Problems with power transformer...need help! - Page 1
Forum section: electronics / beginners

author: Praxis
author_rank: Contributor
timestamp: August 12, 2011, 06:07:20 pm
content: I recently acquired this transformer from Jameco:https://www.jameco.com/Jameco/Products/ProdDS/104417.pdfNow, this is the first time I've ever used a transformer with multiple primaries. I thought I had it wired correctly -- neutral to the middle two tabs and hot to the outer two tabs on the primary side, but when that's plugged in (with no load -- nothing's connected to the secondary taps) the thing passes tons of current -- it blew a 5A fuse in under a second -- not to mention heating up to the point where you get that hot-electronics, immanent-thermal-destruction smell. So now I'm not too sure I'm reading that data sheet correctly. Any help would be appreciated, thanks.

author: Zero999
author_rank: Super Contributor
timestamp: August 12, 2011, 07:48:21 pm
content: It's blown the fuse because you've just short circuited the primary.The schematic on the datasheet appears to be incorrect. Try measuring the resistance between 2 & 3, I'm guessing it should be open circuit.If that's true, for 120V operation, 1 & 3 should both be connected neutral and 2 & 4 should be connected to 120VAC.

author: ciccio
author_rank: Frequent Contributor
timestamp: August 12, 2011, 08:02:24 pm
content: Hi praxis,I think that you are reading the data sheet correctly, but the data sheet is wrong:It is not possible that a transformer primary is wired as the data sheet's drawing.If you had such an high current, the primary was short-circuited.A dual-primary transformer has twoseparatewinding, in your case they should be:pin 1 : 0 V - pin 2: 115 Vpin 3 : 0 V - pin 4: 115 V(this will require this wiring: neutral to pins 1+3, phase to pins 2+4)or, in alternative, that willmayresult in a simpler wiring (and is the wiring standard from my favorite supplier)pin 1 : 0 V - pin 3: 115 Vpin 2 : 0 V - pin 4: 115 V(this will require this wiring: neutral to pins 1+2, phase to pins 3+4)or, last possibility (really stupid)pin 1 : 0 V - pin 4: 115 Vpin 2 : 0 V - pin 3: 115 V(this will require this wiring: neutral to pins 1+2, phase to pins 3+4) :this is how you wired at your first attempt, so I will exclude this possibility).Please check if there is continuity between all the pins: there should be two insulated windings.After this, if the transformer is still alive after your previous test, and you have access to a function generator or an audio oscillator and a scope, test the different wirings by the primary with the oscillator, set at 60 Hz, 10 V rms, and check the secondary with the scope. Only one of the above wirings will give you some secondary waveform: all the others will result in out of phase primaries, with no secondary output.Good luck...

author: Praxis
author_rank: Contributor
timestamp: August 12, 2011, 09:30:08 pm
content: Arrrgh. I -thought- the diagram of the winding was a little weird, but to be honest, transformers are black boxes to me at this point and I don't really know enough to assume right off that the datasheet was wrong.I think it's still alive. I don't have an AF oscillator, but how about hooking it into a (verified working) 13-ish volt transformer's output?

author: Zero999
author_rank: Super Contributor
timestamp: August 12, 2011, 09:40:38 pm
content: Connecting it to 13V is a good way of testing it. If it's connected correctly, you should get around 4V on the secondary.

author: Praxis
author_rank: Contributor
timestamp: August 13, 2011, 07:21:36 am
content: Turns out it was 1&3 and 2&4, and I didn't kill the transformer, whoot!

author: Zero999
author_rank: Super Contributor
timestamp: August 13, 2011, 09:56:37 am
content: Good, I'm glad it worked. That transformer configuration is pretty standard and is common for transformers which are sold to both 120V and 230V countries. It's also not uncommon for there to be another separate 20V primary winding, enabling the transformer to be used on US 208V three phase or Japanese 100V single phase systems.

author: IanB
author_rank: Super Contributor
timestamp: August 13, 2011, 07:07:20 pm
content: Quote from: Hero999 on August 13, 2011, 09:56:37 amFor 100 V supply, you would wire the 20 V winding in series with the 120 V primary, but with opposing polarity?

author: ejeffrey
author_rank: Super Contributor
timestamp: August 13, 2011, 08:03:33 pm
content: Yes. Transformers are awesome like that.