CHAPTER 11 ■ POWER ON!
- When you touch the red test probe tip to the positive terminal of the battery, the
multimeter should indicate the battery’s voltage as expected. My battery is 9.15 V. - Touch the red test probe tip to the clip at the other end of the red alligator
jumper. You shouldn’t see any change in voltage. Remember, the jumper wire is
just a pipe; it shouldn’t use up any voltage. - Touch the red test probe tip to the top resistor lead. You still shouldn’t see any
change in voltage. The resistor lead is also a pipe. - Touch the red test probe tip to the lower lead of the resistor (see Figure 11-5).
Finally some action! The voltage should drop significantly. My circuit is 1.8 V at
this point.
What happened? The resistor did its job. It reduced the voltage to an amount acceptable to the LED
coming up. (That’s not exactly a technically accurate explanation, but good enough for now.)
- Touch the red test probe tip to each of the green alligator clips and then to the
anode of the red LED. The voltage should still be 1.8 V (or whatever you saw
earlier). The green jumper and anode wire are pipes. They aren’t using up
voltage. - Touch the red test probe tip to the cathode of the red LED. The voltage should
drop to zero at this point. The red LED used up all of the remaining voltage. - Touch the red test probe tip to the black alligator clips. Zero voltage. Touch the
red test probe tip to the negative terminal of the battery (same as the black test
probe hook). Zero voltage. No matter what parts you install in a circuit, by the
time the electricity reaches the other end of the battery, the voltage is always
zero.
Measuring Voltage “At” a Point
The test method you’ve just been using answers the question, “What’s the voltage at such-and-such a
point?” To check the voltage at a particular point, always connect the multimeter’s black test probe to the
negative terminal of the battery and touch the red test probe to the point in question.
As you make your way down the circuit, the voltage decreases as each major part uses up some voltage.
Figure 11-5. Red probe detecting the voltage at the point after the resistor