Resistances measurement indicator on PCB - Page 1
Forum section: electronics / repair

author: kraka
author_rank: Contributor
timestamp: October 19, 2024, 11:33:04 am
content: 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)

author: DavidAlfa
author_rank: Super Contributor
timestamp: October 19, 2024, 12:05:28 pm
content: Of course you'll get weird readings with the resistors placed in circuit, lots of other parts connected to them, basically creating parallel networks.

author: tooki
author_rank: Super Contributor
timestamp: October 19, 2024, 01:07:25 pm
content: Quote from: kraka on October 19, 2024, 11:33:04 amIn-circuit resistor measurements are fraught with peril.Quote from: kraka on October 19, 2024, 11:33:04 amThat 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 ohmsR2 = 7.6 ohmsWe 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 ohmsAs 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) = 0.016mAIR2= 10mA*(7.588 ohms/7.6 ohms) = 9.984mASo as you can see, whilemostof the current flows through the low resistance,someof 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.

author: kraka
author_rank: Contributor
timestamp: October 19, 2024, 01:17:45 pm
content: Quote from: DavidAlfa on October 19, 2024, 12:05:28 pmThis 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 ?

author: DavidAlfa
author_rank: Super Contributor
timestamp: October 19, 2024, 01:33:37 pm
content: It'll barely affect the readings if the resistor value is much lower than those parts it's connected to.

author: tooki
author_rank: Super Contributor
timestamp: October 19, 2024, 02:12:40 pm
content: Quote from: DavidAlfa on October 19, 2024, 01:33:37 pmYou mean higher, right?

author: tooki
author_rank: Super Contributor
timestamp: October 19, 2024, 02:17:11 pm
content: Quote from: kraka on October 19, 2024, 01:17:45 pmWithout having a known-good, working board whose measurements you can take and compare to (and knowing that these measurements are only valid and comparable when using the same multimeter!), no way to say. And even with that, no guarantees.So no, I would not assume it is indicative of a short. If it were practically 0 ohms, then maybe. But with the 7.6 ohms you have… it’s hard to say without understanding the circuit. Could be normal, could be bad.

author: kraka
author_rank: Contributor
timestamp: October 19, 2024, 02:40:58 pm
content: Before i read your answer i was intended to re-solder resistors to the PCB and then de-solder the IC, to see if these readings will change.(Is not strange that only these two resistors, from a total of 50 or more, gave these readings ?)After your answer i am not sure that if i remove the IC and receive different readings, lets say the ones that should be (4700 Ω), this would mean that the IC have an inside short.Or if i receive the same readings (7.60 Ω and 40 Ω) this would mean that there is a defective component somewhere else.What is your opinion ?It is better for me, to hear some other opinions before de-solder the IC, because it will not be easy at all, especially if later needs to re-solder it back.

author: tooki
author_rank: Super Contributor
timestamp: October 19, 2024, 02:47:38 pm
content: Quote from: kraka on October 19, 2024, 02:40:58 pmAgain: impossible to say without adeepunderstanding of the circuit.Quote from: kraka on October 19, 2024, 02:40:58 pmSame thing: no generalization is possible. One needs to understand the circuit to be able to determine what measurements might make sense.We just had a huge thread here about this very problem a week or two ago with a repairman who refused to even attempt to understand this. In-circuit resistance measurements are a generally bad idea, and the numbers one gets are ONLY useful when you have reference values to compare to from a working unit, with the same test gear. (An in-circuit resistance measurement is not really a true measurement of resistance, it’s a sort of fantasy value based on how the circuit responds to the particular multimeter’s test current.)

author: kraka
author_rank: Contributor
timestamp: October 19, 2024, 03:11:14 pm
content: OK I have understood it. No measurements on board, for resistors.I have removed them, as i have already described in my Question, and these two resistors are not defective.Now i have a PCB with a component that is getting very hot and i do not have a schematic.Good nigh ...

author: tooki
author_rank: Super Contributor
timestamp: October 19, 2024, 06:00:50 pm
content: Troubleshooting is a process of elimination. You’ve now eliminated bad resistors as the cause of the problem, so it’s time to investigate other leads. I just don’t want you to think of resistance measurement as a good way to measure things other than resistors. (Yes, it is sometimes useful, but with big caveats.)Your overheating chip is likely due to one of three things that come to mind:- it’s getting too much voltage from the power supply (which is why “thou shalt check your power rails” is the first commandment in electronics troubleshooting) or too high voltage on a signal input- it’s got an excessive load on an output (load drawing too much current)- the chip has failed and is causing one of those problems to itselfHopefully those should give you some clues as to what avenues to check out.And see if you can find a schematic. They’re extremely helpful if available. Even better if there’s an actual service manual…

author: kraka
author_rank: Contributor
timestamp: October 19, 2024, 06:59:04 pm
content: OKI need a little more help.The third avenue is completely closed because, i cannot replace this IC. (not technically but physically).Two much Voltage from the power supply : You do not mean the socket in my wall... ? And the PSU of my PC is a relatively new one, except that all other parts of my PC is working with no problems.So if this is the case, there is something on the PCB that cause this high Voltage. The only thing i know is what pins of the PCI is for -12V,+12V,+5V,+3V,Ground,IOPWR. But i do not know, exactly, how to use them to trace the current flow. I have a device by which i can "inject" Volts to the PCI, but from there, except from the connections, i am not sure how to continue to check if the power rails have a problem. I never did it.For the excessive load on an output (load drawing too much current), what do you mean with load ? A device inside the IC (diode, resistor etc) or a device outside it, or both ? If it is a device outside of the IC, will it be heated up , too ?I have the schematic of this IC

author: 807
author_rank: Frequent Contributor
timestamp: October 21, 2024, 12:08:46 am
content: It will help if you gave us more information.What is the PCI card? Make/model?What is the number on the IC that's getting hot?Is it getting too hot to touch, or just warm?You say that you have the schematic of the IC. Measure the voltage on the supply pin. Is it within spec?

author: kraka
author_rank: Contributor
timestamp: November 07, 2024, 10:27:23 pm
content: 1) A friend gave me an identical PCI card which is functional, and the measurement on both resistors (marked 4.7K) really gives 4.7K.2) After removing the overheated PLD (on the faulty PCI card), I remeasured these two resistors and they both give 4.7K.Can we draw any conclusions?

author: tooki
author_rank: Super Contributor
timestamp: November 08, 2024, 04:27:07 pm
content: Not really. You were asked a bunch of specific questions and you ignored them. We are not clairvoyant. A couple of resistance measurements tell us nothing.The comparison between good and bad board simply gives you an idea of where the problem may or may not be. Where you get the same result on both boards, the problem likely is not.

author: kraka
author_rank: Contributor
timestamp: November 09, 2024, 10:21:00 am
content: Quote from: tooki on November 08, 2024, 04:27:07 pmI am sorry , i did not saw this questions. I never ignore somebody who try to help. But as i see, this questions made by 807, not by you.So my apology goes to 807 not to you. If 807 has a problem with me, I would like to discuss it with him, not with others. If his question is important in solving my problem (and it is) he knows better than anyone else and has both the mind and fluency to express it to me himself. No one is a clairvoyant here, just as no one is an interrogator here.But this information is no longer useful because the problem has been solved, as I will explain in my next reply.I would like to thank you for trying to help me.

author: kraka
author_rank: Contributor
timestamp: November 09, 2024, 10:30:03 am
content: Finally, after a long period of trying, the problem was solved. Although the original question in this post of mine is not directly related, it is indirectly related to the repair of a PCI card.This card was repaired by "transplanting" from a donor card.Α forum member gave me this idea and he really helped. It was very difficult to find such a similar card. I found it in an old shop and although it was sold only for spare parts, it gave me its PLD and replacing with it the short-circuited PLD of my own original card I made this card work again.

author: tooki
author_rank: Super Contributor
timestamp: November 09, 2024, 11:49:57 am
content: Quote from: kraka on November 09, 2024, 10:21:00 amI wasn’t seeking an apology for anyone. Just informing you that it’s difficult or impossible to help people who withhold basic information. (Which is a common thing, oddly. With some people, getting essential information is like pulling teeth.)You also misunderstand the dynamics of asking for help on a forum. When 807 asked for information, he was not asking forhim, he was asking foreveryonewho participates in the thread. It’s a fundamentally communal affair. Even though that is the baseline expectation, 807 even worded it as “It will help if you giveusmore information.”

author: kraka
author_rank: Contributor
timestamp: November 09, 2024, 03:08:46 pm
content: Quote from: tooki on November 09, 2024, 11:49:57 amThank you for your explanations. Next time we will be more careful so that we are all satisfied. Both us and you.From all of us, to all of you:Good Night.