Stepdown autotransformer - Page 1
Forum section: electronics / beginners

author: Architect_1077
author_rank: Regular Contributor
timestamp: May 18, 2012, 11:57:28 am
content: Hey folks.I have a 220v -> 120v step-down auto-transformer with no earth ground. To be clear, the 120v output has no earth ground connection. Hypothetically speaking, would it be safe to add an earth connection from the 220v wall outlet to the 120v output outlet?Here's the reason for my question and what (I think) I know about auto-transformers:I'm trying to fix an auto-transformer I have at home. The 120v output plug (no ground) is damaged and I wanted to replace it and add a ground connection to output side of the transformer while I'm at it.HOWEVER, given that the Euro Schuko plugs are not polarized there is a 50% chance of having the "L" and "N" leads on the transformer switched, which would cause a full 220v voltage drop between output "N" and ground. So my understanding is that there could be a potential to blow stuff up connected to the output side of the transformer.Is this correct? Please give me some insight/confirmation so I avoid pretty fireworks at home

author: Flávio V
author_rank: Regular Contributor
timestamp: May 18, 2012, 12:44:13 pm
content: Because alternating current reverses and switches it hasn't a real polarity,so "live" and "neutral" are the same...About the earth ground...not really sure but the earth ground isn't any voltage so well...it might be safe and equal...

author: Architect_1077
author_rank: Regular Contributor
timestamp: May 18, 2012, 01:34:54 pm
content: QuoteI don't agree with the 2nd half of your statement in that it is only partially true. One lead - "L" - carries the 220v potential (relative to ground), the other - "N" - has the same potential as ground. Where this transformer is concerned, I do think itdoesmatter how it is plugged in, because as it is not an isolated transformer, there is a 50% chance of getting a 220v potential on one of theoutputleads (relative to ground). And that is the nature of my concern with adding an earth ground to the output side. I'm wondering if this potential could cause a 120v device (that is, one that is meant to be grounded) plugged in to the transformer to blow up (depending of course on the type of device and how it's wired up internally, etc).Hope that clears it up a bit

author: Psi
author_rank: Super Contributor
timestamp: May 18, 2012, 02:00:34 pm
content: You seem to be concerned that the input plug (due to design) can go around either polarity and this may reverse phase and neutral on the output. So i'm guessing its a 2pin P & N plug Correct?Because the input plug needs an earth pin to provide the earth for the case and the output plug.So this should mean that you can only connect the input plug to the wall in one polarity?You should not hardware an earth (from some other location) onto the output plug while leaving the input plug as a 2pin design. Because as you have said, it will mean in some situations the earthed output plug will have Phase and Neutral reversed. This in-itself doesn't effect operation but it is a safety issue. The first reason that comes to mind is that the appliance maybe fused on phase and if you reverse it the fuse moves to neutral which means the device stays live after the fuse blows.Here's an auto transformer diagram with earth.

author: Architect_1077
author_rank: Regular Contributor
timestamp: May 18, 2012, 02:09:43 pm
content: QuotePrecisely. From my understanding (I'm not saying that it's correct), a polarity switch can cause two distinct situations on the output side. The output will either have L = 120v and N = 0v, OR, N = 220v and L = 100v (making the 120v across both leads). The possibility of a 220v potential on the output side is concerning me, and I'm hoping to get confirmation on whether I'm right or wrong.

author: Psi
author_rank: Super Contributor
timestamp: May 18, 2012, 02:18:19 pm
content: Quote from: Architect_1077 on May 18, 2012, 02:09:43 pmI hadn't though of that but yes, definitely.If the tap moves to the neutral side it will cause an Earth-Neutral voltage as well as a Phase-Neutral voltageI'm actually not quite sure what effect that will have on powering various devices, but it's definitely not good from a safety perspective and it's most definitely illegal (or it would be in NZ).Just put an earthed plug on the input side as well, then you wont have the issue.Note: i added a few more bits to my previous post after you made your post.

author: Architect_1077
author_rank: Regular Contributor
timestamp: May 18, 2012, 02:37:18 pm
content: Yes Psi, you're pretty much spot on with what my concern is.The transformer "as is" has no grounding what so ever built in to it. What that means is I can't plug 120v devices into it that require an earth grounding plug. So, seeing as I have to fix the transformer, I was wondering if I could add earth ground to itsafely. I would accomplish this by exchanging the transformer input cable and plug to include the earth connection as well of course. Earth would be connected directly to the output side. This was the initial idea until I realized the problem that would arise from this modification, given the transformer's design.Devices that don't have earth grounding in their design won't be affected either way, but any device built with earth grounding in mind might suffer something nasty.So, is there anysafeway to add earth ground to this type of transformer so I can safely ground the 120v devices connected to it?

author: Architect_1077
author_rank: Regular Contributor
timestamp: May 18, 2012, 02:41:17 pm
content: QuoteUps, didn't notice you edited your posts while I was typing up a replyAnyway, if you read my reply above, I'm not so sure if that alone would work. The potential is still there, right?

author: Psi
author_rank: Super Contributor
timestamp: May 18, 2012, 02:49:20 pm
content: I'm not sure about the shape of plugs in other countries.If you wired a 3pin plug onto your auto transformer (The side that connects to the wall socket) can that still plug in around the wrong way?

author: Kremmen
author_rank: Super Contributor
timestamp: May 18, 2012, 02:57:03 pm
content: If the distribution system is anything like TN-S commonly used in Europe (and US i think)http://en.wikipedia.org/wiki/Earthing_systemthen i don't really see the issue.PE (ground) _must_ be a separate conductor in a wall socket. The autotransformer won't care about the polarity so just connect the L and N so that whatever polarity goes in, comes out the same way. In Europe even this would not matter because device safety must not depend on any particular polarity. That is why the Schuko is unpolarized.As to ground it is a separate conductor and has nothing directly to do with L and N wires at this point. If you wish to be extra safe then connect it to the transformer core.All of the above assumes that N and PE wires are separate all the way to the entry panel.

author: SeanB
author_rank: Super Contributor
timestamp: May 18, 2012, 03:08:11 pm
content: Just as an aside, I have scrapped a few of these converters. The input and output use identical sockets, so they are actually illegal to connect to the mains here, and probably elsewhere as well, as the input is via a mains plug and not a socket with the strong possibility of touching a live plug.The transformer in all I have dismantled has a 3 part winding, with it being 55VAC, 110VAC and 55VAC respectively. Thus the case if 220VAC is applied to the unit is the output has one pin at 55VAC and the other is at 165VAC, with 110VAC between them. This voltage is with respect to neutral ( which is basically the same as the protective earth potential wise, a slight variance due to loading and the finite impedance of the wiring in the house being a few volts on most cases, basically the same). This happens irrespective of how the input is wired, the pins just swap, but either way they are not SELV levels.The other way, going from 110VAC to 220VAC, the 220VAC output is either 165VAC or 55VAC above the ground terminal.Thus the best is to make a dedicated 3 core lead to bring power in, and have a 3 core lead to a socket to take power out. Connect the green/yellow leads together inside the converter, and add a short wire to a lug under one of the transformer bolts to ground the frame. Your output will still be non isolated, and add a notice that the output is non isolated, and should be treated with the same respect as 220V wiring and appliances.

author: Psi
author_rank: Super Contributor
timestamp: May 18, 2012, 03:11:49 pm
content: I think having a consistence phase/neutral polarity over all sockets is just an extra level of protection.It in-itself doesn't do much but when other faults occur as well it can prevent dangerous situations.

author: Architect_1077
author_rank: Regular Contributor
timestamp: May 18, 2012, 03:28:29 pm
content: Kremmen, check this link I found. It should help to clarify what my question is all about:http://www.gson.org/stepdown/So, how common is it for devices to have neutral tied in with ground??

author: SeanB
author_rank: Super Contributor
timestamp: May 18, 2012, 04:27:49 pm
content: Neutral is tied to ground, but never in an appliance. Only in the distribution system, at the consumer meter, with a grounding rod directly to the earth right there.If any appliance has neutral tied to ground, then, if the plug is removed with the power on, and the neutral pin disconnects first the user, attached to the metal case, will be shocked.

author: Architect_1077
author_rank: Regular Contributor
timestamp: May 18, 2012, 04:51:33 pm
content: QuoteThat's always been my understanding on the matter, until I saw the link I posted above and began to wonder...

author: Kremmen
author_rank: Super Contributor
timestamp: May 18, 2012, 06:09:39 pm
content: Quote from: Architect_1077 on May 18, 2012, 03:28:29 pmOK the incident in the link is related to the fact that the MOV was dimensioned to handle surges between Neutral and PE/ground. Normally there would be negligible voltage there, so the MOV knee voltage is likely far lower than the other MOVs. You _cannot_ do this if the plugs are unpolarized and this is the only problem i can see here. Actually i have designed fixed installation EMI filters where the same design principle was applied.If you don't particularly care about the orientation of L and N wires then it is not a problem if the wires get crossed inside the device. On the other hand if you have something like the circuit in the link then orientation must be maintained. Won't work for Schuko plugs though. It is a question of design principle more than anything else. where Schuko sovkets/plugs are used the reasoning is that you must not implement portable devices so that reversing L and N creates a safety hazard. Because people will do their own power cords etc whatever the code says (and here it is actually allowed).As already stated, conecting neutral and ground in a device or wall socket is expressly forbidden. That is the TN-C distribution system, nowadays considered too unsafe for domestic installations. Again, i can speak only regarding EU and not all of that even.

author: Architect_1077
author_rank: Regular Contributor
timestamp: May 18, 2012, 06:19:10 pm
content: Point taken. It's not something "normal" to do. That's what I wanted to know. Thanks

author: IanB
author_rank: Super Contributor
timestamp: May 18, 2012, 06:46:24 pm
content: Quote from: Architect_1077 on May 18, 2012, 03:28:29 pmJust to be clear, no devices in that link have neutral tied in with ground. The article there describes a mishap with a surge protector when it was used in a situation it was not designed or tested for.

author: IanB
author_rank: Super Contributor
timestamp: May 18, 2012, 06:48:42 pm
content: Quote from: Architect_1077 on May 18, 2012, 11:57:28 amYou absolutely have to avoid repeating the scenario described in the web site you linked to. Otherwise there is a definite risk of fireworks.

author: Kremmen
author_rank: Super Contributor
timestamp: May 18, 2012, 07:57:44 pm
content: The 3rd picture in the linked document is, if not wrong, at least misleading. It is stated that by crossing the wires, there is suddenly 220V between Neutral and ground. This is not correct. The _circuit_ has not been changed in any way. It is only the wire positions in a socket or plug that are reversed. If you rely on a certain ordering of L and N in the socket / plug then there could be trouble, as in the polarized MOV protector thing. Precisely this scenario will be prevented if you, by design, _can't_ know what the plug orientation will be. There are of course other reasons why polarizing would be beneficial.

author: Architect_1077
author_rank: Regular Contributor
timestamp: May 18, 2012, 08:07:53 pm
content: Thanks IanB.I know. I was just trying to understand how common this type of situation is with electronic appliances/gadgets.@KremmenActually, you are wrong (no offense meant). I was just testing this theory with another step-down I have (I have 3 of them, all the same). Sure enough, plugged in one way and you get 120/0, plug it in the other way and you get 220/100v, to ground. The potential is there... Although, in theory, this should never be a problem.

author: Kremmen
author_rank: Super Contributor
timestamp: May 18, 2012, 08:20:02 pm
content: Actually, now that i think about it more carefully, you are quite correct. The output voltage is referenced against the common wire. If the common wire is L then the "neutral" is L/2 or whatever the transformer ratio is. And you get a significant voltage against ground. I didn't think it through at first because in an unpolarzied system it makes no difference. Since you cannot assume that "N" really is neutral inside a device, this situation won't matter. Or it is not supposed to matter. In a Schuko power strip you would never see the MOV between ground and other wires, or at least the dimensioning will be the same as between L and N.

author: IanB
author_rank: Super Contributor
timestamp: May 18, 2012, 08:37:19 pm
content: Quote from: Kremmen on May 18, 2012, 08:20:02 pmThe issue here was that someone took a surge suppressor that was tested and certified for use in America on 120 V systems and transported that surge suppressor to a country with a 220 V mains supply. When they inadvertently exposed that surge suppressor to the full 220 V it failed.It is worth noting thatgoodsurge suppressors have a circuit breaker in the input side before the MOVs. If the MOVs fail in a low resistance mode due to handling an excess surge the breaker will trip and prevent a fire. It is very important when buying a surge suppressor to check that it has this feature.

author: Kremmen
author_rank: Super Contributor
timestamp: May 18, 2012, 09:01:02 pm
content: Yes that was a kind of given. In the US i understand the grounded sockets are also polarized so you cannot fall into this trap.Also correct - and a design error in this case not to provide it - you need a fuse or sufficiently fast circuit breaker in front of the MOV. The surge protector can only handle so many joules and for the case of prolonged overvoltage there must be a mechanism to break the circuit.

author: Tube_Dude
author_rank: Regular Contributor
timestamp: May 18, 2012, 10:33:09 pm
content: Hi Architect_1077If the device you are powering via the autotransformer have a input transformer (110 V ), you can safely connect your SAFETY ground.The input transformer is a balanced device, so it don't care if the neutral and the live are reversed. In the output side of the inside box 110 volts transformer, you will have the same AC voltage anyway round and always floating from SAFETY ground.

author: Architect_1077
author_rank: Regular Contributor
timestamp: May 18, 2012, 11:59:57 pm
content: Thanks to all of you who replied. You helped me clear a few things up. Cheers!