Faulty AC-DC Adapter?

[MrMiniBeast]

I bought a Trisonic TS-506B AC-DC adapter that allows you to choose the ouput voltage: 1.5-3-4.5-6-7.5-9-12. The purpose of this was to power my breadboard and to also be used for an arduino board every now and then. I cut off the tip to expose the wires, and after pulling out my multimeter have found that the voltages are highly innaccurate. At first I would see voltages of about 5.5 DC when the setting was on 3, but now no matter how I have it set I measure approx 12V. Is there a problem with the adapter?
I'd also like to point out that there are switches for polarity and also for input voltage (100-120 AC or 200-240AC). I've tried both so could I have put too much voltage in it or something? I'm using a standard wall outlet.
If anyone could provide guidance I'd appreciate it.

 

[Harald Kapp]

Throw away this one and get a decent lab power supply. These adapters are typically not very accurate. They also lack a solid current limiting circuit.
Or build one yourself.

 

[KrisBlueNZ]

It sounds like this adapter is a mains transformer type (i.e. the heavy, old style, not a light-weight switching type) with multiple secondary voltage taps to choose the output voltage, and no output regulation.

These are inherently poorly regulated; the output can have a lot of ripple at twice your AC mains frequency, and the output voltage can often be 20% different from the specification, or even more. But most importantly, the output voltage is specified at rated maximum load current.

If your load circuit isn't drawing the rated maximum load current, the output voltage will be significantly higher than it should be - sometimes as much as twice the specified voltage.

And the problem that you're seeing will occur if you measure the adapter's output voltage with no load. When you set the output to the highest voltage, the smoothing capacitor charges up to that voltage. When you choose a lower voltage, the smoothing capacitor will not discharge, because there's no significant load current (only a microamp or so from your multimeter), and you will still measure the maximum output voltage you did before.

To test whether this is the problem, unplug the adapter, short the output to discharge the smoothing capacitor, select the lower voltage, and power it up again. Now you should measure a lower voltage.

That type of adapter has been superseded for most applications by switching supplies, which are much more compact, much lighter, and properly regulated. You can get switching adapters with switch-selectable output voltages.

 

[MrMiniBeast]

Alright. I was able to take better measurements and I wrote them down because they were very innaccurate, but at least it was variable.
Also I'm really interested in the idea of building my own power supply in the future. Thanks for the help.

 

[cjdelphi]

Basically you have/had an old iron core transformer, I like them for powering circuits to be honest as I'm always in contact, the last thing you want is your 110/240vac to 12vdc is to fail...

Cheap supplies can kill.

As to why, it needs to be under load to supply the correct voltage, 2nd there's no regulation.

Sounds like you shorted it out.