G4AKW

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

This capacitor bank is currently wired with 22 2200uF capacitors in parallel. Maximum DC voltage is 350v and the total energy stored is approx. 3000 watt-secs (Joules). A discharge resistance low enough to achieve a 1nS pulse equates to 3TW peak pulse power. However, the ESR of the capacitors alone will limit the discharge time to around 100us, hence the peak pulse power will only be about 22MWatt. To confirm the marked value of the caps (2.200uF) a 17.5v DC supply was connected to one via a 100k resistor and the time taken to charge to 1v was measured. 15sec, gives a dv/dt=1/15 v/sec. The charging current (fairly constant) = 17/100k and the capacitance is therefore: C = i (const)/dv/dt = 17/100k / 1/15 = 17 * 15 / 100000 = 2550 uF