6 – Locating and Drilling Holes in the Heatsink, for the Transistors
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6 – Locating and Drilling Holes in the
Heatsink, for the Transistors
Figure 14. Preparations for marking the 3-mm holes
required for the transistors.
Before mounting any parts on the PCB, locate the position of the holes
for the six screws to secure the transistors to the heatsink. Figure 14
shows the PCB with the 6 power stage transistors temporarily fitted
(i.e., not soldered) to permit marking out the positions of the six 3-mm
holes. Also, the ceramic insulation pads must be placed between the
transistors and the heatsink since this can affect the exact position of
the holes. In this prototype, the four holes for the power transistors are
approximately 8.8 mm from the edge of the heatsink, and the smaller
transistors, at 11.5 mm. However, this can vary, and the exact location
depends on the exact position of the bend in the transistor leads. It’s
unlikely that a drilling reference for the heatsink indicates the correct
positions of the transistors. Mark (score) the six holes using a 3-mm
drill bit. Using your fingers and with some force, turn the bit counter-
clockwise through the aligned holes in the transistors and pads.
