Testing Resistor -
How to accurately check resistors on board
Usually when a resistor fail they either
increase in value or open up at all. You can check the resistance of a resistor with an ohmmeter. If the
resistor is in circuit, you will generally have to isolate the resistor so you are measuring only the resistor,
not other components in the circuit. Always be aware of possible back( parallel) circuits when performing
in-circuit resistance measurements.
As a repairer, most of the times we want to
troubleshoot and solve problems as fast as possible thus removing all resistors from the board and check the
resistors one by one will take up a lot of our precious time. There have to be a simple way to check resistor on
Using analog meter to check resistor on
board often produced a wrong reading. This is due to the reason that the output from the analog
meter is from 3 volt to 12 volt. The voltages are quite high and it can trigger the semiconductor
devices around the resistors such as diode, transistor and ICs. Do you know that semiconductors only need
voltage of 0.6v in order to conduct. Since the output voltage from the analog meter is higher than
the semiconductors, checking the resistor in circuit won't give you an accurate reading!
In order to measure resistors while it still
in circuit, you need to get a digital multimeter that have the output of less than 0.6v. This is to avoid
conducting the semiconductor devices around the circuit that you want to check. Currently i' m using the
Greenlee digital meter that have output around 0.2volt. Though it cannot give me a 100% accurate result at least
it can help me to speed up my troubleshooting job. Why not 100%? This is due to that some circuit have resistors
that is directly parallel to each other.
Testing time- If you connect your digital
meter leads across a resistor in a circuit and it measures higher than it should, then you know the resistor is
either open or has gone up in value. Other circuit components cannot possibly increase the value of a resistor;
any parallel circuit could only make the resistance reading lower. In rare cases, sometimes an undischarge
capacitor can cause the measurement higher than it should be. Only through more practice will make you know when
you should remove the resistor and check it off board.
Click here to learn testing electronic components like a Pro
Author By : Jestine Yong