Home-made Transformer - Primary Resistance

[ultinator]

Hi all,

This is my first post, so forgive me if it is in the wrong section but here goes nothing.

I was considering making a transformer myself and basically if I were to power it through a plug socket, would the resistance of the primary have to be high enough to keep the current below 13A? so 230v/13A = 17ohm, this seems to be quite high for a primary winding so therefore is the only way to keep current low enough to add resistors before the primary?

It's not so much the implementation that is my concern more the theory behind it as it seems I may be overlooking something here?

Kind Regards.

 

[micknew]

Yes your overlooking back emf and design of the iron core etc.
Interesting theory to study and understand, go for it.

 

[ultinator]

I know the construction of the iron core would need to be laminated in order to reduce eddy currents and hysteresis would also need to be taken into consideration. I just mean, say I had an 'ideal' core and wound 100 turns on one side and 10 on the other. If the resistance of the 100 turns was say 2 ohm and I plugged it into my domestic ring would it not trip my breaker as it is effectively short-circuiting it and will trip with a current of 120A (if 240V is present)?
Or is the current going to vary enough with back e.m.f to not trip, also I don't wish to have to use large wires for the primary as I don't have any lol!

 

[Marvo]

http://www.giangrandi.ch/electronics/trafo/trafo.shtml

 

[Knobhead]

What are you wanting this transformer to do?
What output etc.
For gods sake don’t start playing about with 240V unsupervised.

 

[ultinator]

This is most helpful, thank you kindly. I shall have a play over the weekend! I was also looking into a making a tesla coil, but why is it people recommend using a NST or an MOT is this simply because they are already made up?

 

[Strima]

I shall have a play over the weekend!

Sounds like fun, can I watch, I haven't seen the 240 tango in a while...

 

[Marvo]

For gods sake don’t start playing about with 240V unsupervised.

I would also suggest safety first, if you're unsure of the risks, don't do it.

 

[ultinator]

The values I used were just as an example. From reading what I have written and the questions I have asked I see the cause for concern, however I would just like to build upon my understanding (or apparent lack of) transformers and electrical separation. I know I could get better transformers for cheaper and less hassle than constructing it myself, but again it is the theory I wish to practically consolidate on. I like to fiddle and I am aware of potential dangers that could arise in this present situation. I would like a 12/24 V output if I were to implement the idea.

 

[Marvo]

I like to fiddle and I am aware of potential dangers that could arise in this present situation. I would like a 12/24 V output if I were to implement the idea.

The output voltage is just the pri-sec winding ratio. The worry is the experimentation with mains voltage plus depending on your winding configurations any protective devices such as RCD's that would normally prevent you getting killed might not protect you from a shock coming from the secondary coil.

As already stated be careful and take any necessary steps to protect yourself from the possible hazards.

 

[MarkieSparkie]

This is most helpful, thank you kindly. I shall have a play over the weekend! I was also looking into a making a tesla coil, but why is it people recommend using a NST or an MOT is this simply because they are already made up?

To get a sufficient output voltage for a Tesla coil (in excess of 500,000v) you need a starting voltage that is considerably higher than the normal household mains. The first step is often to boost the mains voltage by using a conventional HV (high voltage) transformer.
The main reason that Microwave oven Transformers (MOTs), Neon Sign transformers (NSTs) and Oil Burning Ignition Transformers (OBITs) are often the starting point for simple HT Tesla coils is because of their wide spread availability and their inherent current limiting characteristics. With most transformers you will need to have a ballast added to stop excessive current being drawn through the transformer's windings when the secondary is short-circuited. This is not needed with NSTs (virtually all NSTs except some very old types). Because NSTs are actually designed to operate with virtually a short circuit across their output at times, they are wound onto a core that is designed to self limit the maximum amount of current that can flow (so are most OBITs by the way).
If you use other types of transformers you will need to add some ballast (usually in the form of an inductor) in series with the primary (or secondary) winding of the transformer to limit the current that can flow.
MOTs do have some in built ballast in the form of shunts similar to most NSTs, but they are pretty ineffectual, and you will most likely need some additional external ballast with these as well.
Experimentation with Tesla coils is NOT for the novice, overloading of coils and/or capacitors can cause devastating explosions and the sparks can jump many metres, often unpredictably, even with well designed coils, TESLA COILS CAN KILL!