12v power supply, output fluctuating when NOT under load - is this normal?

[xboxgamer]

Hi everyone

So I have a weird "issue". My satellite receiver wouldn't turn on so I did basic troubleshooting with measuring voltage coming out of my cheap, chinese AC to DC power adapter. It was fluctuating 0-9-10-12-0... very fast so I said - it's broken. Also it was giving this very quiet hissing sound every second like it's trying to power on and shuts down.  I spent 2 days replacing capacitors and checking inside and it was still happening.

Then I discovered that there was a lose cable inside receiver itself and after fixing that I powered it on using another power adapter that was giving stable 12v on the output when not connected to anything besides multimeter. Just for fun I connected the "broken" adapter back only to discover that when connected to device, it gives stable 12.35V on the output. But as soon as I disconnect it from device it fluctuates again.

What gives!? All my life I thought that power supply MUST keep stable 12v even without connecting to anything.

Is this a fault? Is there a way to stabilize it in case it's not normal?

Power supply looks like this https://m.media-amazon.com/images/I/61JAjYzklDL._AC_SL1000_.jpg

[johankj]

Voltage for any power supply will change with load. I'm not surprised a cheap Chinese brick is all over the place when unloaded. I've seen this behavior too.

[JohanH]

Some power supplies need minimum load to give stable output. Put a resistor as load to draw 5-10 mA current if you are concerned.

Besides, many devices don't need a precise input voltage because of internal regulators. Cheap devices often come with a 12V adapter that isn't regulated and have higher voltage when not loaded. Also, I have accidentally powered "12V" devices with 5V adapters and they have worked fine.

[Gyro]

The hissing sound is a dead giveaway that the PSU isn't stable under zero load. It is either cycle skipping or just generally instability depending on quality. As others have said, put a load on it and it will settle down.

[Zero999]

As mentioned above, some power supplies need a minimum load to be stable.

It's possible it's a cheap, unregulated power supply. Most switched mode power supplies are regulated, but some really cheap ones aren't, or are very poorly regulated. Your satellite receiver probably doesn't need a regulated power supply. It will have various regulators on its power inputs to five different voltage i.e. 5V and 3.3V.