Make Electronics

Experiment 9: Time and Capacitors

70 Chapter 2

difference between one end of the resistor and the other will be zero (assum-

ing that we ignore little imperfections in the components). Figure 2-80 may

help to clarify this concept.

1K 1K

12v

DC

6

volts

6

volts

1K 9K

12v

DC

1.2

volts

10.8

volts

1K 99K

12v

DC

0.12

volts

11.88

volts

1K

infinite

resistance

12v

DC

0

volts

12

volts

Figure 2-80. When two resistances are in series, the larger one drops the voltage more

than the smaller one. If the larger resistance becomes infinite (as in the case of a capaci-

tor), the smaller one no longer has any measurable contribution to the voltage drop, and

the voltage is almost exactly the same at both ends.

You should try this using real resistors and capacitors—although if you do, you

will run into a little problem. When you use your meter in its “DC volts” mode, it

diverts a little of the current in the circuit—just a tiny taste—in the process of

measuring it. The meter steals such a small amount, it doesn’t affect the read-

ing significantly when you are checking voltage across a resistor. The internal

resistance of the meter is higher than the values of most resistors. However,

remember that the internal resistance of a capacitor is almost infinite. Now the

internal resistance of the meter becomes significant. Because you can never

have an ideal meter, any more than you can have an ideal capacitor or resistor,

your meter will always interfere with the circuit slightly, and you will get only

an approximate indication.

If you try to measure the voltage on a capacitor that has been charged but is

now not connected to anything else, you’ll see the number slowly falling, as

the capacitor discharges itself through the meter.