Just a simple one. How do you measure the ripple on the output of a capacitor smooothed PSU? Hopefully the ripple voltage will be in the millivolt range but the rail voltage will usually be several tens of volts. Can you do this with a DMM or do you need a CRO?
G'day, mate 😉
Whichever way you want. Just make an AC measurement. If you're not certain about the capabilities of your measuring device, put a capacitor of maybe 100uF or something in series, charged to the PSU output voltage. With the fairly high input impedance of your measurement equipment, you'll have no problems measuring the AC ripple fairly accurate.
Potential problem is that not all DMM's measure "True RMS", but assume that they're seeing a sine shape AC, which is by no means the case for this type of measurement. this gives a faulty read-out of the signal.
Jennice
Whichever way you want. Just make an AC measurement. If you're not certain about the capabilities of your measuring device, put a capacitor of maybe 100uF or something in series, charged to the PSU output voltage. With the fairly high input impedance of your measurement equipment, you'll have no problems measuring the AC ripple fairly accurate.
Potential problem is that not all DMM's measure "True RMS", but assume that they're seeing a sine shape AC, which is by no means the case for this type of measurement. this gives a faulty read-out of the signal.
Jennice
Jennice said:Potential problem is that not all DMM's measure "True RMS", but assume that they're seeing a sine shape AC, which is by no means the case for this type of measurement. this gives a faulty read-out of the signal.
Jennice
Even relatively inexpensive DMM's will measure a.c. voltages to a few hundred hertz -- the problem arises if you are trying to measure the ripple in a switching power supply -- but you'll use a scope for this anyway.
if you need a good RMS measuring instrument, consider getting a used HP3403C -- or an analog 3400 --
math??
What ever happened to mathmatics??
If you know how much current the power supply outputs, and you know what the capacitor values are, and the frequency of AC Line with rectifier, 50-60Hz or 100-120Hz(full wave), then you should be able to calculate the ripple voltage. I think it is
I(load) / (C * f) but I am not sure. You may want to look up the formula.
What ever happened to mathmatics??
If you know how much current the power supply outputs, and you know what the capacitor values are, and the frequency of AC Line with rectifier, 50-60Hz or 100-120Hz(full wave), then you should be able to calculate the ripple voltage. I think it is
I(load) / (C * f) but I am not sure. You may want to look up the formula.

Crest Factor
Hi jackinnj,
Jennice was refering to "crest factor". The ratio of peak voltage to rms voltage. Any deviation from a sine wave has an amount of crest factor. This can be approximated and corrected for if you really know your meter.
BTW, inexpensive meters I have calibrated often don't meet their own published specs, or hold cal. very well. Many fail out of the box! I guess it depends on how accurate you want your readings to be.
-Chris
Hi jackinnj,
Jennice was refering to "crest factor". The ratio of peak voltage to rms voltage. Any deviation from a sine wave has an amount of crest factor. This can be approximated and corrected for if you really know your meter.
BTW, inexpensive meters I have calibrated often don't meet their own published specs, or hold cal. very well. Many fail out of the box! I guess it depends on how accurate you want your readings to be.
-Chris
ditto that ! I wonder what a meter would make of the lower trace here;JensRasmussen said:One measurement with a scope is worth a thousand equations!
\Jens
An externally hosted image should be here but it was not working when we last tested it.
Oh yeah! Especially a digital 'scope that will do the math & tell you the answer.
Mark25, I think many meters would just make an answer up.
-Chris
Mark25, I think many meters would just make an answer up.

-Chris
Thanks to all of you. I note the comment about meters making up answers. My wife's favourite "one liner" concerns the man who went to a shop to buy a digital pocket calculator and the salesman proudly showed him a new model and announced that, "This is the Economists model sir.......it guesses to 12 decimal places!"
Hi Jonathan,
That's funny!
Okay, I'll admit to being spoiled here. I use an HP34401A for a meter. It's paid for itself many times over. If you work a lot with a bench meter, I'd recommend it. For portable meters, it's hard to beat a good Fluke.
Just throwing some more recommendations out.
That's funny!
Okay, I'll admit to being spoiled here. I use an HP34401A for a meter. It's paid for itself many times over. If you work a lot with a bench meter, I'd recommend it. For portable meters, it's hard to beat a good Fluke.
Just throwing some more recommendations out.
Can I have another try at this one? I may not have expressed myself clearly with the first post or I have missed something in the responses. If I have a PSU that is quite standard. IE full wave rectifier followed by a smoothing cap I will then get a rail voltage of, for example 30 volts with a ripple of a few tens of millivolts on top of the 30 v. Now given that, how do I measure the ripple? If I put the meter across the output it will give me 30 volts with some fluctuation in the third digit that will not really be meaningful. What I am interested in is the variation about (+/-) the rail voltage which will require the meter to operate in the millivolt range. I guess I am asking where do you put the meter to measure ripple?
You place the probes in the same positions as for measuring the rail voltage, but you switch the meter to the AC Volts range. It will then measure the ac content of the rail volts, ie the ripple.
Multiply the reading by 3 to convert the "rms" meter reading to peak - to - peak.
The other posts have said only that the reading you get will not accurately represent the ripple voltage, for the reasons given, but you will get some idea of it. Keep the meter physically away from the transformer when you do this, because you can get false readings from interfering magnetic fields.
A 'scope is best, get one.🙂
Multiply the reading by 3 to convert the "rms" meter reading to peak - to - peak.
The other posts have said only that the reading you get will not accurately represent the ripple voltage, for the reasons given, but you will get some idea of it. Keep the meter physically away from the transformer when you do this, because you can get false readings from interfering magnetic fields.
A 'scope is best, get one.🙂
Quite right. Keep the DC load constant or the voltage will go up & down, adding a false AC component.
JensRasmussen said:One measurement with a scope is worth a thousand equations!
\Jens
Some of us aren't fortunate to own a scope.
We have to rely on other methods...
Hi cunningham,
Know anyone with a service shop (audio)? When I had my shop, I'd have a few guys over some nights to use my gear. It kept me company and it was no skin off my nose.
-Chris
Know anyone with a service shop (audio)? When I had my shop, I'd have a few guys over some nights to use my gear. It kept me company and it was no skin off my nose.
-Chris
cunningham said:
Some of us aren't fortunate to own a scope.
We have to rely on other methods...
with EBay there is no excuse for not owning a 5 MHz Heath Labscope, there are Tektronix scopes which are almost given away --
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