# Home built digital design station -> Help with PSU?

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#### Gandledorf

##### New Member
I've been recently thinking about building my own design station, similar to the P505 mentioned in another thread (https://www.electro-tech-online.com/threads/pspice-question-rectifing-ac.6268/). The cost on these items are prohibitively large for my current means, yet I'd like to have the level of functionality that they provide.

Here is my current design sketch:
I'd like to have a square wave function generator, uC controlled, so this is easy enough (I already have one designed for my use at home). I'm leaving off the triangular and sine wave generators, as I'm not 100% sure how I would manage this.

Power binding posts, hooked up to a variable powersupply. One of the posts set to GND, another fixed @ +5V, and the other two variable from 1.5-15V, and -1.5 - -15V.

A frequency counter (again I've already built this), as I don't yet have a scope.

Switches and buttons, logic indicators, and an LCD (all simple enough), some POTs, and a speaker.

As defined, my major problem is my PSU. Since I'll be running from the mains, is it worth my time and effort to design and build my own? Looking through Jameco, I found the following PSU's which are dirt cheap (less than \$4). It strikes me as odd that they would be so inexpensive, is there some problem with them I am not seeing?

Output voltage (VDC/A): +3.3@10.5; +5V@10.5A; +12V@0.65A
Watts: 70.0

Output voltage (VDC/A): +3.3V@4A; +5V@2A; +12V@3A; +32V@0.5A
Watts: 65.0

It seems to me that the UPS61-1004-T ought to be the best for my purposes. The 32V line is perfect for powering a LM317, to generate my variable voltages, the 5V seems like it is perfect for using as my +5V line, and for cooling the PSU, and powering the internals.

I do have one question though: I'd like to limit the current going into my regulators and the board in general to 0.5A, what is the best way to do this? Simply a current limiting resisitor? Maybe I ought to use the +12V@3A supply, pass it through a 22ohm resistor, and then into a LM7805?

Thanks for all the help!

#### Sebi

##### Active Member
Use L200 ajustable regulator: it have current limit pin.

#### Gandledorf

##### New Member
I couldn't find an L200 regulator, but I did draw up the following design, would this work? I can't figure out how to generate a negative adjustable voltage with what I have from the pre-built PSU though.

#### plot

##### New Member
i would still find some type of relugator insead of using a resistor. resistors are only ideal if your load isn't going to change -- and i'm definatly presuming it will.

i would also recommend finding someway to do sine waves on the function generator, really comes in handy. i'm sure it is possible with a uC, and someone here probably has some type of code that you can adapt to the cause.

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