EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: frampy on October 01, 2012, 09:20:04 am
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Hey guys,
I'm looking into building a lab power supply similar to the linear supply dave constructed in his tutorial series. Being a big fan of recycling old crap, I busted open an old ATX power supply I have lying around, with the intention of using one of the transformers for my supply. My hope is that the supply would have a 230->12V transformer to drive the 12V rail. What I really need is a transformer with two isolated outputs, or simply two transformers. So I can build two isolated supplies for lab use.
I did a few google searches for the big one - looks like it's an ERL35, but can't turn up a datasheet for it. Can anyone help me out?
Cheers.
(http://i.imgur.com/d1TJZ.jpg)
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All ATX power supplies are switch mode supplies. They do not have standard line transformers in them and their high frequency transformers can't be used without the switching circuitry surrounding it or similar. Designing a line powered SMPS is a serious project and one you should not attempt if you are asking these questions.
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As said those transformers will not work for what you want. Devices that contain the transformers you need are things like old audio amplifiers, something from the 1990's would be perfect, after that and into the last couple years audio is likely to use smps . I got a really nice, 24VAC@3A transformer from a logitech pc sound system.
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You can buy cheap toroidal transformers from eBay ... something capable of about 100-200VA can be found at around 20-30$.
Otherwise, you could try going to TV/radio repair guys and see if they have something they're willing to give you for free or for a pack of cigarettes or something like that.
My parents had problems with a TV and I went with my father to one of those guys with the TV and was chatting with him and slipped that I was interested in amplifiers so he gave me a TDA2040 based amp board from a big TV along with a nice transformer for free.
On Farnell and Digikey, you can get higher rated transformers for 50-60$...
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Thanks for the suggestions guys. There's a big car-parts traders show coming up here in Christchurch in two weeks time. There tends to me lots of random junk there so hopefully I'll be able to turn up some gems amongst the junk. In the mean time I'll have a look on ebay.
I'm currently tossing up between using a transformer, or just using an off-the-shelf DC power pack with isolation. The downside of the latter is that I'll need a separate DC supply for each lab supply output.
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Best source of nice mains transformers are halogen lighting transformers. They usually output 12VAC. If you live in 230V mains country you can get a trafo from 110V main country and that way get 24VAC secondacy which makes a nice base for a lab power supply.
Otherwise you can rewind the secondary yourself, as it usually doesn't need many turns and can be wound by hand (although manual winding of toroidal trafos induces heavy minfu.ck with complications). You can get those very cheap and in high power ratings. I remember that I got 2 of those: 400VA and 350VA together for $15 :).
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Oh yeah... the post above reminded me of this.
http://www.amazon.co.uk/ALASKA-DHI1504-Halogen-Adjustable-Desk/dp/B0078R9KYU (http://www.amazon.co.uk/ALASKA-DHI1504-Halogen-Adjustable-Desk/dp/B0078R9KYU)
I've bought one of this for my desk from an European store chain called Praktiker (a home improvement/diy type store), only it was dark green and sold under the "Budget".
The lamp has a 24v halogen bulb so the bottom actually has a 24v 0.9-ish A transformer at the base. Quite heavy and solid looking one.
The lamp was about 15 dollars and it's quite good for my desk, but I naturally opened it to check if the wiring is properly done inside, if it's safe. It is.
edit: I don't know why I felt it's 24v ... correcting myself, it's the standard G4 halogen bulb running at 12v ... so quite ok to get a 12vac 20w transformer out of such lamp.