voltage adapter concerns

[BodomHatecrew666]

hey guys sorry about all this bs but i have a 9v dc adapter 300mA 2.7VA
the thing is i hooked it up to my Voltage meter and she comes out at an amaizingly bad 20V to the output i tested a few other adapters to see if i was doing something wrong or if my voltage meter was wacked out but they all measured to there voltage give or take 1v but damn close and they seem to work fine but this one is messed is this thing completely gone or can i fix it it is a regular class 2 trans and is the standard input of 120v 60hz 50mA can i still use this maby with a voltage regulator circuit or something to bring it down to something usefull.
I also found a 12v AC 100ma  AC/AC adaptor but i would like to drop this to around 9V for an old effects pedal that uses AC since i cant find any reasonable priced if any 9v AC/AC adaptors localy im doing some searches for some voltage drop circuits right now but can i just through maby a resistor on the + to drop it down. i made a cool little Y-splitter for my effects pedals so i can run them both of one Adaptor it works fine but i still would like to get the other two in working condition for my aplications as back ups and to get my other effects running again.

[cd]

That adapter is probably unregulated.  If you had something that could suck the full 300mA capability off it, it would probably be closer to 9V - you're measuring it unloaded with your meter.

See www.geofex.com for a nice pedalboard power supply.

[niftydog]

i have a 9v dc adapter 300mA 2.7VA the thing is i hooked it up to my Voltage meter and she comes out at an amaizingly bad 20V to the output

Hook up a load accross the output and measure it again.

The minimum load resistor you should use is given by ohms law and the power equations;

load resistor = 9 / 0.3
load resistor = 30 ohms.

But;

Power = (9x9) / 30
Power = 2.7W  :shock:

So, if you can get a hold of a 5W 30ohm resistor, that'll test the power supply to it's rating limits.

Otherwise, work backwards from what you've got. I'm guessing you might only have 1/4W resistors? So;

Load Resistor = (9x9) / 0.25
Load Resistor = 324 ohms

Use something close to 330 or higher and you'll be ok. This SHOULD bring the supply volts back down to something approaching 9V.  :!:  Be aware, however, that these calcs are based on 9V, if the supply continues to maintain 20V you might cook your resistor, or worse, the supply might cark it.  :!:

Best to start with a much higher resistor and work your way down, monitoring the voltage as you go.

I also found a 12v AC 100ma AC/AC adaptor but i would like to drop this to around 9V

A voltage divider will work fine, but again you have to work through the power equations to make sure you won't cook your resistors. You'll need to work out how much current the pedal draws, the exact output voltage of the supply, the voltage drop accross the resistors in the divider and then finally, the power dissipated by said resistors.