Usually someone will ask such a question: the collector of a NPN type transistor is connected to the positive electrode, the emitter is connected to the negative pole, the base is not connected to any end of the power supply, and the collector and the base voltage is large enough.

In fact, the NPN type transistor collector connected to the power supply cathode emitter is connected with the negative electrode of the power transistor "there will be a small current through, generally in the microampere level, due to manufacturing materials and its size will be different. This is known as the penetration current, which is measured at a prescribed set of radio voltage.

The transistor characteristic is the base to control the collector current, so a NPN type transistor collector connected to the positive electrode, emitter electrode, the base is not connected to any end of the power supply, or the negative electrode, the collector and emitter. Such as the collector and emitter current through, the transistor leakage or damage.

The base current can be controlled in a certain range, the size of the current is amplified, and the range is enlarged. When the base current is increased, the tube collector current will reach saturation, and the increase of the base current can not increase the injection current, and the saturation current is controlled by the external circuit.

When the external power supply, such as signal, DC, and the emitter of the potential to be the same, when the charged triode is below 0.6V or negative cutoff.

We concluded that the NPN type of transistor in the base voltage up to 0.7V, start with current guide, as the base voltage increases, the transistor into the amplification area, the collector current is increased; the base current increases to a certain value, the triode amplification into the saturation region, and then increase the base voltage, increasing collector current is very small. While the base voltage is less than 0.7V, or 0, negative, there is no current through the transistor, called off.

Using the above characteristics of the transistor, we can electronic switches, through and through.

To pay attention to any time the transistor should not be at the maximum current and maximum voltage (of course, can not be the maximum power), otherwise very easy permanent damage.