Isolated Variac Project Update - Page 1
Forum section: electronics / projects

author: zanfar
author_rank: Contributor
timestamp: August 17, 2024, 01:31:37 am
content: There was a lot of feedback on my initial schematics for this project, which I really appreciated. Since then, I've refined the schematic, as well as sourced most of the components. I've been having trouble finding an enclosure that will fit well, and am now working under the assumption that I will custom-fabricate the enclosure. To that end, I've been working on the layout with specific attention to the front panel.Again, this is somewhere between a project log and an RFC. Any input or questions on the layout is appreciated.I know it is a bit busy, but I love it. I take a lot of pleasure in sourcing and assembling project components, and I really like how "complicated" this has become. Critiques of that design choice are welcome, but know that I have a strong bias towards the current set of controls.Cutting to the chase, here are some updated documents and renderings:(The original schematic review post is athttps://www.eevblog.com/forum/projects/isolated-variac-schematic-review/)

author: dazz1
author_rank: Frequent Contributor
timestamp: September 23, 2025, 02:57:55 am
content: HiI recently purchased an 8amp variac for a price too good to refuse. I am now looking at doing what you are doing.Some talking points.I will not include a isolating transformer. Too expensive and not sufficient use. I will be looking at including an RCD to do a similar job.I intend using thermal overloads on the input and output, to protect the variac, not the connected equipment.I plan on using a digital power meter, the same as the one you show. These are cheap, useful, and look cool with a blue backlight.I will probably use something like aLM3914(obsolete) LED bar graph display for current.I haven't looked yet, but I want to include programmable/user adjustable current protection device for the connected device.My variac is heavy, but I will mount it on the front panel like you show in yours. Unlike yours, the mounting screws will be on the rear panel, and the dial will poke through the front panel.I intend to use Aluminium Composite Panel for the enclosure.I will include a mains step down transformer on the variac secondary for LV AC. I have suitable 1A transformers.I will be using natural cooling, lots of holes in the enclosure, no fan.My feedback on your design is that I think you should consider what will happen if the relays malfunction, especially for the 50ohm resistors.I don't think a latching relay is necessary on the variac power switch. If the variac and connected load was safe before a power cut, then it should be safe after a power cut. When we have a power cut (very rare), I just go around the house and switch everything off.I'd avoid relays if practical. They are just another point of failure.I don't see the need for an e-stop button. It is a form of manual over voltage/current protection, and as such, too slow to be of any benefit.I think you should include handles on your enclosure. I have purchased plastic recessed handles that will be fitted on each side of the enclosure.How is your project going? Update?

author: coppercone2
author_rank: Super Contributor
timestamp: September 23, 2025, 05:14:11 am
content: What I am interested in is EMI filter on Variac. Most people say No, because the transformer takes cares of it. I put one on my various variac projects, because I had alot of inlet filter in my junk box.It would be interesting if someone actually bothered to measure it.

author: dazz1
author_rank: Frequent Contributor
timestamp: September 23, 2025, 10:01:02 am
content: Quote from: coppercone2 on September 23, 2025, 05:14:11 amI already have a filtered IEC power socket assigned for my variac project, but too early to take any measurements.

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: September 23, 2025, 07:09:02 pm
content: I have found that having several AC plugs on the front is helpful (direct and isolated).Jeff

author: coppercone2
author_rank: Super Contributor
timestamp: September 23, 2025, 08:38:21 pm
content: Quote from: dazz1 on September 23, 2025, 10:01:02 amif you wanna do it "right" you need to pick the right one. Not sure if its that possible because the load is unknown, so I wonder what the best 'middle ground' is for the filter type.I.e. I think you can get C(par) or L(d) or L(c) or c(d) output typeThis highlights what I mean, you can get the same thing with or without 3rd Cxhttps://www.mycoiltech.com/single-phase-double-stages-emi-power-line-noise-filter_p1078.htmlDo you want the last Cx there if there is a 60hz transformer immediately after the filter? Because normally after a 60hz transformer you have a linear supply, so that means they want a C there, I guess. But in this case it could be something else there since its a variac. But I am thinking the best bet might be to go for a C at the end.

author: dazz1
author_rank: Frequent Contributor
timestamp: September 24, 2025, 12:13:55 am
content: HiSome more feedback:What are you going to use for indicator lamps? Standard industrial controls are 24V AC or DC. I think adding a simple and reliable transformer/rectifier to get 24VDC would be justified to avoid using mains relays and indicator lamps.One of the reasons why I will be using ACP is that it is easy and not too expensive to get a sign writer to do the panel graphics in full colour. This would allow the front panel to effectively be the instruction manual.You have included a ESD banana socket. I think in practice, if you need and ESD ground, you would already have one on the work bench. In addition an ESD earth on the variac would require at least a 1Mohm resistor in series for personal safety.I plan on using a RCD which means the output is never isolated from the mains. I would not add a switched earth.Your circuit diagram shows a switched earth. Under what scenario would you see this switch being used? With MEN power distribution, a connection to earth is also a connection to Neutral, so closing the switch would mean the output is no longer isolated. Using an RCD will require earth connection on the output side, so I won't be including an earth switch. I don't see this being a problem, unless I incorrectly connect a scope probe, in which case the RCD would trip. For safety, the enclosure should be earthed.I can't think of a scenario where an isolating transformer would be better than an RCD? Equal yes, better no.I think an LED bar graph showing current would be useful. The modern replacement for the LM3914 to create LED bar graphs requires a micro controller and something like an LP5012 used with addressable tri-colour smd LEDs.In industrial control systems, green = safe, red = danger. Following that scheme, all of your indicator lamps should be red.

author: BILLPOD
author_rank: Frequent Contributor
timestamp: September 24, 2025, 02:24:35 pm
content: Quote from: dazz1 on September 24, 2025, 12:13:55 amGood Morning Dazz1, when I built my Variac project, the digital voltage/current display lit up, as the OPhas. No need for on/off indicator. Or use an illuminated switch.

author: dazz1
author_rank: Frequent Contributor
timestamp: September 25, 2025, 04:12:03 am
content: Quote from: BILLPOD on September 24, 2025, 02:24:35 pmHiI agree. My not-yet-ready design doesn't have a power indicator lamp, because the V,I,W display has a blue back light. OK it isn't red for danger, but it is obvious and unambiguous. I am trying to avoid the need for a 24V supply but I may need one for the inrush protection circuit. My Variac is 2kVA and it makes the lights dim when turned on. It is way too big but also it was way too cheap to let pass.[EDIT: I figured out I need a 24V supply for the in-rush relay, so I have now included a power-on indicator to show power is applied to the unit, even if the circuit protection has tripped. Still working through the scenarios.]

author: JohanH
author_rank: Frequent Contributor
timestamp: September 25, 2025, 06:53:21 am
content: Quote from: dazz1 on September 25, 2025, 04:12:03 amIf you don't want to build one, you can find these amplifier soft start modules on ebay with high current capability.(I'd like to have a 2 kVA variac. I've been looking at the smaller ones (<1 kVA), because they fit better in a small enclosure and are more affordable.)

author: dazz1
author_rank: Frequent Contributor
timestamp: September 25, 2025, 07:55:50 am
content: Quote from: JohanH on September 25, 2025, 06:53:21 amIf I didn't have my variac, I think an electronic variac would be cheaper and better.Quote from: JohanH on September 25, 2025, 06:53:21 amMy variac is old-new stock, unused, still in the original packaging. I only paid 2% of new price. The new price is eye-watering.

author: dazz1
author_rank: Frequent Contributor
timestamp: September 25, 2025, 11:51:42 am
content: HiThis is my draft design so far.Some notes:1. It includes thermal circuit breakers on the primary and secondary. These are quite coarse and relatively slow. They are there to protect the variac, not the load.2. I have included a mains power on indicator LED because I want to know if power is applied. The power meter will show if the secondary is active.3. The circuit includes a soft start design guide found here:https://neurochrome.com/pages/the-ultimate-guide-to-soft-start-design4. I have used a mains powered relay for the power-on delay because it does not rely on other parts of the circuit to work.5. I haven't yet figured out what I am going to do for fast current measure/monitoring.6. Isolating transformers are very expensive here so I have opted for a RCD-type A. That should be as good as an isolating transformer.

author: andrewtaylor
author_rank: Contributor
timestamp: September 25, 2025, 12:41:35 pm
content: Quote from: dazz1 on September 23, 2025, 02:57:55 amWell, I`d stay with the latching relay because I think about the kind of loadA heater or light bulb is no problem.But motor or air blower, electrical safe but if still acivated will/might start unwanted when mains power returns. A latching (or what we call it in my country "Selbsthaltung") will prevent accidents which might happen when the motor start, but you are still on the walk back from "ROUND THE HOUSE".YMMV

author: dazz1
author_rank: Frequent Contributor
timestamp: September 25, 2025, 08:07:04 pm
content: Quote from: andrewtaylor on September 25, 2025, 12:41:35 pmI have done VFD conversions on machine tools in my workshop and those definitely have latching start switches. If a machine tool suddenly came to life when power was restored, the consequences could be severe. If I was running a motor, a heater or some other device off the Variac, then yes I would include latching start. In my use case, the added complexity is not justified. Your use case might be different but you make a good point.

author: dazz1
author_rank: Frequent Contributor
timestamp: September 25, 2025, 11:25:58 pm
content: HiLooking for a solution to provide high speed qualitative measure of current. Specifically an indication of current value and fluctuations. I could do a linear array of 80 leds (10 led per amp) but that is not a convenient form factor.I am looking at concentric circular arrays. The outside = Amps. The inside = Deci-Amps.This would be in addition to the power meter with digital readings. No multi-meter resolution, but sufficient.I found these products:https://nz.element14.com/seeed-studio/104020128/led-ring-board-rgb-arduino-board/dp/4007792So new, they are not yet available.https://nz.element14.com/kitronik/35135/circle-led-board-bbc-micro-bit/dp/3021740Connecting these to a esp-32 micro controller and analog input(s) to measure either current or volts. Doing that would require a custom PCB design. If I did that, I wouldn't need to buy the linked products.If I did that, I would also make the outside LED array a 282 deg arc of 17 LEDs. I would make the inside circle 10 LEDs. I would add a single LED to indicate power on/enabled.It would look something like the attached drawing. A much more convenient form factor and high resolution/range with only 28 LEDs. In addition the addressable tri-colour LEDs would allow for many possibilities to convey information for other applications.To read this, each LED on the outside ring could be 1Amp or 0.5Amp, and each LED on the inside could be 0.1Amp. Zero at the bottom.[EDIT: Just after posting this, I found this:https://maniacallabs.com/2015/05/04/review-code-adafruit-dotstar-disk/The dotstar is not suitable for metering, but it demonstrates what a metering version I proposed could do.]

author: dazz1
author_rank: Frequent Contributor
timestamp: September 29, 2025, 01:49:43 am
content: HiI have been doing some more work on the Variac design. There are some problems still to solve.When I first switched on the variac, the lights dimmed so I set out to measure the in-rush current.The inrush current limiter requires a delay on relay. The relay I can find has a maximum current rating of 16A. I suspect the inrush current is significantly more that that. I think I will need to have the delay relay in series with a high current relay.During my tests the inrush current stopped in-rushing. That is a new problem to look at.I want to meter both the mains AC and the lower voltage 0-30VAC output. The switching required to do that with one LCD digital power meter is more complex and costly than two power meters.Noting the comments about how slow these power meters are, I also want to include my LED meter. That's three meters. Probably overkill.The advantage of my ESP32 driven LED meter is that I can switch between different sensors and the sensors can be different. I can have Volts, Amps, Temperature, switches and something else in any combination.The LED meter is intended to compliment the LCD power meters, not replace them. So I am looking at including 2x LCD digital power meters and 1x switchable LED analog LED meter. The size of the LED meter is set by the space needed for the connectors.The design is not yet complete.

author: andrewtaylor
author_rank: Contributor
timestamp: September 29, 2025, 11:42:51 am
content: Quote from: dazz1 on September 25, 2025, 11:25:58 pmSo something simialr to this on (sorry 12V):https://www.amazon.de/keenso-12V-Digital-Voltmeter-Display