increase in voltage on wall adapter

[KevinW]

I have this 5vdc wall adapter that works with an smps transformer and after a year or more it has increased in voltage anywhere from 3 to seven volts.
Now I'm aware that some of these require a load to operate at their normal voltage but this one tested 5 maybe 5.2 volts without a load when new and has now jumped to 14 vdc and has settled at 8 vdc without a load.
I was using it for a clock and it killed the voltage regulator when it climbed to 14 vdc.
I'm wondering what would cause such an increase in voltage?

 

[Diver300]

I don't know the exact circuit, but it sounds like the feedback device has failed.

Power supplies like that often have a transformer with a primary and two secondaries. The transformer operates in flyback mode, where during one phase of operation, the current increases in the primary and there is no current in the secondaries, as it is blocked with diodes. In the other phase of operation, there is no current in the primary and the stored energy goes into the secondaries.

The two secondary voltages are in approximate proportion to their number of turns.

On secondary feeds the main 5 V output and the other supplies the control chip on the high-voltage side.

There is a feedback, so when 5 V is reached, that is signaled through and opto-isolator, and the control chip turns off.

If there is no feedback, the control chip keeps on running and the output voltage gets too large, which is what i think happened to your power supply.

In that state, the supply to the control chip may be too large as well due to the other secondary now having a larger voltage. A lot of control chips will detect that and turn off, which is hy the voltage isn't huge.

 

[KevinW]

I compared the primary voltage to other similar devices and the voltage is pretty much the same, this device has a single secondary but I can only check the voltage beyond that point and it is pretty constant throughout that side of the circuit.
I've tested most of the components and they all seem fine so I'm wondering if the transformer may be responsible.