RESISTOR ON BOARD VALUES - Page 1
Forum section: electronics / projects

author: kfc_n
author_rank: Contributor
timestamp: May 20, 2014, 03:34:52 am
content: Good day,

author: kfc_n
author_rank: Contributor
timestamp: May 20, 2014, 03:35:40 am
content: Good day,

author: kfc_n
author_rank: Contributor
timestamp: May 20, 2014, 03:37:26 am
content: Sorry for the previous post, by the way, I would like to ask, what could be the culprit in my circuit.. Whenever I mount the resistor on its pad in PCB, its value changes, from 10k to 6ohms respectively, anyone knows why? thanks...sorry, newbie here

author: TVman
author_rank: Frequent Contributor
timestamp: May 20, 2014, 03:53:07 am
content: Broken resistor?

author: Richard Crowley
author_rank: Super Contributor
timestamp: May 20, 2014, 03:53:36 am
content: The most likely cause is the difference between measuring the ISOLATED resistor gives you its actual value.But when you put it into the circuit, the resistance from one end to the other is affected by the rest of the circuit.In general, "in-circuit" measurements of components are rarely the same as isolated measurements.Do you have the circuit diagram ("schematic") for your board?P.S: You can go back and delete the false-start messages. There should be a link in the upper right corner that says "Remove"

author: TVman
author_rank: Frequent Contributor
timestamp: May 20, 2014, 03:57:10 am
content: Quote from: Richard Crowley on May 20, 2014, 03:53:36 amOh I didn't think of that....I'm still learning.

author: TheBorg
author_rank: Frequent Contributor
timestamp: May 20, 2014, 04:02:59 am
content: Quote from: TVman on May 20, 2014, 03:57:10 amHappens all the time in computer repair. Pain in the rear actually, because it means removing everything to find a problem, and if you've already removed it, it's just easier to replace it!

author: TVman
author_rank: Frequent Contributor
timestamp: May 20, 2014, 04:10:48 am
content: Quote from: TheBorg on May 20, 2014, 04:02:59 amSo true, so true........

author: kfc_n
author_rank: Contributor
timestamp: May 20, 2014, 04:45:43 am
content: Quote from: Richard Crowley on May 20, 2014, 03:53:36 amI'll post the circuit later, by the way, it is like 18 inputs with each input has optocoupler(transistor) with pull up on input resistor and output..only one pullup resistor from the output side of optocoupler changes "in-circuit" resistance change. I have tried changing the optocoupler together with its pullup resistors with a new one..still, there is an in-circuit resistance change.. I even tried looking for shorted patterns in PCB, haven't found any.

author: kfc_n
author_rank: Contributor
timestamp: May 20, 2014, 05:04:51 am
content: Attached is the input of schematic diagram, wherein I am having problems..... All optocouplers output "S" is 5V, but S0 in IN1, the ouput is only 2V.As per the resistor values R28-R32, it is constant at 10k ohms, while R27 drops to 6 ohms.... any ideas?thanks...

author: Richard Crowley
author_rank: Super Contributor
timestamp: May 20, 2014, 05:29:44 am
content: My first guess would be that U2 is bad (that output transistor is nearly shorted, etc.)Can you swap U2 and U3 to see if that is the problem?

author: kfc_n
author_rank: Contributor
timestamp: May 20, 2014, 05:44:41 am
content: I tried replacing U2 and R27 new components, still having in-board resistance change... and the output of optotransistor is still 2V, ideally, must be 5V when, input varies it would drop to 0V, will invert voltage. 3.3V-5V is the high state voltage for the output while less than that is low state...meaning my U2 is always a low state.

author: kizzap
author_rank: Unknown
timestamp: May 20, 2014, 09:47:47 am
content: what is the other side of the resistor connected to?I assume you are measuring across the resistor only? or straight to ground? Is there a capacitor attached to the bus anywhere? 6 ohms is in the range of there being a short across something. If the thing is driven by an IC, there is a chance you have blown the output transistor, thus giving you the extremely low resistance

author: David_AVD
author_rank: Super Contributor
timestamp: May 20, 2014, 10:55:45 am
content: Are you trying to measure resistance with power still applied to the circuit?

author: retrolefty
author_rank: Super Contributor
timestamp: May 20, 2014, 11:15:48 am
content: Quote from: TheBorg on May 20, 2014, 04:02:59 amThat is because replacing or removing and measurement of random components is rarely a proper troubleshooting method. Even if a specific component like a resistor is visually FUBAR, there is some reason excessive current was forced through the resistor. Effective and efficient component level troubleshooting requires proper test equipment, proper documentation, and circuit understanding.

author: tszaboo
author_rank: Super Contributor
timestamp: May 20, 2014, 11:27:16 am
content: Quote from: TVman on May 20, 2014, 03:57:10 amThat is why it is wise to listen to other people, ask, and when you acquired enough knowledge to answer other peoples question, only answer them then. Actually when you start learning engineering, there is an oath, not to accept any job (answering question in this case) which doesn't match your capabilities.