Did you ever get to the bottom of this problem? (I have the same issue with a Fluke 75)

Mod Note:
This thread moved from -- https://forum.allaboutcircuits.com/...ts-and-counts-when-measuring-low-ohms.186882/

 

Welcome to AAC.

Please describe the issue you're having. You MIGHT have the same exact problem or you might have a similar problem. Or it could be in the way you're using it (not suggesting you don't know how to use a meter, but from time to time we do get people in here who don't know which end of a lightbulb screws into the socket. OK, that's over the top. But we need to be confident you're doing things right.

Also, that thread was more than 2 years old. It's possible the original poster or thread starter (TS) may not be looking back and might not answer your question. The TS hasn't been seen since Aug 14, 2023.

 

After reviewing some of my comments it's possible you have a weak battery. It's possible you might be testing a capacitor on the ohms scale. In that case the numbers will increase until it goes over limit ( OL ).

 

A weak battery is not my problem. It is 9.40V, and only drops 10mV when I try to measure the resistance of a short circuit. The resistance drops from 50-60 ohms over time (but I've never waited long-enough to see if it gets to zero)

 

9V drops to 0.01V when you measure a short circuit? Measuring voltage on a short circuit is like measuring the voltage of a nail point to the nail head. It's a dead short. That you see 10mV suggests you're reading the resistance of the test leads themselves.

My guess is that you're not making good contact with the device you're measuring. I've also seen poor connections INSIDE the meter where the test lead sockets are soldered to the PCB. Fractured solder joints can cause all kinds of problems with measurements.

I had a neighbor who's sprinkler controller quit working. The failure was in the selector switch where the switch was soldered to the PCB. Broken solder joints are an often overlooked problem.

 

You have misunderstood. The battery voltage drops *by* 10mV (ie from 9.40V to 9.39V) (not drops *to* 10mV).

 

A weak battery is not my problem. It is 9.40V, and only drops 10mV when I try to measure the resistance of a short circuit.

A short circuit such as Pan describes should drop the reading all the way to zero volts. Since you're trying to measure the resistance of a short then I can predict that the resistance should very quickly read zero ohms, or at least very close to zero ohms. Your test leads themselves have a very small resistance which the meter should already take account of.

One thing confusing me is the statement that when you're measuring the resistance "the voltage drops 10mV". You shouldn't be reading voltage at all when measuring resistance. Assuming 9.4V dropping to 9.39V suggests a very high resistance. So now it's my turn to ask a dumb question: Are you using the meter to test for ohmage using the ohms scale or the volts scale?

 

I'm sorry, I'm not explaining myself well at all.

I am trying to measure a short-circuit using the resistance setting.

There was a lot of talk about weak meter batteries being a problem because the meter uses more current when trying to measure a short circuit. I then used another meter to measure the battery voltage of the Fluke 75 when I shorted the probes together. The Fluke 75's battery voltage dropped from 9.40V to 9.39V when I shorted it's leads together (on the resistance setting).

This experiment was meant to demonstrate that a weak battery is not the problem by Fluke 75 is suffering from when producing a short circuit resistance reading of 60-or-so ohms that slowly drops. (It is not the leads either - I have shorted the connection directly on the circuit board.)

 

So you're shorting the leads while testing the test meter's battery? Why? If I understand you correctly, it makes sense. You're sending current down one lead and back on another. The battery in the meter IS going to drop slightly. Still having trouble figuring out what you're doing. So is the meter set properly? The meter is set to what? What is the meter set to? What are you testing? A short circuit? Why? If it's a short circuit then its resistance is basically zero ohms. Not volts, OHMS.

Please be very clear about what you are setting the meter to and what exactly it is you're testing.

 

I am shorting the leads while testing the meter's battery because some earlier post speculated that while measuring a low resistance, the meter uses more current and the voltage of a weak battery might fall to the point where the meter is not functioning correctly. This test was to demonstrate that this is not the problem in my case - the unit's battery barely shifts while measuring a short circuit.

I am trying to test for a short circuit because I am probing tracks for continuity on a printed circuit board (ie I am looking for a short circuit) - and discovered that the meter is not measuring a sensible value (ie less than 1 ohm) when I measure a short circuit - it might start at 60 ohms or more and then slowly start falling in value.

I have a Fluke 75 and I am setting it to the resistance measuring switch position (marked with an ohms symbol).

 

...meter is not measuring a sensible value (ie less than 1 ohm) when I measure a short circuit - it might start at 60 ohms or more and then slowly start falling in value.

The battery is a likely culprit, but it's possible that it might be a failing test lead. Do you have a different set of test leads to try?

 

As per my tests, the battery voltage stays around 9.4V, so it can't be the battery. I have shorted the test lead sockets on the PCB directly - ie it's not the leads.

 

Have you made sure that the PSU is effectively disconnected from the circuit before reading? Or are you trying to measure reasistance with power connected?

 

I am a retired electronic engineer with 40+ years experience.