Electronics > Repair
Resistances measurement indicator on PCB
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kraka:
My question is about measurements i made on the resistors of a PCB, which is a component of my PC. All the readings I made in all resistors, one by one, agree with the labels on the resistors, except for two of them. In one the label on the resistors is 4700 Ω, but my measurement shows 40 Ω, and in the other, the label is again 4700 Ω, but my measurement shows 7.60 Ω.

The measurements were made with the resistors on the board.

The strange thing (at least for me who don't have the background as an electronics engineer) is that when I removed these two resistors from the board, and measured them, they gave the correct readings, i.e. 4700 Ω so it means that they are not broken.

After their detachment from the board, if I measure the resistance between the two empty pads of resistor, I find the same reading as when I was measuring, with the resistors on. It is as if the multi-meter bypasses the resistors and chooses a path with the least resistance. I know that current flows in the path with the least resistance, so is this a proof that there is a short circuit somewhere ?

My question is, whether this is an indication that there is a short circuit somewhere in the circuit. Near these resistors there is an IC which is overheated. Is there any chance that the short circuit (if there is) is not in the IC, but somewhere else ? And how can I locate it ? I don't have the schematic.

note : both of these resistors are connected to one of the I/O pins of the IC (not the same pin)
DavidAlfa:
Of course you'll get weird readings with the resistors placed in circuit, lots of other parts connected to them, basically creating parallel networks.
tooki:

--- Quote from: kraka on October 19, 2024, 11:33:04 am ---My question is about measurements i made on the resistors of a PCB, which is a component of my PC. All the readings I made in all resistors, one by one, agree with the labels on the resistors, except for two of them. In one the label on the resistors is 4700 Ω, but my measurement shows 40 Ω, and in the other, the label is again 4700 Ω, but my measurement shows 7.60 Ω.

The measurements were made with the resistors on the board.

--- End quote ---
In-circuit resistor measurements are fraught with peril.


--- Quote from: kraka on October 19, 2024, 11:33:04 am ---After their detachment from the board, if I measure the resistance between the two empty pads of resistor, I find the same reading as when I was measuring, with the resistors on. It is as if the multi-meter bypasses the resistors and chooses a path with the least resistance. I know that current flows in the path with the least resistance, so is this a proof that there is a short circuit somewhere ?

--- End quote ---
That is a widely held myth. It often looks that way when one resistance is dramatically lower than the other, but fundamentally, current divides up among all available paths, in proportion to each path’s conductivity.

For example, if we take the 4700 ohm and 7.6 ohm resistances in parallel:

R1 = 4700 ohms
R2 = 7.6 ohms

We need the total resistance of the parallel resistors for the later formula, so we calculate the parallel resistance first:

Rtot = 1/(1/R1)+(1/R2))
Rtot = (1/(1/4700)+(1/7.6)) ohms = 7.588 ohms

As you can see, because the one resistor’s value is so much larger, its effect on the total resistance is tiny — but not zero.

Now we calculate tue current across each branch:

IR1 = Itot*(Rtot/R1)
IR2 = Itot*(Rtot/R2)

If we assume your multimeter applies a constant 10mA for resistance measurement:
IR1 = 10mA*(7.588 ohms/4700 ohms) = 0.016mA
IR2 = 10mA*(7.588 ohms/7.6 ohms) = 9.984mA

So as you can see, while most of the current flows through the low resistance, some of the current flows through the high resistance, too. It’s just that because of the huge difference in resistances, you end up with numbers so close to the total that you mistakenly think it’s rounding error.
kraka:

--- Quote from: DavidAlfa on October 19, 2024, 12:05:28 pm ---Of course you'll get weird readings with the resistors placed in circuit, lots of other parts connected to them, basically creating parallel networks.

--- End quote ---
This means that except the two resistors with the "wrong" readings, all the other 50 or more resistors which i have measured on board,  and gave me the correct reading (approximately the one that is on their label), are not connected to lots of other parts (?)
But my question is  mainly for, if this readings (much more low than it should be) is an indicator of a short and if this related to the IC or not.
Please read the section where i mention that the readings of the measurement across the two side of the resistors on board,  is exactly the same with the measurement readings on the two empty pads and when the resistors are removed from the board.
And please notice that these two resistors are connected with the IC.
Do you have to say something about this ?
DavidAlfa:
It'll barely affect the readings if the resistor value is much lower than those parts it's connected to.
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