1970s TV transformer as an isolation transformer - Page 1
Forum section: electronics / beginners

author: lordvader88
author_rank: Frequent Contributor
timestamp: September 21, 2017, 10:31:22 pm
content: Its from a big floor model, with transistors and even a view ICs, I scrapped it, too bad it was smashed, its pretty hard to find old electronics where I live.I checked the winding on Ohms an found 4 sets and even separated out the 2'ndry in order.With 120V in on a solid black pair of wires, , on the main secondary set I get0V145V220248253274350or 0V,76,97,102,130,205Then there's a winding with 115V on it, its resistance was about 0.34Ohm. It has 2 grey wires. The Blk pair had 5.6O and I put mains 120V on on its winding.Does the black sound more like Primary ?Then on a separate sec.wind. there's 5.5V , I assume the CRT heater windingThere was another little transformer in this TV, 120V to 5.7V. so I assume thats run of that gray pair of 115V ? and is an audio transformer I guess.I don't have the schematic tho, but that sounds like the gray winding is only for small power for audio output, and not suitable above say 5-10W ? IDKSo for now I could use 102V (or well what ever combo) and try to power up a small vacuum tube radioSo this thing with some variac and switches and light blub limiter should work great

author: Zero999
author_rank: Super Contributor
timestamp: September 21, 2017, 10:39:25 pm
content: You shouldn't rely on it to provide any protection against electric shock.

author: Electro Detective
author_rank: Super Contributor
timestamp: September 21, 2017, 10:49:37 pm
content: If you are going to play with it, make sure the 120v to 115-120v is truly isolated,and work out how much current it can safely deliver without cooking the transformerSuggest an inline fuse/s on primary and secondary for cheap insurance

author: Brumby
author_rank: Unknown
timestamp: September 22, 2017, 01:17:13 am
content: I echo the previous comments with one highlight:As well as having some age to it, such a transformer was never designed to be a true isolation transformer for a workbench. It may not have the same level of protection as a purpose-built one.The second thing is, you should never treat an isolation transformer as a "magical" solution to make working on any equipment "safe". Unless you know what you are doing, you can actually make the situation more dangerous - because some of the dangers will be less obvious.It is my belief that using an isolation transformer should only be when youknowyou need one - notthink, butknow... and if you do head down that road, do your homework. This includes finding out what can go wrong when using one.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 22, 2017, 06:01:51 am
content: Quote from: Brumby on September 22, 2017, 01:17:13 amTransformer design is a mature technology, they worked out insulation ratings & so on, decades ago, so such a transformer, in good condition, is unlikely to break down causing injury or damage.That said, transformersdofail, & you need to be aware of that.As a TV transformer, its output current may be less than you require.The old BW TV ones in Oz were rated at around 300mA from the main 300v (or so) HT winding.

author: Brumby
author_rank: Unknown
timestamp: September 22, 2017, 06:27:06 am
content: Quote from: vk6zgo on September 22, 2017, 06:01:51 amWhile I do not dispute what you are saying, I would like to point out we are not just talking about technology that was mature decades ago - but of a unit that was built decades ago with materials from decades ago and pressed into service decades ago.As long as there wasn't a cheap-skate manufacturer who built the TV with a transformer on the edge of its ratings, then there should be a reasonable expectation on its performance - but you can't treat it as new.QuoteDefinitely. Be aware.

author: lordvader88
author_rank: Frequent Contributor
timestamp: September 23, 2017, 01:34:57 am
content: Its from a Zenith Chroma Deluxe 2 floor version, I have the CRT tube sticker, but I'm not actually sure what model the TV is. I'm still not sure the average Wattage of similar TVs from back then.My DC resistance measurements, and voltages and calculations of the few equations I know of this matches up on the little 20:1 transformer.From about 1mA to 100mA its 2nd voltage drops from 7 to 5.3V and the power goes from whatever low up to +1/2W with 5.3V*100mA. That was with 50OhmImpedance reflections for equivalent circuits, just learned of that last night...I know 2-3-4yr university maths of for some stuff so thats no problem.................if/once I learn the eq/remember itI have not tested loads on the big tran's yet...........since I don't know the little one yet (and surely better too leanr on it)I have almost nothing to hook up to this yet anyway that needs it.So yup generally black wires on a trans. of the era before SMPS are the mains.So I have a 3-prong ATX plug, and this thing only has 1 primary winding, and no taps. Someone I trust said its fine to solder the solder the earth 3rd prong wire/nut bolt hole thing??? and bolt it to the transformer chassis.So thats what I'll do unless I'm missing something