I recently bought a multiple voltage DC adapter. I'm interested to know how it works. It has a switch to select 3, 4.5, 6, 7.5, 9, and 12V. It's big, hefty, and the voltages are unregulated when unloaded so its not a SMPS.

How does it produce the different voltages? With a multi-tap transformer? It also lists a max current of 500mA. Which voltage can supply the maximum current?

 

For unregulated AC/DC adapters, they all do the voltage switching via selecting different transformer taps on the secondary side. Then the AC voltage is fed to a simple bridge rectifier and smoothing capacitor of some ~1,000uF.

The unloaded output voltage could be as high as 1.4 times the switch selection, i.e. 8.4V for 6V selection. This drops to 6V if one puts a 500mA load on the output.

If the adapter been labeled as 500mA, then it can provide 500mA regardless of which voltage tapping user had selected. You will get the same amount of current on every tapping so there is no particular voltage that gives current "maximum" as per your question.

 

Thanks very much.