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I am fixing my mom's flat hair iron, I already figured out that problem is in one capacitor which should be 470nF, but currently has only 200nF capacity. It is rated for 400V, but I do not know whether it is DC or AC.

I reverse engineered schematic. That capacitor is marked as C1. I understand how this circuit works, and why it doesn't work with lower capacity. enter image description here

Photo of capacitor: enter image description here

I am confused. I googled that sort of capacitor and somewhere it is described as DC capacitor. But it is used as AC in this case, right?

So, is this capacitor DC or AC? Or what type of capacitor it is? I have to find proper replacement.

ANSWER:
According to comments and answers, this capacitor is polyester (or polypropylene), with rating 400V DC.

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    \$\begingroup\$ A capacitor is a capacitor, it doesn't care particularly about what kinds of signals go through it, as long as you stay within its SOA. \$\endgroup\$ – PlasmaHH Feb 1 '17 at 10:57
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    \$\begingroup\$ PlasmaHH, there is some reason why some caps are rated 400VDC and some 400VAC, or 250VDC/250VAC. Isn't it? \$\endgroup\$ – Chupacabras Feb 1 '17 at 11:00
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    \$\begingroup\$ go to digikey or whatever and have a look at the voltage rating that you can search for there. Doesn't say a bit about DC or AC. The only thing that might be relevant is whether they are polarized or not. \$\endgroup\$ – PlasmaHH Feb 1 '17 at 11:08
  • \$\begingroup\$ I searched digikey before, and I am not able to find 470uF 400V AC (well, there is one, only one!, without picture). But there is plenty of 470uF 400V DC. \$\endgroup\$ – Chupacabras Feb 1 '17 at 11:51
  • \$\begingroup\$ Funny, your digikey must be different than mine, as I have no AC/DC filter on capacitors there. \$\endgroup\$ – PlasmaHH Feb 1 '17 at 11:55
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So, is this capacitor DC or AC?

It's AC because it connects to the AC power mains directly (unlike an electrolytic which is polarized and should only be used on a DC system). It looks like one of these: -

enter image description here

Available from this not very reputable source

Or what type of capacitor it is?

It's called a "plastic film capacitor" (possibly polypropylene) and if you google search "474k capacitor" you'll get lots of similar hits especially if you search "images".

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  • \$\begingroup\$ So, you say, that capacitor rating is not important? And it does not matter whether rating says 400VDC or 400VAC? \$\endgroup\$ – Chupacabras Feb 1 '17 at 11:52
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    \$\begingroup\$ I never said that and you never asked that! \$\endgroup\$ – Andy aka Feb 1 '17 at 11:55
  • \$\begingroup\$ I am not able to find such capacitor rated for 400V AC. Only ones rated 400V DC, that look like my one. \$\endgroup\$ – Chupacabras Feb 1 '17 at 12:27
  • \$\begingroup\$ @Chupacabras Use Digikey. Several to choose from! Also, you need at least 400 V, but you can go as high as you want in voltage rating. The capacitance should be kept the same as before. Please read up on what the term "polarized" mean. \$\endgroup\$ – winny Feb 1 '17 at 12:29
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    \$\begingroup\$ @Chupacabras if your supply is 230 V AC then the peak voltage can be as high as 325 volts. Add (maybe) 20% for powergen issues and you get to 390 volts peak. A 400 V DC capacitor is OK but 450 V would be better and maybe, that is possibly why the 400 V DC capacitor failed. \$\endgroup\$ – Andy aka Feb 1 '17 at 12:39
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Your capacitor has plastic insulation layer. It has no defined polarity like the electrolytic capacitors have. In this application only an unpolarized and high voltage capacitor is ok because it must stand the mains AC voltage. 400 V is quite a low safety margin selection for 230VAC except if it's stated in capacitor's datasheet that 400 V means "continuously stands 400VAC".

It may be somewhere be called "DC capacitor" because it has a remarkable job in this DC power supply.

Maybe good to know:

  • Thislike power supplies are bad because for 5V output they drain at least 45 times as much AC current as a proper power supply with a transformer or a SMPS. The vast majority of that current only visits a moment as capacitive reactive power and during the same mains AC cycle it returns to the power network. The only final result is loss during the trip and maybe it helps to grow your reactive power sanction in your electric bill.
  • Thislike power supplies and their loads must be safely housed and stay unreachable behind the isulator because there is a calvanic contact between the load and the mains voltage.

Addendum. No voltage spec for DC quarantees the safety for AC due the continuous AC current in the insulator of the capacitor. A spec for AC is a must if it must stand AC. The insulator gradually detoriates in AC field just like metals detoriate under forth and back vibrating tension.

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  • \$\begingroup\$ Sure I know that this kind of power supply is far from ideal. I did not design it, I just fix it ;) I have do datasheet for that cap. I wouldn't ask otherwise. \$\endgroup\$ – Chupacabras Feb 1 '17 at 12:29
  • \$\begingroup\$ So, in this case (230V mains), I could use whether cap rated for 250V AC, or 400V DC? (230V rms -> 325V peak) Margin looks OK to me. \$\endgroup\$ – Chupacabras Feb 1 '17 at 12:33
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    \$\begingroup\$ @chupacapras continuous230VAC makes more stress to the insulator than permanent 325V constant DC. It must be specified for AC to be safe. AC means continuous forth and back current in the insulator. Search for electric polarization current. \$\endgroup\$ – user287001 Feb 1 '17 at 13:25
  • \$\begingroup\$ You are right, proper capacitor for this application should be rated for AC (like 250V AC or higher). Unfortunately there is so little space, just enough for this 400V DC cap. Capacitors rated for AC are bigger and won't fit inside the product I am fixing. So, I will have to use cap with 400V DC rating :( \$\endgroup\$ – Chupacabras Feb 1 '17 at 15:22

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