This DIY Variable Isolation Transformer Has Looks to Die For, But Safety First and Foremost!

Professional test equipment doesn't need to break the bank or any mirrors! This DIY variable isolation transformer is both safe and sublime!

Tom Fleet
3 years agoHW101

Good test equipment is the staple part of any decent electronics workspace.

We all know just how much we fear someone walking off with our Fluke meter, or how much we dread the idea of having to replace what is usually a very, very expensive power supply or similar.

Yeah, we are all far too familiar with the many figures that are often found on the price tag of a famed out factory made product.

Lajt, owner of Lajtronix, has redefined our expectations of what can be produced on our own workbenches, with their take on a variable isolation transformer that not only aims to stop you from dying on the job, but also has looks to die for!

What is a VIT?

Well, in a nutshell, perhaps the easiest set of definitions to grok are the bullet points listed by Lajt as the prerequisite points for his project, emboldened to drive a point on the ones that would be part of the dictionary definition of such a device.

  • 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.

Such a device might be described by the following, simplified schematic (with modules shown in place of the low level devices, such as the power meter, etc.)!

So, let's follow along with Lajt and break down the building blocks of this beautiful looking bit of test equipment!

Arduino at ❤️

While it might not share the well known shade of teal that the official 'duino boards boast, a clone 'duino is still an Arduino and the Keyestudio UNO variant — with its venerable community favourite MCU, the Microchip ATmega328P at it's center — is no exception!

Chosen for its ability to handle a 12V DC input, 8-bit MCU development board proves that you don't need a 32-bit Arm for everything — safety critical operation of a device is often built into the circuits that are interfaced to the DUT — the test kit needs to remain safe, even in the event that the MCU decides to go for a nap / crashes!

Powering up the front panel mounted power meter!

A number of the panel-mount power meters that are seen on the market develop there operating voltage by siphoning off from the rail they are measuring.

This is handy for simplifying module deployment in general use cases, saving the designer form the request of a separate supply rail.

However, it means that the modules that do this are usually limited, in that they can't sense a voltage that is lower than that required to run the meter module itself!

One quick modification to remove the Zener diode that normally would provide the power to the module, and instead replace it with a 5V regulator and 12V feed, and the module functionality is now independent of the current and voltage being measured — it will function regardless of how low a voltage or current it is sensing.

More modifications to modules!

If you too want to measure the flow of current in a circuit, you might find yourself using a circuit such as the one below.

This is OK if you are able to safely operate your circuits at voltages that do not pose much risk!

But with the safety critical nature of this VIT, a non-contact current sense approach is essential.

Handy rules from Flemming!

You and I likely both know for a given current in a wire, a corresponding - and proportional — electromagnetic force will be generated. This is commonly known as Flemmings Left Hand Rule.

This EMF can be measured in turn, by placing a coil of wire around the conductor in which we wish to measure the flow of current, and then measuring the resultant voltage that is generated in response to the EMF that is generated by the initial flow of current that we wish to measure... a.ka. Flemmings Right Hand Rule. Phew!

In effect, it's a non-contact, air-gapped way of detecting the flow of current within a circuit — allowing the sensing electronics to be decoupled from the potentially hazardous line voltages that may be present in the circuit under test!.

Modules make things easy.

The board pictured above is based around a TI LM358 — likely configured as a comparator — and is designed to "trip" at a pre-set current (controlled by the variable potentiometer, visible at the top of the image above).

When the voltage developed by the sense coil (the black round object mounted on the PCB above) rises above the threshold set by the potentiometer, the relay is disengaged, and the flow of current though the circuit under test is interrupted.

Again, a few modifications are employed in order to slightly adjust the function of this module, and make it more suitable for use within this project!

First up, having initially ordered a 5V variant of this module, Lajt later decided to move over to a 12V supply for power distribution within the VIT.

With a 12V volt version of the module available, there was little sense in placing yet another AliExpress order, as the only difference between the two variants was the rated voltage of the relay.

The solution? Loose the relay — instead choosing to take the signal that would normally drive the relay coil, and instead output it, to be used to trigger an input on the VIT's power distribution board.

Secondly, that potentiometer we mentioned in the paragraph above is taken off-board, and extended to the front panel through some twisted wiring.

Finally, 12V is injected into the board via some flying leads — the in-line ferrite coil is worthy of note — this will help stabilize any fluctuations or noise on the power fails of this comparator board, which could save unwarranted trip events (AKA ghosts in the machine...).

AliExpress — Is it worth it? Lemme re-work it...

With a need to control the potentially high voltages and currents present in this piece of test equipment, Lajt turns to a bank of relays, again mounted on a module soured from AliExpress.

Thing is, these relays *need* to function correctly, with safe operation of the VIT implicitly calling for them to make, and more importantly, break the circuit as needed!

One of the most overlooked parts of a relay is the contact set. These are the bits of metal that are actuated open and closed by the electromagnetic coil inside.

A huge amount of work in the areas of physics, chemistry and metallurgy has been preformed over the years in finding suitable metallic surfaces that make good electrical contact, but also resist arcing and other such damage that can occur when interrupting the flow of enough current / voltage.

Put that board down, flip it and reverse it!

I won't beat around the bush — the supply chains that feed the manufacturers that produce these cheap modules are not the most sanitised, and can often be compromised by the introduction of cheaper, or even counterfeit components.

With safety in mind, Lajt takes no chances, and springs for some genuine Panasonic relay devices, with the appropriate ratings and construction to ensure that they don't weld themselves in the on position when it's least needed — when they are tasked with interrupting the flow of power as fast as possible.

It's a bit of a hack, but owing to the somewhat common, symmetrical footprint, and indeed construction of these relays, it's possible to fit the oversized Panasonic packages into the plated through holes, by mounting them into the rear of the PCB. Clever!

The main power switching relay is a heftily sized Weidmuller part, and in the unfortunate even that the Weidmuller welds itself together, or otherwise fails (these electromechanically actuated parts are only rated for so many contact cycles after all!), the inclusion of a socket means that it's a breeze to swap out and fix any failed part with a factory fresh replacement!

Front panel

With a number of the safety aspects of this design touched upon already, now our focus turns to the finesse and filigree in which it is finished!

First and foremost, the fine handiwork that has gone into the machining and fitment of the front panel. If you've never had to hand tool an enclosure, pay close attention to the writeup linked at the end of this article.

With little more than a hand drill, and assortment of various bits, and a set of files, this sheet of aluminum is transformed from a monolithic slab of metal, into the meticulously and beautifully finished front panel, visible below.

With the addition of some toner transfer markings, the front panel is ready to accept the instrumentation and connections that comprise the user-facing part of this particular bit of test gear!

With some tidy cable management going on, and good attention paid to the routing and placement of the parts within this project, the results are simply stunning, while suitably safe for use under the conditions envisioned - and confident that it will safely cope with any conditions outside those!

You can clock a better view of the VIT below, but really, you should do yourself a favour and head on over to the Lajtronix site, where there is a comprehensive writeup that we could never do justice by, over here on the news feed!

Indeed, I feel I've already gone a bit overboard, so kudos to you if you're still with me here.

Now, go and check out the full project write-up, and hopefully see you back her next time, with some newfound inspiration in how to present your next project in a professional fashion!

Tom Fleet
Hi, I'm Tom! I create content for Hackster News, allowing us to showcase your latest and greatest projects for the world to see!
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