how much current can a reservoir capacitor handle without exploding?

[paul_ma7]

i intend to use an 860mf capacitor to handle around 10 amp at 10v at 50 or 60 hertz, but i'm worried it might explode,
because of this article i read at the wikipedia i think i should use ceramic capacitors instead of regular electrolitics,
could anybody tell me a formula so that i know what is safe and what is not?

 

[Leftyretro]

It's hard to guide you without an actual circuit to look at, however most AC voltage applications require a non-polarized capacitor, not a electrolytic type.

 

[Ubergeek63]

measure or calculate the RMS current and compare it to the spec. Usually you can get away with typicals and normally it will only reduce the life. Of course extremes will cause it to over heat and blow out.

 

[dknguyen]

What are you trying to do?
You may be going about it the wrong way, as the capacitor you are asking for requires a ceramic type because it needs to be non-polarized, but is also way out of the capacitance range available with ceramics (meaning you need electrolytics which are polarized and will explode if you send a bipolar AC through it). UNless you get bipolar electrolytics which are strange, expensive, and rare.

Is the AC unipolar or bipolar?

Do you mean 860uF? or 860mF? The first is microfarads and the second is millifarads. Either way, both are very large for ceramic capacitors.

 

[kchriste]

i intend to use an 860mf capacitor to handle around 10 amp at 10v at 50 or 60 hertz

Are you talking about 10 amps of ripple current when using it as a smoothing capacitor in a power supply? If so, the frequency will typically be between 100-120Hz if you are using full wave rectification.

 

[paul_ma7]

frequency

Are you talking about 10 amps of ripple current when using it as a smoothing capacitor in a power supply? If so, the frequency will typically be between 100-120Hz if you are using full wave rectification.

yes a reservoir capacitor is a smoothing capacitor.
i think you are wrong about the 100-120hz, could you explain that to me?

 

[paul_ma7]

What are you trying to do?
You may be going about it the wrong way, as the capacitor you are asking for requires a ceramic type because it needs to be non-polarized, but is also way out of the capacitance range available with ceramics (meaning you need electrolytics which are polarized and will explode if you send a bipolar AC through it). UNless you get bipolar electrolytics which are strange, expensive, and rare.

Is the AC unipolar or bipolar?

Do you mean 860uF? or 860mF? The first is microfarads and the second is millifarads. Either way, both are very large for ceramic capacitors.

I meant 820uf.
I want to know about this for 2 different projects, one is a power inverter, and the other is to charge a car battery,
i think what both projects are using is called pulsated dc,
i know that capacitors can get hot an explode if they are to handle high currents or frequencies but how do i calculate what is a high current for a certain capacitor?

 

[whiz115]

hi...

if you need a smoothing capacitor for 10V and 10A then 860uF is nothing!
unless you mean 860mF which is very big capacitance.

a capacitor can charge up to it's rated voltage and keep a charge up to it's rated capacitance... 10A you can get even from the smallest capacitor but the duration of time it can give you 10A is very small.

we don't use on AC electrolytic capacitors unless they are non polar because as it is stated above it might blow up.

you can make a non polar capacitor by connecting two electrolytics back to back. (be careful how you connect them!!!)

 

[kchriste]

yes a reservoir capacitor is a smoothing capacitor.
i think you are wrong about the 100-120hz, could you explain that to me?

After fullwave rectification, the 50-60Hz AC becomes 100-120Hz pulsating DC:
**broken link removed**
Read about the bit about the "smoothing capacitor" near the bottom of the page. That is why you'll see the ripple current of most electrolytic capacitors spec'd at 100-120Hz:
https://www.electro-tech-online.com/custompdfs/2008/11/CGR.pdf
If you look at the ratings charts for the various capacitors you'll see they are all spec'd at 120Hz which is the most common frequency encountered in a standard capacitor input power supply.

 

[Menticol]

Now when we are talking about capacitors, waht do you think about the technique I have read recently:

A way to turn polarized capacitors into non polarized ones, is connecting two of them in series, with the - and - leads together

Absurd, or reality?

_________________________

Edit: When I was going to close the window, I have read the whiz115 post, where he already answers my question. Thank you!

 

[JimRodgers]

Capacitors can be complicated!

Ooops, I made a duplicate.

 

[JimRodgers]

Capacitors can be complicated!

Hey there Paul,

Be careful taking advice from forums. I am frequently amazed at what folks sometimes say. I noticed that "kchriste" has his act together, so there's some good advice for you.

The link provided by kchriste for the Full Wave Rectifier is very good. The link provided to the electrolytic capacitor datasheet also is quite excellent. I'm not sure you will get all the answers you need from just these links, so keep on asking!

Bt the way, electronic and electrical units of measure used to be abbreviated differently than they are today. Once upon a time, "microfarads" were abbreviated "mfd" and sometimes "mf" -- and picofarads were abbreviated "mmfd" or "mmf." Nowadays, we use "µF" or "uF" to indicate the same thing. Occasionally, one sees "nF" which means nanofarad; 1.nF is equal to 0.001uF or 1000pF. Personally, I wish everyone would stick to uF and pF! If you see "mf" or "mF," then you cannot be 100% certain; however, the likely intended meaning was microfarad ("µF"). The "F" is capitalized because it is derived from a person name (Faraday). The "u" is really a Greek "µ" but no one cares.

I've done advanced research and development in component engineering and the reliability of electronic systems. So let me help you with a couple of points.

First of all, you DO want an electrolytic capacitor for power supply applications like these (called a filter capacitor or smoothing capacitor, etc.). If you are following the good published plans for something like this, the parts list should be clear about the type of capacitor you need.

Large-valued capacitors like these generally have a spec for ESR (equivalent series resistance) in ohms. When a current flows though a capacitor the ESR value helps to predict the internal heating that results. Too many amps through too many ohms means too many watts of heat is being generated within the capacitor. And, yep, they can explode because of this. However, it is not a beginner's task to predict how much heat a device can take.

The "ripple current" in power supply applications refers to combination of the current "rushing" into the capacitor (at one point in time) from the rectifier plus the current draining out of the capacitor into the load (at another point in time). This can be confusing to talk about. Lets just say this: Your "smoothing" capacitor needs to be large enough (in uF) so the ripple VOLTAGE meets your needs.

If you look at the voltage on the capacitor with no load attached yet, you should see approximately NO ripple. (This requires an AC Voltmeter or an Oscilloscope.) But when you attach the load, the ripple appears. The load "keeps trying" to drain all of the charge out of the capacitor, thereby causing the voltage on the capacitor to drop. Some number of milliseconds later, the voltage has dropped far enough so that the rectifier will "kick in" and start raising the voltage back up again. During this time, the rectifier supplies current both to the load AND to the capacitor. This is only one of the reasons why the current rating of the diode must be greater than the load current.

Choosing larger and larger values for capacitance will allow more and more current to flow into the capacitor -- and more current to flow into the load -- without allowing the voltage to droop down as far as a "smaller" capacitor.

There's a lot more to this, actually. Many factors like the transformer must be considered. I saw one product where the engineer designed-in a giant capacitor to keep the ripple down, but something exploded everytime he turned it on (either the cap or a diode)! He was using a super high performance cap called a Tantalum Electrolytic and other hi perf components. Sure, if the product didn't explode right when he turned it on, it would last forever because there was virtually no ripple even with the load attached. But, there is a special situation when the power first turns-on: the voltage in the capacitor has to ramp-up from ZERO. This causes a one-time in-rush of current MUCH larger than the ripple current -- enough to blow something up. In that product, we switched the large tantalum for a small one (why we needed this would be off topic here), but then also added a large-valued aluminum electrolytic cap to do most of the smoothing work. Normally, a BFC (big fat capacitor) is all that is needed, and rarely does one need to use anything but the usual aluminum electrolytic for this. Sometimes, two filter caps and a resistor are used in a Pi Network for really good smoothing -- that's a good way to make things work well and not cost too much.

So, how does one design one of these supplies? Most engineers worry about this less than you would think. Generally, if the RIPPLE VOLTAGE IS LOW ENOUGH, you are filtering enough. In an audio application, for example, you can hear a buzz if the ripple voltage is too high. Larger caps (more uF) may bring down the ripple voltage, and the ripple current with it, but they also INCREASE the startup in-rush current.

My advice is this: when you build projects from plans, stick to the plans -- unless you know both the math and the component engineering issues involved. Also, if you can choose between this kind of DC supply circuit (sometimes called a linear DC supply) and a so-called "switching supply," you should use the switching supply instead. They are MUCH, MUCH more difficult to design, and sometimes they can be unreliable when not designed right. But, they are Greener: they are way more efficient and emit much less heat. They, too, require filter caps, and filter chokes as well.

When you hear about using ceramics instead of electrolytics, the discussion usually involves COUPLING capacitors. These caps allow signals (which are AC) to pass though them without letting the DC voltage go on the other side as well. If the cap is too small (lower uF), then some lower frequencies might not pass though because they are too "close" to DC (0Hz). On the other hand, bigger caps (more uF) may have undesirable properties that distort the signal. In most circuits these coupling caps are so small (0.001uF to 0.1uF) that ceramic or polystyrene or polypropylene [or whatever] are used. They don't even make electrolytics that small. Most of these [non-electrolytic] caps have few issues with "undesirable properties." In certain applications, however, an idealistic cap is hard to find. This often happens, for example, in the loudspeaker circuit of audio applications; especially in the "crossover networks." This is one common place where "non-polar electrolytic" capacitors ("NP") are used. Making an NP by putting two polar electrolytics back-to-back is controversial, but it is common advice. I say, don't do it. Insofar as power supply caps are concerned, you should NEVER use NPs or put polar electrolytics back-to-back. A backwards-installed polar electrolytic capacitor WILL EXPLODE EVERYTIME!

Not only do we engineers sometimes have disagreements among ourselves, but there is the whole world of Audiophiles out there, and their beliefs frequently defy explanation in engineering terms. If you really want to launch a war on the web, then unlease the Audiophile dogs. I am both an engineer and an Audiophile, so I have had to defend both sides. None of this should come up, however, when we talk about DC supply filter caps.

Good Luck!

 

[JimRodgers]

Some more remarks about design and filter cap choice...

I noticed above that whiz115 mentioned "if you need a smoothing capacitor for 10V and 10A then 860uF is nothing!" He is absolutely right about that! Of course, it depends on how much ripple voltage is okay for you.

When calculating much load current versus how much capacitance (uF), a few formulas come in handy. The voltage on a cap (Vc) equals the charge in the cap (Qc) divided by the capacitance (C). Lets look at a 1000uF cap with 10VDC across it and do some "thumbnail" math. The charge stored in the cap is Qc=Vc*C which is 0.010 coulombs. Now, if we draw a constant 10A for 10ms, that's 0.1 coulombs of charge right there, which is ten times more than is available in the cap! If our goal is, say, 10mV droop in 10ms, then we need a capacitor that will droop no more than 10mV when it gives up 0.1 coulombs. This would be C=Q/V or 1F. That's ONE FRIGGING FARAD my friend, or 1,000,000uF (a million microfarads).

Who needs this? Frankly, I'm not sure; but you can get them. I seem to recall Best Buy and audio stores sell these to folks with killer car stereos. Why? Because voltage is at a premium in car electrical systems, and loudspeakers are usually wired for low impedances (2 ohms to 4 ohms). Consequently, the currents in the amp outputs are in the 1A to 10A range. Thus, so is the requirement for the battery to supply these amps. Cars only recently have alternators that can supply more than 40A or 60A, maybe 80A tops. The advertised power rating of some car stereos are utterly bogus, but they may, in fact, need help with the power required to drive them.

I don't want to go down this path very far, but let me console you with this observation: If you are pulling 10A to go into one pair of loudspeakers, you ain't gonna hear 10mV of ripple! I've never seen a good solid amp that really needed more than 100,000uF in the filter caps. 10, 20, and 50,000uF are more common.

When one has a choice, high current should be avoided. Using 8 ohms to 16 ohms for loudspeaker loads makes more sense, even if the voltages are higher than 13.8VDC (car regulate to this). The amps should use switching supplies anyway, and these can get you to any voltage to drive higher impedance loads. At the end of the day, cars should not be expected to source this without a special generator or alternator and the wiring and fuses this requires.

Was this more helpful?

 

[paul_ma7]

After fullwave rectification, the 50-60Hz AC becomes 100-120Hz pulsating DC:
**broken link removed**
Read about the bit about the "smoothing capacitor" near the bottom of the page. That is why you'll see the ripple current of most electrolytic capacitors spec'd at 100-120Hz:
https://www.electro-tech-online.com/custompdfs/2008/11/CGR-1.pdf
If you look at the ratings charts for the various capacitors you'll see they are all spec'd at 120Hz which is the most common frequency encountered in a standard capacitor input power supply.

i just read the article, and it says what you say,
though it makes more sense to me that the half wave rectifier would output a frequency of 25 and the full wave a frequency of 50.

 

[paul_ma7]

hi...

if you need a smoothing capacitor for 10V and 10A then 860uF is nothing!
unless you mean 860mF which is very big capacitance.

a capacitor can charge up to it's rated voltage and keep a charge up to it's rated capacitance... 10A you can get even from the smallest capacitor but the duration of time it can give you 10A is very small.

we don't use on AC electrolytic capacitors unless they are non polar because as it is stated above it might blow up.

you can make a non polar capacitor by connecting two electrolytics back to back. (be careful how you connect them!!!)

The 820uf is for the inverter project, for the battery i would have to use like three 10 000uf capacitors

 

[saturn1bguy]

...though it makes more sense to me that the half wave rectifier would output a frequency of 25 and the full wave a frequency of 50.

A single, full cycle has a positive AND a negative half wave each. If one rectifies said cycle with a full-wave rectifier, one gets two half cycles, and the count is thus doubled (Fifty positive half cycles, 50 negative half cycles become positive, leaving 100 positive half cycles:the count is doubled).

A half-wave rectifier discards one half of the waveform, and the count thus remains the same. (Fifty positive half cycles, 50 discarded negative half cycles, leaving 50 positive half cycles: the freq remains the same.)

 

[JimRodgers]

the ripple current looks nothing like the ripple voltage

About the frequency of the ripple voltage and current.

There's not much you can do with 50 or 60 or 100 or 120Hz. This is not very useful or important except for knowing if a half wave rectifier is used -- in which case you know the caps have to be bigger (so why not always use full wave rectifiers?).

If you want to be really accurate and you intend to calculate something, then you must consider that fact that there are MANY, MANY harmonics in there, too; not just 120Hz or whatever.

If one calculates a Fourier Transform of the time data (where you see the rectified halves of sine waves), you will see there are tons of harmonics of 120Hz (240, 360, 480, etc.). If the waveform were perfectly symmetrical (like a square wave, for example), then there would be only ODD harmonics (120, 360, 600, 840, etc.). Just a curiousity. The rectified sine wave, however, is VERY MUCH NOT symmetrical, so there will be plenty of 240, 480, etc., as well.

Since the current in the cap is

i(t) = C * (dv/dt)

and the voltage is full of harmonics, the ripple current should be much greater at higher frequencies. If the the fourth harmonic of the voltage were

vfourth(t) = Vf * sin( 2Π*480Hz*t )

so

ifourth(t) = c * Vf * 2Π * 480 cos(...)

See, the frequency comes out front as a magnitude when you do the derivative like that.

...but who cares, really...

Computers normally are used.

When the filter cap in a linear power supply is too small in an audio power amp, then the worst case is, of course, when you peg the volume. When you "run the output voltage from rail-to-rail," the ripple voltage can be huge. Plus that ripple is right on the voltage rails, so you will hear the huge ripple (buzz) coming right out of the loudspeakers if you crank it.

Perhaps the main observation is that the ripple current looks nothing like the ripple voltage once you sum together all the current harmonics. The current signal will have MORE of the higher harmonics than the voltage did.

 

[tiny747]

I hope this is a good thread for my problem. I am trying to build a power supply. I tapped off the secondary windings on my cars alternator. So now I have 3 wires coming out which supply 3 phase power to a voltage doubler that I made. The doubler uses capacitors to double the voltage and it also has a 3 phase bridge rectifier on it. The capacitors serve 2 porpuses. 1 - I use them to help double the voltage and 2- I use them to limit the current. I don't want any more that 10amps flowing through my project. the amount of current that is allowed to flow changes with the rpm or frequency of my alternator. (This is ok. The higher the rpm's the more current I want to flow anyway.) The problem I have is that I need a 5,000 uf cap. These are extremely expensive if they are ceramic. But I can buy a 10,000uf cap on digikey for $5.00 and put them back to back to get 5,000 uf. But as Jim Rodgers pointed out this can be dangerous. My question is this. How can I put them (2 caps back to back) with out using more diodes? Is it possible for me to use a small tantalum or ceramic cap and then use the big electrolytic caps as someone mentioned earlier? Or to use a resister in series with the electrolytic caps? I don't know if I could use some more diodes with my electrolytic so that the current only goes one direction. It might interfere with my voltage doubler. Here is my voltage doubler circuit. **broken link removed** I have one of those doublers on each phase coming out of my alternator. All of your help would be greatly appreciated.

 

[Sceadwian]

Post in your own thread tiny. You'll be able to create new threads after you've responded to other's therads. Do not post questions in someone elses threads.