How Much Current Does It Take To Blow Up An LED?

This project follows the Scientific Method. Review the steps before you begin.

Preparation

If you have never done an electronics project before, this project will be much easier if you learn how to do three things before you start: use a breadboard, use a multimeter, and read resistor color codes. You can learn how to use a breadboard and a multimeter by watching the following two videos. Your resistor kit may have come with a guide or key showing you how to identify the resistance values based on the colored bands. You can also refer to this SparkFun page about decoding resistor markings.

https://www.youtube.com/watch?v=MtiJz7gh1VU

https://www.youtube.com/watch?v=ts0EVc9vXcs

Practice Building a Circuit

Put your safety glasses on before you continue.

Before you start blowing up LEDs on purpose, you should practice building a circuit on your breadboard and taking measurements with a multimeter. Build a circuit like the one in Figure 2, starting out with a large resistor value, like 1 kilo-ohm (kΩ), which will ensure that you do not burn out your LED. This is a simple circuit with a battery, switch, resistor, and LED all in series. Sliding the switch lets you turn the LED on and off. If you watched the breadboard tutorial video, you should know that this is not the only "correct" way to build the circuit on the breadboard. If a different configuration makes more sense to you, you can build the circuit differently. However you build the circuit, make sure you can turn the LED on and off with the switch before you continue. Here is a list of steps if you want to follow our diagram exactly:

1. Put the switch's three pins in breadboard rows 25, 26, and 27. Make sure the switch is in the "down" position, toward the bottom of the breadboard.
2. Use a jumper wire to connect the switch's middle pin to the breadboard's left-side power bus.
3. Put the LED's long leg (positive) in hole E6 and the short leg (negative) in hole F6.
4. Put one end of the resistor in hole J6 and the other end in the breadboard's right-side ground bus.
5. Use a jumper wire to connect hole D25 (the switch's top pin) to hole D6 (the LED's positive side).
6. Put the snap connector on the battery.
   1. Connect the positive wire to the left-side power bus.
   2. Connect the negative wire to the right-side ground bus.
7. Slide the switch to the "up" position. Your LED should turn on.
   1. If the LED does not turn on, double-check all of your breadboard connections. Remember that adjacent rows in the breadboard are not connected to each other. Re-watch the breadboard video if you still do not understand how a breadboard works.
   2. If you see or smell smoke, immediately disconnect the battery. You either have a short circuit somewhere or have accidentally used a resistor that is too small (like 1 Ω or 10 Ω instead of 1,000 Ω). Double-check your circuit and try again.

Image Credit: Ben Finio, Science Buddies / Science Buddies
Figure 2. Breadboard diagram for a basic circuit with a battery, switch, resistor, and LED. Diagram made with Tinkercad Circuits.

Practice Using a Multimeter

Once your circuit is working, you can practice using your multimeter to take measurements. Connect your multimeter as shown in Figure 3. For hands-free operation, you can use alligator clips to connect your multimeter's probes to jumper wires and put those wires in the breadboard, or connect the alligator clips directly to the resistor leads. Your goal is to connect the multimeter to measure the voltage across the resistor. You will then use Ohm's Law to calculate the current through the resistor (which is the same as the current through the LED since they are in series). This approach is safer than putting the multimeter in series with the rest of the circuit to measure the current directly, because there is lower risk of accidentally blowing your multimeter's fuse.

1. Make sure your multimeter set to measure DC voltage (not current!). If you have a manual-ranging multimeter, select the 20 V range.
2. Plug the multimeter's black probe into the COM port.
3. Connect the black probe to the breadboard's right-side ground bus (using an alligator clip and jumper wire).
4. Plug the multimeter's red probe into the V port (labels may vary depending on your multimeter, see multimeter tutorial in the bibliography).
5. Connect the red probe to the other side of the resistor (hole I6 in Figure 3), again using an alligator clip and jumper wire.
6. You should see a voltage reading on your multimeter. For a circuit with a fresh 9 V battery and a red LED, the voltage should be about 7 V.
7. Practice calculating the LED current. You can do this by rearranging Ohm's Law to solve for current. Note that the voltage in this equation is the voltage you measured with your multimeter, not the battery voltage.
   
   Equation 3:  $I = \frac{V}{R}$
8. Make sure you keep track of your units. For an example circuit with a 1 kΩ (1,000 Ω) resistor and a measured voltage of 7 V, the current is:
   
   $I = \frac{7 V}{1,000 \Omega} = 0.007 A = 7 mA$

Image Credit: Ben Finio, Science Buddies / Science Buddies
Figure 3. Breadboard diagram showing multimeter connected to measure voltage across the resistor.

Burning Out LEDs On Purpose!

Make sure you still have your safety glasses on.

Once you have your circuit and multimeter working, you are ready to burn out some LEDs for science! Make a copy of Table 1 in your lab notebook. You can add more rows as needed.

1. Pick an LED color to test. Look up the rated current for that color. This is the maximum continuous current you should send through the LED according to the manufacturer. Depending on where you bought your LEDs, you may be able to find this information directly on the website, in a datasheet linked from the website, or with the physical packaging that came with the LEDs. If you cannot find the value anywhere, assume a value of 20 mA (this is the most common rating for most 5 mm LEDs). Write this value down in your data table.
2. Write down your resistor value in ohms (this should still be the value from the circuit you built in the previous section).
3. Write down the measured resistor voltage in volts (you already measured this for the first resistor in the previous section).
4. Write down the calculated current in milliamps (you already calculated this for the first resistor in the previous section).
5. Write down any observations you make, for example, about the appearance of the LED. Use all your senses! Do you hear or smell anything?
6. Turn your power switch off. Use tweezers to remove the LED and resistor from the circuit (they are probably not hot yet, but it is good to start practicing this now). Set them aside so you do not get them mixed up with your unused parts.
7. Put a new LED of the same color and a new resistor of the same value in the circuit. Since there is some variation in the manufacturing of resistors and LEDs, you should use new ones for each trial.
8. Turn the power switch back on and repeat your measurements. Do a total of at least three trials for this LED/resistor combination.
9. This is where things get interesting. Move down to the next smallest resistor value available in your kit. If you started out with 1 kΩ, this might be something like 680 Ω or 470 Ω. Repeat steps 2–8 for your new, lower resistor value. Make sure you keep your safety glasses on and use tweezers to remove parts from the breadboard since they may be hot.
10. Continue this process, testing smaller and smaller resistor values, until you are down to the smallest resistor in your kit. As a final step, you can test the circuit with no resistor at all! What do you observe as the resistor value gets smaller? If you see or smell smoke, or notice any plastic on the breadboard starting to melt, immediately turn the power switch off, then write down your observations. Note that it may be difficult to get a voltage reading for LEDs that burn out immediately (if the LED "fails open," or becomes an open circuit, then current will stop flowing, and the voltage across the resistor will be zero). It is OK if you cannot calculate a current value for some of the smaller resistor values.
11. Analyze your data.
    1. Make a scatter plot of your calculated current vs. the resistor value.
    2. What is the largest current value you were able to measure before the LEDs started burning out?
    3. How does this value compare to the rated current value?
    4. Even if your measurement exceeded the rated value, do you think it is a good idea to send that much current through the LED? What would happen if you left it on for a long time?