Do very high frequencies through a bypass capacitor cause a short, and therefore (near) infinite current?

Question

From my understanding, capacitors pass high frequency and act as shorts for very high frequencies based upon 1/(2pi*f*c). I see circuits diagrams with these bypass or decoupling capacitors going to ground. So, my question is, wouldn t a signal with a very high frequency noise (enough to overcome a small capacitance value in the denominator and cause the impedance to become virtually zero) basically be shorted to ground, causing infinite current? I hear people say that "the high frequency will be shorted to ground," but I thought that was bad. Or isn t there ever a circumstance where there will be a frequency high enough?
On another note, is it the first resistor in a passive low pass filter that prevents this from happening through the capacitor in the filter?

Thanks,
Nick

Answer 1

A bypass capacitor's intended function is to provide a short circuit across a circuit resistance to prevent a Voltage from being developed across that resistance at the unwanted higher frequencies. The Voltage source for those undesired high frequencies usually has a very high internal impedance at the higher frequencies and it is that high internal impedance that limits the current through the bypass capacitor rather than the reactance of the capacitor at higher frequencies.

Note: The maximum short circuit current that can be drawn from any Voltage source at any particular frequency is determined by the internal impedance of that Voltage source at that particular frequency.

Answer 2

https://en.wikipedia.org/wiki/Decoupling_capacitor



Note that for any size capacitor, there is a resonant frequency, above which it is inductive. This is why designers use a variety of decoupling capacitors.



Also, any voltage source has a source impedance, which will limit the current if the capacitor impedance does not.

Answer 3

NO ! Xc NEVER BECOME ZERO EVEN IT CLOSES TO ZERO.

BESIDES, HIGH FRQUENCY POWER SOURCE ITSELF HAS A VERY LIMITED POWER, THEREFORE, INFINITY CURRENT NEVER HAPPEN EVEN CLOSE TO ZERO REACTANCE IN THE PATH.

Answer 4

The current would be infinite if

A. The capacitor had zero resistance.

B. The high frequency source can deliver an infinite current.

B is the more unlikely, because the unwanted frequency comes from a very low power source. Also sources have a resistance, which limits the current they can deliver. An infinite current is a theoretical concept rather than a reality.

Answer 5

Capacitors do not pass DC, current only flows in a capacitor when a change in voltage occurs. Given that

1/Xc = 1/2Pi F C

You can see that as the supply frequency rises Xc gets smaller. Xc is the capacitive reactance and is measured in Ohms, and just like resistance, when it is lower, higher currents will flow. This calculation works only for sinusoidal supplies, other wave shapes and square fast rising edges will cause huge currents to flow, but only for milliseconds.

Answer 6

Generally the frequencies being shorted by a bypass cap have very low voltage and the circuit impedances are moderate so the current is low in the mA or uA range.

You must keep in mind that although the capacitive reactance is 'zero' the inductive reactance of the conductors is large and both are frequency dependent.

Answer 7

VHF? Very High Frequencies. Are you indicating a band in radio or some undefined concept? Everything is about the resultant impedance which is pretty much the driving impedance of the source into the load which you are referring to as the ground connected cap. If the frequency is high connection of a cap to ground may not ground at all do to the impedance of the ground. It is the impedance of each section of the entire loop that matters where that loop current flows.

Answer 8

If you short a battery out with zero resistance do you get infinite current (and thus infinite power)? Of course not because the battery (source) has some internal resistance. And the source of these high frequencies would also have resistance which would limit current flow.

Answer 9

You have to look at the circuit as a whole. Generally, yes, a capacitor shunting to ground will pull the higher frequency component of the input signal to ground. But it won't be infinite current: the circuit as a whole still has impedance, even at the higher frequencies.



If you're talking audio signals, there's not a lot of energy in the higher frequencies.