CHAPTER 11 ■ POWER ON!
In an earlier chapter, you tested the LED using the diode mode of the multimeter. The amount of
voltage being used up by the LED in a circuit will usually be a little higher than the multimeter diode mode
indicated.
■ Caution Don’t test the LED with the multimeter’s diode mode while the LED is in the circuit!! Only use
voltage mode in a circuit, not diode mode. Otherwise, you might damage your meter.
In diode mode, the multimeter actually supplies its own voltage to the test diode, just like it supplies its own
voltage to test resistors in W mode. External voltage in the diode may be high enough to melt unprepared
circuits inside the meter.
Summarizing Circuit Voltage
The wires (alligator jumpers and component leads) didn’t use any voltage. The LED used about the amount
of voltage the meter’s diode mode said it would. The resistor used up the remaining voltage.
Depending on where you place the test probes, the multimeter can measure the voltage at a particular
point or the voltage used across a single part.
Measuring Current Flow
The second most common test you’ll perform on a circuit is current flow. Voltage measures how much
force each drop has; current is a count of how many drops are going through the circuit. Current is a vital
measurement for determining how long your batteries are going to last.
- Disconnect the multimeter test probes if they are presently connected to the
circuit. - As always, the black test lead is connected to the COM terminal on the
multimeter. - But, the red test lead now needs to be unplugged from the V terminal and
connected to the A or mA terminal on the multimeter (see Figure 11-8). Check
your meter manual for exact instructions.
Figure 11-7. Red and black probe detecting the voltage dropped by the LED