensure the current doesn’t exceed 20 Ma. So we will have to calculate which resistor suits us best, to do that, we just divide the required voltage drop of seven volts by the resistor value of 330 ohms to get 0.021 amps.
And 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 they both will work. But to be safe, we will choose a 390 Ohm resistor as our Led will therefore last longer. We will also need to choose the resistor power rating. We can calculate this using the formula – power = current (squared) multiplied by resistance. So 0.18 amps (squared) multiplied by 390 ohms gives us 0.126 Watts. So a 1/4 Watt rated resistor will be fine for this circuit. How long will this battery power our circuit for?
Let’s say this battery is rated for a typical 500 Ma. 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 very maximum it would power our circuit for in reality, it probably will not achieve this. .
