hello,
i designed a power supply to get from it +12,+5,-12,-5 V ... it worked fine in the simulation but when i tried to get +3V from it using voltage divider... it did't give me the required +3V

my question is how to get +3v from this circuit?

http://img690.imageshack.us/i/21076410150220676729179.jpg/

also i want to make this power supply to drive a 2 ampere load, is all what i have to do is to buy a 2A transformer and a 2A rectifier ??

 

Here is your image reposted...

How accurate does the 3V need to be?

A standard computer power supply has most of these voltages.

The regulators you have selected are spec'd at 1A.

 

I agree with bill. With an ATX power supply, you can get +12, -12, +5, -5, and +3.3 all vDC

 

Perform a Google search for "ATX Bench supply" for lots of ideas on converting an ATX form factor computer power supply to a bench supply.

 

MP Jones has mini ATX power supply for $5.95. It's 150 watts and puts out all the voltage you ask for. Only downside is the -5v & -12v outputs are only 100ma.
+3.3v/6a
+5v/12a
+12v/5a
-5v/0.1a
-12v/0.1a
+5vSB/1.5a

They have many to pick from up to 680 watts for $46.00

 

thank you guys for your ideas, i have one more question if am going to do +12V,+5V,-12V,-5V how i can make it to accept a load 2A , i mean is there a certain regulator for that? or only the rectifier and the transformer must be 2A ?

 

If you use a 2A transformer on that circuit, and your 3v load requires 2A current, then you won't have any current left for your other regulators. Also, the regulator you use will become VERY hot, as it will be dissipating most of the power.

 

Both, and more. If each voltage is outputting 2A, you will need the sum of all those currents for the unregulated power supply. If these are linear regulators as your diagram showed their will be lots of heat, it will be an oven (over 100W of heat may be possible).

A switching mode power supply (SMPS, remember this anagram, you will see it forever) is much more efficient, and can drastically reduce the base load on the unregulated side. In this case it is about wattages and efficiencies. Wattage is voltage times current. So, with a 90% converter SMPS will output 5V at 2A (for 10W) will take 11W in (whatever the power supply is).

I was typing at the same time as Wookie, he just hit enter faster.

 

If you use a 2A transformer on that circuit, and your 3v load requires 2A current, then you won't have any current left for your other regulators. Also, the regulator you use will become VERY hot, as it will be dissipating most of the power.

can u suggest any solution ?

 

All of us already have, a computer power supply. It has everything you want in a metal package and a power cord included.

BTW, it is a great example of a SMPS.

 

You can add an external power transistor to each regulator to boost its current output. See the bottom of page 14:

http://www.datasheetcatalog.org/datasheet/motorola/MC7809.pdf

Figure out the max power dissipation for each regulator using (Vin-Vout) * Iload at maximum load current. Size your heatsinks accordingly.