Data Sheet

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21. Kirchhoff’s Second Law

Kirchhoff’s second law states: The sum of the voltages around

a closed network is zero.

Build the circuit to the left. If a drop in voltage is considered

as a negative voltage and a rise in voltage a positive voltage,

then the following equation is a mathematical representation

of Kirchhoff’s second law:

+ V

= 0

This shows that the voltage drop across the battery module

(91) of 4.5V must equal the voltage drop across the LED (69)

plus the voltage drop across the motor (95). From project

#19 we saw that the voltage drop across the LED (69) was

high, and thus the voltage drop across the motor (95) was

small, which is why the motor spun slowly. But a similar

Kirchhoff equation shows that the voltage drop across the

lamp (76) is the full 4.5V, which is why the lamp (76) is bright.

22. The Resettable Fuse

Positive-Temperature-Coefcient, or PTC, thermistors – also

known as resettable fuses are devices that have very low

resistance until a current is reached, then they get warm and

the resistance changes limit the current.

Build the circuit to the left. Notice that the lamp (76) is not

lit and the motor (95) does not spin. In this circuit the red

glowing LED (69) is acting like a warm activated PTC fuse by

limiting the current to the motor (95) and lamp (76). Replace

the LED (69) with a 4-wire (4) and the lamp (76) and motor

(95) will now get the current they need to operate properly.

1st level