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Transformers and current

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Cazzo

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yooooooooooooooooooooooooOoo

How do you wire a transformer, or re wind one to provide low volts, high current??

the basic principle is???

My main reason, i want to know if i can wire (or re wind, which ever is right) a transformer and powder it with a battery to get high current.

I know batteries are DC and transformers dont work, so how can i get a AC source, from a DC battery to run my high current transformer??

thanks bro
 

Reloadron

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You do not mention the voltages and current? You also don't mention what the final output should be as to AC or DC?

While it can be done, winding your own transformer is not an easy task. Several members here have done so. It sounds like what you are after is an inverting power supply where a battery of for example 12 VDC produces a line voltage output of for example 220 VAC 50 Hz. While units like this can be built they are easier bought in just about all cases. So what exactly are you looking for as to input / output?

Ron
 

Diver300

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It all comes down to how much current is "high", what voltage that needs, and what battery you have.

The basic principle is that a transformer has some turns of wire on the primary, and some on the secondary. The ratio between the number of turns is called the "turns ratio". The voltage will be reduced and the current increased by the turns ratio.

If you have a 240 V input, 12 V output transformer, it will have a turns ratio of about 20:1. You will get about 20 times as much current out as you put in. The power going in and the power going out will be about the same. The 240 V winding is called the primary winding, and the 12 V winding is called the secondary, but there is nothing to stop them being used in reverse.

There are a lot more details in the design that you need to worry about if you want to make a practical transformer. However, if you need to rewind just the secondary, and keep the primary the same, that is a lot simpler. If you halved the number of turns, and used wire twice as thick, you would end up with a 6 V transformer with twice the current rating, without having to worry about lots of the other details.

There are inverter circuits that will produce AC from DC. In the switch-mode power supplies that run most electronic appliances, the mains voltage is rectified to DC first, then put into a "flyback" circuit to make it into AC again, but at a higher frequency. The higher frequency allows for a much smaller transformer.

Also in the inverters that produce mains from a battery, a similar circuit is used to produce a few hundred volts DC. That is then chopped to produce a stepped wave which is good enough for most AC appliances. You might be able to rewind the secondary of the transformer from one of those to produce a lower voltage rather than a higher one. However, most such circuits rely of feedback to control the output voltage, so you would have to get that working.

If you don't need isolation, you can use a circuit called a "buck converter" or "buck regulator" that converts a higher DC voltage into a lower voltage, but increases the current at the same time.
 

Cazzo

Member
Thank you both

I am not really intending on re winding small wall outlet transformers. I am just trying to learn how they work, and what they can do or be used for.

I found some videos of people using MOT's (Microwave Oven Transformers) and rewinding the secondary with thick low gauge wire, and only like 2-6 turns. This boosts the current somehow and they only put out like 2-3v.

i have some questions i need you professionals to answer before i kill something.

A transformers secondary windings are 1 piece of wire, they dont directly connect to any mains voltage or anything directly. they just wrap around the transformer core. they are getting their power through the electromagnetic energy flowing etc etc watever so if you short them out, will the saftey switch in ur house trip? because technically your not shorting out wires like on a battery of mains.

ALSO - If i am wiring transformers for mains voltage testing or usage. How do i wire them??? its AC so polarity dont matter. do i connect earth to the transformer metal housing? or the metal case on the think im making.
 
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Reloadron

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This link is a good start with transformer theory. There is quite a bit to all of it. Yes, the old microwave transformer has abecome a gold mine for experimenters who want to roll their own transformers. This is a good starting link for such a project and a Google of "Microwave Transformer Hacks" should yield a dozen more.

A transformers secondary windings are 1 piece of wire, they dont directly connect to any mains voltage or anything directly. they just wrap around the transformer core. they are getting their power through the electromagnetic energy flowing etc etc watever so if you short them out, will the saftey switch in ur house trip? because technically your not shorting out wires like on a battery of mains.
Yes, in a case like that the secondary is "isolated" from the primary. Also, to trip a mains breaker you need to exceed it's rated current. So, no, they will not always trip. Right off the top, and I am sure you are aware, be careful when working with and around mains voltages. Bad things can happen. Aside from shock there can be a fire risk as well. There is no way to cover transformer theory or all the different types of transformers in a single post, thus the links. Be careful! :)

Ron
 

Diver300

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http://en.wikipedia.org/wiki/Soldering_gun Those have secondary windings that are single turn of wire, although "wire" is the not word that springs to mind for the chunks of copper that are used.

You are right that there no electrical connection from primary to secondary. That is one of the main reasons to use a transformer.

The isolation that a transformer provides means that you cannot trip an ELCB (Translates to GFI in American, I don't know what it is called in Australia) if you short the output of a transformer to ground. It is usual to connect one side of a transformer secondary to ground. The isolation also means that you cannot get a shock from the output of a low voltage transformer, no matter which way the input is wired.

You can get over-current trips from a transformer. However, it needs to be big transformers. If you have a 100 W transformer, which weighs 1 kg or more, it will only take about 10 times its rating if you short it out. That is about 1 kW, so it will draw about 4 A from a 240 V supply. Obviously it will burn out in a few minutes. 4 A won't trip house wiring. If there is a suitably rated fuse in the transformer supply, that will protect the transformer, but that would have to be feeding just the transformer.

I have tripped a house circuit breaker by shorting a transformer, but that is only on big power amplifiers where the transformers are 1 kW or more, and weigh 10 kg or more. I guess that an MOT would trip a house circuit breaker, but I'm not sure. MOTs are not rated for continuous use, so they will have a lower short-circuit power than a continuously rated transformer of the same power.
 

4pyros

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Can MOT,s be run continuously if rewound for a low voltage output say around 12 volts?
 

jpanhalt

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Heating might be a problem, but I have a nice sized MOT rewound with 4AWG battery cable for spot welding and have not noticed it getting warm while idling. The welding periods I use are quite short. I don't know what would happen if one used it continuously for something like an induction furnace.

One caution with MOT's or any large transformer is the in-rush current when they are first plugged in, even with the secondary open-circuit. I have a large Variac that will blow the fuse of a 15A power strip occasionally.

John
 

Diver300

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MOTs run close to or a bit over saturation, so they will get warm with no secondary current. You would need to add a few turns to the primary to reduce the heating at idle.

I don't know how much power you could take to run the continuously. It has little to do with the output voltage. It is the output power that would affect the heating of the transformer.
 

4pyros

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I don't know how much power you could take to run the continuously. It has little to do with the output voltage. It is the output power that would affect the heating of the transformer.
So if you keep the output power low it may be OK without modification?
I was thinking MOTs mite be nice for power supply's.
 

Reloadron

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This is a recent thread where a forum member completed making his own 24-0-24 volt step down transformer hatched from a microwave oven transformer. As can be seen Gary did a real nice job but as also can be seen he was well tooled up to do the work.

Ron
 
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