Evaluating 9V PS

[Anding]

Hello,

I have a new but inexpensive (a few $) 9V wall-wart power supply.  Checking the voltage with a multimeter, I saw that it was fluctuating, and made another measurement with the scope (attached)

May I ask, is the power supply performing adequately?  Will the voltage fluctuations affect my projects?

[Andy Chee]

That fluctuation is concerning.

What load did you have connected whilst measuring the waveform?

[Anding]

No load at all Andy.  Open terminals.

I actually bought two identical PS units and this observed performance is the same with both.

I plan to use the PS in projects instead of a 9V battery, no my expectations are not too high

[squadchannel]

This is terrible; secondary side filter is missing.

[BrianHG]

This can happen with some switching power supplies which require a minimum load of something like 10% their rated power to get rid of that pulsing.

Just add a load resistor and re-measure.

More expensive or more sophisticated supplies will not pulse like that with a 0 load.

[beatman]

Just connect some load on psu and measure again you see better waveform BUT all cheap psus use crap caps and ripple is high.I have a lot experiments with cheap and expensive psus.New 5 volt 1 A adapter for audio device shows 35 mv ripple under load i change the output filter caps with panasonic fr series (i have always in  stock) and the ripple goes down to 12 mv.Lesson is a good cap is always a good cap for a reason.

[Anding]

Thank you all - "test under load" - I see!

Here it is with a 330 Ohm supplying 27mA

[PGPG]

This is terrible; secondary side filter is missing.

Filter for 1s pulses?

[squadchannel]

This is terrible; secondary side filter is missing.

Filter for 1s pulses?

Ouch, I thought it was more of a short pulse.