I have a collection of "wall wart" (others may call them "transformers")
and I'd like to use one to cobble a little power supply. I've never
opened any to see what's really in them (and suspect it'll be quite a
chore), but I'm quite sure they are not ALL "transformers", because some
claim AC and some claim DC output, yet the first I grabbed out of the
junkbox claimed DC AND ALSO provided a HEALTHY AC output on the same
two wires! Furthermore, NONE of them seem to be voltage regulated!
(But if one says "X volts at Y milliamps", then loading it to Y mils
usually gives an output close to X volts.)

So what IS in them? Do the really light-weight ones, for example,
use a capacitor for relatively low-loss voltage dropping? And
why would one give *BOTH* AC and DC? Bad diode(s)?

--
--Myron A. Calhoun.
Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge
NRA Life Member and Rifle, Pistol, & Home Firearm Safety Certified Instructor
Certified Instructor for the Kansas Concealed-Carry Handgun license

wrote:
I have a collection of "wall wart" (others may call them "transformers")
and I'd like to use one to cobble a little power supply. I've never
opened any to see what's really in them (and suspect it'll be quite a
chore), but I'm quite sure they are not ALL "transformers", because some
claim AC and some claim DC output, yet the first I grabbed out of the
junkbox claimed DC AND ALSO provided a HEALTHY AC output on the same
two wires! Furthermore, NONE of them seem to be voltage regulated!
(But if one says "X volts at Y milliamps", then loading it to Y mils
usually gives an output close to X volts.)

So what IS in them? Do the really light-weight ones, for example,
use a capacitor for relatively low-loss voltage dropping? And
why would one give *BOTH* AC and DC? Bad diode(s)?

--
--Myron A. Calhoun.
Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge
NRA Life Member and Rifle, Pistol, & Home Firearm Safety Certified Instructor
Certified Instructor for the Kansas Concealed-Carry Handgun license

It depends...

The AC ones are just a transformer.

The DC ones contain a transformer and diode(s).

The cheapies usually don't contain any capacitors or regulators, hence
there is significant AC on the DC.


--
Jim Pennino

Remove .spam.sux to reply.

wrote:

wrote:

I have a collection of "wall wart" (others may call them "transformers")
and I'd like to use one to cobble a little power supply. I've never
opened any to see what's really in them (and suspect it'll be quite a
chore), but I'm quite sure they are not ALL "transformers", because some
claim AC and some claim DC output, yet the first I grabbed out of the
junkbox claimed DC AND ALSO provided a HEALTHY AC output on the same
two wires! Furthermore, NONE of them seem to be voltage regulated!
(But if one says "X volts at Y milliamps", then loading it to Y mils
usually gives an output close to X volts.)


So what IS in them? Do the really light-weight ones, for example,
use a capacitor for relatively low-loss voltage dropping? And
why would one give *BOTH* AC and DC? Bad diode(s)?


--
--Myron A. Calhoun.
Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge
NRA Life Member and Rifle, Pistol, & Home Firearm Safety Certified Instructor
Certified Instructor for the Kansas Concealed-Carry Handgun license


It depends...

The AC ones are just a transformer.

The DC ones contain a transformer and diode(s).

The cheapies usually don't contain any capacitors or regulators, hence
there is significant AC on the DC.


And AFAIK most of them achieve current limiting by intentionally making
the windings lossy, so you can't burn down your house by shorting the
leads together.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

There are also some transformer-input types that produce well-regulated DC
outputs using linear regulator technology, and a growing number of units
that use switching regulator technology to make the transfoprmer so small
that it's hard to tell from the outside that there's even a transformer in
it.. The latter are easily distinguishable because they put out quite a bit
of power, yet are smaller and much lighter in weight than traditional units
with equivalent power capability.

Joe
W3JDR


wrote in message
...
wrote:
I have a collection of "wall wart" (others may call them "transformers")
and I'd like to use one to cobble a little power supply. I've never
opened any to see what's really in them (and suspect it'll be quite a
chore), but I'm quite sure they are not ALL "transformers", because some
claim AC and some claim DC output, yet the first I grabbed out of the
junkbox claimed DC AND ALSO provided a HEALTHY AC output on the same
two wires! Furthermore, NONE of them seem to be voltage regulated!
(But if one says "X volts at Y milliamps", then loading it to Y mils
usually gives an output close to X volts.)

So what IS in them? Do the really light-weight ones, for example,
use a capacitor for relatively low-loss voltage dropping? And
why would one give *BOTH* AC and DC? Bad diode(s)?

--
--Myron A. Calhoun.
Five boxes preserve our freedoms: soap, ballot, witness, jury, and
cartridge
NRA Life Member and Rifle, Pistol, & Home Firearm Safety Certified
Instructor
Certified Instructor for the Kansas Concealed-Carry Handgun license

It depends...

The AC ones are just a transformer.

The DC ones contain a transformer and diode(s).

The cheapies usually don't contain any capacitors or regulators, hence
there is significant AC on the DC.


--
Jim Pennino

Remove .spam.sux to reply.

W3JDR wrote:
There are also some transformer-input types that produce well-regulated DC
outputs using linear regulator technology, and a growing number of units
that use switching regulator technology to make the transfoprmer so small
that it's hard to tell from the outside that there's even a transformer in
it.. The latter are easily distinguishable because they put out quite a bit
of power, yet are smaller and much lighter in weight than traditional units
with equivalent power capability.

Joe
W3JDR


wrote in message
...
wrote:
I have a collection of "wall wart" (others may call them "transformers")
and I'd like to use one to cobble a little power supply. I've never
opened any to see what's really in them (and suspect it'll be quite a
chore), but I'm quite sure they are not ALL "transformers", because some
claim AC and some claim DC output, yet the first I grabbed out of the
junkbox claimed DC AND ALSO provided a HEALTHY AC output on the same
two wires! Furthermore, NONE of them seem to be voltage regulated!
(But if one says "X volts at Y milliamps", then loading it to Y mils
usually gives an output close to X volts.)
So what IS in them? Do the really light-weight ones, for example,
use a capacitor for relatively low-loss voltage dropping? And
why would one give *BOTH* AC and DC? Bad diode(s)?
--
--Myron A. Calhoun.
Five boxes preserve our freedoms: soap, ballot, witness, jury, and
cartridge
NRA Life Member and Rifle, Pistol, & Home Firearm Safety Certified
Instructor
Certified Instructor for the Kansas Concealed-Carry Handgun license
It depends...

The AC ones are just a transformer.

The DC ones contain a transformer and diode(s).

The cheapies usually don't contain any capacitors or regulators, hence
there is significant AC on the DC.


--
Jim Pennino

Remove .spam.sux to reply.


Such units actually have a bridge rectifier across the AC mains and
supply DC to a switching regulator running at 20-50khz. There is an
isolation transformer running at this frequency (it's core is just a
small bit of lightweight ferrite). The output of the isolation
transformer feeds another bridge rectifier and filter (at ~40khz not
much of a filter is needed, small caps and chokes!). There may then
be a linear or switching regulator. If the later, it may actually
regulate by changing the switching frequency/duty cycle of the
original switcher at the AC mains input. In this case the feedback
is by means of an opto-isolator for safety.

In article , wrote:

I have a collection of "wall wart" (others may call them "transformers")
and I'd like to use one to cobble a little power supply. I've never
opened any to see what's really in them (and suspect it'll be quite a
chore), but I'm quite sure they are not ALL "transformers", because some
claim AC and some claim DC output, yet the first I grabbed out of the
junkbox claimed DC AND ALSO provided a HEALTHY AC output on the same
two wires! Furthermore, NONE of them seem to be voltage regulated!
(But if one says "X volts at Y milliamps", then loading it to Y mils
usually gives an output close to X volts.)

So what IS in them? Do the really light-weight ones, for example,
use a capacitor for relatively low-loss voltage dropping? And
why would one give *BOTH* AC and DC? Bad diode(s)?

There are four types I have come across:

1: transformer only for AC
2: transformer and diode bridge for DC
3: transformer, diode bridge and linear regulator
4: switching regulator

The voltage output and the current output of the first two type depend
upon the load. So for a type 2, for example, if the nameplate says 6
volts and 500 mA. That's exactly what it means. With a laod of 500 mA,
the DC voltage will be 6 volts. a lighter load will result in a higher
voltage, and a higher load will result in a lower voltage.

The last two will provide you with a fixed voltage for which they are
rated. So, for 6 volts and 500 mA, the voltage will always be close to 6
volts as long as the current does not exceed 500 mA. Switchers may
require a minimum current draw of about 10% of their maximum current
rating. Thus one rated ate 500 mA needs a minmum load of 50 mA to
provide the rated voltage output.

I have disassembled wall warts and used their guts for power supplies in
projects. It works quite well. As our town's recycling center is full of
old electronics, I have assemble quite a nice selection of wall warts
that I use as PSs in projects. And the price is right, zero, except for
your time.

Al

Al ) writes:
I have disassembled wall warts and used their guts for power supplies in
projects. It works quite well. As our town's recycling center is full of
old electronics, I have assemble quite a nice selection of wall warts
that I use as PSs in projects. And the price is right, zero, except for
your time.

The ones I've opened up (mostly to see if the problem was fixable) have
never seemed too adaptable. The the transformers are too integrated
to the plug, or at the very least have no means of mounting them.

I admit that they are useful for projects unopened.

But I find consumer electronics (VCRs, radios, whatever) are often
a better source of small transformers. They at least come with mounting
holes. Of course, a fair number of VCRs I've opened up recently are
using switching supplies.

I tend to bring home switching supplies from computers I see lying
on the sidewalk waiting for the garbage truck to pick them up. But
ironically, I take out the switching supply board, and build analog
supplies inside, using parts I've salvaged from other equipment.
Rare is the time I actually need the current of a switching supply,
or the noise, yet the cases make quite nice boxes for analog supplies.
I leave the AC socket in place, and then just use one of those cords
that plug in, they now being in plentiful supply from other junk
I've collected.


They'd even make nice construction boxes, covering the back panel
with a piece of metal or circuit board.

On the other hand, when I got an older Powerbook some years back,
once I'd scraped up the details for the missing AC adaptor, I pulled
the switching supply out of an inkjet printer and it was the 24 volt
supply I needed, and I had the needed AC adaptor.

Michael VE2BVW

wrote:
I have a collection of "wall wart" (others may call them "transformers")
and I'd like to use one to cobble a little power supply. I've never
opened any to see what's really in them (and suspect it'll be quite a
chore), but I'm quite sure they are not ALL "transformers", because some
claim AC and some claim DC output, yet the first I grabbed out of the
junkbox claimed DC AND ALSO provided a HEALTHY AC output on the same
two wires! Furthermore, NONE of them seem to be voltage regulated!
(But if one says "X volts at Y milliamps", then loading it to Y mils
usually gives an output close to X volts.)

So what IS in them? Do the really light-weight ones, for example,
use a capacitor for relatively low-loss voltage dropping? And
why would one give *BOTH* AC and DC? Bad diode(s)?


Uhh,why a Transformer,of course..
The ones with the DC output will have a rectifier diode or two (or maybe
even a FWB,if you're lucky) and usually a filter cap,but not always,IME.

The AC ones are usually just the transformer,and that's it.

Some "wall warts" I've come across were actually really tiny SMPS's,you
can usually identify these fairly easily,they're really light compared
to the "iron core transformer" type.. These may be regulated,the
iron-core types above almost never are.

AC and DC output?
Are you sure you're not measuring the ripple? They usually have very
poor filtering,plus after time the cap(s) dry out,and make it even worse.
Or,it could be the diode(s),they've been known to fail aswell.

PhattyMo writes:

wrote:
I have a collection of "wall wart" ....

....[snip]....
AC and DC output?
Are you sure you're not measuring the ripple? They usually have very
poor filtering,plus after time the cap(s) dry out,and make it even worse.
Or,it could be the diode(s),they've been known to fail aswell.

Well, its "rating" is "9VDC 200MA",
but my Simpson 260 VOM gives it's output as 15V DC AND 15V AC.

I've NOT looked at the output with a 'scope (yet).

--Myron.
--
--Myron A. Calhoun.
Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge
NRA Life Member and Rifle, Pistol, & Home Firearm Safety Certified Instructor
Certified Instructor for the Kansas Concealed-Carry Handgun license

ken scharf wrote:
W3JDR wrote:
There are also some transformer-input types that produce well-regulated DC
outputs using linear regulator technology, and a growing number of units
that use switching regulator technology to make the transfoprmer so small
that it's hard to tell from the outside that there's even a transformer in
it.. The latter are easily distinguishable because they put out quite a bit
of power, yet are smaller and much lighter in weight than traditional units
with equivalent power capability.

Joe
W3JDR
....
Such units actually have a bridge rectifier across the AC mains and
supply DC to a switching regulator running at 20-50khz. There is an
isolation transformer running at this frequency (it's core is just a
small bit of lightweight ferrite). The output of the isolation
transformer feeds another bridge rectifier and filter (at ~40khz not
much of a filter is needed, small caps and chokes!). There may then
be a linear or switching regulator. If the later, it may actually
regulate by changing the switching frequency/duty cycle of the
original switcher at the AC mains input. In this case the feedback
is by means of an opto-isolator for safety.

In small ones (not a whole lot of power; things like 5V at 1A like I
have in front of me), the switcher is commonly a flyback type, and the
regulation is provided by monitoring how far the primary flies back.
The secondary side is a single rectifier diode feeding a filter cap.
Knowing the turns ratio and the expected losses in parts like the
secondary rectifier, you can achieve pretty decent regulation that way.
They commonly run at much higher than 50kHz these days. There are
some small ICs that make the job very easy. One characteristic of the
switchers is that they commonly run on 100-240V, 50-60Hz input, though
not always. It's rarely the case that a mains-frequency transformer
model will operate properly over such a wide voltage range. Wide input
range switchers do make travel to countries with different mains
voltages more convenient.

Cheers,
Tom