Dear members,
I have a 9V AC to DC adapter and there's something wrong with it.
When I use a multimeter to measure its output, the multimeter couldn't read the voltage. The multimeter initially was reading very large unstable voltage(moving up and down) and then after that it indicates couldn't read the voltage(too high).
BUT if I put my finger on the jack/plug(output) terminal, the multimeter could read the voltage exactly. Once I let go, it went back to the abnormal readings.

My question is why is this happening? Thank you.

 

Could be a poor connection, or a bad design that cannot control the output without some load on it. HArd to tell without more info.

 

and what setting did you have your multimeter on?

 

Some switching adapters do need a small load before they will provide a voltage.

 

I guess it's the tree falling in the forest with no one to hear it. To me it would make sense to load the supply internally just enough to bring it into regulation, but that would cause an unnecessary vampire load, so I see the logic in not doing that.

 

Keep in mind the older AC/DC adaptors were unregulated, they typically showed 60% higher open circuit voltage and relied on the rated load to bring them down to the stated voltage, for > 5yrs now, most have been regulated supplies and so far I have found them to be pretty well regulated.
Max.

 

It's the high freq noise from the switchmode supply, the resistance of your finger is likely enough load to kill the bulk of HF spike energy and let your meter read the average DC voltage.

I don't think it's a minimum load issue like Tubeguy said, although that is common on large internal PSUs, on SMPS wallwarts like 9v adapters they are always designed to be stable (and at regulated voltage) with a no load condition.