Isolation and Current Limit Project - Page 1
Forum section: electronics / projects

author: drtaylor
author_rank: Regular Contributor
timestamp: February 19, 2025, 10:45:05 pm
content: I am in the assembly stage of my ridiculously expensive Isolation Transformer, Light Bulb Current Limiter, and Power Center. It has a 1000W Toroidal Isolation Transformer. It has Light Bulb current limiting adjustable in 20W increments from 20W to 300W, controlled by relays. All power is controlled by a Circuit consisting of high power relays. It has connections to add an external Variac, which can be bypassed. There are two circuit boards: 1 relay and control board with 7 small tact switches for control, and a circuit board that mounts all the lamps. It uses 20W and 40W oven incandescent lamps, which are exempted from bans and should be available for a long time.It has three non-isolated duplex Receptacles, two of which are switched by relay. The unswitched Duplex Receptacle is a GFCI which also protects the switched outlets. There are two isolated Duplex Receptacles, one full voltage, and one that has the current limiting lamps and can have an external variac switched in or bypassed. I used medical grade ground isolated duplex Receptacles for those, as most US receptacles ground through the frame, and I didn't want to depend on insulation on the screws for isolation. However, within the individual duplex outlets, the grounds are tied together, nothing I can do about that without tearing apart the part. I don't consider that a problem as usually only one device will be plugged in, and in most cases it won't hurt anything or pose a safety hazard anyway. The current limited part receptacle is ground isolated from the fixed isolated receptacle. The design also uses one of those Amazon AC monitoring modules for voltage and power. All the power connections are into 7.5mm Wago Lever Type Terminal Blocks. There is Circuit Breaker overload protection on the input and isolated output sides.I designed a very robust enclosure and had it laser cut and the front panel power coated. Grounds are fully isolated on the secondary side, no connection anywhere that can lead back to neutral. I'm trying a new way to label all the connectors and controls, more on that after I try it.This design is for 115VAC mains due to the transformer. The relay circuitry will handle 25A at 240VAC, so it would be possible to change the design for 240V countries by specifying a new transformer and changing the receptacles to your countries standard.After years of just using a light bulb in a socket, I got tired of how primitive it looked and how inconvenient it was to change the wattage limit. Also large incandescent light bulbs are going the way of the Dodo. I needed an Isolation transformer for my Tube Amp repair and design work. Plus I have some old instrumentation that I'm going to be restoring, and the Isolation Transformer / Variac / Current limiter should be very useful. I call this project ridiculously expensive because it has added up to a fairly large amount of money. But 1000W isolation transformers are not cheap unless you luck upon a good used one. Being the instrumentation designer I am, I engineered this as I would a commercial project. So everything is documented enough that I could make it a product. But I won't go there, because I will never spring for UL approval testing. But if it all works out, I will be happy to share the design with anyone else who wants to make it on a hobbyist basis. I have spare unstuffed circuit boards, that are mixed SMD and leaded components. Money can be saved if you opt for a lower power transformer and/or can construct your own enclosure.

author: Benta
author_rank: Super Contributor
timestamp: February 20, 2025, 12:25:46 am
content: I have difficulties in understanding what you're trying to achieve. A startup circuit would normally have NTC characteristics, but incandescent bulbs are PTCs.Could you show an example circuit?

author: moffy
author_rank: Super Contributor
timestamp: February 20, 2025, 02:31:46 am
content: Light bulbs are sometimes used in series with a mains powered item to be tested, as a fail safe, current limiting device. They normally only light in the case of a fault, hence the wattage selection by the OP to be able to test devices of different powers. A variac input is also another way to bring the voltage up in mains powered devices for fault testing.The project must weigh a bit with that 1000W transformer and not suprised it cost a pretty penny or two.

author: bdunham7
author_rank: Super Contributor
timestamp: February 20, 2025, 02:40:05 am
content: Quote from: Benta on February 20, 2025, 12:25:46 amOne of the main features of the light bulb setup is saving expensive and rare power transformers in case there is a short in the secondary circuit somewhere. Bringing things up slowly with a variac is another option, but there are examples of things that don't react well to a gradual power ramp up.

author: ledtester
author_rank: Super Contributor
timestamp: February 20, 2025, 06:46:06 am
content: Quote from: drtaylor on February 19, 2025, 10:45:05 pmWould love to see a pic of it - or even a design drawing / 3D CAD rendition / ...

author: coppercone2
author_rank: Super Contributor
timestamp: February 20, 2025, 06:58:17 am
content: I want to make a big glass tube filled with inert gas that has a whole bunch of tungsten in there to make a custom limiter but I have little use for such a devicethe ultimate version would be a incandescent rheostatmost people on the forum seem to hate the idea thoughI am glad to finally run into someone that finds it useful. technically its a minimum thermal mass high temperature coefficient variable resistor . I imagine it would be complicated to makebut its not as bad as tubes, because light bulbs are not vacuum devices. I think a design could be like serviceableIf you just use tungsten elements that are fairly granular, it just seems like you can build it kind of like a beefy switch (like a old decade resistor). If you make it very fine to adjust like a rheostat/variac, the adjustment mechanism concept gets complicated. I do wonder what a good one would look likeAnd I am not working on it or anything, I don't really even like filaments, and I think you can get a way more practical solution with some transistors or something, if you really need it, instead of going hard in the paint over tungsten physics etc, but its somewhat interesting

author: drtaylor
author_rank: Regular Contributor
timestamp: February 20, 2025, 07:12:26 am
content: Pictures and design details to follow. Right now I'm drilling and tapping the side extrusions. The PCBs will be assembled by Monday. Barring any problems, I expect I will have it all together in two to three weeks, as I can only work on it part time.

author: pardo-bsso
author_rank: Frequent Contributor
timestamp: February 20, 2025, 05:38:31 pm
content: Quote from: coppercone2 on February 20, 2025, 06:58:17 amSounds a lot like a barreter, but using tungsten instead.That had very useful real life applications.

author: coppercone2
author_rank: Super Contributor
timestamp: February 20, 2025, 09:55:56 pm
content: It would require a gas tight possibly pressure rated bearing, a special interconnect to connect tungsten wires to a wiper. You would need to figure out how small you can make a element to get maximum resolution from it and how it can be mounted (spacing between elements) and it would probably get funky because of gas-flow from heating in a larger chamber, even if its somewhat rareified (gas baffles?)Perhaps its a device that could have been, but not necessarily will ever be made.And the wear would be uneven for the elementsWhats interesting about making it smaller and integrated is that its maximum frequency ability goes up. I assume it would be low but it would be higher then a bunch of light bulbs, maybe.But I do think that im fairly sure it would look kind of like a gas mantle lamp or catalyst screen when running at high powersome kind of tungsten grid comes to mind too, if you branch it off to a bunch of different circuit paths ?

author: Terry Bites
author_rank: Super Contributor
timestamp: February 21, 2025, 08:47:46 am
content: This has foxed me a few times. See this though.https://www.ametherm.com/inrush-current/ptc-thermistors-for-inrush-current-limiting

author: drtaylor
author_rank: Regular Contributor
timestamp: February 21, 2025, 08:24:46 pm
content: Some people have missed the point of this project. The purpose is to provide a safe, isolated AC output for working on Instrumentation and Audio Amplifiers, especially Tube Amps. It is NOT for limiting inrush current. The Variac that is easily switched in and out by the circuitry is how technicians safely turn on a possibly bad UUT and limit inrush current. This is common practice. No point in this project to use NTC inrush current limiters as it is intended for a universal AC supply for products up to 1000VA.So to summarize: 1) Provides totally isolated-from-the-mains AC output at up to 1000VA. Two outlets not current limited, two outlets are current limited with Variac Voltage Control. The isolated outlets are on the front panel. 2) Provides switchable current limiting from 20W to 300W which covers most instrumentation. This section also has a test switch to prove the bulbs are functional. The bulbs are visible from the front panel. It also has a kill switch to quickly disconnect power if bulbs are lighting up. 3) Provides an un-isolated power center with GFCI and relay switched outlets. Six outlets, 4 switched, 2 always on. These are on the rear panel. 4) Has an input and output connector to attach any variac. The variac output is fed from the output of the isolation transformer and therefore isolated from ground. The Variac is relay controlled to feed the Current limited outlets, or to bypass the Variac at the CL outlets. The isolated Variac output is also brought out on two banana jacks. 5) Has an AC Monitor (all AC Parameters) on the isolated outputs. I just used one from Amazon...so much cheaper than I could design. Only downside is that it only rated down to 50VAC, but I haven't tested that yet.

author: bdunham7
author_rank: Super Contributor
timestamp: February 21, 2025, 09:38:24 pm
content: Quote from: drtaylor on February 21, 2025, 08:24:46 pmYou may need to limit the inrush from powering up that transformer. A 1kVA has a good chance of occasionally tripping a breaker.

author: Benta
author_rank: Super Contributor
timestamp: February 21, 2025, 10:07:32 pm
content: Quote from: drtaylor on February 21, 2025, 08:24:46 pmA drawing/schematic replaces a thousand words. Verbosity doesn't.

author: coppercone2
author_rank: Super Contributor
timestamp: February 22, 2025, 02:29:00 am
content: Well the idea is if you really want the best variac in the world, having an analog dial to regulate inrush control would be like the best possible solution. You put alot of effort into making the most advanced inrush control that is practical to implement.Another advanced thing that could be added is a adjustable reactor of some kind, maybe there is situations where it would be useful lol

author: drtaylor
author_rank: Regular Contributor
timestamp: March 05, 2025, 03:33:56 am
content: Enclosure assembly is complete. Attached are pictures of project as far as I have gotten. Missing is the PCB that controls everything. I'm not ready to share the schematic yet as I still need to place parts on the PCB and test.

author: moffy
author_rank: Super Contributor
timestamp: March 05, 2025, 05:02:41 am
content: Beautiful case and construction, what are all the heatsinks for along the sides of the case, I would think a fan airflow would be more effective?

author: drtaylor
author_rank: Regular Contributor
timestamp: March 05, 2025, 06:43:53 am
content: The Heat Sinks aren't really necessary at all. I originally planned to make an Audio Amp with this construction method. I do, however, plan to put Linear regulators on the heat sinks, but I haven't decided if I will do that. There is a high power Bridge Rectifier bolted to one side for the ±24V output. Mostly, the heatsinks are used as the structural elements, with all panels screwed into them via tapped holes. I don't expect this design to run very warm except on the rare instances I hook up a really high power amp, or some of the instrumentation I am restoring. Thanks for the kind words moffy.

author: drtaylor
author_rank: Regular Contributor
timestamp: April 27, 2025, 01:27:55 am
content: Project Completed (more or less). I have completed testing and charecterization of my Isolation Transformer / Bulb Filament Current Limiter Instrument.This device will probably be of interest only to people who work on Tube Amplifiers, or restore old electronics. It is a safety device that isolates devices entirely from the AC Mains, and provides a current limited output that can prevent things from blowing up or smoking.It consists of two circuit boards, a Relay controller (no software, just good old logic), and a Lamp board. It contains a large 1000VA toroidal isolation transformer for isolation. There are three duplex AC receptacles on the rear panel that are not isolated, one being a 20A GFCI. The other two are switched on/off by the front panel. There are two ground isolated duplex AC Receptacles on the front panel. One is directly connected to the Isolation secondary, the other is bulb filament current limited by selectable incandescent bulbs controlled by relays. There is an input and output for connecting a Variac to use in conjunction with the current limiting section.Since the transformer had low voltage windings, I rectified and filtered them to provide fixed DC of around ±24V. I just hate wasting windings. The variac is also connected to a pair of Banana Jacks in case I need an variable AC source.There is a Chinese made Power meter that monitors the isolated outputs.The build was fairly flawless. I handplaced the SMD parts using a solderpaste stencil, and reflowed them in a modified toaster oven. Everything soldered fine, but I had a couple of bridges on a TSSOP part that I had to clear. The LEDs are too bright at only 5mA, so I might go back and change all the LED resistors. I used RGB LEDs for all of them, and pinned out all three LEDs. I could make any color based on which resistors I placed. I mixed some colors by using more than 1 resistor on them. The LEDs are Right Angle Light Piped to the front panel.The Lamp Board uses E17 sockets for the incandescent bulbs. I designed this with 20W and 40W E17 bulbs which will not be discontinued as they are oven light bulbs which are exempt from being discontinued. I did discover that a 40W bulb really only draws 32W bulb at 120V which kind of threw my nomenclature into error. However, I was able to kludge in "Calibration" bulbs to get it closer to the desired levels. You can see the kludged bulbs on the back of the lamp board in the open top picture attached. I added a 40W (really 32W) to the 160W bank which then totaled actually 160W. I added a 15W E17 to the 80W bank (two "40"W bulbs) which then actually totaled 79W. I'm going to live with the 40W setting being 32W. The 20W actually was darn close when I used a 25W bulb there. I should have anticipated that 40W bulbs are 20% low. This is consistant no matter what brand I tried.The final build had a lot of wiring to hook up. Mostly wired with 14AWG Elastomeric Insulated hookup wire. I used ferrules in all the terminal block wiring. I used lever Terminal Blocks from WAGO. The picture showing the open top was before I dressed the wiring up neater. There is a lot of wiring in there!The enclosure panels were supplied by the excellent SENDCUTSEND company and were laser cut. Front, top, and bottom panels are 100mil Aluminum, and the rear panel is 70mil aluminum (because some of the press-fit connectors I used on the back would not work in 100mils). The Front panel is powder coated, also by SENDCUTSEND. I brushed the other panels. The side panels are heat sinks provided by HeatSinksUSA. They are extrusions that are thick enough at their base to drill and tap all the holes that hold the whole thing together. The price for doing all the aluminum was surprisingly low, and far more accurate than I could manage with drill press, not to mention I have no good way of cutting thick aluminum panels.The labeling on the front and rear panels is Vinyl originally designed for transfers to things like cups or fabric. But it allows full color, quite detailed nomenclature. I'm really happy how that turned out and it seems quite durable. The labeling was made by Ninjatransfers with their UV DTF Gang Sheet product. I supplied art from Adobe Illustrator.I plan on posting the build on Tube Amp and Guitar Amp DIY forums. I have spare circuit boards. If anyone wants to make one (for private use only), I am willing to share all the files for the enclosures, the BOM, the art, etc., and I have left over raw PCBs. I have a version designed for 230V systems for a German friend of mine, but I'm waiting for him to pick the receptacles and other connectors before I can publish the design of that.Attached illustrations are the front panel with control explanations, a view of the construction interior (before I did some wire dressing to clean it up some), the rear panel, and the schematic.

author: Xsaia
author_rank: Newbie
timestamp: September 19, 2025, 12:43:22 pm
content: Hello, drtaylor, I would like to contact with you, not about this particular topic theme, but about understanding principles of work of some old fluke DMM MAC IC (I read your old post from 2013). I wanted to send you email thru forum, but was unable to do it for some reason.In short words, I get my hands on Fluke 8060 a, in great visual condition, but with one con - it's not powering on correctly, I made some measurements on board, looks like it's power unit not working correctly, and not forming VDD and VSS, problem is , part of it is inside 40 - pin MAC IC and I don't know anything about it - because it's unique.Can you help me troubleshoot this malfunction ? I still hope this is a external component problem, not a MAC burnout.