Unlabelled transformers - Page 1
Forum section: electronics / beginners

author: Dave Turner
author_rank: Frequent Contributor
timestamp: August 02, 2022, 05:42:31 pm
content: I've purchased a 300VA (115-115 x2 primary - 115-115 x2 secondary) toroidal transformer.I know from the markings which are the primary and secondary windings.The plan is to use this as a part of a dim bulb tester for 115-230 input to 115-230V output. Yes 120 - 240v depending on local variation.My question is how do I determine/measure how to correctly se the windings.regardsDave

author: bob91343
author_rank: Super Contributor
timestamp: August 02, 2022, 05:51:47 pm
content: For 240 V input, put the primary coils in series. For 120V input, put them in parallel.For 240 V output, put the secondary coils in series. For 120 V output, put them in parallel.Be careful to get the phasing correct or you will have fireworks.This setup gives you isolation only. If you want more features, you will need to add them.

author: Seekonk
author_rank: Super Contributor
timestamp: August 02, 2022, 05:58:29 pm
content: Use a light bulb for that too! Unknown transformers can be tested with a lamp in series. If in parallel or series and the phasing is wrong, the lamp lights and no damage. You can also determine the voltage of an unknown winding because transformer will saturate and turn into a resistor at that point. Best seen on a scope.

author: ledtester
author_rank: Super Contributor
timestamp: August 02, 2022, 08:17:05 pm
content: Someone should make a battery powered "transformer tester" which will characterize an unknown transformer.It would be very helpful to have when you visit surplus stores where you have loads of unknown transformers salvaged from old equipment or NOS.Update: Found this video of an "automatic transformer tester":200 KHz Automatic Transformer Tester (VE2729xA) -- vasavielectronicshttps://youtu.be/eXwt2gxgRd8Unfortunately it isn't battery powered.

author: Kyle_from_somewhere
author_rank: Regular Contributor
timestamp: August 02, 2022, 08:24:30 pm
content: Quote from: bob91343 on August 02, 2022, 05:51:47 pmYou have to tell him how to tell if the phasing is correct or not.

author: andy3055
author_rank: Super Contributor
timestamp: August 02, 2022, 09:13:47 pm
content: Quote from: Dave Turner on August 02, 2022, 05:42:31 pmPost pictures of the labels you have to get better help.

author: jmelson
author_rank: Super Contributor
timestamp: August 02, 2022, 11:18:35 pm
content: Quote from: Kyle_from_somewhere on August 02, 2022, 08:24:30 pmYes, connect the primary in series, any which way. Check one of the secondary windings, it should give half voltage if primary is correctly phased, roughly zero if phasing is wrong. If primary is wrong, reverse ONE of the primary windings. Now, connect secondary in series, any which way. Measure voltage across far ends of secondary. If zero, reverse one winding, then you should get full mains voltage across both windings. Now, you can put either primary or secondary in parallel, if needed to step up or down. Work carefully and methodically, this testing could be a bit dangerous if careless. But, testing with both primary and secondary in series will prevent fireworks. If the phasing is wrong in a paralllel connection, you will get fault currents.Jon

author: MrAl
author_rank: Super Contributor
timestamp: August 03, 2022, 12:01:41 am
content: Testing for 240v or 120v output or even input is different for series 240v or parallel 120vac.You could power the primary or secondary with 120v, applying to only one winding.If you put any two windings in series and you get zero voltage at the two free ends, mark the two connected ends "1" and the two loose ends "2".To connect in parallel, connect the two loose ends together so you will have two #1 ends together and two #2 ends together.To connect in series, reverse one winding so that one #1 end connects to the other winding #2 and use the two free ends one other #1 and other #2 as the 240vac winding.That's about the simplest way.So in short, to connect in parallel for 120vac connect both #1 ends and connect both #2 ends, and for series 240vac connect either #1 to the other winding #2 and use the free #1 and #2 as the 240vac input or output.One caution. You should really connect a fuse between the line voltage and any winding(s) just in case you get something wrong. For example, if you connect a #1 to a #2 and connect the other #1 to the other #2, you end up creating a short which could cause a wire to melt very quickly and burst into a spew of molten copper. That's dangerous and that's not the only thing that can go wrong. With a fuse the fuse will pop and you know you did something wrong. The fuse rating can be as low as 10 amps although it should probably be a slow blow type.When you do a test, you can connect the windings and the meter, then stand away and plug the unit into the line or use a power strip with a switch.Alternately if you have a variac you can run the voltage up slowly and test that way. If you dont have a variac you may have an AC wall wart maybe 12vac you can use that instead for the testing. It must be an AC wall wart though not the more common DC wall warts. It should be able to do maybe 1 amp too so it can power the transformer properly.

author: Zero999
author_rank: Super Contributor
timestamp: August 03, 2022, 12:16:42 pm
content: Note that it's unlikely to be exactly 1:1 turns ratio.To make up for the voltage drop under load, the secondary windings will have slightly more turns than the primary. This means the secondary voltage will be slightly higher, than the primary, when unloaded. If the secondary voltage is slightly lower than the primary when unloaded, it means you've got the primary and secondary windings backwards.If you don't need any isolation, this transformer can be used as an auto-transformer, to increase, or decrease the voltage. If you connect the secondary in series with the phase conductor, it can step up the mains from 240V to 360V, or 480V. The same connection can obviously step down 480V, or 360V to 240V or 240V to 160V. This can be useful for stress testing designs to ensure they can survive overvoltage. Extra caution is required for higher voltages and the fact it's no longer isolated.Another thing to bear in mind is that whilst an isolation transformer does reduce the risk of shock, by making it so one has to touch two conductors instead of one, it defeats GFCI/RCD protection. If the circuit does become connected to earth, then it's possible to get a shock by touching one side of it. Possible solutions include: ensuring there is no earth connection within the vicinity of the isolation transformer, or anything connected to it, or a monitor circuit which warns you if the circuit has become earthed.

author: JohanH
author_rank: Frequent Contributor
timestamp: August 03, 2022, 12:29:01 pm
content: If you have a signal generator, a safer way is to set a signal generator at 50 Hz, connect it to primary and measure the output voltage on secondary side (using voltmeter or oscilloscope).An inductance meter can also be used. When connected correct way in series, inductance will be larger.

author: BrokenYugo
author_rank: Super Contributor
timestamp: August 03, 2022, 03:19:15 pm
content: Quote from: Zero999 on August 03, 2022, 12:16:42 pmOr just put a RCD on the secondary.

author: themadhippy
author_rank: Super Contributor
timestamp: August 03, 2022, 03:28:13 pm
content: QuoteHows that gonna work on an isolated secondary?

author: Zero999
author_rank: Super Contributor
timestamp: August 03, 2022, 05:06:30 pm
content: Quote from: themadhippy on August 03, 2022, 03:28:13 pmIt won't. An RCD requires the power supply to be earthed to work. Note that this doesn't apply to the appliance. An RCD will work with an unearthed appliance, but it still relies on the supply being referenced to earth.

author: themadhippy
author_rank: Super Contributor
timestamp: August 03, 2022, 05:37:24 pm
content: QuoteNope,it requires an imbalance,it couldnt care less how that imbalance happens.,whether its to earth or via another circuit.

author: Zero999
author_rank: Super Contributor
timestamp: August 03, 2022, 05:51:24 pm
content: Quote from: themadhippy on August 03, 2022, 05:37:24 pmSuch an imbalance is virtually impossible without the power supply being earthed. You'd have to touch the live on one side of the RCD and the neutral on the other, which is very unlikely.For all intents and purposes, an RCD on the secondary of an unearthed isolation transformer is useless.

author: Dave Turner
author_rank: Frequent Contributor
timestamp: August 04, 2022, 03:10:52 pm
content: Thanks for the replies and the warnings.The sig gen and scope idea worked perfectly and confirmed the markings on the transformer.