BJT's from PC SMPS's - OK to use as final ouputs on bench PSU's? - Page 1
Forum section: electronics / beginners

author: Cliff Matthews
author_rank: Unknown
timestamp: March 20, 2015, 01:42:35 am
content: Hello all, since spring is approaching the northern hemisphere, I thought I'd clean up and nab some parts off some old PC power supplies. After sweeping up a few days worth of pcb clippings, I sorted the loot and found almost 200 E13007, E13009 and 2SC2625 power BJT's. All of them are NPN and good for at least 8 amps, 400volts, and about 80watts dissipation. On the board here, I've seen several big bench PSU's using several parallel 2N3055 NPN's - So..?? If I wanted to use six or eight 2SC2625's instead, would that be applicable? (provided I gain-match them?)I also plan to cut costs by using 4 old 24vac 8-amp panel transformers and short pieces of 30-gauge nichrome toaster wire instead of costly ceramic emitter resistors. All the rectifiers employed will be the schottky power tab units also pulled from the PC power supplies, and ditto for many of the big caps, diodes, mov's and filter chokes.

author: SeanB
author_rank: Super Contributor
timestamp: March 20, 2015, 04:39:36 am
content: Yes they will work there, it does not take much to replace a 2N3055.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 20, 2015, 07:32:41 am
content: Hmmm, with the shitty specs of a 2N3055, they'll probably still meet the standard (packaging aside)...Mind that hFE is very much lower, and second breakdown is probably much closer than you'd expect, despite the high voltage rating. Don't expect too much of them.Tim

author: Cliff Matthews
author_rank: Unknown
timestamp: March 20, 2015, 10:51:05 am
content: Thanks for the replies! - T3sl4co1l the specific circuit I saw, used the LM317L as a tracker/pre-driver so perhaps a heat-sinked 317T would avoid that second breakdown?What I'm a little foggy on is the gain slope. If these BJT's look more like switches, won't that also make the LM317's "gyrations" to control a bunch of them in parallel to be more switch-like? So maybe the final output will need better filtering - like two stages of caps separated by a choke?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 20, 2015, 02:53:45 pm
content: "Switching" transistors mean they're optimized for the dynamics of switching, not that their behavior is, in some fundamental way, transformed towards switching alone.Typically, the difference lies in optimizing for a low voltage drop, at expense to gain and capacitance, or current density versus safe operating area (SOA), 2nd breakdown and the like. In other words, that switching transistors are only intended to dissipate very high power levels (like kW+) for very short pulses, not continuously. And that it may take a lot of drive to cross that linear region (between "on" and "off"), but because the power gain is still high (or it can be dealt with regeneratively -- BJT switching circuits are generally built with positive feedback, so a small fraction of output power is coupled into the base circuit to enhance drive and achieve lower saturation voltage), that's a more than acceptable price to pay.Voltage drop is voltage drop, and an LM317 can't dissipate much power; I don't see how a single one would perform any better than one or more of those transistors, even for being "switching" types as they are. What kind of voltage, current and power were you expecting to get, and what example circuits were you looking at?Tim

author: Cliff Matthews
author_rank: Unknown
timestamp: March 20, 2015, 04:32:05 pm
content: Tim, I have some ATC-Frost 24v@8amp class-2 panel transformers, so I'd like to build 4 x 30volt bench PSU's (2 with integrated 45deg. proto boards up front). The design I've seen is where the BJT's take the full load and the LM317 does the tracking.http://electronics-diy.com/electronic_schematic.php?id=654My mods are really minor - based on harvesting of the aforementioned parts. I've already bought the LM317T's. As well I bought 0.1 ohm 25watt shunt resistors, and some LM339 comparators to trip 556 timers driving 30amp Omron relay's I pulled from a huge UPS. This I will use for a basic current control (complete cut-off and an alert tone/light for some period).In the attached photo, some of the almost ubiquitous E13007's are on the bottom left, the 2SC2625's (TO267) are to the right, and at the back is some more of the loot with the relays and transformers. The other packages were what I was tempted to buy in the local parts store (I don't visit much, my wife gets cranky..). They have the 2N3055's but they want $8 each - hence my query on these types of BJT's as final outputs.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 21, 2015, 12:37:03 am
content: That's not really regulated... the 317 doesn't know what the output is doing, so the output resistance depends on the transistors. So it's always going to be a little spongy. Like, give or take a volt, rather than millivolts. It also needs a current limiter, since that 8A transformer will deliver transistor-crackling power under short circuit conditions. Let alone if you were hoping for an adjustable current limit or the like.FWIW, if you were to go out and buy transistors... something MJ15xxx would be a good replacement for the damned 3055, but there are even better options out there (though I don't remember offhand which ones are best). The price does tend to creep up with quality/youth of parts.Tim

author: Whales
author_rank: Super Contributor
timestamp: March 21, 2015, 01:10:58 am
content: Your pile of PSUs and processed parts look beautiful.QuotePresumably the temperature coeff of the nichrome would aid the stability more in this situation?

author: Cliff Matthews
author_rank: Unknown
timestamp: March 21, 2015, 04:16:37 am
content: Quote from: Whales on March 21, 2015, 01:10:58 amYeah, I just have to put all those parts to some good use.As for the nichrome, I really do have some, but it's a bit of a joke at being cheap. I did figure out that a 0.75inch length is all I'd need to pass 1 or 2 amps and still not get much above 150C. Kind of scary though if 3 of 6 transistors blow open... any ideas on home grown resistors?

author: Cliff Matthews
author_rank: Unknown
timestamp: March 21, 2015, 04:38:25 am
content: Quote from: T3sl4co1l on March 21, 2015, 12:37:03 amTim, I see what you mean by "not regulated", honestly I did see it (more or less) but blind trust filled-in the gaps. I'll find another design and continue to sift through the rest of these parts. Hey, I've got most of what I need to get going so a little more research is due. Thanks again.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 21, 2015, 09:37:16 am
content: Just from those parts alone, you ought to have some TL431s and LM324s, or something like that... add in a few transistors (of the C945-C1815-2N3904 range) for level shifting and you've got all the gain you need, and a not-terrible reference (better than a three terminal regulator, anyway). You may have some current sense resistors, but probably not enough of the same values.Tim