How to measure ripple from supply using DVM

[Welchs101]

Hi,

I have an external power supply ....two of them......one of the supplies causes caps to heat up and the other does not. I am thinking this is due to "ripple" in the output of the power supply.

Is there a way to measure this voltage ripple using a Digital Volt Meter?

[etnietering]

Re: How to measure ripple from supply using DVM

Well, you really need an oscilloscope to measure ripple well, but there is something you can ghetto-rig together that should give you some sort of idea. Use the attached schematic to rig up a high pass filter to the voltage lines (it says +12v, but after you measure +12 you could hook it up to +5, +3.3, +5sb).
Since you want to measure as much noise as possible, you want to have the cutoff frequency to be pretty low (at least one order of magnitude lower than the 50/60Hz mains freq). Use the equation fc = 1/ (2 * Pi * R1 * C1) to determine the cutoff frequency. For C1 = 100uF and R1 = 1Kohm, you get fc = 1.59Hz, which would be perfect. You can play with different values depending on what parts you have available. Then set your DMM to AC voltage, and hook it up as indicated. The readings you get probably won't be perfectly accurate, but they should give you an idea as to the relative difference between the two supplies. Hope this helps!

Oh yeah, watch the polarity of the cap (reverse it if you're measure -12v or -5v), and be sure to use a 16v or greater cap. Here's the schematic:

[NxB]

Re: How to measure ripple from supply using DVM

This is neat. I will try it. Still to poor for an oscilloscope.