G4AKW

The photo shows the completed class D 600W 160m and 80m amplifier, mains power supply and antenna change over board. The front pannel connectors provide a connection to the transmit o/c output from the drive transciever (FT817ND, background right). All RF connections and made via the back pannel. The white plastic box houses the slow start circuit from a microwave oven and the rewound oven transformer is on the left. All mains wiring (should be) double insulated. Fuses (a) rear pannel mains input fuse 10A (b) slowstart fuse 10A (c) 24v mosfet fuse 30A.

The photo shows a prototype class D power amplifier using 4 Hexfets in class D operation. Power output is 575W from 24 volt power supply. Input requirement is a few milliwatts. The single turn primary of the 5:1 output transformer is centre tapped and the quad complimentary hexfet pairs alternatively short respective half turns. 24v is therefore alternatively applied to the 1/2 turn putting a maximum voltage of 48v on the hexfet drains. The secondary voltage is a squarewave with maximum peak amplitude Vpeak=i/j x Vcc where i/j is the turns ratio which in this case is 10, and Vcc is 24v. The fundamental frequency component of this squarewave is selected by the o/p filter. Measured values are slightly above theoretical.

This script reads the external IP address, stored in file ext_ip_add.txt (full path), then gets the new external IP address and compares the two, then overwrites previous stored address. Replace LT with proper less than sign and GT with greater than sign. Following cron job pipes output to a text to speech converter. #!usr/bin/python # # Program gets your external IP address # # This program reads the title from # myip.dk website and prints out from # 12th char after < # # Prog checks for change in ip address # # The ip address is written to file # ext_ip_add.txt (overwritten) # # Version 1.0 Geoff Robinson July 09 fileobject1=open('/home/geoff/python/ext_ip_add.txt','r') oldipaddress = fileobject1.readline() fileobject1.close import urllib url = urllib.URLopener() resp = url.open('http://myip.dk') html = resp.read() start = html.find("LTtitleGT") + 12 end = html.find("LT/titleGT") newipaddress = html[start:e...

This very simple 2 Fet power amplifier easily achieves 250W output with an FT817 5W drive. The key design details as follows: 3:1 broadband input transformer matches the 5.5 ohm gate load resistor (4 x 22 ohms in parallel) to the 50 ohms required by the FT817 . The 4:1 output broadband transformer presents 3 ohms (16:1 impedance ratio) to the balanced HEXFET pair each mounted on a 3mm copper heat spreader which is insulated from the 2 1w/degC heatsinks. These are blown cool by a fan underneath. The power supply required is 28v at 30 amps. The amp is around 50% efficient with a standing 750mAmp temperature compensated bias. An IC 703, with 10watts output will drive the output to around 400 watts. The output filter shown is a 5 pole topband filter with T130-2 torroids and 400v silver mica caps. Peak output voltage on 160 metres with 5 watts drive is 160v or 320v p-p in 50 ohms equating to 250watts. This is slightly higher than the reading on the 3kw MFJ power meter. The inline F...

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