I am trying to replicate this circuit:


Taken from AN679 [PDF link here]

I have a question regarding C17 - 0.47 uF. This is the capacitor which drops most of the voltage in the power supply part.

In application note, they have mentioned the voltage to be 630 V. I want to know the minimum safe voltage for the capacitor that I can choose for 220 VAC line (which can go up to 250 V in some cases). The objective is to reduce the size of the components and I feel that I might be able to find smaller capacitors if I go for a lower voltage.

Also, they have taken phase as the reference (GND?) while making the circuit. I know that we are dealing with ac circuit and this doesn't matter but is it a preferred way to do things? Should I invert the connections (phase and neutral entering at points K3 and K4) or leave it like that?

  • 2
    \$\begingroup\$ 250 V AC is 353 V peak so 400 V DC peak voltage might just be enough. Do also consider the class (and thus failure mode) for this capacitor, see: powerblog.vicorpower.com/2013/06/what-are-y-capacitors and make R21 a fusable resistor so that if C17 fails and shorts, R21 blows and prevents a fire. \$\endgroup\$ Jan 10, 2017 at 13:00
  • \$\begingroup\$ @fake - Thanks. On digikey, there is an option to select ac voltage for capacitors. Can I go for 300 VAC? \$\endgroup\$ Jan 10, 2017 at 13:02
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    \$\begingroup\$ I do not think connecting GND to either phase or neutral makes much of a difference, I consider the whole circuit live so no touchy-touchy. I would not expect neutral to be at ground potential anyway. Since the shunt resistor needs to be in the live line (makes sense, if it breaks all devices will be on neutral) I think the circuit as it is makes sense (live and neutral wise). \$\endgroup\$ Jan 10, 2017 at 13:03
  • \$\begingroup\$ Yes 300 VAC should be OK yes, but do check the datasheet that that is actually 300 VAC and that the maximum DC voltage is indeed 400 V DC. \$\endgroup\$ Jan 10, 2017 at 13:05
  • \$\begingroup\$ Thanks a lot @fake. You have answered all of my questions. Have a great day ahead :) \$\endgroup\$ Jan 10, 2017 at 13:17

1 Answer 1


You not only need a voltage rating, you need a type of capacitor, one that is rated to be connected directly to mains.

Choose a 250v X2 rated capacitor.

Now, although the voltage on this part doesn't look like enough for 240v connection, it is rated to be connected to UK mains, which can suffer frequent transients of 1500v, and infrequently much more. In addition, it is 'self healing', in that if a large over voltage causes a breakdown of the dielectric, the metalisation is segmented so that just one small portion will short circuit, and disconnect itself from the rest of the capacitor.

To get the equivalent performance and safety in a non mains rated capacitor, you would need one rated for several kV.

  • \$\begingroup\$ And that X2 means: Can withstand 2.5KV impulse voltage indeed a good (sane) choice. \$\endgroup\$ Jan 10, 2017 at 13:22

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