Vpp

[Rob Northen]

Hello folks!

First, please don't butcher me for the questions, that might seem stupid.
I just bought a cheap DSO (Rigol), needed some new toy

1. Could someone advise me on measuring on ex. a PSU, to see if it has a stable and output, ie. its not full of noise.

2. When measuring on a 12v supply, i get 1.2v Vpp. I guess when measuring on DC, its best to get a very low Vpp reading ???
1.2V on 12V DC seems a bit high, is it noise ??

[steve2]

Re: Vpp

I haven't used my scope in a few years, but a 12vpp (volts peak to peak) dc should read 12 vdc on your scope. Do you have the leads on X1 or X10? It seems to me that a setting is wrong or your reading the height of the sine wave wrong.

[Rob Northen]

Re: Vpp

I haven't used my scope in a few years, but a 12vpp (volts peak to peak) dc should read 12 vdc on your scope. Do you have the leads on X1 or X10? It seems to me that a setting is wrong or your reading the height of the sine wave wrong.

Its a 12 DC I'am measuring, and if I understand things right, only AC should should show peak peak voltage. When its a DC, then there should be no peaks, right ?
Or have I missed something ??

[severach]

Re: Vpp

The DC setting on the scope allows the DC through. You'll need to select a scale like 2v so that there's enough room on the screen that 12v can show up. Set the scope to ground and adjust the line position to a low line on the screen. Set the scope back to DC and the line should move 6 spaces up: 6 divisions * 2v/division = 12v. The line will be very flat because on the 2v scale there isn't enough ripple to be seen along the 12v line. If there is you have something to fix.

The AC setting uses an internal capacitor to block the average DC. The average voltage 12v is subtracted off leaving only the AC ripple. No matter how large the average voltage is the line will always be centered so now we can use a really low voltage scale and see the ripple that is on top of the 12v wave. If we tried the 50mv scale on the DC setting, the wave would be 240 divisions up and no scope has that many divisions. For a very good supply, we might need to use the 5mv scale and the wave would be 2400 divisions up. I doubt that even Frank's 2000 Inch TV could have that many divisions turned up on its side.

Set the scope to ground and center the line in the display. Set the scope to AC and the vertical scale to about 50mv. If the ripple doesn't appear keep going down 20mv, 10mv, 5mv, 2mv, ... until it does. You may also need to adjust the frequency time/div up or down to get a good picture of the waveform. If you are new to oscilloscopes or the kids have been playing with the buttons there may be other buttons that need pressing to make the waveform clear.

All SMPS have a ripple though some are so good that the scope barely picks it up on the lowest sensitivity.

And welcome to the Matrix, where dropping ESR from 0.08 to 0.01 turns failure into success and you have the tool that can tell the difference.

[Rob Northen]

Re: Vpp

The DC setting on the scope allows the DC through. You'll need to select a scale like 2v so that there's enough room on the screen that 12v can show up. Set the scope to ground and adjust the line position to a low line on the screen. Set the scope back to DC and the line should move 6 spaces up: 6 divisions * 2v/division = 12v. The line will be very flat because on the 2v scale there isn't enough ripple to be seen along the 12v line. If there is you have something to fix.

The AC setting uses an internal capacitor to block the average DC. The average voltage 12v is subtracted off leaving only the AC ripple. No matter how large the average voltage is the line will always be centered so now we can use a really low voltage scale and see the ripple that is on top of the 12v wave. If we tried the 50mv scale on the DC setting, the wave would be 240 divisions up and no scope has that many divisions. For a very good supply, we might need to use the 5mv scale and the wave would be 2400 divisions up. I doubt that even Frank's 2000 Inch TV could have that many divisions turned up on its side.

Set the scope to ground and center the line in the display. Set the scope to AC and the vertical scale to about 50mv. If the ripple doesn't appear keep going down 20mv, 10mv, 5mv, 2mv, ... until it does. You may also need to adjust the frequency time/div up or down to get a good picture of the waveform. If you are new to oscilloscopes or the kids have been playing with the buttons there may be other buttons that need pressing to make the waveform clear.

All SMPS have a ripple though some are so good that the scope barely picks it up on the lowest sensitivity.

And welcome to the Matrix, where dropping ESR from 0.08 to 0.01 turns failure into success and you have the tool that can tell the difference.

Wow, great guide!!
Followed it, and tried inserting diffrent caps over the output, to see diffrence.
It of course makes good sense to set scope to AC, to filter out the dc line

Thanks, great advice