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Testing PCB Components

Printed circuit boards (PCBs) are composed of a variety of complex electrical elements, including resistors, capacitors, diodes, transistors and fuses. In order for a PCB to function properly, each component must play its part: the result should be a PCB consisting of an array of conductive pathways, etched on copper sheets, which works to mechanically support a larger electronic device. If one component fails, the PCB may fail. Therefore, it’s important that each component be monitored and tested to ensure maximum performance.

Resistors

ResistorOne of the key elements of a PCB, resistors are small electronic devices with two terminals that transmit electric current to produce voltage. To test them, an ohmmeter can be used. First, the resistor must be isolated so that test results are only indicative of the resistor and aren’t compromised by signals from other components. A digital multimeter is preferred over an analog meter because the results tend to be more accurate. Using a multimeter with an output value no higher than 0.6 volts will help avoid causing other surrounding resistors to accidentally conduct. However, if there are other parallel resistors, the results may still be somewhat skewed.

Before testing a resistor, first establish what an accurate reading for the specific resistor should be. Next, connect the multimeter leads to the specific resistor and run a test. If the results are too high, this may be indicative of an open resistor, or a resistor that has acquired a higher value. Other circuit components typically cause the reading to decrease, so a higher value should be interpreted as a possible sign of a problem.

Capacitors

25 Volt CapacitorCapacitors are electronic devices which essentially store energy as an electrostatic field, and are composed of an insulating material placed between two conductive plates. In PCBs, they can block the flow of direct current while enabling the flow of indirect current. When DC voltage is applied to a capacitor, the electric charge is stored by each conductive plate. Current flows while the capacitor is storing energy—when the capacitor is full, current stops flowing. The type of material used as the insulating material (a dielectric material) determines capacitor type. Common insulator materials include ceramic, polycarbonate and silver mica. In PCBs, the board itself often creates a capacitor, with alternating layers of metal conductive areas, ground conductor and powder conductor, creating a stable capacitor. Within PCBs, decoupling capacitors can be found, which serve to reduce the noise and effects of other elements on the rest of the circuit board by routing such noise through the capacitor which can then store the excess energy.

To test a capacitor in a PCB, one end of the capacitor should be removed from the circuit. The power supply of DC voltage should match the range of the capacitor so as not to overload the device. When voltage is applied several outcomes are possible: if the capacitor has shorted the meter will simply reflect the output voltage of the power supply; if the capacitor is leaking the meter reading will jump high and then drop low again (but not all the way to zero); if the meter registers no jump at all, the capacitor is either open or capacitance is too low to register a result.

Diodes

Zener DiodesDiodes are electrical devices that only transmit current in one direction, and are comprised of a semi-conductive material between two terminals (a p-type and n-type semiconductor material on each end). While enabling current in one direction, diodes block current in the opposing direction. However, because of their composition, diodes’ behavior can be manipulated.

Testing a diode is a somewhat sensitive operation, and if done incorrectly can cause irreparable damage. It is always best to consult a professional before attempting to test any electrical equipment or related components. When ready to test the diode, disconnect one end of the diode from the circuit board. Using an analog meter (a digital meter sometimes doesn’t register a leaky diode), identify the red and black meter probes. Connect the black probe to the cathode and the red probe to the anode, and set the meter between one and 10 ohms. If the meter registers no reading, the diode could be open. If the meter registers two readings, the diode is likely leaky. If the meter registers some resistance, then the diode is forward biased.


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