why resistor is open in this circuit

Thread Starter

Benengineer

Joined Feb 6, 2016
133
Hello, can anyone help me with resistor issue in this circuit
Please see resistor R149, the value is about 1K. However, when I turn on the power, the resistor is open when I measure it with multi-meter. Does anyone know the reason?
The other circuit for LINE, LINE_AFT_SHUNT and NEUTRAL is below.


Thank you for your help.
 

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#12

Joined Nov 30, 2010
18,210
You are not supposed to try to measure ohms with the power on.
Turning the power on doesn't magically make the resistor turn into infinity ohms and turning the power off doesn't magically make the resistor 1K again.
 

Thread Starter

Benengineer

Joined Feb 6, 2016
133
You are not supposed to try to measure ohms with the power on.
Turning the power on doesn't magically make the resistor turn into infinity ohms and turning the power off doesn't magically make the resistor 1K again.
Thank you. I mean its value is Mohms, not 1K when the power is off. I guess the esd damage the resistor. Am I right?
 

#12

Joined Nov 30, 2010
18,210
Resistors are mostly immune to electrostatic discharge, but if it measures as too much resistance with the power off, it's wrong. More likely is you have a mistake in something else.
 

RichardO

Joined May 4, 2013
2,271
It seems to me the next questions should be: "Why did the resistor fail?". I am guesing there is something wrong in the "LINE_AFT_SHUNT" circuit.

edit:
And the next question is: "What else has failed?"
 

ian field

Joined Oct 27, 2012
6,539
You are not supposed to try to measure ohms with the power on.
Turning the power on doesn't magically make the resistor turn into infinity ohms and turning the power off doesn't magically make the resistor 1K again.
My first ever DMM was provided by someone doing that.

A Hung Chang obscure model that I rescued from the bin, the resistance and ACV ranges were dead.

Nothing to lose I opened it up - a complex looking LSI chip and a socketed op-amp - take a wild guess at what I tried next.

There was a preset pot on the board, so I borrowed a meter with a recent calibration certificate and made sure the restored ranges were at least somewhere close to where they should be.
 
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