LM723 Based Power Supply

[Anand P. Paralkar]

Hi Everybody,

I need to build my own power supply. I need a proven circuit for 0 to
30V DC variable power supply with minimum 2.5 A current output.

I thought I had hit on something good here:


http://www.next.gr/inside-circuits/0-30-Volts-2-5A-Variable-Power-Supply-l4107.html

But then, I see some comments on this page saying that there could be
errors in the circuit.

Could some expert(s) please confirm that this circuit would work fine?
(Or point to something better.)

Thanks for your help.

Thanks,
Anand

 

[Rich Webb]

Hi Everybody,

I need to build my own power supply. I need a proven circuit for 0 to
30V DC variable power supply with minimum 2.5 A current output.

I thought I had hit on something good here:


http://www.next.gr/inside-circuits/0-30-Volts-2-5A-Variable-Power-Supply-l4107.html

But then, I see some comments on this page saying that there could be
errors in the circuit.

Could some expert(s) please confirm that this circuit would work fine?
(Or point to something better.)

The datasheet <http://www.national.com/mpf/LM/LM723.html#Overview> has
some example designs that can give you a place to start.

 

[Nico Coesel]

Hi Everybody,

I need to build my own power supply. I need a proven circuit for 0 to
30V DC variable power supply with minimum 2.5 A current output.

(Or point to something better.)

IIRC there is also an LM350. Much easier but it won't go below 1.25V.
Another problem is power dissipation. You probably need some circuitry
to switch the secondary transformer turns.

 

[Hammy]

[email protected]
m:

If it needs to be 'proven', you need to BUY, not build.
30VDC output means at least 33V input, and at low-voltage
that puts out 33V x 2.5A = 82W, which means a single
inexpensive transistor isn't gonna handle the output.

For such small power pretty well any cheap bjt or fet in say a
to247,TO-3PN or ISOPLUS247 is fine you'll need two maybe three for
the pass device. Notice I didnt say T0-3 and similiar because
heatsinks are more widely avilable for the other packages. A $20 or
less heatsink should be good enough.Mount them on something like
this

http://canada.newark.com/h-s-marston/43dn-01000-a-200/heat-sink-
extrusion/dp/46WX664.

I was easily dissapateing 120W useing two T0-3PN fets on a heatsink
from a microwave oven with a $5 42CFM fan. The case temp was at
about 90C.The heatsink was less then half the size of the one in the
link.

The only time you are going to dissapate 82W would be in the event
of catastophic failure. The 723 has foldback protection if it fails
the supply should be fused anyway.

There's a half dozen tricks that will help: the input power
can be transformer-tap-selected to lower the waste, or
the pass transistors can be substituted with regulator
ICs (which have inbuilt thermal limiting and won't burn up),
or you can use two regulators with different voltage
ranges, and diode-isolate so they share the output pin.

The easiest thing to do, though, is just to make a brute-force
DC power supply, and plug it into a Variac when you want to
adjust it. That works, it's efficient, and the Variac (variable
autotransformer) comes in handy for lots of other things.

A 160VA or better variac is pretty pricey though and bulky, they are
definetly useful though.

 

[Tim Williams]

It's a good ... transistor

A laughable claim!

At least use MJ15001 for starters. 2N3055 is old and should be forgotten.

Tim

 

[Hammy]

com...

A laughable claim!

At least use MJ15001 for starters. 2N3055 is old and should be
forgotten.

Tim

Whats wrong with a P-FET. $8 AT Digikey 3 pages of 300W plus
pacakages to choose from.

http://ixdev.ixys.com/DataSheet/DS99986A(IXTH-T90P10P).pdf

IXYS makes a linear line of fets too,nice. ;-)

http://tinyurl.com/39jq5yq

The next high power regulator I make is going to be N-FETS for the
pass device; much larger selection, cheaper heatsinks with a larger
selection and no stability concerns at least compared to a PFET or
PNP.

 

[Hammy]

[snip]

Has anybody mentioned switchers in this thread?
Seems so much 'cooler'.

He wants 0-30 out I've seen variable output flybacks in that power
range but they cant vary that widely.

This one here is 18-24V @85W out.

http://www.onsemi.com/pub/Collateral/DN06038-D.PDF

You could use an SMPS on the front end to step down the line voltge.
That way you can save the size and expense of a 50/60Hz transformer.
Then just use an op-amp based regulator driving a fet pass device on a
brick heatsink for your variable output.

Given the size of some of the higher power 300W plus benchtop power
supplies today they have to be SMPS's at least the front end.

 

[Hammy]

[snip]

Has anybody mentioned switchers in this thread?
Seems so much 'cooler'.

He wants 0-30 out I've seen variable output flybacks in that power
range but they cant vary that widely.

But my SEPIC can:
http://panteltje.com/panteltje/pic/pwr_pic/
well at least to 24 V, with a bit more input, say 18V it could do
30 I think.

Nice project. What is the ripple like?

I see your bench is nice and clean just like mine.;-)

Yes, nice.

There actually is an other way, back in the sixties I did a linear
regulator with a thyristor pre-regulator.
That pre-regulator just kept about 5V over the series transistor
at all times. So dissipation was always rather low.
Very simple circuit, one UJT, about 5 transistors,
had current foldback too.

That is the ideal minimze the Vi-Vout difference.The only thing is
you still need that big 50/60Hz transformer. They get really big and
expensive once you get into the 500VA range for say a 300W output.

I've been noticing a lot of the newer variable output benchtop
supplies are now SMPS. I'm guessing they just use the SMPS as a
tracking pre-regulator feed that to an opamp based linear regulator.

 

[Hammy]

@n13g2000vbn.googlegroups.co
m:

Yep, a MJ15001 would do the job with one pass element.


2N3055: four for $8; MJ15001: two for $8

Also, to get a low-voltage output (2V at 2.5A) your drive
requirement on that high-side pass PMOS transistor would require
a negative power supply (-10V to be safe) in addition to the
positive one.

How so? The gate voltage is derived from the input voltage not the
output. The drain is connected to the load.

You will need a negative voltage anyways though if you want to get
down to 0V other then into a short.

I did some more looking their are some P-FETS at digikey in 300W or
greater packages at $4.

To use an NMOS, your 30V output would require
drive power 40V or so.

Charge pump IC's are cheap you can even use a 555 timer. Granted its
a few more components but it will give better performance and will
give you a much wider selection in pass devices. I think their are
even some integrated highside N-FET LDO controllers out their that
can be used.

 

[Tim Williams]

Also, to get a low-voltage output (2V at 2.5A) your drive
requirement on that high-side pass PMOS transistor would require
a negative power supply (-10V to be safe) in addition to the
positive one. To use an NMOS, your 30V output would require drive
power 40V or so.

Nahh, PFET is an LDO style thing. Annoying to compensate, but that's just
something to do.

Tim

 

[Tim Williams]

What if you have hundreds of surplus 2N3055 you got for nothing?

Scrap iron is doing good these days. ;-)

Tim

 

[Tim Williams]

Iron? These are mostly tinned copper.

Even better. I'll pay you 10% over shipping for the whole load.

Tim

 

[Fred Abse]

Drop off at DC? They aren't useless unless you don't know shit about
linear power supplies. There were literally millions of linear power
supplies built with 2N3055. A lot of companies made their entire income
from the LM723 & 2N3055.

A fair few of those suffered from HF instability, as in "I can hear you
three megs up". Layout was everything, otherwise the only cure was to
reduce the loop gain and to hell with the degradation in regulation.

Glad to be rid of both devices.

 

[F. Bertolazzi]

Michael A. Terrell:

Keyboards are easy to clean. Stand it against the wall and spray a
fine mist of household ammonia or Windex. Bet it stand there a few
minutes and spray it again. A few more minutes and spray it with tap
water, or even better, use distilled water. Let it air dry oer night
and it should be as clean as new. I've done hundreds this way.

Have you ever tried http://tinyurl.com/25kgvxf moist with plain water?

 

[F. Bertolazzi]

Michael A. Terrell:

The full link:

<http://solutions.3m.com/wps/portal/...E3E02LECIE20S4K7_nid=7WN0Q227GJbeC8M57VBG46gl>

Useful onli for big, big screens. ;-)

I use ammonia to remove body oils from the keycaps.

That cloth is incredibly effective with finger grease.
Also very useful if you are victim of friends and colleagues that have the
need of pointing at the screen with oily fingers.

How are you going to use a cleaning cloth to clean between the keys
without spending hours? I can clean a dozen at a time with my method.

I only have two keyboards.

 

[Rich Grise]

The full link:

<http://solutions.3m.com/wps/portal/...E3E02LECIE20S4K7_nid=7WN0Q227GJbeC8M57VBG46gl>

I use ammonia to remove body oils from the keycaps. How are you
going to use a cleaning cloth to clean between the keys without spending
hours? I can clean a dozen at a time with my method.

I've used dish soap and a brush with long bristles. Well, first I blow out
the dust and ashes with shop air, then scrub with dish soap, rinse with
clear water, blow dry with shop air, and let it sit for a day. Works
like a champ!

Cheers!
Rich

 

[Rich Grise]

Why work harder than you have to?

I needed the scrub brush because the keytops were so thick with finger
crud that you couldn't see the letters. I touch-type, so it was no big
deal to me, but one day I had to have everybody come into my office to
set their email password, and they uniformly complained about not being
able to see the keys.

And the brush was definitely less work than popping and cleaning
individual key tops. ;-)

Cheers!
Rich

 

[F. Bertolazzi]

Michael A. Terrell:

Useful onli [sic. !] for big, big screens. ;-)
I only have two keyboards.

I have hundreds.

Bigger and "plentyer". What a guy! ;-)

 

[Rich Grise]

So is the ammonia.

 

[Warren]

Tim Williams expounded in

groups.com...

A laughable claim!

At least use MJ15001 for starters. 2N3055 is old and
should be forgotten.

Tim

Mouser lists MJ15001 as "End of Life: Scheduled for
obsolescence and will be discontinued by the supplier.".

I suspect that the 2N3055 will still be around still, long
after it is gone.

Warren