Microwave Oven Transformer

Microwave ovens - The circuit shown is typical of bottom range microwave oven. Protection of the recifier diode and inrush current limiting is by a soft start circuit which puts a 10 watt 25 ohm resistor in the primary circuit for around a second. This slow start cct and the noise filter components are usually on a small pcb which is good without modification in home brew power supplies. The fan?, high voltage diode and transformer are useful for low voltage (eg 35v 30A - rewind the secondary)

- picture shows a 700W transformer (output rating - transformer is >1kW rated) with the secondary removed and rewound with 2.5mm square mains wire to give 23v rms. After rectification with a 40A bridge rectifier and smoothing with 66,000uF @40V this gives a no load voltage of 31.6v measured. Note 23 x 1.414 = 32.5v minus two diode volt drops gives 31.3v. On full load with the 600W topband transmitter the volts drop to 20.9v (some mains volt drop with this measurement owing to shed being at bottom of garden) and the transformer temperature reached 85deg C. this does cause the mains wire secondary windings to stick together but otherwise all ok. Bridge rect is type CM40010 farnel order code 9380841. The 66,000uF cap is 2 x 33,000uF 9A ripple 40v caps from farnell order code 8820732. Note importance of fitting discharge resistor to the power supply. 2k ohms at 1W is ok for 3min ish discharge time -

or high voltage (2.5kV) PSUs. Measurements were made on 4 transformers in the 700 - 800W range. A 12.3V ac input to the primary winding gave secondary voltages of between 112 v and 120v ac. The primary resistance measure around 2 ohms and the secondary resistance 100 ohms.

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