Measuring ripple on Fluke 123 Scopemeter

[Cohibaman]

Either I'm doing it wrong, or a Fluke 123 Scope meter can't show descent resolution of AC ripple on a DC line.

I set it to VAC & 5V/d. As expected, I see a nice 12VDC line with very little ripple resolution. I then increase resolution to 2V/d and the trace moves up and off screen, so I move the trace back into view. Still not a lot of AC ripple resolution at 2V/d so I try 1V/d. Now it moves off screen and I can't move the trace back into view.

So...how do you set the Fluke 123 to measure AC on the DC line with any amount of resolution?

[stj]

Re: Measuring ripple on Fluke 123 Scopemeter

what happens if you set it to DC ?

i had a PM97 years ago and i know you can see ripple, but cant remember how.

[Cohibaman]

Re: Measuring ripple on Fluke 123 Scopemeter

Same thing.

[SteveNielsen]

Re: Measuring ripple on Fluke 123 Scopemeter

I thought the AC setting should decouple DC from the scope input. As I recall in the olden days we used a cap on the end of the proble to remove DC offset.

[Cohibaman]

Re: Measuring ripple on Fluke 123 Scopemeter

I've done that as a workaround, but thought (and was hoping) the meter should without a cap.

[Cohibaman]

Re: Measuring ripple on Fluke 123 Scopemeter

Thanks btw

[SteveNielsen]

Re: Measuring ripple on Fluke 123 Scopemeter

I would think it should.

"AC/DC select. When this is set to "AC" the DC part of the signal is filtered out by a capacitor placed in series between the signal input and the scope. When the selector is set to "ground", the beam will move to zero volts. When the selector is set to "DC", the entire signal will be displayed on the scope. "

http://hyperphysics.phy-astr.gsu.edu...opec.html#c316

[budm]

Re: Measuring ripple on Fluke 123 Scopemeter

The AC ripple voltage will be in the mV range and will increase as the more load is connected.
Set the scope to AC because you are looking at AC signal riding on top of DC.
what is the power supply switching frequency?

[Cohibaman]

Re: Measuring ripple on Fluke 123 Scopemeter

The AC ripple voltage will be in the mV range and will increase as the more load is connected.
Set the scope to AC because you are looking at AC signal riding on top of DC.
what is the power supply switching frequency?

Yes, that's exactly what I AM doing - I set to meter to AC auto and I see a thick line (the ripple on top of the DC). The problem is when I manually increase resolution (a lower voltage / division) from 5V/division to 1V/division, the trace moves off the screen. I can measure the ripple with the cap in place.

[Cohibaman]

Re: Measuring ripple on Fluke 123 Scopemeter

I would think it should.

"AC/DC select. When this is set to "AC" the DC part of the signal is filtered out by a capacitor placed in series between the signal input and the scope. When the selector is set to "ground", the beam will move to zero volts. When the selector is set to "DC", the entire signal will be displayed on the scope. "

http://hyperphysics.phy-astr.gsu.edu...opec.html#c316

The trace looks identical whether AC or DC is selected. The only thing that changes is the digital numeric display and AC or DC (depending on which is selected).

[budm]

Re: Measuring ripple on Fluke 123 Scopemeter

Looks to me you have it set to AC+DC mode, see page 17 of the user manual for example. it should be on AC only.

Do you have user manual?

[Cohibaman]

Re: Measuring ripple on Fluke 123 Scopemeter

Argggg! You were right, kinda. There's two places to select VAC+DC; 1) Under measurement (where I always select either AC or DC; never vac+dc), then 2) under meter options as coupling. Meter coupling was set to AC+DC rather than AC only. It defaults to AC+DC coupling when I hit auto. Doh!

Thanks all!