Modification of Seventeam ST-230WHF SMPS PC PSU Click on the pictures for better resolution
Beneath you will find a thorough modification concept for a specific PC AT-SMPS Seventeam ST-230WHF.
DO NOT OPEN A SMPS UNIT WITH MAINS CONNECTED AND DO NOT TEST THE SMPS WITHOUT THE COVER IN PLACE. WAIT A MINUTE AFTER DISCONNECTING MAINS TO OPEN THE UNIT - THERE ARE LETHAL VOLTAGES INSIDE UP TO 325 VOLT.
To get an idea of SMPS function I will recommend DL2YEO's circuit explanation at his home page - find the link beneath.
The SMPS can deliver 13.8V and 16A continuous and up to 24A shortly (less than 20 sec) sufficient for a 100W SSB-transceiver (50W FM/AM/RTTY/PSK).
You will need few components for this mod: 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.
ST-230WHF SMPS - approx. originally schematic OBS 800 kB
ST-230WHF PSU - modified and simplified schematic OBS 300 kB
This SMPS - pic 1 - is very easy to modify and you can possibly find one antiquated on a recycling station. The advantage to modify a SMPS from Seventeam is they are well-equipped - 30A diode for 5V, two heat sinks, well-known Pulse Width Generator IR3M02, LM339 comparator, two-transistor SCR and the schematic is available.
We don't "touch" the 230V circuit but the low voltage circuit will be changed radically.
Let's start: Remove the box cover - pic 2.
Remove following components:
- PCB, - blue, white, orange and yellow cables from PCB, - if you don't use the SMPS on 115V remove the switch at rear side, - the toroid L4, - capacitor C16 and C17 (2200uf/10V), - L7, - L8, - R16, - C18, - C19,- R18, - J4, - J5, - D7, - D8, - D9, - D10, - R24, - R25, - R26, - R28, - R29, - J7, - R19, - R20, - R21, - R22, - R23, - D14, - ZD1, - R39, - R40, - C21, - Q7, - C12, - C13, - R14 and - R15.
Cut red and black output cables ca 12 cm over PCB
Remove carefully the diode BD2 (12V) from heat sink and PCB - pic 3.
Assembly of new components:
ZD1 = 13V zener diode BZX79C 13 (over voltage protection).
R40 = 18K and R39 = 330k both determine the output voltage; R40 coarse - R39 fine adjustment; higher value higher voltage.
As fixed load connect an 82R/5W from R18 hole to C15 gnd (+) - pic 4.
C16 and C17 = 2200uF/16V 105° [Elfa # 67-234-15] with low ESR, special made for SMPS, remember same diameter as the former.
Between the holes C20/R23 and the hole nearby R21 mount 1 kohm to "fasten" the low voltage end of the zener - pic 4.
Add extra red cables to the 12V output pad (10 back and 10 red).
Short-circuit 5V and 12V output pads.
Remove all wires from toroid L4 and clean it with fine sandpaper and fill it with 27 turns of 2 x 1mm isolated transformer thread alternatively 4 x 0.75mm length 1.5m each.
The transformer need to bee modified - it is constructed with a separate 5V winding and a separate 7V winding each with its own diode CTB34M (BD1) for 5V and CTB24L (BD2) for 7V - these two voltages are added and then 12V appear.
The transformer legs are divided to 4 pads - two nearest print edge are the end of the 7V winding  and  the other two pads are the end of 5V winding ,  and , . Cut the foil as shown at pic 5.
As mentioned before 7V is added to 5V and the center tap of the 7V winding is the black cable from the transformer (CT2) - carefully cut the isolation near the transformer don't scratch the threads - you have 4 isolated threads - split it up to the transformer.
By ohm-ing find the two which are connected together in the other end of the winding and check the other two - no connection between the two double threads; twist each pair together.
Cut the two thread-pairs 7 cm from the transformer and remove the lacquer 1.5 cm from the free end - solder ends - pic 6.
Drill two holes up in the small print pad near transformer leg /  and the hole where leg 7 could be up to 1.8mm and drill through the transformer form - pic 5.
Bring the two threads which are connected to transformer leg  (0 ohm) through the two up-drilled holes and solder them to leg /  - pic 7/8.
Bring the two threads which are connected to transformer leg  (0 ohm) through the bigger hole  and solder them to leg /  - pic 7/9.
The two windings (5V and 7V) are now in serial and connections to the former 5V rectifier BD1 (CTB34M) are missing. Connect a 1mm isolated thread from  to the diode's left side and another from  to the right side.
Furthermore connect a thread from the diode center pad to the former 12V diode center pad to supply the IC's via D11 with necessarily power - pic 7.
Check the PCB for unwanted short-circuits, long thread legs etc.
Adjust length of red and black output wires and terminate with 8mm2 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.
Caused by emission of weak harmonics of the switch frequency the each output terminal should be "grounded" by a short-legs 470nF and a 10nF capacitor to chassis to reduce common-mode emission.
Mount the two toroid's on the internal mains cable; both cables 4 times through the toroid's. - pic 14.
Test of the circuit.
Connect a variable PSU (~13V) to the output terminals without mains connected and an oscilloscope or a voltmeter to pin 8 of IR3M02; connect pin 4 to ground only under this pretest to simulate normal operation.
With a voltage lower than 13.8V the pulse width at pin8 is maximal and is the voltage higher there are no pulses. The corresponding DC signals on pin 8 are ca. 1.35V and 2.0V - the point where you observe the shift in pulse length or voltage shift correspond with the expected output voltage.
If you can recognize this remove the shortcut to pin4.
If you raise the voltage to 15-16V the over voltage protection shall go in action (high on pin4) - 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 voltage drop over 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 (R39); 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.
I have loaded the SMPS with 15A for many hours without problems and the 4 modified units of same type have all a current limit at 24.5A.
At last I recommend you to "ground" the unit either via the mains plug or directly to a ground connection - if not the box will be at 115V AC caused by the internal mains filter and this may give you problems.
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.
Do you have comments send a mail to: mail link
DL2YEO's home page with explanation of how a SMPS works
OZ2CPU's home page with PSU mods
OZ2CPU's interesting home page with lot of stuff
A Czech site with lots of SMPS schematics
Updated September 15, 2011