Impedance protected?

[Robert Reed]

After years of pitching out defunct or useless electrical devices, I always saved their Wall Warts and maybe like many of you I have collected a closet full. Several years ago I started using these in prototypes when a quick need arose for a suitable low VA power transformer. I just carefully hacksaw off the case, clean up and devise a quick method to install. Since I have never seen any type of safety device in these barebones units,got me to wondering how they are protected ( or at all)?. With the half dozen that I "converted" and a half dozen (original condition) that are plugged into outlets constantly, this started to concern me. Does any one know if they derive their protection by virtue of impedance protected primary windings?

PS - on one that I opened up it actually had a tiny fuse in the DC secondary and this was a regulated output. Since 95% of the ones I have are either raw DC or pure AC outputs, I have to assume that fusing is a rare occasion. Also I could have missed something in my investigation

[Bigglez]

Since I have never seen any type of safety device in these barebones units,got me to wondering how they are protected ( or at all)?

Expect to find a thermal over-current fuse protection in the
primary. Often built into the device to monitor
core temps. Many 'dead' Wall Warts (of the coil-core)
variety die with open primaries.

[Janitor Tzap]

On some of the wall warts I have pulled a part.
Just to see why it had failed.
They had a thermal fuse on the primary winding 105c.
These fuses were covered up under the paper, or tape rapper.
Making them hard to get at.

But mostly I found that there was nothing on either side of the transformer windings.
Especially on the cheap chinese junk.


Signed: Janitor Tzap

[MrAl]

Hello Robert,

You know it is interesting that you have chosen the subject
about one of the important things about many wall warts.

Impedance protected is the right phrase, because many wall warts
have to have double isolation to meet the class spec and this often
(if not always) means mounting the primary on one bobbin and
the secondary on a totally separate bobbin, or using a split bobbin.
(Think about what would happen if the secondary was wound over
top of the primary winding...if a short were to occur the primary could
short out to the secondary totally defeating the isolation property,
and layers of tape arent enough as the winding could melt through
that.)

It's interesting that the side effect from separating the bobbins
means the leakage inductance goes up, and one of the effects of
this leakage inductance is to limit primary current during a short.
Achieving this with winding resistance alone makes for a poor design with
bad output regulation.

A thermal fuse is a great idea but this doesnt appear on all
wall warts, too bad, and all of the ones i ever took apart had
nothing else in them.

Wall warts are often tested in the factory by making them run
overnight with full load or a short.

So i guess the answer is "Yes", because it is impedance protected
to some degree.

As a side note just in case anyone is interested...
Did anyone ever notice that there doesnt appear to
be any glue on the wall wart case where the top section meets the
bottom section?
This is because the wall wart top is sealed to the bottom using
ultrasonics, meaning it's an ultrasonic weld.

[Robert Reed]

Sure enough, the first time I 'dissasembled' one, I was looking for that magic glue joint to pry on for starters. It looks like a glue joint , but as you said its a completely homogeneous seam - enter the hacksaw! In the past I have seen small motors and transformers that were touted as being impedance protected and never delved into the internals of them - just assumed that the windings had such a high impedance to their supply voltage that they could sustain a secondary short or locked rotor condition. your split core dissertation has enlightened me on that. As a matter of interest, the utility company I used to work for had several small impedance protected transformers in their system that were fed by Hi-Line drops in remote locations (small being 200 to 500 KVA). Never had a chance to investigate the whys and whats of these devices. And as you and Jan T mentioned , most of the time their appears to be no external protection.

[gerty]

The bad ones that I've performed autopsies on had a small single strand of wire, much smaller than the windings, connected from the ac plug to the windings. I have soldered in replacements when I was in a bind, but the case always looks like crap when I'm done.

[MrAl]

Hi gerty and Robert,


I had one where the secondary opened up on me. I hacked it apart
with a razor knife. Made a mess of the case, so i discarded it.
I couldnt find any obvious reason for the open circuit in the secondary
and the primary still looked ok, so i hacked off the secondary turns
with a hacksaw which left only the primary.

Looped a few turns of heavy wire through the core, and viola,
i had a new current transformer! he he. I used it for years
to monitor current going through my microwave oven.

That was way back then however. Since then, i've found a new way
to hack into these welded cases...

I took a Dremel cutting disc and ground off all the teeth.
This was an actual saw blade about 5/8 inch diameter (not the big ones
made for their cutting tool) and these have teeth just like a big circular
saw blade, just much smaller, and these are made out of some sort
of very thin spring steel, not the composite cutoff discs found for
the Dremel too.
Anyway, after you grind off all the teeth you are left with a simple
disc with a hole in the middle and it is very very thin spring steel.
The hole of course is to mount onto a mandrel, and that gets
chucked in the Dremel chuck.
The thing is, when this type of disc is set to spin around 10k RPM or
higher the disc cuts through plastic like butter, removes very
little material, and leaves a nice clean edge on the plastic.

I found that cutting the little wall wart cases apart at the seams
with this tool not only very simple, but allows them to be glued
back together again if needed.
It may even be possible to use some sort of plastic weld to make
a nice job, but i havent tried this yet. I have used epoxy in the
past though. I havent been overly worried about the appearance
too much when done, but maybe you would be more concerned
about that, so maybe experiment with different glues/compounds.

Last one i did a while back for running a simple dc fan, i used
some epoxy putty to secure the wires coming out of it and seemed
to work pretty well except for the appearance of a blob of it

[k7elp60]

It is possible they fall into the catagory of class II transformers as described below:
What is a Class II Transformer?
A Class II transformer is used to supply Class II circuits. Class II transformers have a maximum VA (Volt-Ampere) rating of less than 100 and a maximum secondary output of 30 VAC. The maximum VA generally offered is 75 and the most common secondary voltage is 24 VAC. All Class II transformers are either inherently or non-inherently limited. This means that the maximum output current of the transformer is limited, either by the intrinsic coil impedance or by a fuse or circuit breaker. These transformers are designed to meet the requirements of U.L. 1585.
Inherently Energy Limited Transformers - Class II transformers up to 50 VA are "Inherently Limited" which means that the transformer, if overloaded, will short itself out and fail safely, not requiring a fuse.
Non-Inherently Energy Limited Transformers - 60 thru 75 VA Class II transformers are generally protected by a resettable circuit breaker or a fuse within the transformer secondary. Without this overload protection, the transformer would not satisfy the safety requirements for a Class II circuit.

I don't remember where I got this but it made sense to me at the time
Ned

[MrAl]

Hi there,

I have various transformers around but all my wall warts are
something like 36VA or smaller. The ones i've taken apart so
far are around 12VA or so.