Hi, I would like to connect an old 100MB IDE hard disk drive to an AT89S52 MCU and mess around with it. Now, It needs both a +5V and a +12V, but I am going to build this circuit on prototype. So I dont want to include the 7812 since it is HUGE!! I read somewhere that using an AT PSU without a load can damage it. The only load I will be connecting to the PSU will be the IDE HDD, I will derive the +5V for the 89S52 board from another power supply. Can I do this without damaging it? 
Posted on 2005-08-10 22:43:49 by x86asm
What is this bull you heard about no load == damage? You have IDE HDD, that is a load!
Posted on 2005-08-11 15:58:21 by comrade
In my uni, we studied how to assemble and fix PCs, learnt that keeping the PSU without load doesn't damage it (I tested it too). Maybe because of the filtering system and the feedback system. A short-circuit won't damage the PSU either :)  (tested it dozens of times too).
A huge 7812.. for how much current? You could use a transistor (CC schem)+7812 for a more compact design, iirc.
Posted on 2005-08-12 15:55:44 by Ultrano
if you are scary, you can clamp a 47 ohm resistor to 5v/12v output.

some designs indeed require a minimum load, but it is often built into the PSU.

the bad things:
-you need two of these resistors.
-they require cooling, or you need a cement version (audio electronics) at least for the 12v load.
-they get hot, and are rather bulky.*

if the HD needs only a small 12v current, you can derive it from 5v, but that's EXPENSIVE.
however, it is a real danger to have two voltages for projects, especially if you use cinch (audio) connectors.
because they are cheap, robust and easy to connect. if you choose 12 volts for supply, you heat away a lot of power by 7805.
they just heat away the difference between 12 volt and 5 volt. if you choose two voltages, and you put 12 volt cinch into 5volt, your MCU roasts up.

you can put them inside the PSU ;)
Posted on 2005-08-22 08:19:36 by akidd86
Btw, today in a music-equipment store I saw a high-power custom-made amplifier, powered by a PSU. Till then I thought PSU power would be too noisy to use for such devices (their pre-amplifiers specifically) ^^".
Posted on 2005-08-26 18:06:25 by Ultrano
ever had psu connectors which came loose?

cinch inner pin is ALWAYS signal, otherwise the "chassis" would get the "noise"

hence, i utilize cinch for all hardware projects. sometimes 12v/5v is unavoidable, but small 200mA fuses and "crowbar" diodes principally secures this.

glass fuses, cinch connectors and audio resistors for digital electronics- and it even works better.
Posted on 2005-08-27 07:11:13 by akidd86