My isolated AC variable Power Supply project
Because one of my latest passion is vintage radios repairs
I started many years ago to look at some repair GURU’s videos on Youtube.
I discovered that to get efficient at
radio repair you need
to own a minimum of RF and supply test
equipment.
many videos I saw on the web were about repairing
AA5 (all american five) radios that had no power transformers
but instead an incoming 120vac line that would supply
5 tube filaments in series and get
rectified to generate a decent B+ voltage.
Well most of these radios had one side of the 120VAC line
tied to the radio chassis !!! Making any
measurements very dangerous
and shock hazardous for any repairmen..
The solution.. An isolated AC power supply.
Many isolated supplies are available for
sale on the web, mostly on eBay.
But they are relatively expensive and
not quite as powerful as what I would need..
So I decided to build my own as a project.
So I browsed my parts bins to check what I could use and found
some parts that would do quite a good job.
First I needed an isolation transformer..
In the past a very good friend of mine
gave me an hacked power supply case
with lots of parts to use. This was an Elenco Precision XP-810 power supply
Elenco Precision XP-810 power
supply
Looking inside the case:
I found a very nice transformer that
could do primary 120VAC to secondary 130VAC @ 4 amp.
Very good transformer for my needs !!
I could also retrieve 2 unbroken front
panel AC female connectors.
Next I needed some way to vary the output voltage ..
Still browsing my parts bins I found a
powerful Powerstat type 21
variable autotransformer 0-140VAC 0-4.5amp
The isolated transformer with this variac will do a very efficient job for this project !
After some verifications
about what would be the maximum allowed
supply voltage/current for this project I
decided that 0-4amp would be well enough
to test most of my future radios. And
because I have a 0-130vac variac
that I could wire to get 0-120vac out of it,
this variac connected
to my isolated transformer primary
120/130v would finally gives 0-130v @ 0-4amp.
Ok now, what about adding some meters to this project
..
Now that I know the overall power supply
ratings I can look at some available meters on the net..
The first would do 0-130VAC and
the second 0-4 AC amp.
Hmmm.. Not
quite easy to find these 2 types of meter..
So I went to some sellers but did not
find anything suitable..
BUT.. I found that the Simpson company
model 1327 would
have a very nice look
Simpson meter
0-100uA shown here
But the problem is that I could not find
a 0-130VAC.
Same problem for a 0-4amp AC.
So I first bought a DC 0-1000uA meter to
do some tests on it..
I built a small rectifying circuit to
get a 0-130vac reading out of 0-1000uA.
That worked perfectly. But I needed to
modify the meter layout to show 0-130VAC instead of 0-1000uA DC.
So I took the meter, removed the
transparent lid, unscrewed
both very small screws that hold the white
layout then very gently removed the white layout.
Please note
that the meters shown are 0-100uA (the one used for the 0-4amp) but the job was
the same for both Volt and Amp meters.
I then scanned the original Simpson
0-1000uA layout at 1200DPI for high resolution.
Now to get an arced graticule of 0-130 to replaced the Simpson’s I used a very handy software named FrontDesigner 3.0
This software is ideal to do front panel
layouts including potentiometers and meters graticules.
I put some values in the graticule maker of the software and after a while,
comparing the original Simpson markings curve
with mine I finaly created this new layout:
But the job was not quite done.. I had to move all the numberings and the bold lines
down a little to fit the original Simpson
markings. I then used a graphic software to move all
the new markings
to my previously scanned Simpson image,
then after some hours and patience I finally went through
all the painstaking steps and created this
new layout to be used:
I then cut it out with an Xacto knife, glued it above the
original Simpson white layout.
Then I screwed back both very tinny
screws in place and.. the
meter was finaly done ! Oooofff !!
Using another Simpson meter but this
time a 0-100uA (I will explain later why this 100uA value),
I did the same type of job for the
0-4amp AC. This is what I ended up with:
OK now.. why this 0-100uA meter value chosen for the final 0-4amp
meter ??
First I had to figure out what means I
would use to read an AC current.
I decided to use a non-intrusive (isolated) way to measure the 0-4amp.
Again, browsing my parts bins I found a
decent CARLO GAVAZZI MI5 current
metering transformer
that could measure 0.5 to 5AC amp and gives
an output voltage of 0.4 to 4Vpeak.
After some tests with numbers of wire
loops inside the transformer I found that
6 loops of wires inside this current
transformer would give nice linear readings.
But the transformer output needs to be
loaded with a 500ohms 1/2w 1% resistor for accurate readings.
7 loops and higher would give more
output voltages but rather bad linearity on the upper scale of the meter.
So I had to decide what meter value I should use.. The
most sensitive.. the better !
So I found on eBay a Simpson the same type (model 1327) of my previous one
But this time 0-100uA DC. Sensitive enough for my needs..
Again, I did a small rectifying circuit
to get a 0-4amp AC reading out of 0-100uA.
Here is a snapshot of my mess doing some
readings and trying to figure out
the best rectifying components for the 0-4A ammeter:
Now the need for a detailed Power supply schematic..
Here is the final detailed circuit:
You will notice that the Powerstat variac primary side has
a DIM bulb load attached
that can be enabled or shorted with the use
of the S2 switch. Using the external Bulb 300w lamp in series
protects the supply AC output from any shorts
that could happen inside a connected old radio or anything else on test..>
There is also a 4A breaker that keeps any excessive current from the AC input
side.
Nothing fancy here.
Ok, now the hardware side..
I first had to choose a nice enclosure
that would hold securely all these components in place.
I chose the venerable, all metal and
sturdy Hammond 1452KE9 enclosure.
Now knowing the final enclosure
dimensions and front panel size I had to create a nice front panel layout.
Again, using FrontDesigner 3.0 front panel layout software I started to
place all the needed layouts in place.
The final result gave this:
I had this layout printed on a
laminated, self-adhesive vinyl and applied to the enclosure metal front plate.
Then I cut and punched all the necessary holes..
Then I screwed back the new placed
layout front panel in place with all the needed components
Next step was to fill all the enclosure
with cables, terminals, connectors, etc..
3 things to be noticed ..
#1
Both small rectifying circuits screwed
to both meters terminals
0-4A Ammeter 0-130VAC voltmeter
#2
The current transformer with its 6
output wiring loops to measure the final output current..
#3
The back side DIM 300w lamp AC
connector..
Finally the complete assembled AC
isolated variable Power Supply
So that’s it !
Thanks for watching and feel free to ask
any questions
about this project using the email below.
If
you’d like to have a look at some of my other projects
Just
go to www.arcenson.com/projects
2018