[sdiy] 'Measuring' PSU ripply without a scope [was: weird voltage regulator problem / question]

[Seb Francis]

harrybissell wrote:

>The 9V should be steady. Measure the input to the regulator on AC
>volts...
>it should be a small number.  Usually these regulator need 3V more than
>the output
>voltage... minimum. You have only 5V... any AC ripple might make the
>regulator
>drop out.
>
>Solutions : Bigger filter cap, or higher input voltage (which makes
>everything
>HOTTER again :^). Recalculate the new power and temperature.
>
>If you have a scope, you can see the ripple voltage directly. If not,
>you have
>to rely on the meter and it is tough to know exactly how it will report
>the
>ripple (AC) voltage.
>
>This variation would usually be 60Hz or 120Hz... more an FM growl than
>a warble, IMHO
>  
>

If you connect the PSU output via a DC blocking capacitor (e.g. 47uF 
electrolytic) to an audio amplifier, you will be able to hear any ripple 
(turn the volume up slowly, don't connect/disconnect with the volume 
turned up or you will get a *loud* thump).

Do this test with the PSU under load (with no load there probably won't 
be any ripple).

Seb