Tips to measure mains transformer and survive? - Page 1
Forum section: electronics / beginners

author: vmallet
author_rank: Regular Contributor
timestamp: November 04, 2019, 01:07:32 am
content: I have this old transformer and I would like to measure its output voltages.Its input is 220V mains (blue wires) and it has two outputs, 12.5V (brown wires) and 18V (red wires). I'd like to measure the actual voltages coming out of the brown and red pairs. This transformer is used to feed an old industrial motherboard.I have a 110V->220V consumer transformer (I am in the US) with a three-prong socket for the 220V output. What would be the right way to proceed and what safety tips should I follow to make sure I don't win a trip to the heavens? Would it be safe to directly connect the two blue wires to phase and neutral (with solid connections) and measure the red & brown outputs with a multimeter?Thanks,Vince.

author: m3vuv
author_rank: Super Contributor
timestamp: November 04, 2019, 02:25:22 am
content: The answer is simple,get someone else to do it!

author: vk6zgo
author_rank: Super Contributor
timestamp: November 04, 2019, 02:31:55 am
content: Quote from: vmallet on November 04, 2019, 01:07:32 amFirstly, you need to connect the primary of the transformer you wish to test to the output of your 220v transformer, making a really solid job of it, as below:-Using a proper power cord, strip the end of it which wil go to the transformer under test.(TUT)Using a screw type terminal block connect the Active ("phase") to one side of the primary of the TUT, & the Neutral to the other.(as your terminal block is probably 120 volt rated, don't use two adjacent teminals-- leave a space)It would be nice if you could mount all this in a plastic box, but you probably haven't got one, so wrap the whole thing securely in good quality insulation tape.--- use plenty!Get another terminal block & connect the secondaries to it--- this prevents the possiblity of you shorting a secondary out whilst farting around.Place the whole thing on an insulating surface like dry wood, or plastic.Plug the 220v plug into the "110-220" transformerPlug that transformer into the 120volt outlet, & step back.If, after several minutes, the transformer under test shows signs of distress, it is, similarly to Monty Python's parrot, an "ex-transformer"'If it looks OK, measure each secondary in turn.Note:- The "no load" secondary voltages will read higher than the labelled voltage, which is determined at rated current.

author: vmallet
author_rank: Regular Contributor
timestamp: November 04, 2019, 02:50:46 pm
content: Thanks a lot @vk6zgo. The TUT is currently mounted on a metal chassis (see original pic); should I connect the ground wire of the 220v cord to the chassis? Or is safer to remove the TUT from the chassis? Or just leave everything as is and proceed with what you outlined?

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: November 04, 2019, 03:40:53 pm
content: Fuses???

author: Fungus
author_rank: Super Contributor
timestamp: November 04, 2019, 03:48:02 pm
content: Quote from: vmallet on November 04, 2019, 01:07:32 ama) Wire it all up with a 1A fuse and long wires on the primary side of the transformer, connect the multimeter to the output.b) Plug the whole thing into a plug strip - one with with a switch on it.c) Plug the plug strip into the wall then use the switch to power on the divice. Don't look directly at it, wear ear plugs.Quote from: vmallet on November 04, 2019, 01:07:32 amOr...Read the label on the side: It says 12.5V and 18Vnb. The voltages will depend on the load you apply to it, you only get that voltage with a particular load. I wonder where it might tell you that information....

author: exe
author_rank: Unknown
timestamp: November 04, 2019, 04:07:18 pm
content: You don't need 220V winding to be connected to 220V, any lower voltage should be fine (but output voltage will be proportionally lower).I wonder if using a siggen would make sense. Ofc you can't do load tests, but if you need to only roughly know the output voltage it might work. Also, by measuring resistance of primary and secondary you can estimate regulation.

author: schmitt trigger
author_rank: Super Contributor
timestamp: November 04, 2019, 04:18:53 pm
content: A simple trick used used by electricians for decades and decades is to place a 60W incandescent lamp in series with the primary.It has to be anincandescent lamp!If anything is shorted, the lamp will glow at full brightness, but nothing will be damaged.OTOH, if the transformer is OK, the lamp will glow dimly, a little voltage will be dropped, which you can measure, and then measure the secondary voltages.It doesn't get any simpler than this.

author: vmallet
author_rank: Regular Contributor
timestamp: November 04, 2019, 05:04:11 pm
content: Thanks all. Armed with this info I went ahead and did my tests, using a remote controlled plug and staying away at a good distance with protection glasses when I turned it on/offNo sparks, no heavens, everything good.Quote from: Fungus on November 04, 2019, 03:48:02 pmI really just wanted to get a sense of what actually comes out of the secondary regardless of what the label says (I did mention in my original post that these were the expected values).Thanks!

author: Fungus
author_rank: Super Contributor
timestamp: November 04, 2019, 08:34:30 pm
content: Quote from: vmallet on November 04, 2019, 05:04:11 pmI know...What you read on a bare transformer will definitely be quite a lot higher then with a load though.

author: vk6zgo
author_rank: Super Contributor
timestamp: November 05, 2019, 12:24:55 am
content: Quote from: Fungus on November 04, 2019, 03:48:02 pmWith respect, ---it's a transformer, not a nuclear device!I will admit that the series fuse is a good idea, but I was more concerned with making sure the Mains to primary connection was secure.Inserting a fuseholder into that simple circuit makes for a less secure connection, unless you go to great lengths to fit the whole assembly in a box.Over many years of working with transformers, I have never seen one which has a direct short across the primary .The usual fault condition is a shorted turn on either primary or secondary (or an open circuit winding).In such cases, the existing turns limit the current drawn from the Mains.Faults with transformers just let the magic smoke out, they don't fly apart.Series wound electric motors, --------now, there's another thing altogether!Quote"Reading the label" doesn't tell you anything about the transformer's condition, though!

author: DTJ
author_rank: Super Contributor
timestamp: November 05, 2019, 02:18:04 am
content: A handy tip for working on mains powered devices is to unplug the power cord and stick it in your pocket when working on it. If the cord plug is in your pocket then you are (generally) safe to poke around moving test leads or wires. It's easy to get side tracked and forget that the gear you are working on is live.Another tip when doing testing like above is to use the back of your hand or fingers to test to see if something is (thermally) hot rather than your fingers which if the surface is live could 'grab' the live material.

author: exe
author_rank: Unknown
timestamp: November 05, 2019, 08:18:59 am
content: Quote from: DTJ on November 05, 2019, 02:18:04 amYeah, happened to me multiple times