Building a transformer

Greetings to the collective.

For my next project I want to build a bench power supply and do away with this old switch mode AT computer PSU that i'm currently using.

The design I have in mind will be pretty beefy, giving a wide range of fixed and variable output voltages (i'm thinking from -50v up to +50) and deliver up to 5amps of current. if my estimates are right i'll need a 600va+ (100v * 5a + overhead) transformer to do the job well. Now i've had a look at some transformers in this range, and they're a little out of my price range. so now i'm considering building my own.

The specifications I need are as follows:

primary: 0-230v @ 50Hz Secondary: 60v-0-60v secondary output current max: 5A

I have an intermediate understanding of transformers, but have never actually built one, hence I am seeking advice from those with experience in this field. So where do I start? what type of core is best for this application? - I have a friend who's blasting off that toroids are best, but i'd like a second opinion, and what kind of wire to use for the primary and secondaries? what are good safety practices regarding insulation? Is this even a feasable project to undertake (moreso on the money side than timewise). Any other advice also greatly appreciated.

regards, Mark

Reply to
Mark Fortune
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Not really worth the effort, IMHO. However, take a look at All Electronics, Hosfelt Electronics, MCM Electronics. There are others also. They have over run surplus trannys with really good prices. The other alternative is try to find an old high power am fm receiver which is kaput and not worth fixing. They have a transformer with the approximate outputs you seek. You're right, if you have the time, building your own power supply is fun and saves lots of loot. regards, Tom

Reply to
t.hoehler

By the time you have bought the core, be it toroid or E I lams, bought the bobbin (if you use E I) bought two lots of wire, insulating fabric tape etc etc etc., you may as well go and buy the right thing.

We make our own up to 3kVA, and we have all the bobbins, wire, varnish, tapes etc., but before you ask, we don't do one-offs. Most of ours are specials that we have designed by an outside consultant. Above 3kVA we go outside for them as it isn't worth keeping all the steel strip, special wire and bigger winders, varnish dip etc etc.

We have a pair of winding machines plus a hand winder which we can wind up to

3mm copper wire or small rectangular tape.

You may have to look around for what you want, but I promise you it will save you a lot of time and trouble.

Peter

-- Peter & Rita Forbes Email: snipped-for-privacy@easynet.co.uk Web:

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Reply to
Peter A Forbes

No, it won't :-) Your math is right. But what do you expect if you decide to draw

1 Volt at 5 amps from that beast ? Who / what is going to dissipate the 600 - 5 = 595 VA for you ?

Here's your advice: don't even think of starting at 600 VA if you want to roll your first transformer :-)

--
Kind regards,
Gerard Bok
Reply to
Gerard Bok

I had 3 toroids made by Tortran a couple years ago for a homemade audio amp, they where very reasonable. I recommend you tell them to give you a non-thermo protected primary.

Reply to
Modat22

In message , dated Tue, 8 Aug

2006, Mark Fortune writes

You need more than 5 A AC to get 5 A DC. The rough rule is that a bridge rectifier needs a factor of 1.6 times, but with modern huge filter caps, think of at least 1.8 times. That's a BIG transformer, with THICK secondary wire. Not a project to learn on, I would say.

It's not even easy to buy parts for transformers now. Maplin used to do kits - bobbins, laminations etc. , but not now.

To learn about transformer design, you need a textbook and one of the best is a classic - Radio Designer's Handbook, by F Langford Smith, which was reprinted by Newnes (Butterworth-Heinemann/Reed) a few years ago. ISBN0 7506 3635 1.

Modern core material (silicon-iron) is better than the materials mentioned in the book. You may be able to get the information on current materials (maximum recommended induction, core loss power/kg, magnetizing VA/kg) from the web.

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OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
2006 is YMMVI- Your mileage may vary immensely.
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Reply to
John Woodgate

Mark Fortune wrote in news:44d8d62e$0$26602$ snipped-for-privacy@news.zen.co.uk:

By a lot of cheap toroidals on eBay, 5A at some low voltage that doesn't sell well. Outputs in series, inputs in parallel. Switch-select to get coarse adjust, variable regulate over one output's voltage range to get any voltage over a large range with little loss.

Reply to
Lostgallifreyan

In message , dated Tue,

8 Aug 2006, t.hoehler writes

A bit difficult for a person based in UK, perhaps. And 60-0-60 V at around 9 A is a BIG receiver or amplifier. Around 300 W/channel.

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OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
2006 is YMMVI- Your mileage may vary immensely.
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Reply to
John Woodgate

Toroids are relatively lightweight and more efficient that most E-I designs. They also have relatively low stray flux which is nice if that's important to you.

For an uncomplicated requirement like that, simply buy one off the shelf. You have no chance of competing on price, never mind your transformer's unlikely to meet safety regs if you don't know what you're doing ( which clearly you don't since you're asking basic questions ).

Graham

Reply to
Eeyore

I would recommend reading up from every source. I have built torroids using the winding which was orginally a variac. These were 110 volt units. You automatically have a primary so all you need to do is wind the secondaries on top. I have also done the other type of transformer, schelacing.the iron plates. I would also recomend what others have, buy surplus on the market. Combine transformers if you can't find them.

greg

Reply to
zekor

Sounds about right.

Years ago I had a neighbor kid who needed tutoring in Algebra.

I couldn't get him really interested until I sweetened the pie by offering to help him build a 400W guitar amplifier IF he got an "A" in Algebra.

He did get the "A".

We built this beautiful thing (in 1975) that was so heavy that it took two people to carry it, and I had to use fans on the heat-sinks ;-)

Put out 400W RMS into 4 x 16 ohm speakers in parallel.

For my effort his father, President of Anthony Pools, built me a swimming pool at a bargain price.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

Do you have lots of time and money and power transistors to blow?

It's not too hard to build a power supply to your specs. It's a lot harder to build one that will have a MTTF of a week or more. Are you sure you can design a power supply that can survive the accidental short circuit, reverse polarity across a car battery, static discharges, inductive kicks, thermal cycles, and all the things that bench power supplies have to tolerate?

In my experience, bench power supplies blow out about once a year. Blown pass transistors, driver transistors, yuck.

You might consider just souping-up a PC power supply. Put a LM350 and a foldback current limiter, switchable to the +5 and +12v outputs. Add a hefty diode and low ESR capacitor to voltage-double the 12 volts to give you a 24VC source. You can even add a pot where the voltage-sensing wire senses the +3.3 at the motherboard end to get a variable voltage power supply. And the PS specs require over-current and foldback protection on all PC power supplies.

Put two in series (with careful separation of the grounds) and you're up to+48 volts at many man y amps, at very low cost and weight. For example, MPJA has nice brand new 180 watt supplies for $8.95 !!!

Reply to
Ancient_Hacker

[snip]

I'm trying to remember what semiconductors I used. But, IIRC,

2N3055's *were* available around that time.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

"Ancient_Hacker" wrote in news: snipped-for-privacy@h48g2000cwc.googlegroups.com:

Therein lies another nice thought. Lots of people use bench supplies, but not everyone who does knows how to repair them. So if you find a decent one broken you might buy it for peanuts and get it working. If you do, you'll have learned a bit, and saved a lot.

Reply to
Lostgallifreyan

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

Was it Ge or Si ?

Reply to
ian field

You can't beat a microwave oven transformer with a secondary transplant!

Reply to
ian field

It was Silicon. I didn't use any Germanium power devices since I boosted my car radio back when I was a junior in high school.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

I would look at cheap battery chargers.

Note that you can put the primaries in parallel and the secondaries in series to get a higher output voltage by using multiple transformers. Check phasing if you do this.

Reply to
mc

I think what you're saying is "try learning on some smaller transformers first" I might just do that, cos it'd be a PITA if I screw up on a big one and have to unwind it all again :]

Thanks for the info, i'll try and get my library to order a copy.

I'll have a look, thanks

Reply to
Mark Fortune

And I see plenty of old microwaves just laying out on the street destined for the scrap heap around here. thanks for the idea.

Mark

Reply to
Mark Fortune

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