My Uni-T 181A is behaving strangely...

Thread Starter

EmptyAtoms

Joined Mar 12, 2020
4
Hi All!

Long time listener, first time caller... So I'm studying electronics part-time, and part of our end of year course is a AD converter. So far the circuit is just a 5v power supply into a Schmitt Trigger, producing a clock signal for a 8-bit counter. All working normally.

8-bit counter feeds an R-2R ladder, this is here it gets hairy. I finished building the R-2R ladder and started testing resistances to ground. My 181A immediately started to confuse me. It shows me 2.5K Ohms between each input and ground.

Screenshot 2020-03-12 13.18.41.png


However it's producing a smooth ramp. I check for shorts or anything odd, for a long, long time. Til I put my Fluke 18B+ from China on the case, and it gives me expected readings.
Screenshot 2020-03-12 13.44.32.pngScreenshot 2020-03-12 13.39.28.png

(Exact same pin as above but with the fluke meter...)
Screenshot 2020-03-12 13.46.19.png


My aim now is to make sure this never happens again. I am bad enough at this without being lead astray by readings like these. So I need to figure out if the UT181A is faulty, or no good in these contexts. I would just love to kno hat's going on.

Thanks all! Your time and attention are hugely appreciated.

-James
 

Thread Starter

EmptyAtoms

Joined Mar 12, 2020
4
Take a 10kΩ resistor not in circuit and measure its resistance with your meter.
Thanks for the suggestion, Mr Chips. I did this. Particularly R3 got a lot of attention, in circuit, it gave me ~5k, so I desoldered it and measured again, I got 10k. This was all measured with the UT181.

I did test R3 again just now just to check, same story, on the 181 I get 5k, on the Fluke, I get 9.96k.

So the questionable readings only happen in circuit, and only happen on one meter.
20200312_141011.jpg20200312_141348.jpg
 

hexreader

Joined Apr 16, 2011
397
First thing that I would do is to remove the socketed ICs. Remove all possibility or suspicion that they ICs might be affecting the measurements in some way.
 

atferrari

Joined Jan 6, 2004
3,793
My UNI-T started recently to give readings higher than expected (voltages and resistance). No time to check but it seems because the battery is down.
 
Since you actually have 2 meters, look at the voltage used when measuring resistance. i.e. put one meter as a voltmeter and the other measuring resistance and vice versa.

Lower readings can occur if you turn on semiconductor junctions of connected parts.
 

Thread Starter

EmptyAtoms

Joined Mar 12, 2020
4
First thing that I would do is to remove the socketed ICs. Remove all possibility or suspicion that they ICs might be affecting the measurements in some way.
Okay. So without either IC I'm getting the expected 20-55k in 5k ohm increments between D0 to D8 and ground, with just the counter, which is a SN74HC590A, removed. Curious now why it affects the 181A There must be something happening there related to the circuits each meter uses to test resistance and the chip.

My UNI-T started recently to give readings higher than expected (voltages and resistance). No time to check but it seems because the battery is down.
Since you actually have 2 meters, look at the voltage used when measuring resistance. i.e. put one meter as a voltmeter and the other measuring resistance and vice versa.

Lower readings can occur if you turn on semiconductor junctions of connected parts.
Thanks for the suggestions. All of these were tested with 0 Volts on the board. With power, on the UNI-T the circuit tests as closed (0 Ohms) from all pins to ground, and on the Fluke; same points test as OL, open. At this point I'm super curious about how Fluke seem to have made a meter that gives accurate readings even when goofs like me don't factor out the counter IC as a path for current.

I'm now charging the UNI-T meter (having just dug out it's power supply). I'll see if that has an effect on the readings later.
 

hexreader

Joined Apr 16, 2011
397
EDIT: Doh.... :( KISS already said this in post 6. Note to self... read first, post after :(

Put Fluke on volts range and UniT on Ohms range. Connect +lead on Fluke to + lead on UniT, - to -

The Fluke is now reading the UniT's test voltage.

Repeat the above, but with "Fluke" and "UniT" reversed, so that Unit measures Fluke test voltage.

My guess is the UniT test voltage is higher - making it a better diode detector, and a worse in-circuit resistance tester
 
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Thread Starter

EmptyAtoms

Joined Mar 12, 2020
4
EDIT: Doh.... :( KISS already said this in post 6. Note to self... read first, post after
I appreciate you re-stating it I wasn't aware the suggestion was to connect the meters to one another. Thanks KISS, excuse my misunderstanding. Ignore my last reply.

My guess is the UniT test voltage is higher - making it a better diode detector, and a worse in-circuit resistance tester
The 181A gave me a -0.641 Volt reading, Fluke gave 0.280... I didn't change the leads around or anything, though. Don't know if I should have? Interesting if it's testing resistance the opposite way around.

Screenshot 2020-03-12 16.58.05.png
Screenshot 2020-03-12 16.57.49.png

Diode checker mode; 181 takes it by ~0.72 volts.
20200312_164149.jpg
Screenshot 2020-03-12 17.17.39.png
Resistance measurement rule
Any resistance measurement of a component while still in-circuit will give questionable readings.
Makes perfect sense. Thank you Mr.Chips for all the help. :)
 
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