Step "UP-DOWN" transformer wired wrong ...? - Page 1
Forum section: electronics / general

author: IvoS
author_rank: Frequent Contributor
timestamp: October 07, 2012, 10:50:04 am
content: When trying to power up my EU version of audio amplifier here in states, from 110V to 220V, I grabbed my meter just to check the voltages inside the amp. To my surprise, I measured both leads, phase and neutral read 110V against the chassis. What the heck? I opened up the step transformer and I was shocked. I think common sense should be to have the transformer taps connected to hot side instead to neutral. All original connection seems backwards. The transformer works but I think it's a violation. If I want to use it for instance as step down transformer from 220V to 110V, then I would have phase "hot" wire on 110V receptacle measuring 220V against ground. Am I right? Is my new connection correct?

author: amyk
author_rank: Super Contributor
timestamp: October 07, 2012, 12:52:42 pm
content: Quote from: IvoS on October 07, 2012, 10:50:04 amNot literally, I hope.Definitely wrong to be switching the neutral.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: October 07, 2012, 12:55:40 pm
content: Quote from: amyk on October 07, 2012, 12:52:42 pmWas wondering that also. Must have been a truly shocking experience.

author: vk6zgo
author_rank: Super Contributor
timestamp: October 07, 2012, 02:28:47 pm
content: Nasty!Actually,I hate the whole idea of it!I'd rather have a plain 240:120V or vice versa isolation transformer.Probably my upbringing in the old days of Oz Electronics,where full transformer isolation was usual,& auto-transformers were looked at very much askance!

author: IvoS
author_rank: Frequent Contributor
timestamp: October 07, 2012, 03:59:08 pm
content: Was the original connection a code violation or not?

author: Pentium100
author_rank: Frequent Contributor
timestamp: October 07, 2012, 07:12:14 pm
content: What's the difference? The amp will have a regular transformer anyway.I have a couple of autotransformers (to step down to 120V) and the difference between the two connection modes (as shown in the circuit diagrams) is the orientation with which I plug it into the wall.Then again, in all of my equipment (that I took apart) both wires are treated equally - as if both were live. After all, any of the wires could be live - just reverse the plug and you will have it. Don't Americans have 240V too, where both wires are actually live?

author: IanB
author_rank: Super Contributor
timestamp: October 07, 2012, 07:30:51 pm
content: I have the 200 W version of that transformer. Prompted by this thread I just checked it out.With the tap selector on the 110 V setting, the 110 V output socket is wired directly through to the input plug and is switched on the L side. However, the 220 V output has theN terminalL terminal in common with the 110 VL terminalN terminal, and the 220 V N terminal is at 220 V relative to ground.So the thing is oddly wired, but it is differently oddly wired compared to your sample. However, when the switch is in the off position there are no voltages on the output sockets. I shall have to take the lid off at some point and check out the internal wiring.

author: IvoS
author_rank: Frequent Contributor
timestamp: October 07, 2012, 11:08:37 pm
content: On top of, the AC cord has blue and brown wires reversed right on the (sealed) receptacle. I think that blue wire should be neutral in Europe and brown is hot. When I plug the cord in the wall outlet, the blue wires comes hot.The transformer plugged in the wall and running unloaded heats up itself like a bastard.

author: IvoS
author_rank: Frequent Contributor
timestamp: October 07, 2012, 11:17:18 pm
content: Quote from: IanB on October 07, 2012, 07:30:51 pmThen you will experience exactly the same thing with 220V as I did. Both wires, neutral and hot on the receptacle will have equal potential to ground, 110V.

author: Unknown
author_rank: Unknown
timestamp: October 07, 2012, 11:29:14 pm
content: Wouldn't this be safer than having 220 V between phase and ground? This way, a single fault can result in max 110 V to ground. I don't see a problem, assuming neutral is treated with the same care as the live wire. Treating neutral the same as live with regards to insulation is standard in most electrical codes, as far as I know. I have no idea if there are any norms regarding wiring of autotransformers.

author: IvoS
author_rank: Frequent Contributor
timestamp: October 07, 2012, 11:34:50 pm
content: Quote from: alm on October 07, 2012, 11:29:14 pmHow about you have another thing plugged down stream the first device (not thinking the transformer itself), when you shut the switch off on the first device, you still have 110V potential to ground (on neutral) running down stream, very dangerous situation that can get someone seriously hurt or killed.

author: IanB
author_rank: Super Contributor
timestamp: October 08, 2012, 12:27:16 am
content: Quote from: IvoS on October 07, 2012, 11:17:18 pmSorry, I made a typo (now corrected). When I plug the transformer into my 120 V mains, the L of the 220 V output socket measures 0 V relative to ground and the N of the 220 V output socket measures 245 V relative to ground.