power supply explanation

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This news group business is a hard path to tread what with cranky
australians and americans with money and magical Radio Shacks that have
just what you want.
Most of the replies were courteous and may have been helpful if I were
into design
My existing power supply I made over fourty years ago with some ca130 op
amps and a 3055.
I did not design the circuit [it came from radio and hobbies or
electronics australia or some such magazine]
I wound the transformer myself on c laminations [I had access to a
winding shop then].
I etched the circuit board myself made my own case out of sheet steel
Calibrated my own meter face.Only paid for components.
It was 0-20 V and 0-2.5 A [you could choose volts or amps to control]
I have lost this circuit and this power supply is starting to become
unstable so I thought now would be the time to make something beefier.
As you can gather I like making things myself from scratch
Perhaps I was silly thinking newsgroups might be where I might find some
clues to a suitable circuit

Re: power supply explanation


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Specific circuits are generally hard to come by on the internet.

Just google "circuit variable voltage supply" and you will probably find
everything there is. Trying to make your own variable voltage supply is
pretty uncommon these days, considering you can by them for $50 - $100.



Re: power supply explanation
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I was trying for controlled current and controlled voltage regulation my
old one had switch so that you could vary the current drawn using a pot
or the voltage from a different pot if the switch was the other way. The
regulated bench power supplies of 0to 20 or 30v up to 4A seem to be $200
or more

Re: power supply explanation
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I have found a circuit which might do me

http://www.electronics-lab.com/projects/power/003/index.html

Re: power supply explanation

----- Original Message -----
Newsgroups:
aus.electronics,alt.binaries.schematics.electronic,sci.electronics.basics
Sent: Wednesday, June 11, 2008 3:57 AM
Subject: power supply explanation


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We shall not name any names, of course!  ;)

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Perhaps at one time they did.  Not any more.

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I'm not familiar with power supplies that have separate controls for voltage
and current regulation, where you would have to disable voltage regulation
in order to achieve current control (limiting).  You don't really need such
a switch for most applications.
Consider the standard, voltage-regulated, current-limited power supply:
when current reaches the setpoint and the current-limiting loop kicks in,
voltage regulation gets cut out of the loop anyway.  So why have a switch?
Here's an example of a voltage regulator with current limiting:
http://www.rason.org/Projects/discreg/discreg.htm
His claims for "performance" are a bit exaggerated.  I think you might get
better line regulation and a flatter temperature response with an LM317 or
LM723.
This link goes over the fundamentals of linear voltage regulation pretty
well:
http://www.national.com/appinfo/power/files/f4.pdf




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the switch on my old supply controlled weather current or voltage were
controlling. the meter showed either current or voltage depending on the
switch. The potentiometers controlled the current or voltage ie if the
current was set at say .5 amps then that is the current regardless of
where the voltage pot was unless it was too low. The voltage was limited
for that current
Visa versa if the switch was other way


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Re: power supply explanation
On Wed, 11 Jun 2008 05:39:12 -0700, "Michael Robinson" <kellrobinson


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---
They're common, and usually called CV/CC supplies:

http://www.alliancetesteq.com/pdf/agilent_hp_6216a.pdf

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Re: power supply explanation
On Wed, 11 Jun 2008 09:05:54 -0500, John Fields

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---
Oops...

You don't need the switch to change modes, it's just there to switch
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Re: power supply explanation

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I'd be tempted to repair the old circuit, first. Things like pots,
caps and switches do eventually wear out, and it's simpler fixing
something you're familiar with. The 3130/3140 is still alive and well
as a replacement part, from distributors like DigiKey. It wouldn't be
the first suspect in troubleshooting flakey behavior, though.

You might be surprized just how similar your old circuit is to the new
kit you've found. Beefing your old circuit up might simply involve
changing a few components and adding a larger heatsink.

RL

Re: power supply explanation
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You've been told what to do. Go to (Australian suppliers, since I read this in
a.e):
<http://www.jaycar.com.au/productView.asp?ID=ZV1630&CATID=&keywords=LM338&form=KEYWORD
or <http://www.altronics.com.au/index.asp?area=item&id=Z0565 or another
supplier, and
get the data sheets from e.g.
<http://www.jaycar.com.au/images_uploaded/LM138.PDF
or the manufacturer: <http://www.national.com/mpf/LM/LM338.html>, and just build
the
damn thing using the information that I found there in 2 minutes searching.

If you want adjustable fold-back current limiting (to protect the devices you're
supplying as opposed to protecting the supply itself) you'll have to look a
little
further, but NatSemi's website really gives all the help you could need. If you
find folk are a little cranky, it's because you seem happy to ask us to write
answers,
but you don't seem willing to read them and take their advice.

Clifford Heath.

Re: power supply explanation
F Murtz wrote private email.... responding here.

I understand what you want better now. When you say "control current",
you mean that you want your supply to act as a current source. That's
the correct term to use when searching for information on that aspect.
Current sources feign infinite impedance, voltage sources feign zero
impedance.

Most power supplies do not act in this way. It's just not what people
want from a power supply very often; and when they do, they often use
a specialised current source circuit, not one that's built into their
power supply. The CC/CV supplies that John mentions are the exception.

What people *do* want is for the power supply to detect overload conditions
and rather than just limit the current, to actually reduce the current down
to near zero. This means that for a given output current, there are two
possible output voltages; the voltage that is set on the control pot, and
the voltage that results in the circuit drawing that amount of current.

This sort of current limiting is called "foldback current limiting", and
is almost certainly what you should be building into your supply. A CC
supply does not do this, and unless used carefully will often blow things
up by delivering an excessive current *and* voltage at the same time.

That said, if you can build a new supply, you can fix the old one.
If you can't, you should buy a new one.

Clifford Heath

Re: power supply explanation

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Well, I _did_ suggest that you check out the ARRL Handbook, which has a
nice adjustable supply project with pretty much those specifications or
better.  I don't know how hard it'd be to get one in Oz, but certainly it
isn't impossible.

And if Radio Shack has more for it than the bannana jacks and some
overpriced, underperforming meters, I'd be surprised.

--
Tim Wescott
Control systems and communications consulting
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Re: power supply explanation
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Radio Shack rescues me regularly. My local one has a nice supply of
overpriced transistors, LEDs, switches, relays, connectors, etc, all stored
neatly in extremely nice metal drawers, and a bunch of sales people who try
to sell me cell phones and radio controlled cars. However, when I need a
widget, and don't want to drive to Frys or wait for mail order, it
occasionally has what I need.

Regards,
  Bob Monsen


Re: power supply explanation

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Yep. I agree. In SoCal, Frys is about the best place that you can still
drive to and get parts, but Radio Shack still can be useful certain ICs,
transistors, leds, switches, connectors, and cheap/inexpensive project
boxes. Of course, neither compares to Digikey.

Bob
--
== NOTE: I automatically delete all Google Group posts due to uncontrolled
SPAM ==



Re: power supply explanation

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Maplin in the UK nowadays.

Chris

--



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Me, too -- but they don't have _everything_.

I was in there last night buying jumpers, and it occurred to me that you
could build entire amateur radio transceivers just from Rat-Shack parts
and some creativity.  It would be a cool contest, if you had the time.

--

Tim Wescott
Wescott Design Services
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Re: power supply explanation

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I smell a new reality tv show. That is one that I would spend my time
watching or even participating in.

Bob
--
== NOTE: I automatically delete all Google Group posts due to uncontrolled
SPAM ==



Re: power supply explanation
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There actually was a very cool reality show that I caught a few times (on
PBS in the US, of course) that pitted groups of scientists against each
other to build something cool out of primitive/everyday parts. Sadly, I
can't remember much about it, other than existence. Perhaps someone else can
fill in details.

Regards,
 bob Monsen



Re: power supply explanation

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I remembered the show, and you got my curiosity going, so I Googled it.  
Here it is:
http://www.shoppbs.org/product/index.jsp?productId19%15658&cp =
2729315.2951549&pg=2&fbx=0&fbn=AS+SEEN+ON+PBS%7CYes&f=PAD%
2FProduction+Year+Range%2F2004-2005&f=PAD%2FAS+SEEN+ON+PBS%2FYes&fbc=1
&fbc=1&parentPage=family





Re: power supply explanation



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Some caps (electrolytics) are likely at their end of life. Just replace
them.

Graham


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