If I obtain the iron core from a microwave oven transformer and strip off all the windings then wind 2 coils, each being 20 turns with 12 gauge wire, will I effectively have made an isolation transformer?
Can I use it with household power without tripping the circuit breaker?
What do I need to do to make sure to not trip the circuit breaker?
Yes, assuming the wire is insulated from itself and adequate creepage and insulation thickness and breakdown rating (for your purposes) is provided between primary and secondary windings. With 20 turns it will only be useful for low voltage AC.
No, it will smoke and/or blow the breaker.
Wind enough turns to reduce the magnetizing current to a reasonable value. That will likely be at least as many as were there originally on the primary for whatever your mains voltage is. The magnetizing current is proportional to the reciprocal of the square of the number of turns.
Yes, you will have effectively made an isolation transformer. However, there are insulation, creepage and clearance as well as electrical design considerations if you want to make it a safe isolation transformer for mains voltage applications. If you don't understand those terms you probably shouldn't be playing with dangerous voltages.
To know if you can use it as an effective isolation transformer (likely not with 20 turns) you either have to calculate or measure the magnetizing inductance to be sure that when you apply mains voltage and frequency across the primary you are not drawing excessive reactive power.
You also need to be sure you are not saturating the core by calculating the max flux density or observing the current as you slowly increase the voltage with a Variac for example.
You can easily start a fire if you don't understand all this, so you should consider buying an off-the-shelf isolation transformer if you need one.
If I obtain the iron core from a microwave oven transformer and strip off all the windings then wind 2 coils, each being 20 turns with 12 gauge wire, will I effectively have made an isolation transformer?
Yes. However, as the core of a typical microwave transformer runs at a maximum of about 1 volt per turn, it's only good for up to 20V AC.
Can I use it with household power without tripping the circuit breaker?
No. It's only good for a maximum of 20v AC.
What do I need to do to make sure to not trip the circuit breaker?
Only strip the HT winding, and the magnetic shunts, and leave the mains winding intact. The mains winding is intended for mains. Then wind your new secondary within the vacated space. You'll need approximately 1 turn per volt. If you want mains-level voltage out, you'll need 115 or 240 turns, maybe a few more. You can measure the volts/turn with a few test turns on the secondary, with the primary energised.
This is quite a popular way to make a DIY spot-welding transformer. With 5 turns of very thick wire on the secondary, you'll have a 5v 200A transformer.
