• Resistors are identified on a circuit using numbers and colours (colour coding) to show the exact value of resistance.
  •  When I was populating PC Boards I have used SM resistors, they use numbering system for identification than colour coding. It can either be 3 or 4 digits, in case of 4 digits resistor like the one shown above the first 3 digits represent the 3 digits in the answer and the fourth digits represent the number of zeros to place after the 3 digits.
  • The value of the resistor shown above in this case the value will be 15KΩ.
  • In the schematic diagram the resistor is represented by a jagged line or margin and the value is always return next to the identifier e.g. R1
  • To reduce number of resistors in a circuit, a set of identical resistors is used called Single-In-Line (SIL) resistor network as shown above.
  • A resistor is a small component designed to provide specific amount of resistance in an electronic circuit. The main purpose of the resistor is to limit the current flowing in the circuit.
  • I have used the resistor to reduce the voltage level to a level required by specific parts in the circuit.
  • To check the resistance value I had to remove it from the circuit since the surrounding components can also affect the reading.
  • However colour coding was also essential.
  • Resistor can also be tested by feeling its temperature rise. If it is getting too hot that you can’t hold your finger on it, this could mean is damaged or destroyed.

  •  Small value capacitors are often identified on a circuit using numbers to show the exact value of capacitance.
  • This number identifies the value of the capacitor in picofarads. The first two digits represent the two digits of the capacitor value and the third digit represent number of zeros to be written after the two digits.

e.g. 474 = 470000pf
  • The SM capacitors have no specific mark to identify, they require a meter.
  • Polarised capacitor such as tantalum and electrolytic have the exact capacitance value written on them unlike non-polarised.
  • A capacitor is used to store electric energy, they are mostly used in electronic devices to maintain power supply.
  • Used to pass AC signal and block DC, by separating the two signals.
  • I also used a capacitor as a snubber by creating a path for the impulse to bypass the contact point.
  • I tested the capacitor for short circuit using a meter.
  • I also tested for leakage by removing it from the PC board and used meter to measure.

  • A transistor can either be NPN or PNP with three leads base, collector and emitter.
  • The emitter describes the flow of convectional current in both NPN and PNP.
  • A transistor is used to control the amount of current flowing in the circuit.

  • I have used the meter to test for the emitter, by setting the meter to diode settings.

  • When working with diodes it is always important to distinguish one end of the diode from the other (anode from cathode).
  • The manufactures use a band to identify the cathode side of the diode.

  • The diode allow current to flow in one direction.
  • It is used to extract modulation from radio signals in radio receivers.
  • Some can also be used to regulate the voltage namely zener diode.

  • I have used the meter to test the diode I set the setting to diode mode and connected the common lead of the meter to the cathode of the diode and positive lead to the anode.


  • A device that contains a coil and mechanically linked to a number of contacts.
  • When the relay is energized is sometimes referred to as relay has picked up.
  • When the relay is de-energized is sometimes referred to as relay down.
  • It consists of front and back contacts.

  • I have used the relays shown above when populating and testing relay interface cards.
  • It uses current to energise the coil to operate the contacts.
  • It was used to control the signal by applying a low power signal.

  • It is important to know the contacts of the relay being used, I have used the meter to test the two relays as shown.
  • Knowing the contacts of the relay I have used the meter to check for resistance between contacts. The reading on NC contact the output was short circuit, the reading on NO is infinite.
  • I applied power to the circuit and check if the relay pick up or vice versa.


  • IC’s are identified on a circuit using numbers to show the type and manufacture.
  • Each IC has specific purpose on different circuit.

  • I have used this devices in all PC Boards I populated they are used in all electronic equipment.

  • It is difficult to test IC because the design of each IC varies with its purpose, when I was repairing I had to replace them if I suspect there are destroyed or damaged.


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