I want to make this circuit: enter image description here

Where Sens2 is mains (233VAC in Switzerland)

I found this resistor that look perfect for the job: RND 155SP12 3W (datasheet).

I don't understand what does max voltage means for a resistor. For example, in the datasheet it says Max working voltage: 250V

Is it AC(RMS) or DC? Can I use this for mains? Is it the voltage drop only that I need to check?

  • \$\begingroup\$ Two resistors in series. Will be 500V. Good enough. \$\endgroup\$
    – user263983
    Nov 3 at 17:30
  • \$\begingroup\$ Thanks @user263983, so it is the voltage drop that I need to take a look only? \$\endgroup\$ Nov 3 at 17:35
  • \$\begingroup\$ If it is proper placement, not too close. Datasheet shows amplitude, RMS is used for calculating power dissipation. And it normal practice in high voltage put resistors in series \$\endgroup\$
    – user263983
    Nov 3 at 17:41
  • \$\begingroup\$ What would mean "not too close?" :) I have 4 rows of 2 resistors next to each other: screenpresso.com/=ZgyTd \$\endgroup\$ Nov 3 at 18:17
  • \$\begingroup\$ Not too close means proper spacing between points where s high voltage appears. Your design looks good but there is no info about PCB base material, coat , etc. Resistors are OK. \$\endgroup\$
    – user263983
    Nov 3 at 19:16

You can't exceed maximum temperature, maximum power dissipation, or maximum voltage across the resistor. In pulse applications, you need to be aware of the joule rating of the resistor. Bulk ceramic resistance elements are used in pulse applications to minimize spot heating within the resistance element. If you have the full 117VAC (165Vpeak) across a 39k resistor, that's about 0.35W, so your 3W resistor should be fine as long as it gets proper cooling. When consdering the maximum working voltage, be conservative and use the peak voltage since this is an arcing failure you are trying to avoid.

Exceeding the max working voltage, even though you are still under the maximum power rating, can cause small arcs within the resistive element. This grows to larger arcs and failure of the resistor. In pulse applications, it's fun to turn off the lights and watch 2 to 3 mm balls of orange flame emanating from resistor body when driven past the maximum voltage (above the max overload voltage).

The other specification is the maximum dielectric withstanding voltage which is what the resistor insulation can handle (arcing to other parts).


The "max working voltage" in the datasheet is any voltage (be it AC or DC). This means it will work properly if it sees no more than 250V. Note the dielectric voltage is much higher, so the working voltage parameter has a margin for safety.

And, since you are using two equal series resistors, any voltage will be equally divided, so if you give it 233V (again AC or DC no matter) then each resistor will only see 117V, which is far below the specification, so you are good.

Using two series resistors is also a good safety practice with high voltage circuits. If one goes bad (short circuit itself), the other is there to protect from excessive current, so in this case the other resistor would see the full 233V and also be good.


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