Wallwarts....how do you make them put out the right Voltage?

[sir_modulus]

I just found some old wall warts that are 200ma @ 9VDC, but they put out odd values (around 12 - 15 V depending on the load) now i know they only put out 9 VDC when the current is 200ma, and i want to power my mini Q-tron off them (it needs 9VDC @ 100ma (well...that's what the EH manual says)). What resistor do i put to get 9VDC @100ma out of that thing. This could be solved with ohm's law, but i don't know wether mini Q-tron has variable power requirements. Could someone please help me?

LM 250+

[R.G.]

With a regulator.

Wall warts vary all over the map.

Be aware that the "regulation" of all small transformers, wall warts included, is poor. "Regulation" as it's applied to transformers means the change in output voltage from no load to full specified load. Since transformers have no active means of regulating voltage, the term simply means the percentage of voltage drop caused by wiring resistance over the load current range.

So why am I nattering about regulation? When a wall wart says "12V @ 1.5A" that means "this transformer will provide *at least* 12V when you pull out a current of 1.5A. It will be more than that at lesser loads."

A wall wart typically has about 10-20% regulation. That means that at no load, you may see a voltage that's 20% higher than the specified voltage under load.

If you attempt to lower the voltage with a resistor,  then your pedal gets the right voltage only at one current, and the series resistance can cause feedback and oscillation problems with the pedal.

The simple thing to do is to use a fixed (7809) or variable (LM317) regulator to make 9V rock solid, out of whatever the wall wart gives you.

Try reading "Power supplies basics" at GEO. And "The Spyder power supply" at GEO. In fact, all the power supply stuff at GEO.

[sir_modulus]

Thanks R.G. Your advice is always useful. Will look into some VR's soon.

[Joe Davisson]

It's pretty easy to hook up a common LM317T adjustable regulator for 9v operation, although you'll have to fiddle with the adjustment resistor (use a trimpot). As long as the measured raw output of the transformer is a volt or two above your target, it'll work out. You don't have to worry about the current, as long as what you're running is lower than the max shown on the transformer, which it is.

-Joe