Let’s clear one thing first. This is not a “Step by step -How to” guide to make your own variable isolation transformer. This is just me sharing my work, comments and ideas while building a piece of much needed equipment for my bench. Maybe it gives you a little push or sparks an idea for doing something similar.
Whether you are diagnosing a broken piece of electrical equipment or testing a repaired one, there comes a moment when you have to plug it into mains. That moment is like playing a lottery. Everything could work out great or it could go BOOM and trip your apartments fuse. Not once did I repair something and only way to test it was plugging it to mains and hoping for the best. Lots of times everything worked out great, but other times, one missed component caused even worse problem and destroying all the new parts.
Tripping the fuse could be avoided by a simple isolation transformer. But why stop with a simple solution when you can complicate your life and spend your time and money on something more versatile.
Variable isolation transformers exist as a commercial unit. One of the more interesting is models is: Sencore PR570
But… there were three big obstacles stopping me from buying a commercial unit:
1. General unavailability in my country
2. Total cost (unit, transport and import taxes)
3. Physical size
Last obstacle was the biggest problem of all (pun intended). Since my work area is very limited so is my bench space. And I just couldn’t find any product that would fit my small desk. So, I decided to build my own. Device, not a desk 😊.
Every component used in a design of this VIT (Variable Isolation Transformer) was a compromise between the physical size of the unit and usability in everyday work.
Biggest supplier of electronic components in my general area is: www.tme.eu . They are not as big as Digikey or Mauser, but they are good enough. And my only financially viable choice.
That is the reason why most of the components used to build this project is linked to their web site. Availability of the certain items like DIY plastic cases and variacs also had a major impact on the final product.
After some time of planning and deciding what I want of the final product, these were my main goals:
- It must fit on my bench
- It must isolate DUT (Device Under Test) from mains
- It must regulate output voltage from cca. 1 – 110%
- It must show main parameters like voltage, current, power, power factor
- It must be safe for the operator and DUT
- It must have current limit regulation
- It must be able to fit multitude of electrical plugs
- It must allow me to choose different modes of operation
Taking all the above in consideration, this was the general idea of the block diagram:
There were two options for the user interface of the device. Buttons with latching mechanism connected directly to its dedicated relay, or processor-controlled environment with momentary switches and more flexible control of the whole unit.
I chose the latter. It complicates things quite a bit but adds a certain degree of flexibility and fine tuning of the final product.
Since I am not an advanced programmer (easily seen as soon as you go through my code) I opted for Arduino Uno R3 clone: Keyestudio UNO R3
After few revisions this is my final schematic for the user interface of the VIT:
Let’s clear some things before we go deeper into hardware details.
I am by no means an electronic PCB designer. And as I said before, I am far, far from coding expert too.
That said, all my builds and devices are based on electronic modules and PCB-s that I am able to source from web shops like eBay, AliExpress, etc. All I do is try to integrate and interconnect simple modules to form my idea of the final product. Now that we’ve cleared that, lets dive into component selection and final product build.
Product components choice:
After measuring my available desk space and checking all available components, I decided to go with:
Z39W KRADEX Enclosure with panel. Dimensions: 294 x 217 x 120mm (11.57 x 8.54 x 4.72in)
Choice for the variac that would fit this case boiled down to:
Variable autotransformer OIEA3 BREVE TUFVASSONS: link
|Primary voltage 230V AC
Output voltage 0…260V
IP rating IP00
Thermal class Ta40B
Isolation transformer was custom made in small local business:
Dimensions: 137 x 59mm
After deciding on three main components, I needed a good power meter. It had to be independently powered, capable of 0 – 300V measurement and it had to have a good resolution in 0-5A current range.
And believe me when I say, it was hard to find one that would roughly fit the purpose. I bought several devices. Some were not sensitive enough; some didn’t work on independent power source and some had an extremely bad LCD visibility angle. Finally, I settled for one that was a good compromise that suited my purpose: 6 in 1 Combo Voltmeter Peacefair: PZEM-018, that can be found on eBay or AliExpress for 7 – 10€
This power meter, same as most of others, uses the voltage it measures as a mean to power itself.
That is not something that works for me because it has input range from 80 – 300V. Under 70V it turns off. For that reason, I had to do some modification. I opened the instrument and disconnected its inbuilt power supply of 5V by removing its rectifier diode. Instead, I soldered 1117-5V LDO, since my main power supply for whole VIT is 12V. This way, power meter was independently powered by external power supply. That enabled it to measure full range from 0 – 300V without shutting off.
After settling for the fourth major component, I could now come closer to my final design. To give you some perspective, this is how I planned the positioning of internal components inside of device:
How I planned dimensions and clearance of the components:
Next in line was finding a solution for relay control and connection to the Arduino controller.
As I mentioned before, I am not very skilled in designing PCB-s and currently don’t really have any time to invest in learning it. That is where products from AliExpress come to rescue.
I bought a 12V-four channel Arduino relay board for 2.5€ total. The most important thing was a complete solution with easy Arduino connectivity, optocoupler protection and flexible configuration.
For 2.5€ in total you can’t expect to get some high-quality relays. And since I need this device to be quite reliable, I decided to buy some quality relays from Panasonic.
Panasonic relays were physically bigger than original ones, so I needed to be creative. Original relays were de-soldered and Panasonic relays were soldered from the bottom side which had more than enough space.
Output relay was the main connect – disconnect device so I opted for WEIDMÜLLER 7760056050 with corresponding socket:
For the current limit solution another AliExpress board was used: “AC Current Detection Sensor Module 5V 12V 24V”
I bought a 5V board, but later in design I opted for 12V power supply. And instead of waiting another month for a 12V board, I reviewed all the pictures from AliExpress and figured out that only difference is the relay itself (5 or 12V). All other components were exactly same.
First revision of the design used the inbuilt relay for disconnecting the circuit if overcurrent event occurred, but later I changed it to processor (Arduino) controlled interrupt event and ditched the relay. To do that, board needed to be adapted a bit. Relay was removed, pullup resistor added, and a sensor wire routed to Arduino interrupt input.
Also, the trimmer resistor was removed from the PCB and extension wires were added so that a potentiometer could be used on the front panel. Screw terminals on the current sensor board were used as a distribution contacts for 12V power supply.
As a cooling solution I used an old Antec 3 speed 12V fan I had laying around from some old computer case.
For the brain of the device, I chose Arduino Uno R3 clone: Keyestudio UNO R3 since it can be powered directly by 12V
Front panel components:
Now that we have most of the components covered, lets get on with assembly
Main issue of VIT is its weight. Isolation transformer + variac contribute to 80% of its weight which is substantial 8kg.
Plastic case can’t handle that weight without deforming so a solution was needed for adding some structural integrity. I stumbled on a piece of aluminium case from and old spectrophotometer. It was a 2mm aluminium piece with a 90° bend in it. So, I measured and cut out a piece that best suited my purpose.
I live in an apartment and have a limited number of tools. Everything you see was done either in my room or on the balcony with basic electric and lot of hand tools (mostly files).
Through the whole process, some components were mounted to check the clearance and alignment to be adjusted if necessary.
Structural Support Plate
A missing piece of aluminium plate was added to fill the hole, and isolation was glued in place where Transformers will be mounted. A good quality isloation sheet was used from an old UPS, APC SU1000I.
Also, holes were drilled for proper ventilation.
Wires and wiring harness
For wiring harness LED, power and reset switch wires were used from old computer cases.
One of my wishes was easy serviceability. So, I designed everything in a manner that every front panel component can be disconnected and taken out if needed.
Front Panel Inside
Back Panel Inside
Front panel Graphis design
I designed front panel graphic and printed it on my laser printer on silver Avery Zweckform label sheet L6013-20. I am not very happy with the quality of the labels since they are very prone to even the gentlest scratches (gentle scrape of fingernail). But I don’t have anything better so it will just have to do.
Most important thing is DUT safety!!!
That is the reason why these conditions had to be met:
- Startup conditions must not risk any unplanned power to Device Under Test.
- Default relay connections were all designed to be in Normally Open state. For example, output relay is OFF by default and DUT is disconnected from any power.
- Internal wiring was done in a way that any accidental turning ON of output, only default mains conditions can be applied (230V with earth connected).
Code was also written in a way to maximize the safety of DUT
- All relays in OFF state
- Output disconnected (Main output is OFF)
- Short press of the button enables the chosen option
- Short press of enabled option turns it off
- If Output is enabled, any change of options automatically turns it OFF
- Output needs to be turned ON after any change (safety reasons)
Current limit (Arduino interrupt)
- Current limit condition has the highest priority
- If current limit condition is true, Output is immediately turned OFF
- If current limit condition still exists Output can’t be enabled (LED warning blinking)
- Only when current limit condition is cleared Output can be re-enabled
That is basically it.
Code is crude and possibly over-complicated, but it works.
Softstart – (600VA transformer is not very powerful but still trips the fuse sometimes)
Video of a VIT in action
Improving current limit ability by adjusting max. range by 3A per full potentiometer turn.
Idea to be added in future:
Saving default startup options in eeprom
IMHO good ideas but not integrated in final product for various reasons:
Serial/Parallel isolation transformer configuration for maximum 115V power
AC-DC output option