The DC Motor Speed Controller is built on two PC boards: a main board coded 09103081 (124 x 118mm) and a display board coded 09103082 (73 x 58mm). These are joined together via a 12-way flat ribbon cable which plugs into a pin header on the main board.

Fig.9: install the parts on the main PC board as shown on this layout diagram, starting with the 17 wire links. The 40EPF06PBF 40A fast recovery diode (D3) is shown in position here but you can also use two MBR20100CT 20A diodes connected in parallel (see Fig.10). Note the alternative positions for VR1.

The main board can be assembled first – see Fig.9. Start by checking the PC board for hairline cracks and for any visible shorts, especially between the ground plane and any adjacent tracks. In addition, check the hole sizes for the larger hardware items by test fitting these parts into position.

That done, begin by installing the 17 wire links. These must go in first, since some of them run under components.

To straighten the link wire, first clamp one end in a vice, then stretch it slightly by pulling on the other end with a pair of pliers. It’s then just a matter of cutting the links to length and bending their leads down through 90° to match the holes in the PC board.

The resistors are next on the list. Table 1 shows the resistor colour codes but you should also check each one using a digital multimeter before installing it on the board, as some colours can be difficult to decipher.

Note that resistor R1 should be 1kΩ if the supply voltage will be higher than or equal to 16V. Alternatively, use a 100Ω resistor if the supply voltage is going to be less than 16V. The 22Ω 1W resistor (on the lefthand side of Fig.9) should be mounted 3-4mm proud of the PC board to aid heat dissipation.