0
\$\begingroup\$

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?

\$\endgroup\$
  • 1
    \$\begingroup\$ At what input voltage do you plan on not saturating the core with just 20 turns? \$\endgroup\$ – winny Apr 24 '18 at 15:37
  • 1
    \$\begingroup\$ Related question and ditto \$\endgroup\$ – Andy aka Apr 24 '18 at 16:19
  • \$\begingroup\$ I would suggest about 10% more turns than the old mains winding so the core is a bit further away from saturation than in the cost sensitive application it was sold into. Two primary bobbins on one transformer core makes a serviceable isolation transformer but it will get a bit hot even on idle. \$\endgroup\$ – KalleMP Apr 24 '18 at 21:34
  • \$\begingroup\$ Consider connecting outputs of two identical transformers together. Line power to the input of one transformer and use the input windings of the second transformer for your load. \$\endgroup\$ – Optionparty Apr 25 '18 at 2:47
6
\$\begingroup\$
  1. 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.

  2. No, it will smoke and/or blow the breaker.

  3. 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.

\$\endgroup\$
  • 2
    \$\begingroup\$ Nice answer. +1 for NOT saying "blow up". \$\endgroup\$ – Marla Apr 24 '18 at 15:36
2
\$\begingroup\$

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.

\$\endgroup\$
1
\$\begingroup\$

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.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.