Posts Tagged ‘power supply’

Replacement power supply build

June 20, 2013

I say build, it’s more a mod.  A long ,long time ago in a college town far away I helped a friend move into his frat house.  After I did this one of the housemates gave me an old monitor and power supply simply because he didn’t need it.  I gladly accepted it, but when I checked to make sure it was the right power adapter I noted it was a 12v 1a one, the display called for a 16v 3a one.  Now, I don’t know exactly the audience I have (yes I do, it’s web spiders) but I’ll tell you right now: there’s no way that will work.  I mentioned this at the time, but was assured that that was the adapter he always used for it (no it wasn’t).  My persistence at wanting this thing to work may seem misguided, but here’s the thing: this isn’t a computer monitor, it’s a TV.  Now I know what you’re thinking  that’s worse right? Right.  But in this case I don’t want better, I want more.  This TV (Phillips 20pf5120/28) has a cable tuner (useless), a composite input (expected), an s-video input (expected, but appreciated), a component input (cool! my first component input), and a dvi input (what?).  The DVI input can only do a resolution of 640×480 (and only digital), but it’s a 20″ 4×3 LCD TV, what more can you expect?  Let’s look at the specs for a new power adapter, shall we? 16v DC: that’s not too hard, if it were 12 or lower you could use a linear reg on a computer power supply to get that, but it’s not too bad.  3a: here we have a problem; that much current usually warrants the power supply to be inside the device it’s powering so those are a bit hard to come by, or at least they were.  I say that because we are now in the era of scrap laptops.  Yes, that’s right, laptop computers are being thrown out left and right for all manner of faults: broken screen, dead battery, won’t boot, broken power adapter (more on this later).  The thing about laptops is that they take a lot of power, sometimes upwards of 4.5a.  The problem now becomes the voltage.  Laptops generally run off of somewhere between 18.5v and 20v to charge their lithium ion batteries.  Let’s see what the junk drawer has to offer.

image taken post mod

We have a nice 19v 3.25a laptop power supply: perfect! Ok, there’s still that voltage problem; there are two ways to tackle this: a switching regulator, or a linear regulator. The switching regulator is a much more efficient design, but requires parts I did not have on hand.  The linear regulators could not pass enough current (1.5a max) but that’s ok, we’ll just put 2 of them in parallel to get that extra current through.

The design of the linear regulator circuit is simple, we use a fixed voltage linear regulator and a voltage divider made out of two specially chosen resistors to set the voltage to whatever we want.  For this one I just pulled out my phone and used electrodroid, but the calculation is simple to do if you want.  The heatsinks are mandatory since I’m running these regulators at the ragged edge of their tolerances.  There you go, a 16v 3a dc power supply that will give you second degree burns if you handle it wrong.

I swear I build these things better when they aren’t just going to be used by me

DIY Power Supply

June 15, 2013

This was one of those projects I knew I had all the parts for and it just took being bored one night to decide to actually do it.  I decided I wanted to make my own power supply, not just a modified computer power supply, but entirely from scratch.  This may not have been the best idea since it’s entirely based on linear regulators (I did put heatsinks on them, but no ventilation).  Anyways, here it is:

I should emphasize that I purchased none of the parts to build this project myself, they were all either donated, salvaged, or found.

The base circuit for this I got from here and I modified it slightly to suit my needs.  I decided that I wanted 5, 12, 3.3, and a variable voltage output (at this point I realize I could have done all of this with a computer power supply, but too bad!).  The second place I got inspiration was this EEVblog post, I decided I needed some more equipment and built some.  Now, some design decisions based on parts on hand.

First, it only goes up to about 14vDC.  it was the best transformer I had on hand at the time (although before rectification I read it as about 12vAC, I have no idea why it’s higher once rectified).

Second, I have fine and coarse knobs since I couldn’t find my 10 turn pots (they had ball bearing based planetary gears).

Third, I opted not to have power cut switches for the regulator inputs (space concerns) but I do have output switches. I originally wanted to cut power to the regulators because they would just be dissipating heat otherwise.

<where my schematic would be if I bothered to make one>

OK, now that that is out of the way I can explain a bit of my design process.  I took the base circuit from Raijuu.net and modified it to suit my needs.  I didn’t put in a fuse since I didn’t have one on hand (well, I didn’t have a panel mount holder).  The power connector, rectifier, and power switch are from a computer power supply, the transformer came out of some old piece of equipment, probably a radio, or amp, or something (it was in my ‘transformers drawer’), the large filter capacitor is of unknown salvaged origin.

As you can probably already see the entire thing is put together with computer power supply wiring and ethernet cable (how else would you know I built it?).

Up to this stage we have a 14vdDC power supply with a switch, now let’s get some useful voltages.  The 5v and 12v legs of the power supply were built exactly according to the Raijuu.net schematic, except I calculated the resistor for 25mA (1.2v red LED) and I put a switch between the output and the LED.  The 3.3v leg was basically the same exact circuit as the Raijuu.net variable leg except with r1 and r2 fixed.

The variable portion of this build was the same as the 3.3v leg, I had some pots lying around from an old equalizer whose transformer blew and figured a 1k and 5k in series would constitute decent fine and coarse pots.  I called that a 6k r2 and set r1 toso the max voltage was just below the point where the lm317 started freaking out about not enough input voltage.

That basically covers the entire circuit, the enclosure is a standard radio shack project box I got from a friend’s box of un-finished projects (I think), the screws for this did not come with the box, I assume the originals were pan head and these are not.

The banana jacks came off an old electronic educational board (the sort of thing that exists so you can build circuits with just banana cables).

The project was compressed onto this proto board as much as possible, but the LEDs and resistors were done hanging.


This has been a simple overview of my power supply (I intended on posting this a long long time ago, but I got sidetracked thinking I was going to make a schematic).