Back to back isolation transformers now in a box, plus other contraptions - Page 1
Forum section: electronics / projects

author: intabits
author_rank: Frequent Contributor
timestamp: November 14, 2019, 08:08:55 am
content: In this thread:https://www.eevblog.com/forum/projects/testing-back-to-back-power-transformers-for-use-as-an-isolation-transformer/msg2398320/#msg2398320I investigated using two transformers wired back to back as a mains power isolation transformer. I've now put the two mid-sized ones that I judged suitable for most general usage in a box.I estimate it's good for about 150VA. The front panel includes the schematic.The 32V internal voltage between the two transformers is made available for convenience.The output earth is isolated, but can be joined to the incoming mains earth if that should be required.The insides, showing the two 240v to 32v@4.75A transformers.In that thread, I also looked at some much larger transformers from an IBM 4381 mainframe computer, and they are very useful for experimenting with mains power at up to 2KVA.But I needed a way to make these heavy (22Kg) beasts a little easier to handle, so I built them into frames with handles.While testing the transformers, I used light bulbs as dummy loads, and have also used them as current limiters in the past.But I wanted to make them easier to use in either of these two modes, and provide some protection for the bulbs. Here's what I did.In Dummy Load mode, the bulbs are in parallel with the output sockets, presenting a load to the input source, that can be increased by any additional load plugged into the output sockets.In Current Limiter mode, the bulbs are in series with the output sockets, limiting the current available to any load plugged into them. The Bypass switch shorts out the bulbs, allowing the output sockets to draw the full load current.A look inside showing the wiring. The sockets are one each of the Edison screw type, and the bayonet type (which are quicker to swap out so as to change the load/limit).Schematic diagram of Mains power Dummy Load / Current Limiter.

author: rvalente
author_rank: Frequent Contributor
timestamp: November 14, 2019, 09:51:19 am
content: Great build, very useful deviceTwo thing's got in my head: why build the enclosure for electrical systems with MDF/MDP?I have not seen any protection, you really should add an input circuit breaker, output circuit breaker with tripping coils attached to two 90°C paralled thersmostats assembled on the trafos cores

author: 001
author_rank: Super Contributor
timestamp: November 14, 2019, 12:40:52 pm
content: Why do You power panel meters with DUT circuit?

author: intabits
author_rank: Frequent Contributor
timestamp: November 14, 2019, 08:46:00 pm
content: Quote from: rvalente on November 14, 2019, 09:51:19 amI used melamine coated particle board because it's easy, quick and cheap. And enables the enclosure to be the size and shape that I want,These devices are my personal use only. They are for short term, interactive use only, and will not be operated unattended.Obviously, I will be monitoring them for problems while in use.I do not envision operating them in conditions that could come anywhere near to being a fire risk.I had planned on thermo-switches and a fan for the isolation transformer, but dropped the idea given the above.Should it become necessary, I can easily retro-fit a fan by cutting four wider side pieces to extend the box depth to accommodate a fan.In fact, one of the first things I want to do with this thing is develop a temperature controlled fan circuit.But I understand your concern, and I probably will end up retro-fitting such a fan and control system.Or after using these things for a while, I may come up with new requirements, or ideas for other configurations and features. Maybe then I could incorporate all of it, your suggestions included, (and maybe the dummy load as well) into one of the larger metal enclosures that I have lying around here...In any case, for higher power levels, I have the big transformers.These things will almost invariably be operated from my variac, which has a circuit breaker and an RCD.Quote from: 001 on November 14, 2019, 12:40:52 pmNot sure what you mean exactly.In current limiter mode, the DUT is after the meter, so not a problem. But it is only useful for measuring current, as it won't be showing the voltage across the load. (though a mod to the meter may allow this)As a dummy load, yes the meter forms part of the DUT/load, but consumes only 1.5W, and can be accounted for.These things represent a starting point that provides what I anticipate needing at the moment, and I expect they will suit my purposes just fine.But nothing stops me chucking all of it and doing it a better way if the experience of using them indicates that changes are needed.