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 board.
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.