# power supply circuit

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I would like to make an adjustable power supply using a transformer with
a 24v 5a single wound secondary I would like the voltage to be
adjustable from lowest to highest possible given transformer I would
like to have regulated current

There is another possibility I also have a larger transformer with two
32v 4.7a secondaries If anyone has a schematic for an adjustable power
supply with adjustable current regulation making the most of the
transformer specs I would be eternally grateful

Re: power supply circuit
Re: power supply circuit

How much current do you want?

Lets just assume you make DC from the AC and then regulate down.

Doing a couple of figures for making 4A into lets say zero volts:

24V * sqrt(2) = 34V

34V * 4A = 136W

The the regulator circuit needs to get rid of 136W.

To do this, you need something like 10 TO-220 packages transistors
screwed down to a quite large chunk of fins.

If you have enough fin area, about 5 to 7 TIP-35 or TIP-36 transistors
would work for you.

Another question is how fast the current limiting or regulation needs
to act.  A lot of very practical designs can make a large spike of
current when suddenly shorted.  The power supply will easily survive
this but if the short was you connecting it to a diode, the diode may
be damaged.

Do you need instant (or nearly so) action on the current limiting?

Take a look at the LT3080:
http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1003,C1040,C1055,P38503

Notice how it lets you set the output voltage all the way down to
zero.  This sort of design is a good place to start your thinking.

You can also think of the circuit as just a non-inverting op-amp
amplifying the output of the pot you use to set the voltage.  This is
another good direction to go.

Re: power supply circuit

Minor points - the transformer isn't rated to deliver 136W, and that 34V is

Re: power supply circuit

When working out the power in the regulator, it is best to assume that
the peak voltage is what you get.  The rated voltage on the
transformer is for a normal line voltage.  In the high line case, you
can get a little more.

Re: power supply circuit

I appreciate that, but my original comments stand.  There is no valid reason to
state "the regulator circuit needs to get rid of 136W" when the transformer is
not rated to deliver that power.

Re: power supply circuit
wrote:

You seem to have a poor understand of what the rating of a transformer
means.

Re: power supply circuit

wrote:

to

On the contrary.  A 24V 5A secondary is rated for 120VA.  Your education
theory.

Re: power supply circuit
wrote:
wrote:

with

is

reason to

is

Yes that is the *rating* in VA and yes that works out to 120/1.580%W
of power from the rectifier.  It however is far below the limit of
what you can actually get out of the transformer.  It is what the
maker says the transformer can do forever on a normal line voltage and
usually with an air temperature of some 50C.  The transformer does not
contain any significant current limiting.  The output impedance is
fairly low.  The heat sinking on the regulator needs to be designed to
withstand the high line case with a short circuit on the output.  This
is a lot more than the rated power of the transformer.

If the OP goes with less than the amount of heat sinking I suggested,
I predict he will be replacing the transistors very early in the power
supplies life.

Re: power supply circuit

Regulated current or regulated voltage?  Or do you want a lab-type
supply with regulated voltage and smooth current limiting?

It sounds like you don't have a tremendous amount of experience with
electronics -- you may find it helpful to get a copy of the ARRL
handbook, which has some nice power supply designs as well as a _lot_ of
good practical basic knowledge.

--

Tim Wescott
Wescott Design Services
We've slightly trimmed the long signature. Click to see the full one.
Re: power supply circuit
Snip
">

My guess is it's for a glow plug in an RC model engine.... so regulated
voltage, current limited.

Re: power supply circuit

Oops, forget I said that. :(

Re: power supply circuit

One thing to watch for:  RMS current being drawn from the transformer
exceeding the DC output current.  The bigger the filter capacitor, the
worse this gets.

Assuming that you will be using an adjustable voltage regulator, you
should be OK with a few volts of filter capacitor ripple, so don't make
the filter capacitors too big.

I would not try for more than 3-4 amps of DC from a 5 amp transformer
unless the rating is for achievable DC with a "capacitor input"
rectifier/filter circuit (capacitor immediately following the rectifier,
as opposed to having a choke between the rectifier and the capacitor).

24 volts times sqr(2) is 33.9 volts.  Allow loss of a couple volts in
a bridge rectifier, a couple volts minimum drop in a voltage regulator
that is doing its job well, a couple more for a current regulator, and
that is about 28 volts.  Allow 4 volts ripple on a moderate filter
capacitor, and you get 24 volts.

As for a filter capacitor value:  4 volts ripple at 4 amps over the
approx. 6 milliseconds or so that the capacitor has to supply current
works out to 6,000 microfarads.  I would not use more.  Greater filter
capacitance makes the current waveform drawn from the transformer spikier,
which makes the RMS current drawn from the transformer greater.  I might
go for 4700 microfarads and only count on drawing 3-3.5 amps DC without
overheating the transformer.

BEWARE - with mild-moderate overloads, transformers this size can take a
couple hours to reach excessive temperature.

- Don Klipstein ( snipped-for-privacy@misty.com)

Re: power supply circuit
"Don Klipstein"

** That last claim is   ABSOLUTE   BULLSHIT  !!!!

Nothing new for this ignorant, septic asshole to make them.

** Make it as big as you like and the rms ripple current will not increase.

** No it does not !!

The whole idea is an idiotic  MYTH  put about by complete fools like
Klipstein.

** 20,000 uF would be a good value to use -  cos increasing the filter
capacitance above the minimum usable value has no effect on the rms current
in the transformer windings.

......   Phil

Re: power supply circuit

Look up the LM338 three terminal regulator. It's rated at 5 amps and
35 volts from output to input. All you need is a couple resistors or a
pot to set the output voltage. Use a 10 amp bridge rectifier to
convert the transformer secondary AC to DC and a 20,000uF, or
thereabouts filter cap. Connect the capacitor to the (+) input of the
regulator and ground and adjust the resistors for the desired output
votage. The regulator also has current limiting, but not sure what it
is. You can get 1.2 volts to maybe 24 volts DC out, but you need a
large heat sink   when the output is set low and drawing 5 amps.

-Bill

Re: power supply circuit

But certainly not at the same time:

http://www.national.com/mpf/LM/LM338.html

Go read the datasheet before you believe this last bit.

Earlier, I posted the simple math that suggests that you want about 10
TO-220 packaged transistors to get that much power.

Here's a bit of ascii art for a way to share the power around:

NPN     R4
-----------------   ----/\/\----
!              Q2 \ /e           !
!                 ---            !
!  R2    PNP       !      R3     !
+-/\/\---   -------+-----/\/\----+
!       e\ / Q1                  !
!        ---     U1              !
!   R1    !      -------         !
--+--\/\/---+-----! LM317 !----+---+---
-------     !
!        !
+-/\/\---
!
\
/
\
!
GND

The capacitors have been left out because I am too lazy to do them.

When U1 draws current R1 develops a voltage due to Ohms law.  Once
Q1's e-b draw is overcome, R2 sets how much voltage is needed on R1
per mA flowing in Q1.

R3 and R4 repeat the trick.  If you set the resistor values correctly,