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Measuring ripple with digital multimeter

Snafu

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#1
does this work "close enough" to get idea if power supply is good (low ripple) ?

"
A ripple voltage is a small AC voltage, which is placed on top of a DC offset. It can be measured using a digital multimeter.

Plug the probes into the digital multimeter. Two probes are normally supplied. Plug the red probe into the positive terminal and the black probe into the negative terminal. Switch on the digital multimeter by turning the dial on the front panel clockwise.

Select the "AC Voltage" by turning the dial on the front to the picture of an oscillatory wave. Bring the probes into contact with the circuit that has the ripple voltage. The multimeter will only measure the AC component of the signal -- i.e., the ripple voltage.
"
https://sciencing.com/read-ripple-voltage-meter-10017535.html

thanks
 

mansr

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#2
That depends on the multimeter. It works with my Fluke, but it also has a mode for measuring DC and AC components simultaneously which is much more useful. A cheap, nasty meter gives totally outrageous readings. It's best to try with a known source first.
 

sergeauckland

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#3
That won't work for an analogue meter, or mansr's 'cheap, nasty meter'. What should work is a capacitor in series with the meter to block the DC component and leave the AC ripple. A 10uF non-polar capacitor of a suitably rated voltage should work, the higher the capacitance the better. If you can't get a non-polar capacitor, that's also OK, but you must make sure the polarity of the capacitor is respected.

10uF has an impedance of 317 ohms at 50Hz, 158 ohms at 100Hz, 132 ohms at 120Hz, so in series with a meter as long as the meter's resistance is more than 1K, it won't result in a serious error, and it won't be a problem if you're doing comparative measurements rather than absolute.

S.
 

DonH56

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#4
For low-ripple supplies a DMM might not read correctly due to meter accuracy and noise picked up by the leads. You can twist the leads together to reduce noise pickup but that may not be enough. Also, for SMPS (switch-mode power supplies), the ripple bandwidth may exceed the meter's measurement bandwidth, especially for inexpensive meters. And finally note where the ground is placed is important; measuring ripple across a decoupling capacitor is usually best. Don't just stick the (-) lead to some remote ground point -- even a small loop can corrupt the readings.

I use coax and either a differential probe or a spectrum analyzer to measure power noise since I am usually looking wideband (DC to 10 GHz) and the noise floor is usually in the 10~100 uV range (integrated noise in the mV range, from a SMPS). Too low for a 'scope to be accurate, too wideband for a DMM.
 

sergeauckland

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#5
For low-ripple supplies a DMM might not read correctly due to meter accuracy and noise picked up by the leads. You can twist the leads together to reduce noise pickup but that may not be enough. Also, for SMPS (switch-mode power supplies), the ripple bandwidth may exceed the meter's measurement bandwidth, especially for inexpensive meters. And finally note where the ground is placed is important; measuring ripple across a decoupling capacitor is usually best. Don't just stick the (-) lead to some remote ground point -- even a small loop can corrupt the readings.

I use coax and either a differential probe or a spectrum analyzer to measure power noise since I am usually looking wideband (DC to 10 GHz) and the noise floor is usually in the 10~100 uV range (integrated noise in the mV range, from a SMPS). Too low for a 'scope to be accurate, too wideband for a DMM.
For me this comes into the category of 'if it's too low to measure, it doesn't matter any more'. ;) I appreciate completely that for some applications it does matter, but I suspect that the OP's question is more as a rough and ready indication of 100 or 120Hz ripple on a conventional sagging supply rather than a low noise stabilised or SMPS.

On balance, I think I would rather measure 100Hz ripple with an analogue multimeter rather than a digital meter as the mechanical movement does a sort of averaging of the waveform whilst a digital instrument's sampling point may be different and therefore not work well with the sawtooth ripple waveform.

S.
 

solderdude

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#6
Here is another tip....

Build this
schak 2.png


and instead of connecting the C'Moy connect it to the input of your soundcard and use a freeware oscilloscope program like this and you can see the waveform as well and look at very small amplitudes as well.
 

DonH56

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#7
For me this comes into the category of 'if it's too low to measure, it doesn't matter any more'. ;) I appreciate completely that for some applications it does matter, but I suspect that the OP's question is more as a rough and ready indication of 100 or 120Hz ripple on a conventional sagging supply rather than a low noise stabilised or SMPS.

On balance, I think I would rather measure 100Hz ripple with an analogue multimeter rather than a digital meter as the mechanical movement does a sort of averaging of the waveform whilst a digital instrument's sampling point may be different and therefore not work well with the sawtooth ripple waveform.

S.
Sure, agree on all the above (natch). Including watching the analog (analogue) meter wiggle. I wish I still had my old HP rms voltmeter.

One other general caution (not to you): many of the cheaper meters target just power-line measurements and have very limited (like 100 Hz) bandwidth. It reduces the noise for measuring the power line, but is inadequate for measuring higher-frequency ripple (or signals -- how many times have we explained why someone's $5 special DMM is not reading accurately at 1 or 20 kHz?)
 

sergeauckland

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#8
Here is another tip....

Build this
View attachment 20611

and instead of connecting the C'Moy connect it to the input of your soundcard and use a freeware oscilloscope program like this and you can see the waveform as well and look at very small amplitudes as well.
What is a C'moy? I've searched for the term, but can't find anything other than the former Director of the US Mint. o_O

As to freeware oscilloscopes, I too use Soundcard Scope. It's a really useful tool.

S.
 

Snafu

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#9
Thank You for the replies, i have basic digital meter ( UNI-T UT33D ~18€ ) to measure power amps tube bias and i came across that page above.

To me it's enough if i can compare which my 5V wall adapter power supply has lowest ripple to be used with D50.

Thanks again
 

solderdude

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#10
What is a C'moy? I've searched for the term, but can't find anything other than the former Director of the US Mint. o_O

As to freeware oscilloscopes, I too use Soundcard Scope. It's a really useful tool.

S.
the standard opamp amplifier which Chu Moy made well known.
 

solderdude

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#11
Thank You for the replies, i have basic digital meter ( UNI-T UT33D ~18€ ) to measure power amps tube bias and i came across that page above.

To me it's enough if i can compare which my 5V wall adapter power supply has lowest ripple to be used with D50.

Thanks again
Most of the ripple on wallwarts is > 20kHz with some of it around 100/120Hz.
The higher ripple is filtered out by the D50 input filter (which it is bound to have)
The soundcard won't show this ripple unless it can sample at over 192kHz.
Those 5V adapters are sort-off regulated devices.
 
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