Measuring Power Supply Ripple Percentage Accurately

Thread Starter

Dynaman

Joined Jan 17, 2008
94
Good day to all AAC'ers:

I am trying to rule out power supply noise bleeding into my circuitry, so as a side task, I want to verify how good my power supplies are. I found this link but I would like someone to verify I did the measurements and calcs correctly:

https://electricalbaba.com/ripple-factor-definition-formula-derivation/

Step 1- Used a 5.5 digit DMM, and measured the DC ouput. For this example, it measured: 12.11 VDC.
Step 2- Used a 5.5 digit DMM and verified with a scope the the A/C component which measured: 230uV rms. Quite low me thinks.

I calculated the ripple factor (not percentage):

upload_2019-4-4_10-21-44.png

Answer for Y=.99999999918

So in the article it says to convert the factor to %, simply multiply by 100. Okay I did...
So now: 99.999999918 %

So that is telling me my ripple is 99.99.........%? I don't think so, it most likely tells me that the percentage is that much in DC.

Now I have to admit in my calculations, on the top line of the equation, the number comes out negative, so I square rooted the absolute value

Where did I go wrong? How do I calculate the actual ripple in percent?

Help very much appreciated.
 

AlbertHall

Joined Jun 4, 2014
10,258
From the link you gave:
Aramsays:
April 4, 2019 at 11:08 pm

Hello:

I am trying to rule out power supply noise bleeding into my circuitry, so as a side task, I want to verify how good my power supplies are. I found this link but I would like someone to verify I did the measurements and calcs correctly:

Step 1- Used a 5.5 digit DMM, and measured the DC ouput. For this example, it measured: 12.11 VDC.
Step 2- Used a 5.5 digit DMM and verified with a scope the the A/C component which measured: 230uV rms. Quite low me thinks.

I calculated the ripple factor (not percentage):

(See last ripple equation you provided above)

Answer for Y=.99999999918

So in the article it says to convert the factor to %, simply multiply by 100. Okay I did…
So now: 99.999999918 %

So that is telling me my ripple is 99.99………%? I don’t think so, it most likely tells me that the percentage is that much in DC.

Now I have to admit in my calculations, on the top line of the equation, the number comes out negative, so I square rooted the absolute value

Where did I go wrong? How do I calculate the actual ripple in percent?

Help very much appreciated.

Reply
  1. adminsays:
    April 4, 2019 at 11:32 pm
    carefully read the definition of ripple factor. It says, it is the ratio of rms value to the average value. Here rms value is the root mean suare value of rectifier output which is having DC and AC mixed. You used DMM in AC mode and measured 230uV, so this reading is not the rms value of the output rather it is rms value of ac component present in the output. Your ripple calculation should be,
    Ripple factor = (230uV/12.11)x100 = 0.0019%.
 

crutschow

Joined Mar 14, 2008
25,274
The stated ripple definition is:

The top definition seems correct but the bottom equation doesn't follow from that, nor the equation you used.
I don't understand why the RMS voltage and current are squared.:confused:

So I would just use the RMS value of the ripple divided by the average (DC) output voltage.
That would give a ripple factor in your case of 230μV / 12.11V = 19e-6 or 1.9e-3%
 

danadak

Joined Mar 10, 2018
4,057
Did you measure the ripple at line frequency or for all noise ? Ripple is
just, I think, supposed to be line freq component of output.

Then how did you verify 230 uV with a scope ? What scope and DMM were you
using ? Model numbers.


Regards, Dana.
 

Thread Starter

Dynaman

Joined Jan 17, 2008
94
"carefully read the definition of ripple factor. It says, it is the ratio of rms value to the average value. Here rms value is the root mean suare value of rectifier output which is having DC and AC mixed. You used DMM in AC mode and measured 230uV, so this reading is not the rms value of the output rather it is rms value of ac component present in the output. Your ripple calculation should be,
Ripple factor = (230uV/12.11)x100 = 0.0019%."

Thank you admin for the explanation. Negating your equation for the sake of argument, are you saying that for my equation above, Vrms should be (230uVrms+Vdc)? If that is correct, then my calculation is .61% ripple. And if you agree that it is correct, then is it a more accurate equation than the one you provided?

I guess the next question

Danadak: DMM Tek 4050, Scope Tek TDS-320 (averaging mode rms reading)
 
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