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I have an AC/DC adaptor, as it has variable voltage setting (1.5, 3.0, 4.5, 6, 7.5,9 and 12 V).

Voltage readings are larger. For 7.5V setting it provides 12V, for 12V it provides about 18V.

So I suspect this might be an unregulated power supply.

I unplugged adaptor from 240V AC power and measured the voltage across terminals. Despite a lot of time has passed I should be able to measure about 11.5 volts between positive and negative terminals.

Is it the expected behavior. If so what might be the correct way to safely discharge it ?

I use this adapter to power a DC 12V 0.60W fan. It is controlled by an IRF540 mosfet and Arduino's PWM signal.

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    \$\begingroup\$ Sounds perfectly normal. \$\endgroup\$ – winny Feb 3 at 22:37
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    \$\begingroup\$ Welcome to EE.SE. Normally there would be a 'bleed' resistor built in to drain off the charge. Usually it is 1K to 2.2k 1/4 watt. \$\endgroup\$ – Sparky256 Feb 3 at 22:38
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Sure it's normal. There's a reasonably large capacitor in there that can only discharge though the load. If there is no load, it can retain its charge for quite a while.

If it worries you, you can simply put a 1k resistor across it as a permanent "minimum load".

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