Modifying of TE SMPS PC PSU

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Modification of a SMPS for radio amateur purpose.

You can easily modify a PC SMPS to deliver 13.8V instead of the "official" PC voltages and normally you can find several SMPS free of charge in the recycling centers.
Why I'm using a specific SMPS is: making one mod instruction is easy but get several equal SMPS's is relative difficult.
To get an idea of SMPS function I will recommend DL2YEO's circuit explanation at his home page

Look here

Beneath you will find a mod instruction of a 300W PC SMPS (Switch Mode Power Supply) look at the label pic0 left. The SMPS can deliver 13.8V and 16A continuous and up to 25A shortly (less than 20 sec) sufficient for a 100W SSB-transceiver.
You will need some smaller components: few resistors, electrolytic capacitors, epoxy glue, output terminals, paint etc.
If you carefully follow this instruction you will end up with a functional SMPS with over voltage and over current protection.

The 230 Volt side is untouched except for the PFC coil mounted in the cover. It is removed caused by the strong hum from it under heavy load. Its two pads on the PCB are short-circuited.
After the modification we use mostly of the 12V circuit of the low voltage side which can deliver the 13.8V. Don't touch the 5VSB supply (delivering power for starting the SMPS) you can submit this voltage to an output terminal (5V/2A).

To have a better PCB overview remove the mains and PFC-connectors, wires for the fan, all external wires except the black and red ones, cut it down to 10 cm length (for the 13.8V output terminals later).

Remove the toroid L4 and carefully remove/cut the 3 thickest threads - the thinnest thread must remain for -12V for the fan.
Wind two 1mm or four 0.75mm (1.5m length each) isolated thread on the toroid 27 turn each.
Remember the thin threads shall through the thin holes in the PCB - the thick (new) threads are spread to both 5V and 12V holes to split the current likely to both diodes.

Remove all components in the 3.3V supply, see pic3 compared with pic5.

Remove carefully the low voltage heat sink with diodes - remove the 12V diode F16C20 - remove the 3.3V diode S20C40 and mount it instead of the 12V diode.
The two 3-leg diodes are now S20C40 (20A/40V) - don't touch the diode nearest the fan - remount the heat sink carefully.

Near the heat sink you will find a 10k NTC (TR2) feeling the heat sink temp. - press the NTC tight to it and glue it with epoxy.

Cut the connection at the PCB between the transformer 4 pads and the 5V diode, see pic8 left of the blue short-circuit.
Parallel the two diodes (5V and 12V) blue and brown short-circuits.
Short-circuit 5V and 12V directly after L4 and between 5V and 12V output pads.
You can now user same number of red output wires compared with the black ones (10 of each).

Remove the link P18 and mount an 8V2 zener instead with the cathode nearest the PCB edge (positive voltage for the fan circuit).
Remove R49 and mount a 15V zener with the cathode nearest the PCB edge (over voltage protection).

Connect the two lowest PCB-leads at pic9 to 0V (earlier 3.3V sense-signals).

Remove D22 and R48.

Near C40 - change R33 to 22k (coarse adjustment) and R34 to 47k in serial with 330k (fine adjustment of 13.8V), see pic3 links.

Connect the point ON/OFF on the PCB to ground to start up the SMPS with "power on" - the short green wire on pic4 (normally done by the computer ON/OFF knob).

Mount a male/female connector between the PCB J2 and the fan wires (easier for you to mount/demount the PCB in a short time - my experience).

Connect 82ohm/5W between +13,8V and ground to have a minimum load for the SMPS, see pic5 (green vertical resistor).

Change the electrolytic capacitors to 2200F/16V with low ESR (Equivalent Serie Resistance) .

Adjust the length of red and black output wires and terminate with 8 mm2 appropriate solder or press-terminals. Use same amount of black and red wires (extra red).
Use a LED as indicator for 13.8V (2.7k serial resistor).
The box is changed a little - remove the female mains connector and cover the hole with an appropriate plate; fasten it with epoxy. Do the same with the hole for the former output cables.
It is important to cover the two holes for sufficient cooling of the components.
Paint the box if you like.

Test of the circuit.

Connect a variable PSU (ca 13V) to the output terminals without mains connected and an oscilloscope or a voltmeter to pin 8 of KA7500B; pin 4 to ground only under this pretest to simulate normal operation.
With a voltage lower than 13.8V the pulse width at pin 8 is maximal and is the voltage higher there are no pulses. The corresponding DC signals on pin8 are 1.1V and 1.5V - the point where you observe the shift in pulse length or voltage shift is the expected output voltage.
Remove the shortcut to pin 4.

If you raise the voltage to 15-16V the over voltage protection shall go in action - when you hereafter reduce the voltage under 13V no activity will appear at pin 8. Remove the variable PSU from the output terminals for few seconds and connect again - now the pulses at pin8 will appear.

Have you experienced something like me you can mount the PCB, fan, mains connector etc. in the box - remember the cover. I normally connect a 25W 230V bulb in serial with the mains to the SMPS under first upstart (chicken) - it bright up shortly and you can expect about 11-13V output if the circuit is OK (the voltage is soft caused by the bulb).
If your test so far is equivalent to mine remove the bulb and connect the SMPS direct to mains - is the output voltage OK, the job is done; if the voltage is too low or high adjust the serial resistors (R34) increase of resistance gives higher output voltage.

If the SMPS has switch off caused by over voltage or over current turn off the main power for 5-10 sec. and it is ready again.

Addendum of April 11, 2005: The fan driver transistor Q7 is rather warm and I has replaced it with a NPN BD135 (uncritical) with the heat sink from the earlier used negative regulator 7905.

Addendum of May 13, 2005: Caused by emission of weak harmonics of the switch frequency each output terminal should be "grounded" by a short-leg 470nF and a 10 nF capacitor; see last pic.

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.


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A Czech site with lots of SMPS schematics

SMPS output wiring/connectors

Updated September 15, 2011