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I've found some 400V 0805 resistors. However, as in my previous question, I noted that almost no power supplies used surface mount components.

One of the things I'm working on is a capacitive power supply. This is connected to the mains and has no isolation, but operates inside a completely sealed box so there is no hazard to the user there. However, I'm not entirely sure if it's okay to use 0805's when dealing with such high voltages... it just seems like arc-over or something similar would happen. And, say there was a fault - would through hole be better at handling a line transient than SMT?

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Thomas O
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4 Answers4

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Creepage is the term you're looking for. You'll want to use that for trace spacing too. If the part is rated for 400V it's good for it. However, you may have more than 400V. 240Vac is actually RMS voltage and has a tolerance. Add 10% to the line voltage (240*.1= 264Vac) to accommodate its tolerance and remember to design your circuit with the possibility of being supplied 10% less of 240V. Convert to peak-peak voltage (264x1.414= 373.3) because flash-overs pay little attention to time. Now, you could fret about aging components, temperature cycling, dust, moisture, bad luck and worse math then decide to add another 10% to the figure (373.3+10%= 410.6Vp-p) "just to be sure." That's cutting it close, but if you throw an MOV across the line (behind the fuse, of course) then maybe that'll be enough to redeem you from omitting that last 10% and keep your house from burning down. Maybe.

Concerning SMT vs. TH transients, the components are rated as they're rated. 400V applies whether you're reading a TH or SMT datasheet.

Joe Frazier
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    The creepage distance for 700V, (or even 350V) is longer than the entire length of the resistor (0805 part). The 400V rating is almost definitely a transient thing, and may require conformal coating to even manage that. – Connor Wolf Nov 24 '10 at 09:37
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    In the EU the specification for the mains supply is 230V (-6%, +10%), 50 Hz (±1%). ie a range of 216.2 - 253V rms – uɐɪ Nov 24 '10 at 12:05
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In the case of resistors that actually have mains across them, use multiple parts in series -resistors are cheap. As well as increasing creepage for safety, it improves reliability, as resistors with high voltage across them are prone to failure. The main difference between TH and SM is thermal mass, i.e. SM parts tend to have lower surge withstand capability.

mikeselectricstuff
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Bear in mind that even though the part is OK for 400V, you may need a slot in the PCB to ensure that there's sufficient clearance between the pads.

In general, axial devices beat SMT for transients simply because they have higher power handling capability.

Power supplies often use tons of surface mount components - cheap, made in Asia commodity power supplies often don't use any SM because they're cheap and commodity products.

(Power supplies that I've help design have hundreds, if not thousands of SM parts - not in the power train, but in control and monitoring.)

Adam Lawrence
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Direct connection of rail to rail

250VAC gives you amplitude of 354V (twice that for peak to peak). So at 100Kohms you are looking at a power dissipation of almost 1 watt peak and the resistor is only rated for 0.125W.

Now since it is actually AC your rms voltage will be about 250V, but even using that number you are still looking at an average power dissipation of .625W.

Other Cases

I can see cases where you would be using a resistor like this in a power supply circuit, but it wouldn't be having 250VAC across the pads, instead it would be some other smaller voltage.

Kellenjb
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    you've got some confusing things going on here between Peak, Average and RMS. RMS is what you need to use for calculating power. When he says 250VAC i would assume that is RMS and RMS != Average. 250Vrms is ~707V peak to peak. – Mark Nov 24 '10 at 04:33
  • The 100k resistor wouldn't be connected straight across the mains - I'd probably use a similar 2.2Mohm resistor. Either way, I need to know if it can handle going straight from live to neutral in some kind of fault condition, say a component shorted. – Thomas O Nov 24 '10 at 08:40
  • @Thomas O your question says you need a part for 250VAC. I think you need to change your question. – Kellenjb Nov 24 '10 at 15:01
  • Well, say the capacitor shorted, this would leave 250VAC across the resistor. I need to know if it can handle this. It doesn't matter too much if it burns out but it's important it doesn't arc. – Thomas O Nov 24 '10 at 15:08