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Modifying a wall-wart

N

Norm Dresner

Jan 1, 1970
0
I have a Elpac wall-wart which is spec'd as follows:
5V @ 380 ma regulated
+-12V @ 180 ma regulated

and I'd like to modify it to change the 5V output to an (unregulated)
voltage somewhere around 8V.

I'm not particularly afraid of opening the thing up and changing the
circuitry but I was wondering if anyone could give me some idea of what I'm
likely to find when I do so I can plan what I'm going to do to it.

As an alternative, I could use a wall-wart or external power supply which
provides 8V and +-12V instead but I haven't found anything in searching
through various catalogs. I think that something as high as 9V would
function too.

TIA
Norm
 
R

Rein Wiehler

Jan 1, 1970
0
Norm said:
I have a Elpac wall-wart which is spec'd as follows:
5V @ 380 ma regulated
+-12V @ 180 ma regulated

and I'd like to modify it to change the 5V output to an (unregulated)
voltage somewhere around 8V.

I'm not particularly afraid of opening the thing up and changing the
circuitry but I was wondering if anyone could give me some idea of what I'm
likely to find when I do so I can plan what I'm going to do to it.

As an alternative, I could use a wall-wart or external power supply which
provides 8V and +-12V instead but I haven't found anything in searching
through various catalogs. I think that something as high as 9V would
function too.

TIA
Norm
I would expect a to220 format regulator like 7805 for the 5 volt
output. Remove and bridge the input to output, maybe you get 8VDC and up
to 15VDC. hard to predict. could also effect the -12Vdc output depending
how it is wired.
rw
 
U

Uns Lider

Jan 1, 1970
0
I have a Elpac wall-wart which is spec'd as follows:
5V @ 380 ma regulated
+-12V @ 180 ma regulated

and I'd like to modify it to change the 5V output to an (unregulated)
voltage somewhere around 8V. ....
As an alternative, I could use a wall-wart or external power supply which
provides 8V and +-12V instead but I haven't found anything in searching
through various catalogs. I think that something as high as 9V would
function too.

For a quick one-off hack how about regulating down the +12V to 9V with
a 7809? If the +12 on your wallwart isn't beefy enough, you could use
an off-the-shelf switching power supply with +-12V@>=500mA.

What's your application, anyway?

-- uns
 
T

Tim Shoppa

Jan 1, 1970
0
Norm Dresner said:
I have a Elpac wall-wart which is spec'd as follows:
5V @ 380 ma regulated
+-12V @ 180 ma regulated

and I'd like to modify it to change the 5V output to an (unregulated)
voltage somewhere around 8V.

A small boost converter on the 5V output, or regulating the +12V output down,
may satisfy your needs without opening it up. How much current do
you need at 8V?

Many of the Elpac's are internally switching regulators. They aren't
particularly cheap, either.
I'm not particularly afraid of opening the thing up and changing the
circuitry but I was wondering if anyone could give me some idea of what I'm
likely to find when I do so I can plan what I'm going to do to it.

As an alternative, I could use a wall-wart or external power supply which
provides 8V and +-12V instead but I haven't found anything in searching
through various catalogs. I think that something as high as 9V would
function too.

I would think that starting with an unregulated wall-wart or
transformer(s) and doing all the regulation yourself would be far
easier than modifying an existing Wall-wart. Doing the modification pretty
much nullifies whatever UL/CSA/ EMI advantages you had by using a wall-wart
in the first place.

Tim.
 
N

Norm Dresner

Jan 1, 1970
0
Tim Shoppa said:
A small boost converter on the 5V output, or regulating the +12V output down,
may satisfy your needs without opening it up. How much current do
you need at 8V?

I have no specs or documentation on the unit as the manufacturer is no
longer in business :-( but I know that this input feeds a TO-7805 with a
minimal heat sink and a TO-220 LM350 (nominal 3A regulator) with only a
slightly larger one. I can't believe that the heat sink for the LM350 would
be adequate for even a few watts of dissipation since there's effectively no
ventilation.

As a first step -- since it's the minimal work path -- I'm going to open the
wall wart and -- if I can -- jumper around the 5V regulator. If this works,
great! If not, I'll create a real power supply with the necessary voltages.

Norm
 
P

petrus bitbyter

Jan 1, 1970
0
Norm Dresner said:
I have a Elpac wall-wart which is spec'd as follows:
5V @ 380 ma regulated
+-12V @ 180 ma regulated

and I'd like to modify it to change the 5V output to an (unregulated)
voltage somewhere around 8V.

I'm not particularly afraid of opening the thing up and changing the
circuitry but I was wondering if anyone could give me some idea of what I'm
likely to find when I do so I can plan what I'm going to do to it.

As an alternative, I could use a wall-wart or external power supply which
provides 8V and +-12V instead but I haven't found anything in searching
through various catalogs. I think that something as high as 9V would
function too.

TIA
Norm

Norm,

This would work with older wall-warts that use a 50/60Hz transformer, a
bridge and a 7805. But I'm pretty sure the type you mentioned has an SMPS
inside which is very hard to modify. You would have to reverse-engineer the
circuit to find out how the output voltage is set and then has to poke
around in a very conpactly build PCB covered with SMD components. Eventually
you may find out that it cannot be modified at all. Best solution I can
imagine is finding a +-12V supply that can provide enough current from the
+12V to power +8V or +9V via a 7808 respectively 7809.

petrus
 
U

Uns Lider

Jan 1, 1970
0
I have no specs or documentation on the unit as the manufacturer is no
longer in business :-( but I know that this input feeds a TO-7805 with a
minimal heat sink and a TO-220 LM350 (nominal 3A regulator) with only a
slightly larger one. I can't believe that the heat sink for the LM350 would
be adequate for even a few watts of dissipation since there's effectively no
ventilation.

It might be instructive to power the thing for a while with an adjustable
power supply, measure the current it uses, and finger-test the heatsinks
after it's been running for a while with various voltages.
As a first step -- since it's the minimal work path -- I'm going to open the
wall wart and -- if I can -- jumper around the 5V regulator. If this works,
great! If not, I'll create a real power supply with the necessary voltages.

It could be the minimal work path, but maybe not. It's high-risk because you
have to reverse-engineer the wallwart before you can know how much work it's
actually going to be. I'd go the route of adding a 7809 because I can know
ahead of time exactly how much work that involves.

-- uns
 
N

Norm Dresner

Jan 1, 1970
0
To recap, I got a piece of equipment (about 2 years ago on eBay) with what
has turned out to be the wrong wall wart power supply. The WW outputs +-12V
@180 mA and +5V @ 380 mA. The two +-12V inputs are used directly but the
third input feeds a pair of IC regulators, an LM7805 and an LM350. From the
resistors on the LM350 I believe that the input should be about 8-9V,
obviously unregulated. I did a quick test and substituted a 9V 350 mA
unregulated wall wart for the third (+5) input and its voltage was pulled
down to around 6.25 V by the combined load of the two regulators and the
circuitry they feed, but the equipment seemed to be closer to working.
From this, I conclude that it's absolutely impossible to use the +12 @180 mA
input to feed the third input as well as its own load too.

I see two -- and only two possibilities -- for using this piece of equipment


1. Open the wall wart and modify it. If I'm lucky, it'll be a linear
supply (Note 1) and I could try jumpering the input to the +5 regulator to
its output and see if that's enough to power the whole equipment. But since
the LM350 is a 3A variable regulator (and they didn't use an LM317), I
suspect that this might not work.

2. (Assuming #1 fails) Construct a completely new power supply. While the
+-12V outputs of the existing wall wart might be salvaged to feed the
corresponding inputs, I'm going to need a completely new lower voltage
supply. I've identified a few candidates and while I'll probably
test-with-a-variable-supply first, I'm also inclined to just get a 9V
transformer, bridge, and capacitor instead.

Approach #1 is the next thing to do and if I find anything interesting
inside, I'll post a follow-up.

Thanks to all who posted suggestions and comments.
Norm

Note 1: The wall wart is rated at 13.5W input but the outputs total 6.22 W,
and since the wall wart is 3.5"x2.5"x2" (17.5 cu.in), I strongly suspect a
transformer and linear regulators inside.
 
W

Wade Hassler

Jan 1, 1970
0
Norm Dresner said:
I have a Elpac wall-wart which is spec'd as follows:
5V @ 380 ma regulated
+-12V @ 180 ma regulated

and I'd like to modify it to change the 5V output to an (unregulated)
voltage somewhere around 8V.

I'm not particularly afraid of opening the thing up and changing the
circuitry but I was wondering if anyone could give me some idea of what I'm
likely to find when I do so I can plan what I'm going to do to it.

As an alternative, I could use a wall-wart or external power supply which
provides 8V and +-12V instead but I haven't found anything in searching
through various catalogs. I think that something as high as 9V would
function too.

TIA
Norm

If the thing is a switching regulator, try this:
Since it's mass-produced, most of the parts used will be standard
values. The divider that samples the output for feedback, however,
will have at least one resistor whose value is dictated by arithmetic.
Find the oddball resistor and replace it with a pot to get a new
value. This has worked for me a couple (all right: exactly two) of
times.
Wade Hassler
 
N

Norm Dresner

Jan 1, 1970
0
Norm Dresner said:
To recap, I got a piece of equipment (about 2 years ago on eBay) with what
has turned out to be the wrong wall wart power supply. The WW outputs +-12V
@180 mA and +5V @ 380 mA. The two +-12V inputs are used directly but the
third input feeds a pair of IC regulators, an LM7805 and an LM350. From the
resistors on the LM350 I believe that the input should be about 8-9V,
obviously unregulated. I did a quick test and substituted a 9V 350 mA
unregulated wall wart for the third (+5) input and its voltage was pulled
down to around 6.25 V by the combined load of the two regulators and the
circuitry they feed, but the equipment seemed to be closer to working.
From this, I conclude that it's absolutely impossible to use the +12 @180 mA
input to feed the third input as well as its own load too.

I finally opened up the wall wart and found visible a transformer, a few
caps, and three TO-220 IC's or transistors. A first guess is that they are
an LM7812, an LM7912, and an LM7805 and it looks like it will be easy to
remove the '05 and jumper its input to the output.

I think that the large physical size of this unit was the key to predicting
that it was a linear device and not a switcher.

Norm
 
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