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:
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:
An externally hosted image should be here but it was not working when we last tested it.
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!"
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.
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