AC caps and DC caps? Are they interchangeable?

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canidoit

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I didn't know there was such things as AC and DC caps. When I was placing orders, the order never mentions of whether these caps are AC or DC. Is it possible to use DC caps in the La2a Drips v3 board? I presume that the Drips Version 3 La2a board is running AC for all the components. I just bought myself these NOS vintage caps and I have been told they are DC.

Can someone please clarify if you can use DC caps for C1, C2, C3, and the other sections that use 0.1 uF, 0.2uF caps on the La2a Version 3 Drips board. Also what sort of damage can occur using DC caps when you should be using AC?
 
canidoit said:
I didn't know there was such things as AC and DC caps. When I was placing orders, the order never mentions of whether these caps are AC or DC. Is it possible to use DC caps in the La2a Drips v3 board? I presume that the Drips Version 3 La2a board is running AC for all the components. I just bought myself these NOS vintage caps and I have been told they are DC.

Can someone please clarify if you can use DC caps for C1, C2, C3, and the other sections that use 0.1 uF, 0.2uF caps on the La2a Version 3 Drips board. Also what sort of damage can occur using DC caps when you should be using AC?

I would worry about "vintage" capacitors. Their values might be off significantly from what the markings indicate,  they could have dried up, etc etc. I don't really see the point.

As for "AC" vs "DC" caps, about all I can think of is that "AC" caps are non- (or bi-) polar and "DC" caps are standard polarized electrolytics.

The 0.1uF caps are probably ceramics and as such are not polarized.

-a
 
I rang up some random tech guy who was kind enough to talk to me on the phone unlike some tech guys out there.

AC caps - mainly used for power supplies. Non polarized. Usually used in the primary of power transformers.

DC caps - polarized. Usually found in amps circuitry and is usually used in the secondary of power transformers where it has been rectified or stepped down. You can also get non polarized capacitors.

He sais you can't use an AC cap where a DC cap goes, but I forgot to find out whether you can use a non-polarised DC cap to replace a AC cap in a circuit or use an AC cap to replace a non-polarized DC cap in a circuit. Does anyone know the answer to this question?

Can someone please confirm that if you hook up 2 DC caps with + to + in series and use the negative leads to connect to the circuit board, you will get a non polarized capacitor. I forgot to ask whether that become a non polarized DC capacitor. Can this be used to replace an AC capacitor?

Also all these caps do not have it written whether they are AC or DC? How come!
 
Where do you find "AC caps"?

All non-electrolytic caps (and non-polar electros) will handle AC.

The difference is: a little AC or big AC?

A motor-start capacitor has to pass hundreds of watts of power while shifting its phase to twist an AC motor into starting. A large array of poly-film caps can do this, at high cost. The motor-start cap is designed for the starting voltage, starting current, and start-up duration, to be just good enough at lowest cost.

Across-the-line caps suppress radio interference. If they short-out, the cord could burn, or the chassis might be connected to the live wire. Electric Safety Agencies issue specific rules and tests for such applications. These caps should cost more than a non-safety-rated cap. However because they are used in EVERYthing, sometimes you can get them cheaper.

AC caps will be rated in AC Volts. There is not any strict translation to DC Volts, or vise versa. A 600V DC Orange Drop can handle 550V DC plus 20V audio AC forever, but 400V pure AC may kill it. A 240V motor-start cap can probably take way more than 240V DC, but how much more? And for how long? (Motor-start duty is just a few seconds, rarely more than a few times per hour, and often not warranted more than a few years.)

Frequency matters. Series resistance which causes slight heating at 60Hz would cause extreme heating for large 10KHz AC curent. Historically, this meant that motor-start caps were all film caps (or nasty wet electrolytic); however recent improvements in dry nonpolar electros make this practical (good-enough and cheaper) in motor-start duty.

In general, caps rated for heavy AC will be more expensive.

In most audio applications, "all" caps will be DC rated. Our DC voltages are much larger than our audio voltages. This includes the large bulk capacitor in the power supply (anyway, a polar electro is perfect here and much cheaper than any alternative).

The main exception would be capacitors across your 120V/230V wall-power line and switch, used to supress radio interference and contact clicks. We used to use plain 400V wax-caps on 120v lines, but these fail in a few years or decades, which is why line-rated caps exist.
 
Yes, two polar caps + to + or - to - will make a bipolar cap.

DC caps - polarized. Usually found in amps circuitry and is usually used in the secondary of power transformers where it has been rectified or stepped down.

Not only stepped down, it HAS to be rectified.  If it isn't then it will cause the cap to have to charge/discharge a lot of actual charge.  This will cause it to heat up significantly and either dry up or pop in this application.

AC caps - mainly used for power supplies. Non polarized. Usually used in the primary of power transformers.

AC (bipolar) caps are used to do a lot of things but the place that they are used most is usually in IEC inlet filters inside of computers.  AC motor start caps are "AC" too.

Also all these caps do not have it written whether they are AC or DC? How come!

Because technically they both can do AC or DC depending on what you are using them for.  You CAN use either in either circuit but I wouldn't waste a more expensive bipolar cap on DC decoupling duties and it won't work so hot, but it will work somewhat.

Look at it this way:

Phantom blocking capacitor.  It has audio going through it and audio is an AC signal.  they can also be biased with phantom voltage which is DC.  In this situation it is exposed to BOTH AC and DC at the same time!  

Yes, ceramics and plastic caps are not polarized.  Some of the old foil caps have a directional marking but this might only be to show the user which lead is attached to the outer portion of the foil.

EDIT:

As I wrote this PRR beat me to it..
 
To shed more mud:

> AC (bipolar) caps

I would say that:

ALL caps are "bipolar"--

EXCEPT electrolytics which do not block reverse voltage.

Normal electros are caps for one polarity, poor shorts for the other polarity. They are almost useless for many classic cap applications, but work fine in DC filters and some types of audio coupling and timing applications. And they are SO much smaller/cheaper than film caps, that we use them a lot.

So much smaller/cheaper, that it is worth building two electos back-to-back. This type will block reverse voltage, one way or the other. And while "all caps are bipolar except...", this is the only type where we routinely say "bipolar". It is the exception to the exception, and has to say so.

So we have good caps (air, vacuum, glass, ceramic, paper, oil, poly) which are always bipolar, can always stand significant AC (though maybe not big AC). And we have electrolytics, which are crummy caps but great in many common uses, AND bipolar electrolytics which solve one of the several limitations of common electrolytics.
 

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