Testing PCB Components: Transistors and Fuses

Printed circuit boards (PCBs) function based on a complex array of devices that must transmit electricity without interfering with other devices’ signals. Because the components of PCB are closely placed, testing each device independently is difficult to conduct without picking up surrounding signals. However, it is essential to ensure that the PCB works properly by testing each component individually. There are several ways to test individual devices, depending on the exact component—diodes, for example, must be handled differently than resistors. Below, we explore processes for testing fuses and transistors within PCB set-ups.

Transistors
 

A transistor’s basic purpose is to amplify current, although they also switch electronic signals. They are semiconductors with three terminals—the additional terminal provides them with greater flexibility when transmitting electricity. As voltage is applied to one set of terminals, the voltage is altered between all pairs of terminals.
 

There are two general types of transistors: NPN and PNP. The letters refer to the main semiconductor material from which the transistor is made—an NPN transistor features two terminals made from N-type material, which is negatively charged, and one terminal made from P-type material, which is positively charged. A PNP, on the other hand, features the opposite configuration: two terminals feature P-type material and one terminal features N-type material. (Typically, P and N type materials are made from doped silicon. Silicon, in its natural state is an insulator. When mixed with certain impurities it becomes semi-conductive, and either assumes a positive or negative charge depending on the nature of the impurity added.)
 

Testing transistors in PCB circuits first requires identifying the type of transistor. Every transistor, regardless of type, has three leads: C, B and E, which stand for collector, base, and emitter. Once the type and the various leads have been determined, the transistor can be tested. When testing a transistor in a PCB circuit, disconnect the base lead from the circuit before attempting to measure current. Emitter current is typically between 0.5 milliampere to 3 milliamperes, where as collector voltage ranges from 3 to 15 volts. An ohmmeter can also be used to check emitter, collector and base leads.
 

Fuses
 

A fuse in a PCB guards against current overload—when too much current is present, a small metal wire begins to melt, which inhibits current from flowing from the wire to the next circuit. Each fuse features two leads or pads: in a PCB, the leads on a fuse will be radial or axial. In other applications, fuses can feature solder pads in place of leads; in semi-enclosed fuses, the fuse wire can be replaced if damaged by operation.
 

Typically, fuse wires are made of aluminum, coated nickel or coated copper. The fuse body is often made of glass and cylindrical in shape, featuring fuse leads or pads on each end. Current range can widely vary, from as low as a few one hundredths of an ampere to several hundred amperes. Fuses can be fast acting or slow acting—fast acting fuses opens quickly when current is exceeded, whereas slow blow fuses are slower to respond, and may not be an adequate choice for high current applications because they may not prevent damage in time.
 

Testing a fuse in a PCB is done with a multimeter. After the PCB is turned off and the fuse is removed, connect the probed of the ohmmeter to both leads, and make sure the multimeter range is as low as possible. A fuse that hasn’t blown will register zero ohms; a blown fuse will register infinity.