Winding a transformer - Page 1
Forum section: electronics / beginners

author: Rick
author_rank: Frequent Contributor
timestamp: August 19, 2012, 07:52:34 pm
content: Hi,I have seen another question on transformers in the general chat section and as I want to build a transformer too (with both primary and secondary coils) I would like to ask my questions about transformers.Somebody on youtube describes how to wind a low current transformer, here:There are a few points I do not understand: At the end of the video when he tests the transformer with a series circuit, he shorts the secondary and the bulb lights up, but normally the transformer with an open secondary too has a quite low primary coil resistance, hasn't it? Therefore I don't quite understand why there is not enough current in that series circuit on the primary to make the bulb light up...Besides that between 9'28" and 9'35" he interchanges the edges of the coils to form the central tap. If he does not do that I guess both 18 volt rails will have the same phase right? The fact of interchanging is to have an 180 degrees phase difference or ...?To get an isolated ground reference, can we just use an adapter without a ground connector to plug a device to the mains? Would this be equivalent to using an isolation transformer?----By adapter, I meant an adaptor plug like A, B or C type on the following page:http://www.american-allergy-supply.com/dn3000/foreign-adapter.htm

author: shebu18
author_rank: Frequent Contributor
timestamp: August 19, 2012, 09:06:46 pm
content: The part at 9min is interesting because he could left it out, he could just put the 2 tips that where there together and not change them.

author: szhighstar
author_rank: Contributor
timestamp: August 21, 2012, 03:15:51 pm
content: At the end of the video when he tests the transformer with a series circuit, he shorts the secondary and the bulb lights up,Sorry, I can not understand the word, can you explain again? why short secondary ?

author: Rick
author_rank: Frequent Contributor
timestamp: August 21, 2012, 03:34:18 pm
content: Well I was myself trying to understand that actually.... My guess is that when he shorts the secondary, the latter's equivalent impedance seen at the primary is lower or null thus letting pass a higher current in the primary circuit (I am not sure at all of what I say here). This is a wild guess, I was hoping somebody would clarify that... I tried to work it out from the transformer's equivalent circuit but could not succeed. Obviously the idle current of THIS transformer is not high enough to let THAT bulb light up. But an exact answer would be welcome.

author: SeanB
author_rank: Super Contributor
timestamp: August 21, 2012, 03:53:28 pm
content: Those adaptors can at best be described as lethal.As to the transformer, what is happening is that it is being used as a magnetic amplifier, when open circuit on the secondary only the magnetising current flows in the primary, normally very low. Short the secondary and the current rises to the value you get when the core is saturated, normally a lot higher than the full load current, as you do not want to run a transformer in this region with full mains applied, as the power dissipated in the core and windings will be very high. In this case though the majority of the voltage is developed across the light bulb, the transformer behaving like a switch.

author: Rick
author_rank: Frequent Contributor
timestamp: August 21, 2012, 04:00:10 pm
content: Quote from: SeanB on August 21, 2012, 03:53:28 pmYou mean that transformer is "lethal" ?Quote from: SeanB on August 21, 2012, 03:53:28 pmThanks a lot

author: vk6zgo
author_rank: Super Contributor
timestamp: August 21, 2012, 04:13:13 pm
content: Quote from: Rick on August 19, 2012, 07:52:34 pmTraditionally,a transformer has been used as part of a device to supply the various voltages required for it to operate,with the only wiring directly connected to the mains being fairly simple connections to the on/off switch & hence the transformer primary.Working on such equipment does not normally require an additional isolation transformer.Switchmode power supplies,on the other hand,have quite complex circuitry which has a common main connection.It is good practice to use an isolation transformer with such devices,especially if you need to look at switching waveforms,as Oscilloscopes normally have one side of their circuitry connected to earth.If you place the earth clip of your 'scope probe on the active side of the mains,you will see a blinding flash,& probably do your 'scope & possibly yourself an injury!

author: vk6zgo
author_rank: Super Contributor
timestamp: August 21, 2012, 04:17:47 pm
content: Quote from: Rick on August 21, 2012, 04:00:10 pm

author: Rick
author_rank: Frequent Contributor
timestamp: August 21, 2012, 04:41:04 pm
content: Quote from: vk6zgo on August 21, 2012, 04:13:13 pmRight, but if you isolate an SMPS from the mains earth using an isolation transformer, wouldn't you face the same risk in the case of the fault you mentioned above?Anyway your answers helped me understand things better. Thanks!

author: Rick
author_rank: Frequent Contributor
timestamp: August 21, 2012, 05:11:40 pm
content: Quote from: vk6zgo on August 21, 2012, 04:13:13 pmOk I think I get it now (or do I?). If we use that adaptor, as the mains earth is usually connected to the neutral, there would still be a "galvanic" connection between the earth and the neutral, whereas with the isolation transformer, this wouldn't be case.

author: vk6zgo
author_rank: Super Contributor
timestamp: August 22, 2012, 03:57:49 am
content: Quote from: Rick on August 21, 2012, 05:11:40 pmBy George! I think you've got it!Yep! Without the isolation transformer,one side of the supply to the switchmode or whatever, is connected to the Earth.Most of the plumbing in the house,& the outside cases/chassis of the other electrical/electronic equipment are also connected to earth to provide the safety function previously discussed.If you use an isolation transformer,both sides are above earth,so there is no voltage between earth & either side of the supply to the device under test.If you really try hard,you can still get zapped,but you have to hang across both sides of the isolation transformer secondary,which is fairly hard to do accidentally.Don't kid yourself that because the neutral is theoretically at the same potential as earth,you can connect the earth clip of your 'scope to that point.DON'T DO THIS!,as you may have a reversed socket or plug,& you will again zap yourself or your scope!

author: Rick
author_rank: Frequent Contributor
timestamp: August 22, 2012, 06:44:16 am
content: Quote from: vk6zgo on August 22, 2012, 03:57:49 amOk thanks a lot.That's because I always think of that earth-neutral connection as a trick used by our local electricians to avoid installing a proper earthing and that in a normal case there shouldn't be one. And a few years ago when I got a real earth connection to one room, I told the guy not to connect the earth to the neutral as I had then a REAL earthing(!).

author: vk6zgo
author_rank: Super Contributor
timestamp: August 23, 2012, 04:47:41 am
content: Quote from: Rick on August 22, 2012, 06:44:16 amIf your earthing isnotconnected to neutral ,it isn't really doing anything!It might help with a lightning strike,but that's about it.

author: IanB
author_rank: Super Contributor
timestamp: August 23, 2012, 05:22:09 am
content: Quote from: vk6zgo on August 23, 2012, 04:47:41 amYes, another name for the safety earth or ground wire in a home is "the alternative current return path". If it isn't connected to neutral it isn't performing any safety function. A proper electrical socket tester will show "fault" if the earth/neutral connection is not good.

author: T4P
author_rank: Super Contributor
timestamp: August 23, 2012, 07:25:20 am
content: O_o the last time i learnt my electrical system the Earth must be SEPARATED from the NeutralIf i had it on the neutral trip goes the breakers ... I always knew my earth here was just to connect to you know, earthIf anything leaked it goes through the earthMaybe we are really using a 2phase system here and i'm not sure about that(240V nominal)

author: Rick
author_rank: Frequent Contributor
timestamp: August 23, 2012, 07:26:55 am
content: But in that case if a person touches the earthing connection when current is flowing he would get some electric shock wouldn't he? If this a return path...

author: IanB
author_rank: Super Contributor
timestamp: August 23, 2012, 07:38:38 am
content: Quote from: Rick on August 23, 2012, 07:26:55 amIfthere was current flowing in the earth wire it might have a dangerous voltage on it.But, there is supposed to be no current flowing in the earth wire under normal circumstances. This is why the earth wire is separated from the neutral wire back to the origination point where the connection is made to the actual earth. Current should only flow in the earth wire under fault conditions, and those fault conditions should quickly trip the breaker.

author: Rick
author_rank: Frequent Contributor
timestamp: August 23, 2012, 11:38:52 am
content: Quote from: IanB on August 23, 2012, 07:38:38 amYou mean very high voltages?I tried to measure (silly of me) the resistance between the earth wire and the ground, the floor with a multimeter:) I think there are special equipment to verify your earthing right?

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 23, 2012, 01:32:01 pm
content: neutral line and earth are connected through different impedance. current return path should be through neutral, not directly low impedance to earth, otherwise the earth leakage circuit breaker will trip, like my body did few months ago.

author: SeanB
author_rank: Super Contributor
timestamp: August 23, 2012, 03:46:57 pm
content: Earth and neutral must be separated all the way through to the supply metering point, where there is a single join to them, and a earth rod or mat attached, as well as a connection to the metallic portions of the structure like water piping, drain piping or steel structural members or concrete reinforcing rods. The supply side is a 2 wire system and is PME ( Protective Multiple Earth) with each service connection being a part of the supply earth grid.The consumer side has 3 wires, one being a protective earth connector to shunt incident current from the fault until a protective device like a fuse or a circuit breaker operates to open the circuit. The supply side only has 4 wires, 3 phases and neutral, and a thin lightning wire across the top between poles ( often the carrier cable for an ABC supply) that is earthed at each tap off, and also connected to the neutral conductor. This is done because it saves cost, and only authorised personnel can connect to it.

author: Rick
author_rank: Frequent Contributor
timestamp: August 23, 2012, 04:01:26 pm
content: Quote from: SeanB on August 23, 2012, 03:46:57 pmWhy only at the supply metering point? Here what they do is that they connect them in each wall plug. I must say this is a house built in 1940's and there was no earthing connection before I got an earthing wire installed in my room.

author: SeanB
author_rank: Super Contributor
timestamp: August 23, 2012, 06:09:49 pm
content: Connecting at the wall means if the neutral goes open circuit ( possible, 50 % chance with 2 wires) then any appliance will be at mains live, and touching it and any conducting surface ( wet wall, puddle on floor, tv antenna, cable lead, telephone cord) can lead to a lethal shock. If you had that in South Africa and it was noted by any electrician he would call the supply authority ( mandatory call) and they would come and remove the supply connection until it is compliant. They generally remove the pole fuse, and for those who reconnect illegally they come with a bulldozer and pull the meter out by the cabling, and blacklist you until you have a certificate.

author: madires
author_rank: Super Contributor
timestamp: August 23, 2012, 06:33:30 pm
content: Quote from: Rick on August 23, 2012, 04:01:26 pmEver tried to install a RCD?

author: Rick
author_rank: Frequent Contributor
timestamp: August 23, 2012, 06:35:44 pm
content: ) It is the electrician who makes that connection at the wall plug;)South Africa is like the western hemisphere (including NZ and Australia) I suppose, I mean as regulated as western Europe.

author: Rick
author_rank: Frequent Contributor
timestamp: August 23, 2012, 06:38:26 pm
content: Quote from: Rick on August 23, 2012, 06:35:44 pmNot yet. But soon I am changing all the cables of the flat (they date back to 1940, with "cloth" type isolation, I don't know the proper word for it). So I may get it installed then.

author: IanB
author_rank: Super Contributor
timestamp: August 23, 2012, 10:48:36 pm
content: Quote from: Rick on August 23, 2012, 04:01:26 pmWhere is "here"? Your location isn't showing up in your profile, so we don't know what country you are in.

author: Rick
author_rank: Frequent Contributor
timestamp: August 23, 2012, 10:53:17 pm
content: Quote from: IanB on August 23, 2012, 10:48:36 pmTurkey