Measuring open circuit voltage from 12 volt AC-DC adaptor I get 18 Volts is this right?

Question

Measuring open circuit voltage from 12 volt AC-DC adaptor I get 18 Volts is this right?

Answer 1

Yes, that can be right... that is why you have to measure the output of wall warts UNDER LOAD.

Answer 2

Yes, that's about right for an unregulated adaptor.

Think of your mains adaptor as an ideal voltage source in series with a resistance.  A voltmeter is designed to have a very high resistance, so it draws almost no current from the voltage source it is measuring.  When there is something drawing current from the adaptor, that current is flowing through  (and therefore developing a voltage across)  its internal resistance, and the voltage at the terminals will be reduced compared to what it was when open-circuit.

Regulated adaptors are designed so that the voltage will be the same anywhere up to the rated maximum current.  There are two types:  either they use a conventional step-down transformer and rectifier, with a zener diode producing a constant voltage and a transistor passing the load current, or they are "switched mode" power supplies which basically work by charging a capacitor from the mains, then discharging it partially into another capacitor -- but stopping once the required voltage is reached, and waiting until it drops again before topping up the output capacitor a bit more.

Answer 3

Cheap adaptors are not regulated. Hopefully if you load it down with the rated current, the output will be close to 12 volts.

.

Answer 4

Yes, that is ok. Always measure a voltage under load as well.  I.e. Put a bulb across the pack.

Answer 5

The 12 volts AC in the adapter will be 18 volts but the DC output will be regulated to 12 volts.

  The bridge and regulator in the system consumes about 6 volts.