Transformer - Number of turns?

[ag273n]

Whats the minimum number of turns possible?
I viewed articles and videos about this, but not one mentioned such item. like, can it work with just 1 turn on secondary and 20 turns on primary?...

And, I've read the frequency of the input voltage matters, but how does this affect the number of turns?...

 

[davenn]

Whats the minimum number of turns possible?
I viewed articles and videos about this, but not one mentioned such item. like, can it work with just 1 turn on secondary and 20 turns on primary?...

And, I've read the frequency of the input voltage matters, but how does this affect the number of turns?...

so what primary and secondary voltages did you want one for

20 turns on primary with either 120VAC or 240VAC main will cause a large bang and cloud of smoke
DONT do it

a mains transformer for either 120 or 240VAC will likely have a couple of thousand turns or so of somewhere around 24 - 26 AWG wire

here's a bunch of links for you to start reading through
http://www.bing.com/search?q=transf...&src=IE-SearchBox&FORM=IENTTR&conversationid=


Dave

 

[ag273n]

thanks Dave.. i've seen someone in youtube plug his car fuel cyclinder ignition coil into the mains and Boom!.. LOL.. so hilarious!. so i've understood transformers work only for some frequency.

one is to step down 230 V AC, 50 - 60 hertz to 30 volts
and another one is to step up about 6 volts (kilohertz frequency range) to thousands of volts.

knowing what these two transformer's turn ratio, and minimum number of turns would give me a bigger picture.

 

[duke37]

Well worth a read is some information by Ludens, see Ludens transformer.
The core of the transformer is critical. Mains transformers use silicon iron laminations, High frequency switch mode supplies use a ferrite core, higher frequencies use iron dust cores. Radio frequencies often use transformers with air cores.
The voltage across each turn is constant so that the output voltage is proportional to the number of turns.
The voltage that the transformer can withstand is proportional to frequency before the core saturates. A car ignition coil is used to transform a pulse which is effectively high frequency so the coil will not have enough turns for mains frequency.

For a specific core, the number of turns is inversly proportional to frequency. High frequencies give low weight transformers.

1. 230V to 30V needs a ratio of 230/30 = 7.7. A normat radio lump may have 8t/V on 50Hz and 6t/V on 60Hz but you need to do the design properly.

2.I have used the ferrite core from a TV scan circuit to generate 400V DC for a transmitter. Here I used 1t/V at 10kHz, it makes the mathemartics easier.Thousands of volts is difficult because of the insulation required. Perhaps a TV LOPT EHT transformer could be used without modification. These run at about 16kHz in the UK with a low loss ferrite core.

There are high frequency transformers in switch mode supplies used to power fluorescent lights, wall warts, LED lights and computers. TV transformers can be dismantled since the core is gapped but many others are glued and I have not found a way of getting the core apart without damage.

 

[ag273n]

i did start read Ludens page and find the info he's giving suitable for a newbie like me
The reading material really mattered whether i'd understand it or not.

For a specific core, the number of turns is inversly proportional to frequency. High frequencies give low weight transformers.

this is probably why CFL's has a puny transformer and a Ni-MH chargers has a bulkier one 4 times bigger.

 

[Minder]

Over the years I have rewound or added secondaries to many sizes of transformers, both EI and toriodal types, in some cases removing the EI lamination's and rewiring the bobbin.
For EI I have found them to be between 2.5 and 4.5 turns/volt.
Toroidal are a tad lower at 1.5 to 2 turns/volt.
These were all 50hz/60hz transformers up to around 1Kva.
M.

 

[ag273n]

thinking about those 220v to 110v step down transformers commercially available, can i modify those to become a 220v to 30v transformer?... i believe its a matters of adding more turns to the secondary - if this is accessible and still has space... or probably just rewinding the secondary using a thinner gauge wire. -- is this a bad idea?..

 

[duke37]

Transformers can be rewound. It can be difficult to take the laminations out without damage and re-.insert them.
If the secondary is on the outside, then the primary can be left intact, Count the number of turns to get the turns/volt and multiply this by thirty.
The space for primary and secondary should be about equal, if you use thinner wire for the secondary, this will limit the current perhaps to a unaccepteble level.
You want to drop voltage by a factor of about seven so you want to increase the current by a factor of seven. The secondary wire should therefore have an area of seven times the primary. It must be laid on the bobbin very neatly.
It is in general much easier to buy what you want. A potted transformer may be impossible to dismantle.

 

[BobK]

You would use fewer turns to convert a 220:110 transformer to 220:30, not more turns.

Bob

 

[ag273n]

You would use fewer turns to convert a 220:110 transformer to 220:30, not more turns.

Thanks Bob and Duke

so it'll be just a matter of accessibility whether the secondary can unwound a bit to have fewer turns. i hope those aren't potted

 

[Minder]

For adding an overwind etc you can't beat a Toroidal type, To find out the T/volt wind on a small number of turns, say 10, power and measure the voltage and it can be calculated from there.
If reducing one, measure, take 10 turns off and re-measure.
M.

 

[ag273n]

The space for primary and secondary should be about equal, if you use thinner wire for the secondary, this will limit the current perhaps to a unaccepteble level.
You want to drop voltage by a factor of about seven so you want to increase the current by a factor of seven. The secondary wire should therefore have an area of seven times the primary. It must be laid on the bobbin very neatly.

Can i get away with a wire thats about 5 times thicker?... my application wont probably reach the rated wattage of the transformer...
I believe the thinner wire would be the weakest link so the durability of this will depend on how long my secondary can sustain the amps.

 

[duke37]

Ludens will tell you the wire thickness necessary for your output current. If the wire is too thin, then there will be too much heat produced and the transformer may fail due to insulation breakdown.
You could get away with a thicker wire (5 times thicker than what?) but may not be able to get enough turns in the space.
5 times thicker is 25 times the area.

 

[ag273n]

the transformer i got off the rack from a local hardware (Ace) unfortunately has the secondary windings overlapping the primary winding. and the E-I sheets are alternating... which means no easy taking apart and i will have some long time modifying it. Gladly, the wires aren't potted

 

[ag273n]

After i get this rewound, how do i test how much amperage it can deliver?

 

[(*steve*)]

You would use fewer turns to convert a 220:110 transformer to 220:30, not more turns.

I read it as an autotransformer. In that case you have exactly the same number of turns, but the tap is in a different position.

Edit: and it's also dangerous.

 

[duke37]

Ludens gives a table of wire diameters, areas, resistances and current capability