Need 7.5V, 2A power supply -- using a computer supply?

[mstacho]

Hi everyone,

I need 7.5V, but 2A.  My Agilent desktop power supply only gives me 0.5A at 7.5V, so I'm kind of screwed.  I'm thinking of just using a computer power supply and regulating the 12V line down to 7.5V and calling it a day.  Problem is...I don't know how to do that.

Does anyone:

a) Have links to tutorials I can use to build it?
b) Have a better option that doesn't use batteries?

Also, I'm assuming I need 2A, I might only need like 0.75, but I figure I should try to get more amperage than less to be safe.

MIKE

[waltr]

Fast and easy is an adjustable 3-terminal linear regulator. Some do handle 2 Amps but ensure enough heat sinking as it will dissipate 9 Watts to drop 12V to 7.5V at 2 A.

If you can not find a regulator to handle 2 Amps a smaller regulator like the LM317 and a large NPN transistor (2N3055) on a heat sink will work. Circuits for this pass transistor can be found in most linear regulator data sheet in the application section.

If you only need 0.75A then the circuit will just not dissipate as much heat (3.4W). I also like having extra current available.

[Soeren]

Hi,

[...] I'm thinking of just using a computer power supply and regulating the 12V line down to 7.5V and calling it a day.  Problem is...I don't know how to do that.

Does anyone:

a) Have links to tutorials I can use to build it?
b) Have a better option that doesn't use batteries?

The few modify-a-PC-PSU-tutorials I've seen have been highly specific towards a particular make and model and if what you've got is different, it won't help you any.

Start by opening the supply - after it has been off for at least an hour and remember to discharge the electrolytics, not by a screwdriver, but with a suitable resistor (you want them to still be in good shape).
Find the switch mode regulator chip. It will have traces to the switching transistors and via a resistor network of some sorts to the output.

Find the datasheet for this chip. Look up at what pin the feedback voltage enters the chip and trace out what's connected to that pin. Usually it will be a potential divider that divides the output down to whatever reference voltage the chip use for comparison.

Calculate the upper transistor in that divider for 7.5V, swap it and Bob's your uncle.


Don't change anything until you have the datasheet and finally, remember a fuse on anything you connect, there's a lot of current in such a beast.

[corrado33]

Hi,

The few modify-a-PC-PSU-tutorials I've seen have been highly specific towards a particular make and model and if what you've got is different, it won't help you any.

I used an old dell power supply to make a desktop power supply.  (I can see you imagining me burning my house down now...) 

It was actually really easy.  The red wires are 5v and yellow are 12V.  There is usually a "sense" wire that has to be connected to 5V for the unit to turn on.  That wire is the hard thing to find.  That is also the wire I switched (and fused) to turn my unit on and off.  It varies on different units.  Sometimes it's blue, sometimes it's grey.  Sometimes it needs 5V, sometimes it needs to be connected to ground.  Sometimes you need to put a load on SOMETHING to make it run at all.  You should be able to find a tutorial on your computer power supply (or at least your brand).  I THINK that was all I had to do.... I don't remember exactly, I did it a few years ago.  I got REALLY nervous plugging it in after I had messed with it.  Turns out computer PSUs have good protection too.  I accidentally had a power wire touching a ground wire when I plugged it in once.  It just refused to turn on.  (Lucky for me).  Now it's all heat shrunked (it's a word... maybe) and tucked inside and good to go.  I haven't had problems with it for years now.  I did have to relocate the fan to the outside of the case though to create room on the inside to tuck the wires into.  (And for the back of the banana jacks, they take up room too.)

[Soeren]

Hi,

I used an old dell power supply to make a desktop power supply.  (I can see you imagining me burning my house down now...) 

Yes, but this is about changing the output voltages, rather than just using the 5V and 12V lines as is.

You should be able to find a tutorial on your computer power supply (or at least your brand).

No - for the same reason as already mentioned.

[...] it's all heat shrunked (it's a word... maybe)

'Heat shrinked if you put heat shrink tubing over them (I bet the wires are still the same size  ).

[mstacho]

Thanks for the info everyone.  I'll see what I can rig up :-P  It was really funny spending about 7 days straight working on the wiring for the robot hand only to realize when I turned it on that my supply can't source enough current :-P

MIKE

[Daanii]

I also had a good experience changing a computer power supply into a desk power supply. But the instruction video I used does not seem to be available anymore.

Although the video was for a different brand of power supply, it was easy to use. My computer power supply said on the case what color wires carried what voltages. That made it easy.

Just like corrado33, I had to move the fan from inside the power supply case to outside. But that was easy enough.

Taking the 12 Volts down to 7.5 Volts may be your hardest problem. And as Soeren says, be careful about safety. The computer power supplies can deliver a lot of power.

[joe61]

Just a suggestion, but Google shows a number of wall warts that don't seem very expensive. For example:

http://www.powerstream.com/power2-3-6.html

Joe

[Soeren]

Hi,

Taking the 12 Volts down to 7.5 Volts may be your hardest problem.

Perhaps I didn't mention it, but (at least in my mind) I was talking about increasing the 5V line (that's where most of the power is and it's usually the 5V line that is regulated - the other lines are just following suit, due to a tapped transformer.