Building a transformer

In message , dated Fri, 11 Aug 2006, Bud-- writes

Pages 93-97 contain assertions that are very much not true. Approach with great caution. The basic problem is that the difference between a rectifier resistively loaded and one with a filter capacitor is totally obscured, and in the later case, the impression is given that the rectifier still conducts for a full half cycle, whereas it conducts only for a brief period around peak supply voltage. As a consequence, the text on harmonics is rubbish.

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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.

John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK

Need to backtrack on the maths a bit first. To get 50v dc regulated you need 50v dc +5v for regulator drop + 3v for rectifier drop + say 15% to allow for mains sag. Thats 66.7v dc.

Now to get 66.7v dc peak you need 66.7 / 1.414 = 47v ac.

To get +-50v dc you need the same transformer, no reason to have 2 windings at all. So you just need one 50v winding.

Now, current. If you play it within all specs you could use 5A x 1.6 =

8A transformer. But I built supplies for years when younger without using that 1.6 factor and never had any problems from the bridges or tfs, and since this is for personal hobby use I'd be tempted to do that. Or if you need something portable you could even go smaller and add fan cooling.

So you need 50v 5A = 250w transformer.

Toroidals are smaller, lighter, and more money. Microwave transformers should be avoided, they just need too many mods to be worth playing with.

I'd be asking myself if I really needed +-50v at 5A, and would be answering no. I once had a massive all singing psu and almost never used it. A small portable bench supply is a lot easier to use than a meatlump. For those occasional jobs that need more power I'd use something much more basic.

These days I'd buy one, or maybe look at modding a pair of pc psus if for some reaosn thats not an option.

NT

** Groper FROM HELL alert

** So a two diode voltage doubler ?? 50/60 Hz ripple ??

Bad idea.

** BOLLOCKS

The power tranny will COOK at full load with a 60% over current.

** BOLLOCKS.

The OP has specified +/- 50 volts at 5 amps = a 1000 watt DC supply.

Means a 1500 VA tranny for continuous operation.

** Finally, some sane advice.

An unregulated +/- 50 volt DC supply operated off a Variac is all the OP needs.

Just a 600 VA tranny with twin 40 volt windings plus bridge and two hefty electros.

...... Phil

"Phil Allison"

** Correction:

The OP has specified +/- 50 volts at 5 amps = a 500 watt DC supply.

Means a 800 VA tranny for continuous operation.

...... Phil

thank you for at least warning me.

dont be dense.

NT

** MASSIVE GOOGLE FUCKWIT ALERT ~!!~

** Uranium is translucent compared to this asinine POS.

........ Phil

In message , dated Sun, 13 Aug 2006, snipped-for-privacy@care2.com writes

You've allowed for all those and forgotten transformer impedance (usually expressed as 'regulation', when it's the opposite). For this sort of transformer, you can expect the open circuit voltage to be 5% higher than the full-load voltage.

Again, that's the voltage you would get with no output current. Calculating what you get with a given output current is difficult, but you can get an approximate value by assuming that the diodes conduct for (these days, with BIG filter caps) a quarter of a cycle. It's difficult to explain the calculation without a diagram and I don't have time to do it at the moment.

If you were 'younger' 30 or more years ago, and operated your transformers outside of an enclosure, you got away with it. Modern transformers, with those big filter caps (resulting in higher r.m.s. diode current), mean higher temperature rises, and you won't get away with it now. Diodes now have lower losses, so they generally don't overheat.

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

John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK

so how do you get +/- 50V from a single 47V secondary

You can call it two half-wave rectifiers but it's the same circuit as a voltage doubler, and it puts a common mode ripple on the power rails

Bye. Jasen

Kinda late but i was playing around with 2n3055's in the late 1960's. Also

2n4400's i think.

--
 JosephKK
 Gegen dummheit kampfen die Gotter Selbst, vergebens.  
  --Schiller

OK. You have used a "doorknob" power device. What the heck did it look like? Where can i find outline drawings? You are about the third person i have heard admit of their existence.

--
 JosephKK
 Gegen dummheit kampfen die Gotter Selbst, vergebens.  
  --Schiller

In message , dated Mon, 14 Aug 2006, jasen writes

What do you mean by 'common-mode ripple'?

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

John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK

I took a look at the ieeta handbook. Yipee, this is something i want. You are right about the heavy physics, not for everybody. Thanks

--
 JosephKK
 Gegen dummheit kampfen die Gotter Selbst, vergebens.  
  --Schiller

2 diodes, half wave rectification. That way the OP can get 5A 50v or 2.5A +/- 50v. Which may or may not be what he wanted, and almost certainly isnt what he now wants, or needs.

NT

see below

Thats what you get under full load, no load V will be higher. Transformers are rated at voltage under full load. Offload their V_out can be well above for tiddlers.

yes, there is that, which I didnt take into account, and that will lower the real world Vout under load.

I was referring to small enclosed transformers I used (typ 6VA). Even without that 1.6 factor they barely got warm.

I once wanted a very small high power device so experimentally I ran a

50VA tf way over its ratings on a heatsink, attaching it to the exterior of the case so it saw good ventilation. This confirmed my suspicion that trannies tend to be conservatively rated. I cant remember for sure, but I think I may have loaded it somewhere around 2.5x its rating for a couple of hours at a time, and it lasted long term. Ran very hot, but survived. V_out sagged of course.

If the OP wanted a beefy supply but also wanted portability, and its only for personal hobby use, pushing a smaller tf is a workable option. SMPS is a better option technically, but then someone that needs to learn about linear supplies wouldnt learn a lot, as the smps would be rather over their head - and a tough project for such person.

Whats good for hobby and whats good for pro design are 2 different things. People play it too safe when it comes to hobby stuff :)

NT

** Same thing - you FUCKING MORON .

.......... Phil

In message , dated Mon,

14 Aug 2006, joseph2k writes

There are also a lot of 'mistakes' in it. Schade's curves, for example, were derived in 1943 for valve/tube rectifier diodes and aren't correct for semiconductor diodes, which have a much lower 'on-state' resistance. Some of the waveform diagrams are positively weird, and don't aid understanding at all.

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

John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK

--
John Fields
Professional Circuit Designer

There were "doorknob" tubes as well - which actually looked like glass doorknobs.

[snip]

I probably have an appropriate data book around here somewhere. I'll look for it.

...Jim Thompson

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