Hello!
I'm trying to make a small gift for a friend, who's going through a rough time in life right now, and could use a little bit of a lift. This gift will need to be powered by a 12v 3a supply. Unfortunately, my budget this month does not permit me to purchase even the absolute cheapest eBay offering ($6.49 + free shipping) and although I do have exactly one suitable supply, I would prefer, since I *only* have one, not to let it go out the door.
Two quick but important points, first...
One, I really would very, very strongly prefer that this not become an economic discussion. My financial situation is controlled, it's just unusually constrained by various factors, and I really would prefer not to have to explain my whole life (which would be somewhat necessary in that context) to deal with the question of "why the heck can't you afford a power brick that's the cost of two lattes and tax at Starbucks?". I have done this in the past, and most people either are unwilling to accept that my circumstances are what they are, or insist that there is something I can and should do to change them (which is not really possible at this time, or I would already be doing it), or they just troll or shame me for what must be my fault because they simply have no room in their minds for the possibility that civilization is not fairly organized, economically or otherwise, and that people can be born into -- or incidentally fall into, in the course of their lives -- circumstances not of their choosing, and my situation is a combination, really, of all three of those. Because this topic is, at best, secondary to the issue at hand, and because it has a remarkable propensity to devolve quite rapidly into a flame war, I will ask that we not go there.
That said, if you absolutely must ask, you may PM me for that -- but you will receive a rather formidable wall of text, and I will get very frustrated, very quickly, very openly, if it becomes apparent that you have not read it in full before responding, because that's who I am.
Further, if you wish to Paypal me the money to buy that brick I mentioned -- sure, PM me for my Paypal address.
Two, mathematics and I have had a very troubled relationship for a very long time. In grade school, I was diagnosed with a "math fluency disorder" -- I was never, to my recollection, given anything more specific than that by way of label. The essential effect, though, is that although I understand math /concepts/ at a very advanced level (I got the idea of squares and square roots in 2nd grade), I struggle to actually execute mathematical problems and formulae to any real degree. Simple addition is something I can handle... multiplication or subtraction involving more than two two-digit numbers, and division essentially in its entirety, are essentially impossible without a calculator. In college, I managed to get through a remedial algebra course, a statistics course, and (somehow) a trig course -- although the latter very nearly ended in rather epic disaster halfway through, and I only survived because, at my tiny liberal arts school, the professor was willing to find enough time in his schedule to one-on-one tutor me for a few hours a week. It was his opinion that, if I wanted to take calculus, I could probably manage, but doing so would "consume my life for a semester". I was pretty sure he was being overly optimistic, and declined... and, sadly, I've lost a lot of what I learned in those courses I *did* take. In my defense, I graduated from the place a bit over ten years ago...!
OK, onto the main part of the show here...
After Googling around a bit, I found a simple DC-to-DC power supply design, consisting of a 7812 and PNP BJT with three passives (two capacitors and a resistor) that I think I'm qualified to build. (The schematic is attached to the end of the post.) I know my limits -- a circuit that I design entirely, or even mostly, myself is probably going to fail to function, especially if it's even slightly more complicated than say a Joule Thief. But this...? Even tacking on a transformer and a diode bridge, which I'm pretty sure is all I need to add... I can't see myself messing this one up. I really can't.
I have some parts from eBay and some parts that I've been able to scavenge up from other places. I have a transformer. I have a TO-3 7812 that I'm about 90% sure does indeed work, extracted from a board someone gave me, that was the controller board of a Commodore 1541 disk drive until the mechanicals were no longer capable of being realigned into functional equipment. I have a KBU1004 bridge out of probably an ATX power supply that had capacitor problems, and two beefy transistors, a B817 and a 2SA1061, both of similar origin, and either of which is at least approximately suitable for this circuit, at least in my estimation. I also have a 0.33uF/330nF polypropylene film "box" capacitor, part # KNB1560, and a whole pile of little yellow eBay-issue 0.1uF/100nF MLCC capacitors... and bags upon bags upon bags of both quarter-watt and half-watt resistors of various values. Most of my resistors are carbon film, but I do have a few old carbon composition ones in there.
I plan to build the circuit deadbug, not because it's easy for me to do it that way (it isn't) but because I have a great whacking spool of either 20awg or 22awg (I forget) solid copper wire with red insulation, that's far better at handling the sort of current loads I'm dealing with, here, than ordinary perfboard would be, which is what I'd use otherwise.
Ultimately, I have three questions that I need answered here.
One. The transformer I have is untraceable, as most transformers are, and it was a gift from another friend who doesn't recall how *he* got it... all he could tell me was "I think it's a power transformer but I don't remember what it's from." I have attached pictures with the transformer in the presence of a ruler, to the end of this post. I have no datasheet. I have no way of obtaining a datasheet. I have no history on this component, and no history is obtainable. In addition to the information present in the photos, I can say that the primary (across the two black wires) is approximately 23 ohms, and that from each secondary to the center tap (from either red wire to the white wire) is about an ohm, and that across the entire secondary (from one red wire to another, with the white wire floating) it's about two ohms. I have not hooked it up to wall power yet -- or /any/ power, for that matter -- so I don't know its voltage characteristics yet... if it becomes readily apparent that that info is needed, however, I can provide it. I can roughly estimate that the power it needs to handle, in this role, is somewhere in the neighborhood of 40-50VA -- the output overall is 36w @ 12vDC, which translates (as I understand it) to 36VA before the rectifier part, but the 7812, being a linear part, is never going to be particularly efficient, so there's going to be a bit of extra load in there to deal with that.
Ultimately... is the transformer I have, based on what little information I can provide, likely to be able to handle the current draw that this supply needs to be able to deal with? or is that utterly impossible to determine without either destroying the transformer, or without going off and building the whole thing and hoping that when I plug it in it won't catch fire and die?
Two. Which is a more suitable transistor for this application, of the two I have? (Again, that's a B817 and a 2SA1061.) or is it somehow a case of, it absolutely doesn't matter in any conceivable way (i.e., there is absolutely no difference to be had between them, operationally or otherwise), or of, neither will work at all, forget it...? Er, in the latter case, I think I have a few TIP122s, one of which I could throw at this if I needed to and it would be suitable. But, IIRC, that's a Darlington, and those are special amplifying transistors and this isn't really the place for that, I'd think.
Three. The schematic gives an example value of three ohms for the resistor. It also gives a pair of (to me) inscrutable formulae that must be used in combination to determine the more correct value of that resistor for a given application. I do not have the requisite electronic or mathematical knowledge to derive /any/ of the values that those formulas require -- can someone run the numbers for me, and work out both the necessary resistance and necessary wattage of that component...? Because I quite literally can't. (Thankfully, the use of a Hakko 926 iron and a spool of 60/40 solder doesn't require all that much in the way of math!)
Thanks everyone in advance
