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.
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.