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

 

IMG_6013

Elenco Precision XP-810 power supply

 

Looking inside the case:

IMG_6014 

 

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.

AC 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

IMG_6008IMG_6009

 

 

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

IMG_6112

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.

AC voltmeter schematic

 

 

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.

 

IMG_6114IMG_6115IMG_6122

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:

meter 0-130 volts

 

 

 

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:
meter 0-130 volts 120 dpi

 

 

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:

meter 0-4 amp120 dpi

 

 

 

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.

AC ampmeter schematic

 

 

Here is a snapshot of my mess doing some readings and trying to figure out
the best rectifying components for the 0-4A ammeter:

IMG_6127

 

 

 

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:

Power supply AC variable final

 

 

 

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..

IMG_6146IMG_6143

 

 

 

Then I screwed back the new placed layout front panel in place with all the needed components

IMG_6148

 

 

Next step was to fill all the enclosure with cables, terminals, connectors, etc..

IMG_6151

 

 

 

3 things to be noticed ..

 

#1

Both small rectifying circuits screwed to both meters terminals

IMG_6149IMG_6150

   0-4A Ammeter                      0-130VAC voltmeter

 

 

#2

The current transformer with its 6 output wiring loops to measure the final output current..

IMG_6152

 

 

#3

The back side DIM 300w lamp AC connector..

IMG_6154

 

 

 

Finally the complete assembled AC isolated variable Power Supply

IMG_6156

 

 

 

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

 

 

 

arcenson negatif

www.arcenson.com

info@arcenson.com

2018