CHAPTER 11 ■ POWER ON!Measuring Voltage “Drop” or Voltage “Across” a Part
Often you’ll be interested in how much voltage a particular part uses up by itself. One way of determining
this is to perform a little math.
Here’s what I calculated for my circuit:
The voltage before the resistor is 9.15 V. The voltage after the resistor is 1.8 V. So, the resistor uses 7.35 V.
The voltage before the LED is 1.8 V. The voltage after the LED is 0 V. So, the LED uses 1.8 V.
You don’t need to calculate these numbers. You can test the voltage usage of an individual part with a
multimeter. The meter dial stays the same from the last voltage measurement, except now the red test probe
goes above the resistor and the black test probe goes below the resistor (see Figure 11-6). The meter should
display 7.35 V (or whatever you calculated for your circuit).
Figure 11-6. Red and black test probes detecting the voltage dropped only by the resistor
■ Caution Don’t test the resistor’s resistance with the multimeter’s W mode while the resistor is in the
circuit!! Only use voltage mode in a circuit, not W mode. Otherwise, you might damage your meter.
In W mode, the multimeter actually supplies its own voltage to the test resistor and measures how much of the
voltage is consumed by the test resistor in comparison to a known-value resistor inside the multimeter. As such,
the multimeter is not expecting voltage to already exist across the test resistor. If a high-enough voltage already
exists, the external electrical pressure can break through the unsuspecting circuit inside the meter.
On the other hand, a multimeter in voltage mode expects external voltage to be in the circuit, because that’s
exactly what the multimeter is supposed to be measuring. So, as long as you dial a reasonably high-enough
voltage range on the multimeter dial, the multimeter’s electronics are designed to expect and thus withstand
external voltage.
Test the voltage used by the LED by placing the red test probe above the LED and the black test probe
below the LED (see Figure 11-7). The meter should display 1.8 V (or whatever you calculated for your
circuit).