Before you startYou can print out an image of the circuit board silkscreen layer (PNG format | PDF format), to use as a guide for installing components.While you look at the board layout, please also take the time to look at the schematic diagram and associate each part with their location in the circuit. While this is not normally required to build a working power supply, one of the opportunities of DIY is to learn about how the circuit works. Try to determine what each part does and why the particular part or value is chosen. There are many web resources to help you with this, including the σ22 PSU forum thread on headwize.com. You will find the overall DIY experience more rewarding as a result. Note: Be sure you have chosen an appropriate power transformer, the proper D5 zener diode and R10 resistor value to provide the desired output rail voltage and current. See the Parts list section for details. Circuit board assembly instructionsIf you use screw-mount onboard MOSFET heatsinks, they must be tapped for imperial #4-40 or metric M3 machine screws, or you may use self-tapping screws in order to secure them to the board. The screw holes for the heatsinks on the board will need to be enlarged slightly. You can either use the same tap or a drill bit for this purpose.
Since each MOSFET's mounting tab is internally connected to the drain pin, it will carry live voltages and should not be shorted to the chassis, ground or other parts of the circuit. If your onboard heatsinks will be in close proximity to the top metal cover when the board is installed in your chassis case, then you should mount the MOSFETs to the heatsinks using TO-220 mounting kits. If you use offboard heatsinks, then TO-220 mounting kits are also required. Otherwise, you may mount the MOSFETs directly to the heatsinks using #4-40 or M3 machine screws and hex nuts, after applying some heatsink thermal paste to the mating surfaces. The MOSFETs are vulnerable to electrostatic discharge damage when you are handling them, so keep their pins plugged into anti-static foam while working with them until you're ready to solder them to the board. Set the heatsinks pre-mounted with the MOSFETs aside for now. The CRD positions (CR1-CR4) have pads to accommodate several case styles. See the illustrations below. 
If you are using the CRxxx (TO-18 or TO-206AA) devices, be aware that the metal body is internally connected to the cathode pin. To prevent a short circuit to the anode pad, you should allow a gap of at least 2mm between the body and the surface of the board. You should also make sure that the body does not touch any other part. Solder the components to the board, starting with the lowest profile parts. This means the resistors and zener diode, then the CRDs, small transistors, small capacitors, followed by the large capacitors, TO-220 rectifier diodes and connectors. Lastly, install the MOSFETs and secure the heatsinks, and solder their pins last. If you use the specified Phoenix Contact 1729128 series terminal blocks, they have tongue-and-groove channels on each side. Adjacent output terminal blocks should be snapped together before they are inserted into the board and soldered. Other terminal block types may be similar. Since the σ22 circuit board holes are plated through, you only need to solder the parts from the bottom of the board. Do not drill or enlarge the holes because that would damage the through-plating. Make sure the correct part goes into each position on the circuit board. Measure each resistor with your multimeter to ensure it's the proper value. Pay attention to the polarity of electrolytic capacitors, diodes, CRDs and transistors. Be sure not to mix up the N-channel and P-channel MOSFETs! If you are using the CR240 and CR470 current regulating diodes, be sure not to let their metal bodies touch each other. Pay special attention to CR2 and CR3 as these are positioned next to each other. Clean up the solder flux residue from the board with isopropyl alcohol (or electronics flux remover) and a brush. Inspect all solder connections carefully, using a magnifying glass, to make sure there are no solder bridges or cold solder joints. Use a multimeter in ohms scale to check for short circuits. As a minimum, you should verify that the V+ and V- outputs are not shorted to ground, or to each other, and that the transformer inputs are not shorted. Correct any mistakes before proceeding to the next phase. Next, proceed to the Wiring & ground section for details about wiring up the connections to the board. Main: σ22 Main | Prev: Parts list | Next: Wiring & ground |
