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We are using a capacitive power supply for one of our applications.

The X2 capacitor (330nF in this case) provides good reliability with respect to failing safely/open and good over-voltage withstanding capability.

So the question came up what to consider, when choosing the parallel bleeding resistor to prevent negating the properties of the X2.

Apart from calculating the resistance value (500k for ~1s discharge choosen) and power rating (0.5W choosen) which is pretty straightforward, what else is there to consider?

What about properties like
* Flame retardant
* Fusable type
* Pulse voltage rating

I would tend to select a fusible type at least. I am unsure about the pulse voltage and peak power ratings though. The circuit will be used in an industrial environment and thus will be tested against 2kV surge L-N and falls into over-voltage category II. Resistors that withstand voltages >2kV are like 20x6mm 4W types, which seems total overkill.

PS: Our customer would prefer not using a varistor for surge protection if possible, due to very limited PCB space. So it may be a good idea to rate the resistor accordingly. And yes, they would really like to have the bleeding resistor because its easy to touch the contacts after being disconnected.

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When surge testing, the X2 capacitor will shunt the bleed resistor significantly so it won't see anything like 2kV. However, the resistor should have an adequate voltage rating to withstand full ac mains voltage as if it were connected directly across the incoming ac power.

You need to choose a resistor that can withstand the peak ac voltage and, if necessary, put two resistors in series to exceed the limit by a decent margin.

If you have any doubts about the 2 kV testing then use a sim tool like LTSpice to see what the peak could be.

I would also consider it proper to have a fuse in series with the ac feed to the voltage reducer if it connects directly to line.

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  • \$\begingroup\$ Thanks for the answer. Considering that the surge voltage should be shunted by the X2, would you say its an option to consider a standard SMD resistor? Something like 1210 0.5W 400V. Or better go wire wound for higher reliability? \$\endgroup\$
    – Rev
    Apr 12 '17 at 8:32
  • \$\begingroup\$ I'd be happy with SMD resistors for this. \$\endgroup\$
    – Andy aka
    Apr 12 '17 at 10:55

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