Homer said:

Thanks but what do you mean by unregulated adapters?

Some "wall warts" contain internal

voltage regulators that keep the output

voltage very close to the stated voltage

on the adapter. If yours were regulated,

it would deliver 18V, regardless of

load, up to its rated current output of

1 ampere.

The unregulated variety have a

non-negligible internal resistance that

causes the output voltage to drop as the

load increases. In a rare gesture of

honesty, the manufacturer actually

promises the rated voltage at full load.

And also if the rating is the voltage at full load then how do we

explain the Voltage Drop with 6 LEDs (that is far less than 1A for

sure)?

Well, the greater the external load, the

greater the voltage that will be

developed across the internal

resistance. This voltage will be

subtracted from the output voltage.

The voltage drop follows Ohm's law,

using the internal resistance and the

load current.

John has calculated the internal

resistance to be four ohms using (22V -

18V)/1A. That assumes a linear internal

resistance but the 0.6V drop with a

current of 0.0094A suggests an internal

resistance of ~63 ohms which would never

allow 18 volts output at 1 A. So maybe

it is non-linear (at least over part of

the output current range) or defective.

It can get complicated to work with an

unregulated supply. For less than a

dollar, you can add a simple IC voltage

regulator and some capacitors and have

your output voltage stay within 50mV of

18V at any load up to 1A.

Chuck