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Dual-Tracking ±19V Power Supply, Pt.2

Last month, we introduced our new Dual Tracking ?19V Power Supply and explained how it works. This month, we show you how to build the PC boards, install them in a case, wire it up and get it going. Both the mains-powered and plugpack-powered versions are covered.

Pt.2: By Nicholas Vinen

As mentioned last month, there are two versions of this supply – one powered via an internal mains transformer and the other via an external AC plugpack. These instructions concentrate on the mains-powered version, which can supply more output current. If you want to build the plugpack version, check the panel at the end of the article.

Main board assembly

Fig.8 shows the parts layout on the main PC board (code 04206101). Start by carefully inspecting the underside of the board for any cracks in the copper or short circuits. That done, install the 20 wire links using 0.71mm tinned copper wire. You can substitute 0Ω resistors for some of the shorter links if you prefer, although none have been specified in the parts list.

After that, mount the six 1N4148 small-signal diodes (D11-D12 & D13-D16). Note that D11 & D12 face in opposite directions.

Next solder in all the 0.25W resistors. If you are building the plugpack version don’t forget to install a 150kΩ resistor instead of the 91kΩ resistor to reduce the current limit appropriately. Check each resistor with a DMM before installing it since the colour codes can sometimes be difficult to read.

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Fig.8: follow this parts layout diagram to build the main PC board. Make sure that all polarised parts are correctly oriented and be careful not to get the regulators mixed up.
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The completed main board prototype differs only slightly from the final version shown in Fig.8. It's connected via a number of cables that terminate in headers and screw terminal blocks.

Follow with the 10 1N4004 power diodes (D1-D10). Six of these have a short lead spacing (0.3-inch or 7.62mm), so their leads must be bent right at the diode bodies so that they sit flat on the board. After that, solder the TVS (or the alternative 5W zener diode) into place. Leave some space between it and the board (about 3mm), as it will get very hot if the 5V output is shorted for more than a couple of seconds.

Now you can install the four DIP ICs. Make sure they go in the right way around – see Fig.8. Sockets have not been specified but can be used if desired. After the ICs, mount the five trimpots. Regular horizontal or Piher-style mini trimpots can be used. They all have the same value and can only go in one way.

Next on the list are the BC549 & BC559 transistors (Q1 & Q2) and the 78L15 & 79L15 regulators (REG3 & REG4). These are all in TO-92 plastic packages, so check their markings carefully to ensure they each go in the correct position. Be sure to orientate them as shown in Fig.8 and use small pliers to bend their leads if necessary, so that they fit the PC board holes.

Once they’re in, install the five 100nF MKT capacitors, then solder the 16-pin IDC socket in place. Line up the notch in the socket as shown on the overlay (ie, towards the top) and check that the socket is sitting flat against the PC board before soldering its pins.

LEDs 1 & 2 can now go in (they are orientated in opposite directions), followed by the three screw terminal blocks (CON1-CON3). Be sure to install the latter with their entry holes facing outwards. The four polarised headers (CON4-CON7) can then be installed with their plastic locking tabs orientated as shown. Again, make sure they sit flat against the PC board before soldering their pins.

Follow these parts with the 10µF and 100µF electrolytic capacitors, taking care to ensure they are correctly orientated. The two 0.1Ω resistors can then be installed. These resistors only dissipate about a 0.25W each, so they can be mounted flush against the surface of the board.

Mounting the regulators

Regulators REG1 (LM317T), REG2 (LM337T) & REG5 (7805T) can now be mounted along the top edge of the board – see Fig.8. Don’t get them mixed up and be sure to push them down all the way into the holes before soldering one pin of each regulator.

That done, check that each tab is at a right angle to the PC board before soldering its remaining leads (if it isn’t, re-melt the solder on the first lead and adjust it).

Once the regulators are in place, all that’s left are the two large electrolytic capacitors. Once again, take care with their orientation.

Front panel board assembly

The smaller front panel board is coded 04206102 and the parts layout is shown in Fig.9.

Begin by installing the four wire links. You can either use 0.71mm tinned copper wire or 0Ω resistors if preferred.

That done, use flat pliers to bend the pins of the two 16mm potentiometers at right angles, so that they project out in the same direction as the shafts. Bend them as close to the potentiometer bodies as possible, then remove the nut and washer from each pot.

Next, take the 5kΩ potentiometer (VR1) and insert the shaft through the indicated hole on the PC board, with the body on the copper side. When it is correctly orientated, the metal tab will fit through the small adjacent hole.

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Fig.9: follow this layout diagram and the photograph below when building the front panel PC board. Note that the binding post terminals are soldered to the board after they have been mounted on the front panel.

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