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.

  • 1
    \$\begingroup\$ Sounds perfectly normal. \$\endgroup\$ – winny Feb 3 at 22:37
  • 1
    \$\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

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".


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.