Toroidal transformer suitability for isolation power supply - Page 1
Forum section: electronics / beginners

author: Cykar
author_rank: Contributor
timestamp: May 07, 2021, 12:44:37 pm
content: Hi,I came across a power amplifier, chucked out by the bins, so scavenged it, as you do. Inside, at least 2 of the large caps looked puffed up, so perhaps it wasn't working so well, or perhaps the owner just didn't want it any more. It's branded Oxford Audio AV2200, but although there is a high end audio place in UK called Oxford Audio selling equipment up to many tens of thousands of pounds, there was nothing about this piece of kit I could find. In fact, the only reference anywhere I could find was an ebay listing sometime in 2017 i think, where it was sold for about £80, so high end or not, it's not worth much so not going to try and fix/sell it, and I don't have any way to test it anyway.But, to the point - inside is a large toroidal transformer. I tested all the windings coming of it, as you can see in the diagram (image attached), wondering if I could use it as an isolation transformer, and possibly a regulated bench power supply too.2 of the windings have red-black-red wires, the black being a centre tap, with about 56V between the two red wires. This is identical on both the red-black-red windings, and corresponds to the info on the transformer itself given that I'm on 240V not 230V (see image). So my first question is about the suitability of this transformer as an isolation power supply if I wire these two winding in series. I've no information about the power of the transformer other than that ebay listing which said it was a 200W amplifier. There is an 8A fuse on the mains input, but I guess that could have been changed so doesn't necessarily tell me anything.Then there are 5 black wires bundled together. 3 of them (black 3, 4, 5 in my diagram) have continuity giving 40V between 3 and 5 with 4 being a centre tap, which seems to correspond to the info on the transformer, again, allowing for the fact that I'm on 240V not 230V. How suitable might this be to power a regulated power supply unit?If I could get an isolation power supply and regulated power supply using this one transformer, that would be pretty good - but I don't really know the pitfalls so, is this suitable at all?I have another question regarding the transformer: the 2 other black wires bundled with the other 3 that gave 40V with a centre tap - what are they? At first I thought they were another winding as they measure next to 0 ohms, but there is no voltage between them when the unit is turned on. There is voltage between both these wires and the other black wires tho, so, what's going on? Is it some sensing component hidden inside the transformer? The info on the transformer says nothing about this and the 5 black wires just all go into one connector and then onto the main board with no info on the board either.Thanks for your help.

author: Robert Smith Eco Warrior
author_rank: Regular Contributor
timestamp: May 07, 2021, 01:06:10 pm
content: Is the other black/black pair a thermistor or something? Maybe put it in the fridge and measure its resistance, then warm it up and measure it again. If its resistance changes I guess some sort of thermal measurement.That lower voltage output has to be useful

author: PKTKS
author_rank: Super Contributor
timestamp: May 07, 2021, 01:19:35 pm
content: Quote from: Cykar on May 07, 2021, 12:44:37 pmFor an ISOLATION BENCH device target to repair stuff...probably using a VARIAC.. no it is not.The power range for such transformer is above 1KVA.For a bench PSU this is very handy.Try to measure how much you can really take fromthis core up to saturation...Toroidal are less leaky than traditional mains ones..Paul

author: Cykar
author_rank: Contributor
timestamp: May 07, 2021, 04:23:08 pm
content: Quote from: PKTKS on May 07, 2021, 01:19:35 pmSo, is it because the voltage in the secondaries will reduce with higher loads? And if that is the case, might it still work for devices which are lower power, say 3A or below?My intention is to use it to repair stuff, mainly so I can use my oscilloscope without blowing things up, and i do not have/use a variac, if that makes any difference.

author: CaptDon
author_rank: Super Contributor
timestamp: May 07, 2021, 06:01:12 pm
content: All of the best quality hospital isolation transformers used INSIDE the operating rooms are toroidal. Due to their low leakage of magnetic flux which would disturb many of the sensitive patient monitoring instruments in the O.R. We did have conventional huge 5kw E/I core isolation transformers outside the O.R.'s with 'LEM' leakage monitors connected to alarm systems.