Experiment 13: Broil an LED.

Chapter Experiment 13: Broil an LED.

Make Electronics Book Learning by Discovery
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Make Electronics Book Learning by Discovery

  • Experiment 13: Broil an LEDChapter 3114Experiment 13: Broil an LEDIn 16,Chapter 1, you saw how an LED can be damaged if too much current flows through it. The electricity caused heat, which melted the LED. Unsurprisingly, you can just as easily melt it by applying too much heat to one of its leads with a soldering iron. The question is: how much heat is too much? Let’s find out. You will need:• 30-watt or 40-watt soldering iron• 15-watt pencil-type soldering iron• A couple of LEDs (that are expendable)• 680Ω resistor• Wire cutters and sharp-nosed pliers• “Helping hand” gadget to hold your workI don’t want you to use alligator clips to join the LED to a power supply, be-cause the alligator clip will divert and absorb some of the heat from your sol-dering iron. Instead, please use some sharp-nosed pliers to bend each of the leads from an LED into little hooks, and do the same thing with the wires on a 680Ω load resistor. Finally bend the new wires on your AC adapter so that they, too, are tiny hooks. Now you can put the hooks together like links in a chain, as shown in Figure 3-66.Grip the plastic body of the LED in your helping hand. Plastic is not a good thermal conductor, so the helping hand shouldn’t siphon too much heat away from our target. The resistor can dangle from one of the leads on the LED, and the wire from the AC adapter can hang from that, a little farther down. Gravity should be sufficient to make this work. Set your AC adapter to deliver 12 volts as before, plug it in, and your LED should be shining brightly. I used a white LED in this experiment, because it’s easier to photograph. Make sure your two soldering irons are really hot. They should have been plugged in for at least five minutes. Now take the pencil-style iron and hold its tip firmly against one of the leads on your glowing LED, while you check the time with a watch. Figure 3-67 shows the setup.I’m betting that you can sustain this contact for a full three minutes without burning out the LED. This is why you use a 15-watt soldering iron for delicate electronics work—it doesn’t endanger the components.Allow your LED wire to cool, and then apply your more powerful soldering iron to the same piece of wire as before. Again, make sure it is completely hot, and I think you’ll find that the LED will go dark after as little as 10 seconds (note, some LEDs can survive higher temperatures than others). This is why you don’t use a 30-watt soldering iron for delicate electronics work. The large iron doesn’t necessarily reach a higher temperature than the small one. It just has a larger heat capacity. In other words, a greater quantity of heat can flow out of it, at a faster rate. 12V DCPowerSupplyFigure 3-66. By literally hooking together the leads from a resistor and a white-light LED, we minimize pathways for heat to escape during the subsequent test.Figure 3-67. Applying heat with a 15-watt soldering iron. A typical LED should withstand this treatment for two or three minutes, but if you substitute a 30-watt soldering iron, the LED is likely to burn out in under 15 seconds.