Transistors that are have good joints can be used as diodes. Thus if we find that base junction/ emitter of transistor is good using a multimeter, the component can be used as a diode, as shown in Figure 1.
The diode characteristics are the same of transistor that will be used. Thus, for transistors of the type commonly used as the BC548 the current is of the order of about 20 mA and a maximum voltage of about 20V.
For medium power transistors such as BD135 and TIP31, the maximum current is of the order of 200 mA and a voltage of about 30V.
For a transistor as high-power 2N3055 current can reach about 500 mA. Note that the current is less than the transistor collector, since normally the transistors the base current is much lower and it is that limit the application in this case.
