Reversing microwave oven transformer...

Thread Starter


Joined Nov 29, 2005
What to expect at the low-turn-primary of a ~1KW salvaged transformer when powering its high-voltage-secondary with 120VAC ? -Ignore the filament-
This is not about rewinding or adding windings.
How to calculate the idle 120VAC current when no load is connected ?


Joined Mar 14, 2008
The output of a microwave transformer is typically about 2200V, giving a turns-ratio of 2200/120 = 18.3/1.
Thus if you connected 120Vac to the output, you would see about 120/18.3 = 6.5Vac

You would need to measure the inductance of the secondary and calculate the magnetizing current from that, which is the idle current.


Joined Sep 17, 2013
Be aware that the high voltage secondary winding usually (always?) has one terminal grounded, so unless you can disconnect that from ground you effectively have an autotransformer with associated risks.


Joined Jun 22, 2012
The 1Kv winding will be from small guage wire, which will have high resistance, so you may be limited to the output current.


Joined Jan 21, 2019
This reminds me of a time when....

My son found a video about building a spot welder from a microwave transformer. We cut the frame apart then replaced the secondary with about 4 winding of some 6 AWG. We were able to spot weld sheet meter but I took it apart because it was very dangerous. I might be able to make a safer version and better electrodes but what we had was pretty shoddy.