Make Electronics

Getting Somewhat More Serious 115

Experiment 13: Broil an LED

Throw away your burned-out LED. Substitute a new one, connected as before,
but add a full-size copper alligator clip to one of the leads up near the body of
the LED, as shown in Figure 3-68. Press the tip of your 30-watt or 40-watt sol-
dering iron against the lead just below the alligator clip. This time, you should
be able to hold the powerful soldering iron in place for a full two minutes
without burning out the LED.

Imagine the heat flowing out through the tip of your soldering iron, into the
wire that leads to the LED—except that the heat meets the alligator clip along
the way, as shown in Figure 3-69. The clip is like an empty vessel waiting to
be filled. It offers much less resistance to heat than the remainder of the wire
leading to the LED, so the heat prefers to flow into the copper clip, leaving the
LED unharmed. At the end of your experiment, if you touch the clip, you’ll find
that it’s hot, while the LED remains relatively cooler.

The alligator clip is known as a heat sink, and it should be made of copper,
because copper is one of the best conductors of heat.

Because the 15-watt soldering iron failed to harm the LED, you may conclude
that the 15-watt iron is completely safe, eliminating all need for a heat sink.
Well, this may be true. The problem is, you don’t really know whether some
semiconductors may be more heat-sensitive than LEDs. Because the con-
sequences of burning out a component are so exasperating, I suggest you
should play it safe and use a heat sink in these circumstances:

• If you apply 15-watt iron extremely close to a semiconductor for 20 sec-
  onds or more.


• If you apply a 30-watt iron near resistors or capacitors for 10 seconds or
  more. (Never use it near semiconductors.)


• If you apply a 30-watt iron near anything meltable for 20 seconds or more.
  Meltable items include insulation on wires, plastic connectors, and plastic
  components inside switches.

Rules for Heat Sinking

1. Full-size copper alligator clips do work better.


2. Clamp the alligator clip as close as possible to the component and as far
   as possible from the joint. (You don’t want to suck too much heat away
   from the joint.)


3. Make sure there is a metal-to-metal connection between the alligator clip
   and the wire to promote good heat transfer.

Figure 3-68. When a copper alligator clip is

used as a heat sink, you should be able to

apply a 30-watt soldering iron (below the

clip) without damaging the LED.

Heat Flow

Soldering Iron

Heat Sink

Figure 3-69. The heat sink intercepts the

heat, sucks it up, and protects the LED

from damage.