Experiment 13: Broil an LED
114 Chapter 3
Experiment 13: Broil an LED
In 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 work
I 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 DC
Power
SupplyFigure 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.