Building a transformer

[John Woodgate]

10mA ? That'll power a pair of 5532s !

How quaint. Try different op-amps. In any case, two 5532s will do four
active filters. Sallen and Keyes are quite good enough, unless you want
to do tricks with phase.

 

[John Woodgate]

I ran across info on rectifier design up through filtering (but not
regulation) at:
http://www.ieeta.pt/~alex/docs/ApplicationNotes/Rectifier Applications
%20Handbook.pdf

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.

 

[[email protected]]

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

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

 

[Phil Allison]

** Groper FROM HELL alert

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.

** So a two diode voltage doubler ??

50/60 Hz ripple ??

Bad idea.

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,

** BOLLOCKS

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

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

** BOLLOCKS.

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

Means a 1500 VA tranny for continuous operation.

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.

** 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]

"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

 

[[email protected]]

** Groper FROM HELL alert

thank you for at least warning me.

** So a two diode voltage doubler ??

dont be dense.


NT

 

[Phil Allison]

** MASSIVE GOOGLE FUCKWIT ALERT ~!!~

dont be dense.

** Uranium is translucent compared to this asinine POS.




......... Phil

 

[John Woodgate]

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.

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.

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

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.

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.

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.

 

[jasen]

dont be dense.

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

 

[joseph2k]

What primitive semiconductors did you use in the output stage, or was it a
toob device? Did we have 2N3055s in '75? I don't memember.

Jim

Kinda late but i was playing around with 2n3055's in the late 1960's. Also
2n4400's i think.

 

[joseph2k]

message
On Tue, 8 Aug 2006 13:44:27 -0700, "RST Engineering \(jw\)"


What primitive semiconductors did you use in the output stage, or was
it a
toob device? Did we have 2N3055s in '75? I don't memember.

Jim


[snip]

I'm trying to remember what semiconductors I used. But, IIRC,
2N3055's *were* available around that time.

...Jim Thompson

Was it Ge or Si ?



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

Correction: it _is_ silicon.
http://www.onsemi.com/pub/Collateral/2N3055-D.PDF.

Digi-Key has them for $2.10 a pop, in the TO-3 case, no less.

I said it was silicon. The Ge device I used in HS was a Delco
doorknob power device. I did 10W class-A ;-)

...Jim Thompson

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.

 

[John Woodgate]

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

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

 

[joseph2k]

I ran across info on rectifier design up through filtering (but not
regulation) at:
http://www.ieeta.pt/~alex/docs/ApplicationNotes/Rectifier Applications Handbook.pdf
It is: Rectifier applications handbook - 272 pg - 2MB
From a brief scan it looked very good and more than anyone here wants
to know about rectifiers, including the physics.

A paperback book for transformers:
Practical Transformer Design Handbook; Eric Lowdon; published by Howard
W Sams; 240 pages; 8x11"; my copy is 1981.
It is aimed at experimenters designing single transformers using
salvaged cores. I thought it was very good. It may be hard to find, but
with the internet who knows.

bud--

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

 

[[email protected]]

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

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

 

[[email protected]]

In message <[email protected]>, dated
Sun, 13 Aug 2006, [email protected] writes

You've allowed for all those and forgotten transformer impedance
(usually expressed as 'regulation', when it's the opposite).

see below

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.

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.

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.

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

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.

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

 

[Phil Allison]

2 diodes, half wave rectification.

** Same thing - you FUCKING MORON .





........... Phil

 

[John Woodgate]

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

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.

 

[John Fields]

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

 

[ian field]

Jim Thompson wrote:
in
message
On Tue, 8 Aug 2006 13:44:27 -0700, "RST Engineering \(jw\)"


What primitive semiconductors did you use in the output stage, or was
it a
toob device? Did we have 2N3055s in '75? I don't memember.

Jim


[snip]

I'm trying to remember what semiconductors I used. But, IIRC,
2N3055's *were* available around that time.

...Jim Thompson

Was it Ge or Si ?



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

Correction: it _is_ silicon.
http://www.onsemi.com/pub/Collateral/2N3055-D.PDF.

Digi-Key has them for $2.10 a pop, in the TO-3 case, no less.

I said it was silicon. The Ge device I used in HS was a Delco
doorknob power device. I did 10W class-A ;-)

...Jim Thompson

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.

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

 

[Jim Thompson]

Jim Thompson wrote:
[snip]

I said it was silicon. The Ge device I used in HS was a Delco
doorknob power device. I did 10W class-A ;-)

...Jim Thompson

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.

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

...Jim Thompson