The Uni-T UT139C and my power supply ripple

[Martini]

I thought I could use the AC range of my multimeter to check for any ripple on the output of a power supply (which I'm working on). Turns out it's not as simple as I thought:




And the difference worsens if I increase the voltage:





I also tried a frenquency measurement, I get 0 Hz (100 Hz on the scope, which is logical since main is 50 Hz here).
All this with no load. It's worse with one.



So, what's your take on that?

[newbrain]

That's a known "feature" of some True RMS DMM, any DC offset will make the measure more or less useless.
Other DMMs might give only the AC RMS value, and still others have a true AC+DC RMS mode (not what you are looking for).

I think your UNI-T falls in the first category...as does mine (61D).
My Fluke 87V*, instead, should ignore the DC offset and give just the AC RMS.
They internally use the same True RMS converter (AD737), the difference is in the way it's coupled to the inputs.

See, for example,  this tread, where more complete explanations are given, and other UNI-T models are also discussed.

A large enough capacitor in series will allow you to read the ripple RMS value by blocking the DC component.

*Brag mode on: "Yes, my other DMM is a Fluke" 

[macboy]

The UT139C is known to be based on the DTM0660 IC. To measure AC RMS, this IC samples the signal and calculates the RMS value. The input signal is DC coupled even when on an AC range, and the DC offset is removed from the reading digitally. This means that in order to accurate measure AC ripple, you need to manually set the range so that the DC voltage is within the range of the meter. Only then can it sample without clipping. The problem with that is that the ripple may only be a few digits of this high range, making the reading rather worthless. To measure small ripple on a large DC voltage, you will need to AC couple the input. The high input impedance of the meter (10 Mohm) means that only a small capacitor is needed. You may get better results too if you shunt a 1 Mohm or even 100 kohm resistor across the input to reduce the input impedance. Some experimentation might be necessary.

[mos6502]

Had the same problem:

https://www.eevblog.com/forum/testgear/ut139c-ac-volts-weirdness/

Lesson learnt, stay away from Uni-Turd.

[Martini]

The UT139C is known to be based on the DTM0660 IC. To measure AC RMS, this IC samples the signal and calculates the RMS value. The input signal is DC coupled even when on an AC range, and the DC offset is removed from the reading digitally. This means that in order to accurate measure AC ripple, you need to manually set the range so that the DC voltage is within the range of the meter. Only then can it sample without clipping. The problem with that is that the ripple may only be a few digits of this high range, making the reading rather worthless.

This is most interesting, thanks!
(But that's because the input is DC coupled on the AC range, not because the chip is a DTM0660, right?)

To measure small ripple on a large DC voltage, you will need to AC couple the input. The high input impedance of the meter (10 Mohm) means that only a small capacitor is needed. You may get better results too if you shunt a 1 Mohm or even 100 kohm resistor across the input to reduce the input impedance. Some experimentation might be necessary.

Where would one learn more about AC coupling caps (how to choose the size)?



mos6502, I'm sorry you had to learn it the hard way.