# Measuring Ripple???

Discussion in 'Electronic Basics' started by [email protected], Jun 7, 2006.

1. ### Guest

I was wondering if there were any way to measure ripple with a DMM or
something? I built a power supply (AC to DC conversion) from a
transformer, full wave rectifies, some capacitors etc... and want to
measure the ripple Any ideas? Thanks, Lucas...

2. ### The PhantomGuest

Which ripple? Ripple voltage on the reservoir capacitor? Ripple current
in the reservoir capacitor? Do you have an oscilloscope? Does your DMM
measure true RMS voltage and current?

I'm sure the people here will have lots of ideas.

3. ### John FieldsGuest

---
Usually, you use an oscilloscope. If you don't have one you can
measure the AC voltage on the DC output of your supply by putting
your DMM on "AC VOLTS" and multiplying what you read by 2.83.

It won't be perfect because the ripple waveform won't be a sine
wave, but it'll be close.

For example, I measured 0.443VRMS (which is 1.25VPP) using a true
RMS meter, and my scope gave me 1.3VPP. Pretty close...

You can also calculate what it'll be by using:

It
Vr = ----
C

Where Vr is the ripple in volts,
I is the load current in amperes,
t is the period of the ripple waveform in seconds, and
C is the capacitance of the reservoir cap in farads.

Let's say that you've built a 12V supply using a transformer
connected to 60Hz mains, a full-wave rectifier and a 10000µF
reservoir capacitor, that you're feeding its output into a 12 ohm
resistor, and that you measured the voltage across the resistor and
found it to be 12V. That means the current into the resistor will
be:

E 12V
I = --- = ----- = 1 ampere
R 12R

Or, you could measure the current directly with your DMM.

In any case, you now know what the load current is and you have what
you need to figure out what the ripple is:

It 1A * 8.33E-3s
Vr = ---- = --------------- = 0.833 VPP
C 1E-2F

That 8.33 milliseconds is because full-wave rectified 60Hz makes
120Hz ripple.

4. ### Guest

Thanks for all the great answers...I really appretiate it. Lucas

5. ### DecaturTxCowboyGuest

Or a high impedance analog voltmeter where the analog meter movement
responds to true RMS.

7. ### DecaturTxCowboyGuest

The RCA "Voltohmist" was reincarnated as the VIZ with solid state
electronics. Had mine for 30 years and still accurate.

8. ### Phil AllisonGuest

"DecaturTxCowboy"

** Huh ?

Analogue volt meters with *moving iron* movements are scarce as hen's
teeth.

........ Phil

9. ### Phil AllisonGuest

"DecaturTxCowboy"

** Ain't " true rms ".

....... Phil

10. ### Phil AllisonGuest

<

** Just use your DMM on AC volts. .

You may need to connect a cap ( 0.1 uF ) in series with the meter.

Multiply the reading by 3 to give the peak to peak ripple voltage.

........ Phil

11. ### DecaturTxCowboyGuest

Perhaps I should have said "RMS" instead of "true RMS".

12. ### Phil AllisonGuest

"DecaturTxCowboy"

** Bout the same difference between a gal being "plump" and "
pregnant".

...... Phil

13. ### DecaturTxCowboyGuest

Still not quite sure what you are saying.

14. ### The PhantomGuest

Perhaps he's not quite sure what *you* meant by:

'Perhaps I should have said "RMS" instead of "true RMS".'

I'm not either. What's the difference?

15. ### Phil AllisonGuest

"The Phantom"
DecaturTxCowboy
Phil Allison wrote:

** An "rms responding" meter is also "true rms" - such meters indicate
the rms value of any continuous waveform within the frequency limits of the
meter.

However, most AC meters are "average responding" types that have been
calibrated to read the *rms* value of a sine wave. Such meters do not
correctly read the *rms* value of other AC waveforms.

......... Phil

16. ### The PhantomGuest

Actually, I already know this, Phil. I was just trying to find out if
DecaturTxCowboy does.

17. ### DecaturTxCowboyGuest

To me, the RMS voltage is the amount of rectified voltage that can heat
up a resistor, move an analog meter movement, etc., in other words, the
voltage under the curve that can actually do work.

What I am not sure of is - Is Phil saying that an analog meter cannot
correctly read the "heating equivalent" of an AC waveform?

18. ### The PhantomGuest

It's not the "voltage under the curve", but the "square of the voltage
under the curve" that does the heating (speaking somewhat loosely, you
understand).
What he is saying is that a (typical) analog meter movement's needle will
be deflected (respond) proportional to the *average* current applied. The
"typical" movement in such meters is the classic D'Arsonval moving coil
movement. When measuring AC, the input voltage is rectified (usually
full-wave) and applied to a multiplier resistor in series with the meter
movement, which then responds to the average current in the movement. To
get the meter to read correctly on AC, the assumption is made that such
meters will usually be measuring a sine wave derived from the power grid.
The ratio of a full-wave rectified sine wave RMS value to its average value
is used as a multiplier in the calibration of the meter.

Thus, the meter will read the correct RMS value of an AC sine wave, but
will be in error if the input waveform is not a sine wave. If, however,
the meter movement weren't a D'Arsonval movement, but rather what is known
as a "moving iron" movement, the meter would read correctly on any AC
waveform (within the frequency limit of the movement), because moving iron
meters inherently respond to the true RMS value of the applied waveform.
But I don't think I've ever seen a high impedance voltmeter using a moving
iron movement. Maybe Phil knows if any such have ever been manufactured.

The ripple voltage on a reservoir capacitor in a power supply is
typically not a sine wave, so a meter whose movement is a D'Arsonval type
won't read the correct RMS value.

converter chip in it rather than just a full wave rectifier, and it would
then read correctly any AC waveform, even with a D'Arsonval movement. But
RMS to DC chips weren't available in the days of analog meters, so those
older analog meters only read the value of an AC voltage correctly if its
waveshape is sinusoidal.

19. ### DecaturTxCowboyGuest

Yep...the the voltage that does the work is "over the curve" to put it
another more inclusive way.
OK, that's what I thought he was saying, but something made me think for
a minute he might have been referring to something I was not even aware of.
Scientific Atlanta rings a bell. And maybe Edmunds had one.
Like the biz end of an SCR controller is not a sine wave, then all bets
are off.

Bottom line, we're on the same page...just a different way of saying it
with more or less artistic over simplification.

20. ### Phil AllisonGuest

"DecaturTxCowboy"

** I certainly was.

YOU were NOT aware of the correct meanings of "true rms" or "rms
responding" .

....... Phil