Electronics repair technicians often use a digital multimeter to test whether a transistor is working properly or not. Simple tests with a digital multimeter will allow the technician to find out quickly if the transistor's internal components, two back-to-back diodes, are working correctly. Savvy technicians, however, don't rely on multimeter tests as the sole basis for determining if a transistor is faulty. A digital multimeter will only tell you whether or not the transistor is functional, that is, will turn on and off.
Obtain a working NPN transistor. Order one online or buy one from an electronics store. Select a common type of silicon NPN transistor, such as a small signal NPN transistor like the 2N3904. Use the transistor's data sheet to determine the location of the transistor's base, emitter and collector leads.
Set your digital multimeter to "Diode Test." Look for the diode symbol on your multimeter and move the function select switch to point to that symbol. Consult the multimeter's user's manual if you can't locate the diode test function.
Connect the digital multimeter to the base and collector leads of the 2N3904 NPN transistor. Connect the multimeter's positive probe to the transistor's base lead. Connect the multimeter's negative probe to the transistor's emitter lead.
Read the measurement on the meter's display. Check to see if the voltage reading is between the minimum and maximum values of the base to emitter saturation voltage given in the manufacturer's data sheet. For the 2N3904, reject the transistor if the meter displays a voltage that is less than 0.5 volts or greater than 0.95 volts.
Connect the digital multimeter's positive lead to the transistor's base lead and the negative lead to the transistor's emitter leads. Check if the meter reading is in the range of 0.5 volts to 0.95 volts. Reject the transistor if the reading is not in this range.
A NPN transistor can be modelled as two back-to-back diodes. There is a diode between the transistor's base and emitter leads and a diode between the transistor's base and collector leads. Each of these diodes' anodes is connected directly to the base of the transistor. The voltage measured in the diode test is the forward on voltage, also called the turn-on voltage or the base-to-emitter voltage. Most silicon diodes have forward voltage drops in the order of 0.5 and 0.7 volts. Germanium diodes have forward voltage drops between 0.2 and 0.3 volts. The 2N3904 transistor is a silicon transistor, so you can expect a forward voltage drop in the range of 0.5 to 0.7 volts. If the transistor you are testing is in a circuit, you will need to remove the transistor from the circuit board. Use a solder gun to melt the solder and a solder sucker to remove the heated solder. Melt the three solder joints that connect the transistor to the board, and then remove the melted solder with the solder sucker. Pull the transistor out gently with some pliers.