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I am working with an existing design and seeing a 200Ω, 2W resistor becoming discolored in the field (customer questioned its condition), but not appearing to fail. I measure resistance and is 203Ω, maximum measured voltage across this resistor 3.5V. That’s .06W, this is a 2W resistor. The resistor is not hot to the touch at all. My thinking was possibly moisture, but there are many other resistors on the board and none look like this.

The purpose for this resistor in the circuit is to prevent the input voltage to the +5VDC liner voltage regulator from exceeding its maximum allowable (35VDC). There is one relay powered off of V+. So drop across said does vary from 2VDC (relay off) to 3.5VDC (relay on).

Has anyone seen this or have any input as to why this is occurring?

enter image description here

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    \$\begingroup\$ It's a power resistor and it is getting hot. The paint components are evaporating and loosing color. \$\endgroup\$
    – Eugene Sh.
    Apr 12, 2018 at 13:39
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    \$\begingroup\$ What is it powering? It is enough to momentarily dissipate a lot of power to get discolourations, like inrush current and similar. \$\endgroup\$
    – PlasmaHH
    Apr 12, 2018 at 13:43
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    \$\begingroup\$ And the solder joint on its right lead looks suspicious. \$\endgroup\$
    – AlmostDone
    Apr 12, 2018 at 13:44
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    \$\begingroup\$ Is your customer using it within it's specification? (of course he is) Have you tested what happens if you stress it beyond your specifications? Will the resistor in question get hot then? And maybe you can share those specifications with us (max current on V+ and max current on 5V) \$\endgroup\$
    – Arsenal
    Apr 12, 2018 at 14:44
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    \$\begingroup\$ Is the polarized cap in backwards? \$\endgroup\$
    – George Herold
    Apr 12, 2018 at 15:01

1 Answer 1

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The power scheme is not presented here but thinking I come to the conclusion that the 36V is pulsing. Otherwise C 100uF would not have been inserted, or at least of less value. In practice, this capacitor is involved in an AC circuit that consumes power at the charge of the capacitor, and this power is dissipated to a limiting resistor of 200ohm. And probably the power dissipation is at the edge as seen. If the power supply is done with a bridge rectifier (and even one half does not work) the pulsations will be (very) large, the capacitor will compensate them at the expense of a higher charge current which causes the resistor to overheat. What a paradox! One half of the bridge does not work and limiting resistor overheated!

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