I've got a 8 year old pair of powered studio monitors that have recently started hissing for about 5-10 minutes after being powered on from being completely off. The culprit was some bad caps that I am replacing. While I was at it, I noticed that there are two large resistors coming from a bridge rectifier that are positioned in such a way that they are 1cm vertical from the board. I was wondering, is there any reason for resistors, or other components for that matter, to be soldered in such a manner?

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    \$\begingroup\$ power dissipation? \$\endgroup\$ – Wesley Lee Oct 14 '16 at 0:03
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    \$\begingroup\$ The outside chimney effect increases convection air flow for cooling. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 14 '16 at 0:13
  • \$\begingroup\$ I think a resistor with a higher wattage rating should be used to minimize heating if space permits \$\endgroup\$ – user116345 Oct 14 '16 at 0:51
  • \$\begingroup\$ Mike - That would do nothing. If a resistor is dissipating 2 watts, it doesn't matter if you use a 2 watt, 5 watt, or 500 watt resistor, it will still dissipate 2 watts. The only thing a larger package would do is have a greater surface area, but then space becomes a problem. \$\endgroup\$ – R Drast Oct 14 '16 at 11:33

The resistors are spaced away from the PC Board so as to allow better cooling.

One side benefit of doing this is that heat damage to the PCB is reduced or eliminated.

It is common to see discolored circuit board under and around resistors that run hot. Spacing the resistor away from the board both helps keep the resistor cool as well as minimizing board discoloration and damage.

  • \$\begingroup\$ Especially on Pertinax boards like these! \$\endgroup\$ – winny Oct 14 '16 at 10:31

Another reason to space resistors off the board like that is isolation. In the picture both resistors have rubber or plastic socks on them near the bridge rectifier. This would reduce the heat dissipation but increase isolation.

A board I worked on had resistors similar to this with line voltage on them. Underneath the resistors were low voltage traces. Under some heavy load conditions the resistors would arc to the low voltage traces. Working with the assembler we spaced the resistors off the board to prevent this condition.

  • \$\begingroup\$ yea I put the 'socks' on those resistors but kept the original height - they looked like they were so close to the rectifier that they might touch the wrong lead and mess something up ;) \$\endgroup\$ – Alex Oct 18 '16 at 3:27

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