AC in the DC - two taste treats

[ethrbunny]

I bought an oscilloscope to better understand whats happening in my builds. For grins on put it on the 18V DC adaptor that I use to power my board. To my horror it showed a dramatic sawtooth AC form. Fortunately the adaptor runs into a splitter where it gets conditioned down to 9V (which is clean).. but.. sheesh.. pretty ugly.

[petemoore]

Do the inputs on Oscilloscopes have a Max Voltage Rating?

[niftydog]

AC ripple is commonly found on DC supplys. However, it's the AMOUNT of AC ripple that is important. If you had the o'scope wound down to 20mV per division, chances are that AC ripple is of little consequence.

Often AC ripple is given as a percentage of the DC supply. So, 0.5VAC RMS ripple on top of 18VDC is said to be 2.7% ripple.

Also, the characteristics of the supply may change dramatically when connected to a load.

I guess what I'm saying is this; to the untrained eye, an o'scope can make things look a lot worse than they actually are due to it's ability to look very closely at low levels signals.

So, take those voltage readings carefully! A waveform display with no reference to the voltage level is not very useful.