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Around 10 years ago we bought 3 dimmers for lighting at a venue. In the last year all three showed the same problem: one or multiple resistors of the dimming circuit (6 channels per dimmer, therefore 6 of those dimming circuits). In the internet I have found a schematic which fits the device. In this excerpt of the schematic R31 is the blown resistor.

I have replaced the resistor and the capacitor (C30) in one of the three and it seems to work fine for now. However I'm concerned about whether the cause is can simply be aging of the components (the resistor or the capacitor) or if there has to be an external cause for the problem (e.g. overloading / overvoltage input from the grid / etc.) as the three showed the same problem in a quite short time-period.

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It might be noteworthy that the devices have never been cleaned and therefore were quite full of dust, so they might have gotten quite warm over time...

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    \$\begingroup\$ Just to exclude: I don't suppose you have replaced the lamps by LED lamps lately? \$\endgroup\$ – Huisman Mar 4 '19 at 19:49
  • \$\begingroup\$ No, we're usually feeding our LEDs power directly from the power grid. Hower I don't always check what my co-workers do. Would that really destroy the dimmers? @Huisman \$\endgroup\$ – aquaatic Mar 4 '19 at 19:57
  • \$\begingroup\$ Yes, "old" dimmers for "old classic" light bulbs cannot be used for LED lamps. \$\endgroup\$ – Huisman Mar 4 '19 at 20:25
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The resistors blowing would be a symptom of a shorted capacitor.

Film capacitors can be damaged by transients on the mains and the damage typically shows up as a short (the capacitors are made with rolled-up thin plastic film that is aluminized and a high voltage can arc through the plastic). Self-healing can take place if there is enough current to blow out the damaged shorted section but the total capacitance drops as a result. There is probably not enough current through the resistor for that to happen, which is probably just as well.

You can try replacing the capacitors with ones with a higher voltage rating so they last longer.

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This could be an intentional (or at least implied) design choice.

All electronic components deteriorate with age, this is a common design consideration with capacitors but the same is true for resistors. The designer might have chosen to run those 1W resistors close enough to their rated temperature, or the devices might be in an environment/load conditions in which this happens. For many resistors this would be a 10000Hr estimated lifetime before failure, but it could be as little as 1000Hrs before problems start to arise.

10 years at 8Hrs/day would be 29000 Hrs. Such lifetime would be roughly consistent with a common resistor running 10°C to 20°C below its rated temperature. If the resistance drifted enough to increase the resistor dissipation to the point of failure (also due to temperature), this would also explain the problem. I believe the relevant standard (IEC 60115-1) only requires tolerance specification at 1000Hrs.

Measure the resistors in the working units (given the circuit, you just have to remove power to do this), and see how much have their values drifted with time. This should give you an indication of how much stress are these under. You can also measure their temperature under normal operating conditions, but this might be harder to do safely and reliably.

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  • \$\begingroup\$ They are not nearly running that long, only on special occasions around 50 times a year in the evening. Most of the time they're just sitting in the storage room... I'll measure the circuit when I have access to it the next time. \$\endgroup\$ – aquaatic Mar 4 '19 at 20:48
  • \$\begingroup\$ @aquaatic Components values also drift during storage. Not as much as during actual use, but if the design was close enough to being marginal (and environmental factors such as humidity are particularly bad) it could be enough to matter. \$\endgroup\$ – Edgar Brown Mar 4 '19 at 20:52
  • \$\begingroup\$ I have measured the component values of both burnt and working resistors and the capacitors to be within specs (+-1ohm / +-0,01uF), so it appears as if there is an external cause... \$\endgroup\$ – aquaatic Mar 13 '19 at 15:37
  • \$\begingroup\$ @aquaatic or a design oversight. The resistor power might be slightly underrated for the application, severely limiting its work life. \$\endgroup\$ – Edgar Brown Mar 13 '19 at 16:09
  • \$\begingroup\$ I highly doubt that. The device is used in the event business where reliability is key. \$\endgroup\$ – aquaatic Mar 13 '19 at 20:21

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