Is it safe to use the 330K, 1206 pack as shown or to use 2x of 115k,1206 resistors in series? Input is 230v AC.
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3\$\begingroup\$ You need to distinguish between whatever definition you have for "correct" and "added safety margin" \$\endgroup\$– PlasmaHHCommented Jul 6, 2018 at 12:34
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2\$\begingroup\$ You will be required to pass 2 kV surge and burst transients for your product in the end, yes? \$\endgroup\$– winnyCommented Jul 6, 2018 at 13:34
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3 Answers
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You have two of these resistor in series, so their voltage capability adds. Each is good for 300 V, so together they are good for 600 V.
You say the input is 230 V AC, presumably sine. That means the peaks are 325 V. For insulation, it's the peak voltage that matters.
Since 325 V is less than 600 V, you are OK in theory. In practice, stuff happens. The 230 V in might actually be 240 V, so the peak would be 340 V. That still gives you 260 V of margin. Power line spikes also happen. You are OK up to a 260 V spike at the peak of the waveform. Is that good enough for you? Only you can answer that. To paraphrase Dirty Harry: Do you feel lucky? Well, do ya, punk?
Another issue is that the 300 V specified by Vishay only applies if you follow all the rules, including the right separation between pads. The board also needs to be clean, and other traces shouldn't be running closer than some minimum for the voltage you are trying to withstand.
Then there are regulatory issues. It depends on how and where the equipment will be used. Different minimum creapage and clearance distances apply depending on what standard you want to claim compatibility with.
answered Jul 6, 2018 at 12:51
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The real issue is what will break down when you get a 1 to 5kV transient online?
How much will dust and humidity degrade breakdown performance and catastrophic follow on current?
Then you’ll realize your proposed solution is inadequate.
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\$\begingroup\$ Yes. Conformal coating adds insulation but also capacitance. \$\endgroup\$ Commented Jul 7, 2018 at 14:37
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As the maximum element voltage is given as 200V I'd suggest to use two resistors in series. 115k is not part of the E12 series, so I'd switch to 120k instead. From a power dissipation point of view you'll have (230V^2*2)/240k=~311mW, so per resistor you'll have appx. 160mW what's okay for the 1206 form factor.
