Hum-free 9V and 18V supply box out of cheap 18V wall mart?

[Steben]

I see Velleman sells 18V dc 1000mA(!) unregulated "wallmarts" at a reasonable price.
Can it be used in front of some regulation and 9V/18V outputs without too much hum? Is it a good choice in order to avoid transformer DIY-ing?


WALLMART
      |
      |
      -----regulated 9V
      |
      -----regulated 9V
      |
      -----regulated 18V
      |
     ...

[brett]

Hi.
Go for it.
All you need are two 7809 regs and a few capacitors (two large ones -about 2200uF, and some small ones -0.1uF).  Search the web or look for a circuit that shows two regs being stacked on top of each other to form a ground half way between the + and - rails of the transformer/wall wart.

cheers

[d95err]

A voltage regulator needs a few volts more than the regulated output (2-3V) to work. I.e. to get 9V DC regulated, you need 11-12V unregulated DC. So, you could get 9V, 12V or 15V regulated from the 18V wall wart, but probably not 18V regulated, unless the unregulated DC is actually something like 20V (under load).

[Steben]

There is a 24V dc 1000mA version too. Guess that will do the job very succesfully then.
Probably 24V will be too high for a 9V regulator without voltage dividing? EDIT: no it seems the 7809 can handle up to 37volts I see. Same as 7818. Why is a 18V reg cheaper BTW?

Do they need cooling strips/fins?
Oh, another thing: is it possible to make a simple current indicator circuit that makes a LED light as soon as for example 900mA's are exceeded?