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I need a resistor to work with 230 VAC voltage. So 400V rating would be appreciated.

However most of the chip resistor with size 1206 and above has only 200 Volts of maximum working voltage. Main of my concern is: why this voltage doesn't rise up with resistor size?

This table taken from Rohms datasheet:

enter image description here

And the similar situation appears in different suppliers.

It looks very weird as 2512 is more than twice as long as 1206 but it has same voltage rating.

This is a bit frustrating... What shoud I use in a circuits working under power line voltages?

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    \$\begingroup\$ Also, when working with 230VAC "400V would be apreciated" is understated. Because it is required. Your Peak voltage at 230VAC is 330V. \$\endgroup\$ – Asmyldof Apr 2 '16 at 11:30
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    \$\begingroup\$ @Asmyldof, 400V equals to 230 VAC + 23% which is pretty pessimistic. This is why I used word "appreciated". I still think that 350V rating would be enough for general use. \$\endgroup\$ – Roman Matveev Apr 2 '16 at 11:49
  • \$\begingroup\$ Could you chain a couple in series to reduce the voltage across any single one? \$\endgroup\$ – tangrs Apr 2 '16 at 12:18
  • \$\begingroup\$ @tangrs this is how I handle this for now. But it looks weird: 2510 resistor is longer than two 1206 put in series. But the voltage rating for 2510 will be twice lower than a series. This is despite the fact that series has two metal plates in between. \$\endgroup\$ – Roman Matveev Apr 2 '16 at 12:45
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    \$\begingroup\$ If you think 20%-ish is pessimistic for crummy, noisy and badly filtered supplies as a residential 230VAC (under which small business falls as well) - spikes and surges and all -, then there's more problems with this project than just availability of resistors. Farnell stocks over 3000 different 400V+ rated SMD resistors, by the way. \$\endgroup\$ – Asmyldof Apr 2 '16 at 15:06
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Generally speaking SMD resistors have lower voltage ratings than through hole .The older larger through hole resistors were used in valve circuits.Whats done these days is placing several SMD resistors in a series string to get the voltage rating.Because SMD parts are machine placed in production the extra component count is not seen as a penalty.Remember that the price of machine placing one part is a very small fraction of the cost of through hole hand placement .Some SMD resistors have laser cuts on them to get the right value .This can mean focused electric fields that reduce the voltage rating.Hence length is not going to always mean a high voltage rating.

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  • \$\begingroup\$ Thanks! How about THICK filf resistors? Could it be better? \$\endgroup\$ – Roman Matveev Apr 2 '16 at 11:35
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I agree with @Autistic and I'd like to add some of my experience.

1) When it comes to sink power from mains (small resistor values). Use through hole resistors with 100% more power dissipation than the calculated value. Also avoid flammable resistors. Avoid carbon, metal film or metal strip for ex. A wirewound resistor is usually a very good choice.

2) When you just have high voltage drops (large resistor values). Use a series network with resistors (for ex: 1206 1/4W). 3-4 1206 resistors in series for 230VAC and 4-5 for 400VAC are good numbers. Again the resistor network voltage drop is higher than the calculated. Also here you can use a wider range of resistor technologies. Metal film for ex.

Reason for both "over-engineering" values is noise and spikes. If your design is for an industrial application it wont last a month with the calculated rated voltages and power.

hoo2

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  • \$\begingroup\$ What if I have a TVS on the mains input to protect the circuit from spikes? As the device should be small and I have half a dozen of those nets. So it will be a pain to place this amount of resistors... \$\endgroup\$ – Roman Matveev Apr 2 '16 at 12:50
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    \$\begingroup\$ 1st. We don't know what you are try to do exactly so we are guessing. 2nd. TVS are very good parts but not magical. They don't protect you from everything. They can fail them selfs also and they are more expensive than the resistor-solution. Make your network safe and put a smaller tvs in a place with lower voltage. (Again, my opinion with no knowledge of what you are trying to do). \$\endgroup\$ – hoo2 Apr 2 '16 at 12:56
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Resistors have three ratings that matter.

  1. Resistance: this is the parameter that tells us how much current will pass at a given voltage. It is usually why we use a resistor

  2. Power dissipation:As we apply a greater voltage, we need to worry about the power \$V^2/R\$. This is a bigger concern for small resistors. Usually you look at the power rating of the resistor to make sure that the calculated power can be safely dissipating without the resistor getting too hot.

  3. Voltage drop: As a larger current flows, the voltage drop increases - and so does the electric field across the surface. If you do not take special precautions, there comes a voltage at which arcing can occur across the surface. The probability of this happening depends on the surface condition (contamination, asperities, ...), the relative humidity, air pressure, and other factors. Since the resistor manufacturers don't control many of these conditions they specify a "safe" voltage at which the package will usually behave itself. Since this has more to do with the form factor than the resistance value, the rated voltage is independent of resistor value.

When you need a certain combination of voltage, power and resistance rating, and you are stuck with a given form factor (SMD package), you may have to use series (spread the voltage drop) or parallel (spread the current/power) equivalent circuits.

Don't design a circuit with components at or near the limit of their rated performance. The fix will cost you far more than just doing it right the first time. Electronics has a way of testing for sloppiness and punishing you...

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Yes, it's limiting. I once disregarded it and it took weeks to understand why the hell the resistors blow up. In my system the voltage was below 30V, but sometimes spikes over 100V appeared.

The reason in in the thin film technology, i guess.

Panasonic has surge resistors that hold higher voltage on same footprint.

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  • \$\begingroup\$ Melf are your friends when it comes to surge ratings. \$\endgroup\$ – Autistic Apr 2 '16 at 11:27
  • \$\begingroup\$ Thank you! Luckily i don't work on that system now :) it was awful evene without the resistors. \$\endgroup\$ – Gregory Kornblum Apr 2 '16 at 11:28
  • \$\begingroup\$ @Autistic, this is not really a surge protection. I just need the circuit to work under regular 230 vac wich means 325 volt repetitive. \$\endgroup\$ – Roman Matveev Apr 2 '16 at 11:36
  • \$\begingroup\$ Still, take higher voltage rated resistors \$\endgroup\$ – Gregory Kornblum Apr 2 '16 at 12:06
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    \$\begingroup\$ @RomanMatveev 325V is pretty optimistic, it is just the theoretical peak AC value, which doesn't take into account two key practical factors: (1) AC nominal value tolerance (that 230Vac nominal can vary during the day and can be higher at night); (2) voltage spikes due to other devices connected to the line (a big motor is switched off and you get easily over that 325 limit for a couple microseconds or worse). You should design-in some higher voltage rating margin if you want your design to be robust. \$\endgroup\$ – Lorenzo Donati Apr 2 '16 at 12:44

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