CHAPTER 11 ■ POWER ON!For the power indicator circuit:(9.15 V - 1.8 V) / (30 mA / 1000) = 245 W
The calculation reveals a 245 W minimum, but earlier I warned you not to go below 470 W. Well, that
provided a bit of safety in case your battery had a little more voltage or your LED used a little less voltage.
Don’t Measure Voltage with Probe in Current Terminal
To test current, you had to pull the red test lead from the V terminal on the multimeter. You put the red test
lead into the mA, A, or amp terminal. This formed a pipe in the multimeter to let the electricity flow through
and get counted.
Let’s say you now decide to test the voltage of the resistor but you forget to put the red test lead back into
the V terminal. You put the red and black test probes above and below the resistor. Poof! Suddenly your light
emitting diode becomes a smoke emitting diode.
■ Note Just kidding. The LED dies without drama.
What happened? Because the multimeter has become a pipe to test for current, the electricity can now
get around the resistor (see Figure 11-13). It’s like the resistor isn’t in the circuit anymore. The LED takes the
full force of the battery, destroying it.
Don’t laugh. You’re going to do it one day.
The point is, be sure to switch your red test lead back to the multimeter’s V terminal before you test for
voltage. Or, an even safer practice is to get in the habit of always putting the meter’s red test lead back into
the V socket and turning the dial to voltage mode as soon as you’re done measuring current.
Figure 11-13. Electricity accidentally getting around the resistor through multimeter amp mode