Modified PC-SMPS's (Switch Mode Power Supply) Click on the pictures for a better resolution
Modified PC SMPS for radio amateur purpose.
The start of my SMPS knowledge began in 1997 when my computer went down - power failure - a new SMPS was bought. I repaired the broken SMPS a Seventeam ST-230WHF AT-type and considered to modify it for radio amateur purpose. The goal was modifying the SMPS to deliver 13.8V and more than 23A for my old IC-730 HF-transceiver.
A schematic from Seventeam can be downloaded here: ST-230WHF SMPS schematic or in a better resolution: ST-230WHF SMPS schematic, NB ca. 800 kB
To modify the SMPS was rather simple - I had most trouble to disassemble the transformer - to dissolve the epoxy or glue - but I succeeded and removed the secondary windings. Hereafter a new winding was made of 0.2mm Cu-foil and as isolation very very thin "flower paper" - totally 2 x 8 windings (16 w. center tapped). The over current protection switch-down the SMPS at 24.5 Amp.
Later I have simplified the procedure by picking up the secondary 7V winding's center tap and split it up into the two winding-parts and then connect each of them in right phase to each end of the 5V winding.
The 12V is a combination of 5V added with 7V; look at the schematic.
This procedure is much easier than dis- and reassembling the transformer but you can only get ca 15A continuous and still 24A peak from the SMPS.
The hereafter mentioned Amps are max load for 20 sec and you can expect a continuous load of 60-70% of the max value.
Inspired of OZ2CPU I bought a 400W ATX SMPS (LT-B400ATX) for 22 Euro just for fun or rather modification. After lots of problems I succeeded and finished with 13.8V/30A. Later I have found the app. schematic: LC-B250ATX PSU-diagram
After a SMPS demonstration in our local radio amateur club OZ2KB Kurt asked me to give a hand with modifying an old AT SMPS (CPI-230C); shortly after he brought another SMPS(PW-9412) and modified they both gave ca 20A.
Few months ago I modified a Seventeam 230WHF for OZ5NC Niels and have afterwards mod a 200W AT-SMPS EXPEN TECH-200 and a Jumbo MAV-200P both with max 22A output.
RF-emission from the SMPS:
Measure of Common Mode currents at the SMPS output and the main cable is made with a home-made current-probe consisting of a ferrite split core with one wound of thin coax as "sniffer" for the Spectrum Analyzer.
Measurements satisfy the requirements of CE regarding RF-Emission between 500 kHz and 30 MHz [CISPR 22 Class B]; limit value is -61 dBm and measured values are below -90 dBm.
IMPORTANT: If you connect your SMPS to mains via a 3 wire mains lead (or the chassis to ground) the internal mains filter will reduce radio noise emission from the unit. Furthermore AC level of the chassis will be zero - no more electric shocks!
OZ2CPU's home page with PSU mods and several interesting electronic stuff
A Czech site with lots of SMPS schematics
SMPS output wiring/connectors
Updated September 15, 2011