# Thread: Do capacitors always block DC ?

1. ## Do capacitors always block DC ?

One thing that has bugged me for some time...

When you calculate a really low corner frequency, like using a really big coupling cap, is it possible to go so big that you let DC through ?

I would first think not based on the function of a cap, but if the capacitor where huge, lets say 1000uf, and you had a resistance to ground of say 38,000hz, your corner frequency theoretically would be a super low .00419hz, so according to the maths result, it's getting close to zero, but I realize it will never actually reach zero, regardless of how big the cap or resistor is. What theory of capacitors allows them to block DC no matter what, (other than exceeding rated voltage or from failure) ?

Thanks for the help in understanding !

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2. Originally Posted by HaroldBrooks
One thing that has bugged me for some time...

When you calculate a really low corner frequency, like using a really big coupling cap, is it possible to go so big that you let DC through ?

I would first think not based on the function of a cap, but if the capacitor where huge, lets say 1000uf, and you had a resistance to ground of say 38,000hz, your corner frequency theoretically would be a super low .00419hz, so according to the maths result, it's getting close to zero, but I realize it will never actually reach zero, regardless of how big the cap or resistor is. What theory of capacitors allows them to block DC no matter what, (other than exceeding rated voltage or from failure) ?

Thanks for the help in understanding !
Well if it doesn't block DC then by definition it is not a capacitor, right?

nosaj

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3. It's all relative. A really long time constant might seem like forever, but it's not. Any decent modeling program will show you that the capacitor is still blocking DC or holding the DC up. Like the multi Farad caps that keep logic circuits going when the power is off, only for so long. Any long time constant requires a big cap that doesn't leak and a big resistor. Time constants over a couple of minutes become troublesome to implement and keep working.

For the most part, we are dealing with audio frequencies. Sometimes we forget about low frequencies like the spaces between notes in slow blues. Some circuits shift bias when you hit them with a big overdrive tone. You think you can just install a really big cap to keep the bias stable, but you can't. It will always drift when you hit the big note and have to recover afterwards.

A friend of mine came up with an interesting circuit to tweek the bias to an op amp. Instead of a cap to hold the offset Voltage, there was a counter connected to a DAC (Digital to Analog Converter). The counter would run and then freeze when the offset was corrected. The DAC was then putting out the right Voltage and would hold it until the power was turned off or the circuit determined that the counter needed to run again. Effectively a really long time constant.

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4. .00419hz is still AC. It might take time, but if you started at say +5v, it would sit there a long time looking like more or less +5v, but come back in an hour and it looks diffrent.

WHen I was a kid I read that a triangle's three angles always add up to 180 degrees. Of course then I didn't believe anything, so I set out to draw a triangle that defied that rule. I very quickly got to some super squeezed out triangle with maybe 179.8 degree on one corner. But of course the other two angles were always so closed up they were just a fraction of a degree.

AC is AC even if it doesn't seem to act like it in a practical sense.

What theory of capacitors allows them to block DC no matter what
Mainly the fact there is no direct connection between the poles.

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5. What theory of capacitors allows them to block DC no matter what, (other than exceeding rated voltage or from failure) ?
All types of capacitors have an isolating material (dielectric) between the metal parts (film, foil, plates) that prevents direct electron flow from one side to the other.
Capacitors transfer signals (AC current or voltage) by repetitive charging and discharging.
The basic formula describing caps is Q = C*V, with Q = stored charge in the cap, V = voltage across the cap. This formula states that as long the voltage is constant (DCV), charge doesn't change and so no current flows.

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6. Though it's not the question being asked, all aluminum electrolytic capacitors leak a tiny amount of DC current. That's why old capacitor checkers that test caps for leakage at full rated voltage have different settings for film/mica and electrolytic caps.

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7. Originally Posted by Helmholtz
All types of capacitors have an isolating material (dielectric) between the metal parts (film, foil, plates) that prevents direct electron flow from one side to the other.
Capacitors transfer signals (AC current or voltage) by repetitive charging and discharging.
The basic formula describing caps is Q = C*V, with Q = stored charge in the cap, V = voltage across the cap. This formula states that as long the voltage is constant (DCV), charge doesn't change and so no current flows.
Thanks, I think with all the responses I understand that DC cannot break through the gap in the cap, it has to be AC if the cap is functioning correctly, and if the voltage is not too high, and of course the frequency of AC that is passed is filtered based on capacitance and resistance.

Another question though : Are caps limited to the current they can carry ? I always hear, and adhear to voltage ratings, but what about the amount of current ? Will a small .01uf coupling cap carry a full amp of current if required to ? Isn't currrent the amount of power in the form of electrons ? Not sure how or if current is a limit for a cap. It would seem there might be a limit based on physical size or structure. Thanks for the understanding so far !

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8. Caps are two parallel conductive surfaces, usually foil or something similar. Those are connected to the outside world via either wire leads or mabe terminals of some sort. Too much current through a cap and you can burn the little connections away. The rolled up foils in a can cap say have tiny little tinsel wires up to the terminals, those can burn open like fuses. That is why, as exciting as it may be, charging up a big cap and then shorting it with a screwdriver to marvel at the big spark and chunk out of your screwdriver blade, is a bad idea, as it just might burn out that inner wire.

Big filter caps often have ripple current ratings in their specs.

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9. Not sure how or if current is a limit for a cap.

Isn't currrent the amount of power in the form of electrons ?
No, current is not power. But current produces power across resistance. And that power produces heat which may cause damage. As termination and foil resistance is typically very low, foil, mica and creramic cap can handle high currents at least at audio frequencies.

HF currents produce dielectric losses (heating the dielectric) reducing current handling capabilities.

In switching applications (SMPS) often very high peak currents (several hundreds of amps) have to be handled and such caps need to be chosen for sufficient dV/dt ratings. The dV/dt limit is specified in the datasheet of suitable caps and corresponds to max. peak current per capacitance.

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