Testing resistors in circuit

z-adamson

Addicted Member
My understanging is that if the resistor is not in parallel with anything, then throwing an ohm meter across the resistor is fine. If there is another resistor in parallel, the reading is not reliable. Do I have this right?
 
Basicly yes. Transistors and diodes will interfere to some extent also. The reading with parallel resistors should be correct, just won't match either resistor. It will be lower than either one due to ohms law.
 
Any decent meter will measure resistance with a voltage lower than about 0.7 volts on most scales, so will not turn on diode or transistor junctions. Thus, you can measure more than you might think with little trouble. Still, there will often be other resistive paths, or large power supply caps that pull the readings down. As c.coyle says, you can't get a false high from paralleled "stuff", with one exception. If there's residual voltage on caps, that can throw things off in either direction, but it's usually obvious. Just measure in both directions. If the answer is wildly different, suspect residual voltages. I sometimes put a jumper right across the filter caps to avoid this, but never forget to remove that jumper! Even with paralleled "stuff", readings should be similar between both channels.
 
Well stated 'conrad'. I've done several boards analysis power on power off and residual caps will hold charge. Effect of same to transistors if you know what you're doing can reveal a bad transistor power off but still charged. I go for the rails after the p.s. testing for power clues. then as explained by 'conrad' both 'stereo' channels should be basically equal. Note that readings on transistors have one channel differ by usually a decimal point. For in test of resistors power on; I've read good resistor values but you have to be good at testing fine points. (oh yes I've cooked a few circuits.) In doubt or hessitation? lift a leg.

"resistance is futile" :D
 
The reason I'll test in circuit first is for speed. Obviously, this works best when you have a pair of identical circuits and you're comparing one to the other. Theory being that any off values should be mirrored side to side. Then, when something doesn't match up, you pull a leg to get an accurate test. Preferably you know that one channel is functioning properly when going this route for quick diagnosing.
 
FWIW, it would be extremely rare for a metal film to drift. It would be quite rare for a carbon film to drift. It would be common for a carbon composition to drift, almost expected in fact. The same for fusible resistors, usually identifiable by appearance. Thus, other than carbon comps, fusibles and anything with visible thermal damage (or damage to the board) I don't worry much about measuring resistor values unless a problem is extremely stubborn. Even then it's almost always something else.
 
A capacitor with a small charge in parallel circuit with a resistor can also lead to confusing readings.
 
Measuring anything in circuit with the ohms function. Make sure no power is applied.:thumbsup:
 
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