Question about adapter!

[Izzy]

Hi I have this Universal adapter which has selectible Voltage from 3v to 12v. But no matter where I keep the voltage switch it gives me 14-18 v??

What is wrong with this adapter? Is there anyway I can make voltage to be constant 9V?

May be by adding 9v Zener Diode in series in Positive terminal?

[GibsonGM]

Hi Izzy,

I see 2 possibilities with your adapter:

1) The switch or something in there is broken or fried

2)  Since you're not using the adapter at the time you measure the voltage, you're operating it "no-load".  A supply with a stiff voltage will normally measure higher with no load on it.   Maybe connect a 1K resistor across it, and measure the voltage across the resistor. This will provide a small load on it, and should make it come down to where it needs to be (maybe!).   The current there would be 12/1000= 12mA, and so should be no problem.    If this gets the voltage closer, it's probably OK.    14-18V all the time DOES seem pretty high!

You COULD use a zener, just look up the appropriate way to use one - probably reverse biased to ground from V+.   Myself, I'd go for a voltage regulator IC, LM317, etc.  Then you could use any voltage over that which is required (within logic). 
Good luck!