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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?

enter image description here

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    \$\begingroup\$ power dissipation? \$\endgroup\$
    – Wesley Lee
    Oct 14, 2016 at 0:03
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    \$\begingroup\$ The outside chimney effect increases convection air flow for cooling. \$\endgroup\$ Oct 14, 2016 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, 2016 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, 2016 at 11:33

2 Answers 2

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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.

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  • \$\begingroup\$ Especially on Pertinax boards like these! \$\endgroup\$
    – winny
    Oct 14, 2016 at 10:31
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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.

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  • \$\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, 2016 at 3:27

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