CHAPTER 16 ■ TRANSISTOR SWITCHES
With the large resistance of 470 kW, R2 doesn’t allow much current to flow through the base. The
amount of current flowing through the base is multiplied by the transistor’s gain to determine the maximum
current that is permitted to flow through the collector. Low gain (if the transistor is connected backwards)
multiplied by low base current means LED1 will be limited to a low maximum current.
Unfortunately, a high-efficiency LED can still illuminate with very little current. So, that’s why you
should use an ordinary LED.
■ Note Usually you’ll want to use high-efficiency or ultra-bright LEDs because their low current appetites
save battery power. These LEDs work perfectly well with transistors. It’s just that in the case of the transistor
testing circuit, we want a current hog.
Building the PNP Transistor Test Circuit
You can build the PNP transistor test circuit to look very much like the schematic. See Figure 16-6 for a
photograph.
Figure 16-6. PNP transistor test circuit built on a solderless breadboard