# In circuit debugging using continuity test on DMM give un-predictable results

I did a continuity test, between a ground point of my PCI card (audio) and a resistance (47Ω SMD). No beeping sound on both ends of the resistance. After I de-soldered the resistance from the board, I received a beeping on one end of the mounting metal plate (where the resistance was soldered). Can somebody explain, what exactly could have produced this behavior? An example diagram would be the best for me to understand.

• Did you check carefully for a solder splash or bridge from your de-soldering that's shorting the pad to ground? In-circuit resistance tests can sometimes fool you due to non-linear junctions and capacitances. Aug 21, 2023 at 21:02
• You tried continuity test on an unknown circuit and it did not beep. Then you changed the circuit by removing a resistor and it beeped. You don't know what the circuit that you are probing does and how it works and you also don't know how your meter determines when to beep or not to beep. We can't draw a diagram of your sound card as we don't know what resistors there are, what they do and which one you removed. It would be easier if you posted photos or diagrams what you did. Aug 21, 2023 at 21:24

## 1 Answer

Your issue is very simple. Here is the explanation I gave previously to someone else with a very similar issue: https://electronics.stackexchange.com/q/676251

In short, your measurement are invalid. You can't measure in circuit resistance.

Edit: Since you improve your question, I'll improve my answer! How does the continuity works on a DMM. There are more then one way they operate, but the most common one is, just like the diode, a constant current source that when the voltage drops bellow a certain value, it beeps. So, on the strange behavior, when it doesn't beep, that means that the voltage on the terminal don't get low enough. Your schematic is clearly oversimplifying and the reason it doesn't beep is hidden there. There is no faulty component in parallel that would prevent a short in DC. So that measurement is invalid.

On the next one, it could beep for a number of reason. One of them is that you are powering your circuit, or that a component failed due to the higher then rated current.

Basically, beep beep test should be ONLY used when you have a bare board. Any thing else will create odd behavior because you can't predict how semiconductors will behave while not powered. Never debug any circuit with an ohm meter or beep beep. It's a bad practice that beginner often use and lead them to false conclusions.

You should power the board and measure the behavior with voltage and current, not ohms.

• The resistance is 47Ω. My multi-meter gives a beep on resistance lower than 30Ω. If i understand your explanation, it means that when i made the first continuity test between GND and the end of the resistance that connected to the ground, multi-meter must have being beeping (because the wire which connect the resistance end with GND has less than 30Ω resistance) but because there are active components in the card, the Ohm's low R=V/I , does not give the right result (<30 Ω), because the Volt provided by multi-meter (and used by it in the calculation of R) is not the one that have produce... Aug 21, 2023 at 23:43
• that have produce the Current that the multi-meter measure (in order to use it in the calculator of R), because there are several amplifications due to existence of active components (the same if the multi-meter provide current and measure Volt) ? But how this can explain the continuity test, after i have removed the resistance from the board ? Why, then i have a beeping between the GND and the one metallic pad ? Does not happen the same "missunderstanding" from the multi-meter due to the existance of active components that amplify the currents or Volt ? Aug 21, 2023 at 23:43
• Sorry. I added my two previous comments before i read your new edit. Now i have no time to read it. When i will read it, and if you have already answered my last comments i will delete them. Aug 21, 2023 at 23:48
• Sorry if I was unclear. The behavior you are seeing is indeed unpredictable because you are using a function of a DMM that CAN'T be used in circuit. Any result it will give you will be trash. You should NEVER use a ohm-meter or continuity tester with semiconductor components. In circuit debug can ONLY be done with a voltmeter or ammeter. Any other function will give you unpredictable results. Aug 22, 2023 at 0:37
• I could figure out why your DMM beep, but what's the point! I will only find the reason why it's invalid! Just the polarity of your probe could make it beep or not! Aug 22, 2023 at 0:39