qustion about unregulated power supplies.

[Boner]

So if you have an unregulated power supply that is rated at 12W, giving 1A at 12Vdc...

does that mean it can do 24Vdc at 0.5amp or say 6Vdc at 2amp?

Depending on the load, amps and voltage change to keep power constant?

[Marcos - Munky]

Nope. The output voltage is 12V DC. The unregulated part means the voltage is about 12V DC. For no load, if you measure the voltage, it'll be probably around 14V or 15V. For a lower current load, you'll have a voltage a bit bigger than 12V. For a load near 1A, the voltage will drop to a bit lower than 12V.

You can turn it into a regulated 9V power supply. That means the voltage will be 9V no matter what is the load (if it's under the current capability). This is the difference between an regulated and an unregulated power supply.

[vigilante397]

So if you have an unregulated power supply that is rated at 12W, giving 1A at 12Vdc...

does that mean it can do 24Vdc at 0.5amp or say 6Vdc at 2amp?

Depending on the load, amps and voltage change to keep power constant?

Definitely not. Do you have a specific power supply you're referencing? A lot of "unregulated" supplies are just not-very-stiffly regulated supplies. Linear regulators will give the same voltage regardless of load (up to their limit, of course), and more expensive switching regulators generally do as well, but cheap switchers will sit close-ish to the specified voltage until they are pulled down by a load, at which point they should be pretty damn near their specified voltage.

I have a cheap 9V wall wart that gives 13V without a load on it. It's rated for 300mA, and it takes about a 100mA load to pull it down to about 9V.

[Boner]

perfect, thank you so much!

[Rob Strand]

For an unregulated wall-wart  the typical *average* DC voltage at full load is about 0.9 times the rated/indicated voltage.       The minimum voltage at the dips of the ripple is less than that.   The ripple corresponds to about a 1000uF filter cap.

The reason they allow the voltage to drop to 0.9 is to keep the unloaded voltage as low as possible. 

IIRC the unloaded voltage is always less than 1.4 or 1.5 the rated voltage;  don't quote me on it though - I posted details in an old post.

There's like a recipe manufacturers used for wall-warts.

[amptramp]

Don't expect unregulated wall wart power supplies to have particularly good filtering.  I have one wall wart that I took apart and it had a transformer, a push-pull rectifier and an electrolytic capacitor with a resistor in series with the capacitor but not in series with the output.  In other words, whatever the internal ESR of the capacitor was, someone had added a 1.5 ohm resistor in series, probably to limit the inrush current.  It was intended for an application where hum and ripple didn't matter.