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Forum section: electronics / general

author: ambie401
author_rank: Newbie
timestamp: March 25, 2013, 12:47:50 am
content: Hello everyone,i am putting together my first lab and cannot afford to purchase a power supplyfor my bench, is there anyone out there willing to let one go for free?Thanks AgainAmber R

author: mariush
author_rank: Super Contributor
timestamp: March 25, 2013, 01:05:08 am
content: Yes, sure, with only 1 post on this forum I'm sure everyone will jump at the chance of giving you something for free. We're all stupid rich here.

author: jh15
author_rank: Frequent Contributor
timestamp: March 28, 2013, 05:16:13 am
content: Hey cut him some slack,I've tossed or paid to get rid of old supplies a beginner would like to have.If he/she made it this far into an engineering forum, then they are welcome to ask.

author: codeboy2k
author_rank: Super Contributor
timestamp: March 28, 2013, 06:10:10 am
content: Quote from: jh15 on March 28, 2013, 05:16:13 amI presume Amber is a she.Also Amber should create new posts with more useful titles.A thread title simply "wanted" in a forum category called "Buy/Sell/Wanted" doesn't help much from outside the thread.And to make my post more constructive, I'll advise Amber to check with a local University or high-school engineering or physics lab, they might have old power supplies just hanging around that they are too lazy or just don't have the time to cleanup.. they might be happy to let you have it.2nd, check on Craigslist for someone getting rid of any VCR's , or ideally an old integrated stereo Amplifier. These have large power transformers, VCR's less so. But either one can get you started building a power supply from any number of schematics online or on this forum.

author: madshaman
author_rank: Frequent Contributor
timestamp: March 28, 2013, 06:20:31 am
content: Quote from: codeboy2k on March 28, 2013, 06:10:10 amHonestly, if she's going to build one, by the time she buys a bridge rectifier, voltage regulator, opamps, pass transistor, heat sinks, filter caps, breadboard, fuse holder and fuse, pots, etc. the transformer isn't going to be that much of the cost. :-(e.g.:http://www.digikey.ca/product-detail/en/VPP24-2330/237-1092-ND/242536I know a lot of that stuff could be scrounged, but...

author: madshaman
author_rank: Frequent Contributor
timestamp: March 28, 2013, 06:39:55 am
content: I take that back actually, thinking about it for half a second, all that stuff can be scrounged without too much difficult, and would probably be educational.With a combination of garbage picking and a couple visits to the thrift store to buy some old electronic toys and maybe some ancient stereo, one would have everything they need. I got too old...

author: madshaman
author_rank: Frequent Contributor
timestamp: March 28, 2013, 06:47:36 am
content: Quote from: codeboy2k on March 28, 2013, 06:10:10 amActually that's a damn good suggestion, I'd bet money you could build an entire bench supply out of the components in just one circa 70s/80s stereo amp. Even better if it's got a built-in radio receiver.

author: codeboy2k
author_rank: Super Contributor
timestamp: March 28, 2013, 07:56:25 am
content: Yep. those old 80's even 90's 5.1 and 7.1 channel amps have 300, 400W of power into 8 ohm loads. That's around 7 Amps, and they usually have positive and negative rails between 50V to 60V DC each.That means there is often 2 isolated 45-50VAC windings for the amplifier power rails, and another one or two isolated windings for the electronics.The transformer would seem custom made for a 2 channel or 3 channel power supply, 0-30VDC @ 5A. 0-6V@1A easyI'm looking to make one myself that wayI've priced out transformers...They are the single most expensive part... upwards of a hundred dollars for a 300VA. Adding more taps and isolated windings make the price go higher, up to $200 or more. The recently reviewed ATTEN PSU has a huge transformer that is likely half the BOM cost.that PSU sells for about $450 USD or so online, and I figure if I can get a hefty transformer for next to nothing from an old integrated receiver/amplifier then I'm way ahead of the game. Often times it will have decent pass transistors and rectifiers as well, as you've said. Most of the parts would be there already, just needed to be ripped out and rebuilt in a new case with regulation and protections.

author: ttp
author_rank: Regular Contributor
timestamp: March 28, 2013, 11:00:57 am
content: Quote from: codeboy2k on March 28, 2013, 06:10:10 amLocation would help too, generally power supplies are not light and shipping/postage can be expensive.

author: casinada
author_rank: Frequent Contributor
timestamp: March 28, 2013, 04:41:20 pm
content: May be Ambie is not of a driving age yet. May be can get the parents involved as we also don't want to be sending gear to a minor without the parent's permission.

author: madshaman
author_rank: Frequent Contributor
timestamp: March 28, 2013, 04:54:03 pm
content: Quote from: codeboy2k on March 28, 2013, 07:56:25 am1) There's a reason I periodically type "toroid winder" into eBay search2) Agree, large transformers with multiple taps aren't cheap, but I figured she'd wanted to start with something like a 20V 1-2A supply.Still, an old audio amp has got everything, from small signal to power devices, even the pots, could even use the speaker hookups for cheesy output terminals, what a great idea :-) I might just make a round to the thrift stores this weekend and have some fun.

author: codeboy2k
author_rank: Super Contributor
timestamp: March 28, 2013, 07:22:10 pm
content: Quote from: madshaman on March 28, 2013, 04:54:03 pmThe OP can get that size transformer easily from any old VCR that craigslisters are always giving away. VCR's seem to have transformers that can provide about 1-2A at 20V-36V or so. Also, I got a nice 3A/40V transformer from a largish boombox once.QuoteYep I have scavenged those speaker hookups for output terminals before. Some amps have binding posts, most have spring clips, but they work ok. The pots can often be rotary encoders. plus you often get a decent vacuum fluorescent display (VFD) to play with. VCR's are loaded with those too.