Beneath you will find a thorough modification concept for a specific PC AT-SMPS Seventeam ST-235GL. DL2YEO's home page with explanation of how a SMPS works OZ2CPU's home page with PSU mods
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 15A continuous and more than 20A 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.
SMPS schematics:
I'm sorry for the missing link in the past!
ST-235GL SMPS - original schematic OBS ~900 kB
This SMPS - photo 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, two-transistor SCR and the schematic is available.
We don't "touch" the 230V circuit but the low voltage circuit will be changed radically.
I recommend you to print out the text and mark your mods.
Removal of components:
- all external cables from PCB except the black and red ones
- if you don't use the SMPS on 115V remove the switch at rear side
- compare your PCB with photo 3 and remove the unnecessary components from the PCB.
Cut red and black output cables 15 cm over PCB.
Remove carefully the heat sink with low voltage diodes BD2-4 from PCB and the big transformer.
Assembly of new components:
D24 = 15V zener diode BZX79C 15 (over voltage protection).
R59 = 15K determine the output voltage; higher value higher voltage.
As fixed load connect a 100R/2W from earlier R5 - now R3.
C37 and C38 = 2200uF/16V 105° [Elfa # 67-234-15] with low ESR, special made for SMPS, remember same diameter as the former.
Install the heat sink with one diode D83-004 (BD2).
The transformer need to bee modified - it is constructed with a separate 5V winding and a separate 7V winding each with its own diode D83-004 (BD2) for 5V and C82-004 (BD3) for 7V - these two voltages are added and then 12V appear.
The transformer has 7 low voltage legs - two nearest print edge are the end of the thin 12V winding [7][6], the next two pads are the end of the thick 7V winding [5][4], the next is the 7V center tab [3]and finally you will find the ends of 5V winding [2], [1], the center tab of this is the thick wire at the transformer's upper side.
As mentioned before 7V is added to 5V and the center tap of the 7V winding is pin 3 - carefully loosen the two outer wires from pin 3 (both in one isolating sleeve). Two 'bottom' wires remain at pin 3.
If you remove the outer 12V windings connected between pin 6, 7 and 1, 2 from the transformer you can easily move the sleeve towards pin 2 - photo 4 and 5. Solder the two loosened wires to pin 2.
The print side of the board needs to be modified - cut as shown at photo 6.
Short circuit pin 1 to pin 3 at print side - now the two windings (5V and 7V) are in serial and connections to the former 5V rectifier BD2 (D83-004) are missing. Connect a 1.5 mm isolated thread from [5] to the diode's left side and another from [4] to the right side.
Add the missing short circuits at print side according photo 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 - you can move the mains switch to the U-shape hole in front and install the output terminals at rear side beneath the main connector. Close the hole where the switch has been situated.
It is important to cover the hole 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.
Test of the circuit.
Connect a variable PSU (10 to 15V) to the output terminals without mains connected and an oscilloscope or a voltmeter to pin 8 of IR3M02.
With a voltage between 11 and 14V the pulse width at pin 8 is maximal - photo 8; if you increase the voltage the pulses disappear. The corresponding DC signals on pin 8 are ca. 1.5V and 2.0V - the point where you observe the shift in pulse length or voltage shift correspond with the expected output voltage.
Adjust VR2 to the desired output voltage (I got a max voltage = 13.65V - sufficient for me).
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 pin 8 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 VR2.
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 over current limit seems to be >20A.
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 with switch mode noise.
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
Good luck.
OZ1DB, Karsten
Links:
Updated September 15, 2011