Skip to main content

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.

Comments

Post a Comment

Popular posts from this blog

Digital Bandpass Filter FIR design - Python

The python code generates the Finite Impulse Response (FIR) filter coefficients for a lowpass filter (LPF) at 10 (Hz) cut off using firwin from scipy.  A highpass filter is then created by subtracting the lowpass filter output(s) from the output of an allpass filter. To do this the coefficients of the LPF are multiplied by -1 and 1 added to the centre tap (to create the allpass filter with subtraction). A second LPF is then created with a cutoff at 15 (Hz) and the bandpass filter formed by addition of the LPF and HPF coefficients. The program also generates a test sine wave of a given amplitude and power and to this noise from a Normal distribution is added.  The graph below shows the signal and nois, and the signal (green) after filtering. The input snr is approximately 3dB. The frequency response below shows the passband centered on 12.5 (Hz), the Nyquist frequency is 50 (Hz). from numpy import cos, sin, pi, absolute, arange from numpy.random import normal fr...
Post a Comment
Read more

GNU Radio Waterfall and CW Filter

The following GNU radio application adds a waterfall spectrogram to the previous CW filter program. The plot show 4 CW signals in the audio band (lower sideband) at 7023 kHz. The 700Hz signal is filtered and output to the laptop headphones by the CW bandpass filter. The frequency display is shown after the script which is as follows: #!/usr/bin/env python from gnuradio import gr from gnuradio import audio from lpf_bpf_class import Bandpass from gnuradio.qtgui import qtgui from PyQt4 import QtGui import sys, sip     class cw_filter(gr.top_block):     def __init__(self):         gr.top_block.__init__(self)           sample_rate = 44100         out_rate = 8000         kaiser = Bandpass()         cw_flr = gr.fir_filter_fff(1, kaiser.bpftaps)         decimate = int...
Post a Comment
Read more