Tranformer Building Question

[fishman94]

I am trying to construct a transformer to step 120VAC down to about 19 or 20VAC. I have a laminate ferrite E-core. My question is how do you prevent shorting of the 120VAC line without a resistor in series with the primary coil winding. Is there a minimum number of turns of wire needed on the primary. I believe I am using 18AWG enamel coated magnet wire. Any help would be very much appreciated.

 

[Nigel Goodwin]

Ferrite is not suitable for low frequency use, you need a laminated iron core, ferrite is only for high frequency switchmode transformers.

You also need LOT'S of turns on the primary for 50/60Hz mains transformers - it's not just a random guess though, you need to calculate it.

Far easier to buy one surely?.

 

[Optikon]

I am trying to construct a transformer to step 120VAC down to about 19 or 20VAC. I have a laminate ferrite E-core. My question is how do you prevent shorting of the 120VAC line without a resistor in series with the primary coil winding. Is there a minimum number of turns of wire needed on the primary. I believe I am using 18AWG enamel coated magnet wire. Any help would be very much appreciated.

Yes there is a minimum number of turns needed. If this is the first time you've designed & built a transformer, I would take Nigels advice and just buy one. You can get one that gives you 20Vac output. Playing with mains devices is asking for trouble if you are inexperienced.

 

[Diver300]

The number of turns depends on how big the transformer is. The larger the transformer, the more the magnetic flux, so the fewer turns needed on the primary.

If you don't have enough turns, the core will saturate, which means that no more flux can circulate in the core, so there is no back voltage, and a huge current flows and blows things.

If you want to mess around with a transformer, mess around with the secondary. All transformers in one manufacturer's range of the same power rating have identical primary windings, so you can customise the secondary without worrying about its affect on the primary.

If you don't touch the primary, you are less likely to kill yourself, and you will have enough turns so there will not be a problem with too much primary current when there is no load.

The larger the transformer, the fewer the turns you need for a particular voltage. A 300W toroidal will give about 1/2 V per turn.

Years ago, you used to be able to buy transformer kits, containing the core, primary windings and former for the secondary. All the user had to do was wind the secondary.