One of the most common question asked by electrical engineering students again and again that’s why do capacitors block DC and allow AC?. To know the exact reason, let’s know what is a capacitor and how it works when connected to DC and then AC supply source.
What is a Capacitor?
Capacitor (also known as condenser) is a two metal plates device separated by an insulating medium such as foil, laminated paper, air etc. It stores the energy in the form of electrostatic filed and released to the circuit when needed in case of AC. It storage ability is measured in Farad “F” and “µF” or “nF” units are used for small capacitors. Keep in mind that capacitor acts as an open circuit in DC i.e. it only operable at AC voltages.
Difference Between AC and DC
DC is a constant value i.e. it doesn’t change the polarity (direction) and magnitude while AC changes its direction and amplitude continuously related to its frequency as shown in fig below.
Now lets connect the capacitor in DC and then AC and see what happens?
Why Does a Capacitor Block DC?
Keep in mind that a capacitor act as a short circuit at initial stage and a fully charged capacitor behave as an open circuit. Capacitors resist a changes in voltage while inductors resist a change in current and acts as a short circuit in DC.
At initial stage when we connect a capacitor to the DC supply, there will a small current of flow will occur until the plates becomes saturated. In other words, the positive terminal of DC supply source will suck the electrons from one terminal and push the electrons to the second terminal until the first plate becomes positively charge and the second one as negatively charged as shown in fig. At this stage, the applied voltage equal to the voltage across capacitor and capacitor plates are saturated and there is no more flow of current. At this stage, capacitor behaves like an open circuit and if we increase the value of applied DC voltage, the capacitor may damage and explode.
Let’s see with a solved example of DC connected capacitor.
We know that there is no frequency i.e. 0Hz frequency in DC supply.
If we put frequency “f = 0″ in the inductive reactance (which is AC resistance in capacitive circuit) formula.
XC = 1 / 2πfC
Putting f = 0
XC = 1 / 2π 0 C
XC = 1/0 = Infinity
It means, theoretically, a capacitor will provide infinite resistant to the flow of current according to its rating. Hence no current flow will occur as current in capacitive circuits are:
I = V / XC
If we put XC as infinity, the value of current would be zero.
I = 0 A
That is the exact reason why a capacitor block DC.
Why Does a Capacitor Pass AC?
When we connect a capacitor across an AC supply source, it starts charge and discharge continuously due to continuous change in the supply voltage. This is due to changes in AC voltage i.e. AC is positive in the initial cycle for “t = 1” and negative in the second cycle “t = 2” as shown in fig below.
In fig 2(a), same happens like in DC connected capacitor at the initial stage i.e. the positive terminal of source sucks the electrons from the connected plat of the capacitor and pushes back to the second terminal. The first plat becomes positive and the other one is negative due to plenty of electrons. This process is known as charging of a capacitor i.e. it store the energy in the form of electric field.
Charging of a capacitor is given by:
VC = VS (1− e (−τ/RC))
or
VC = VS (1 – e–τ/τ)
Where:
- VC = Voltage across the Capacitor
- VS = Source or Applied Voltage
- e = 2.718 (Exponential i.e the base of the natural logarithm)
- τ = R/C = “tau” time constant in seconds
Now, the polarity of applied voltage is reversed i.e. positive becomes negative and vice versa as shown in fig 2(b). Now the negative source terminal attracted to the holes and pushes back the electrons to the holes in the opposite direction. The process remains continuous and current flows due to the continuous flow of electrons. This process is known as discharging of a capacitor i.e. it restores back the stored energy to the circuit.
The discharging of a capacitor is given by:
VC = VS x e (−τ/RC))
Why Capacitor is rated in DC then?
We know that there are different capacitors with different marking rating on its nameplates i.e. 400VDC or 400VAC. If a capacitor blocks DC, why the rating is mentioned in DC?
Well, Its not means that we can’t use capacitors in DC circuits (you already seen them). The value of DC printed on capacitor nameplates are the maximum value of DC voltage which can be safely connected to it. Keep in mind that it is not the value of charging capacity. Polarized capacitors are mostly used in DC while non-polarized are used in AC circuits.
As a rule of thumb;
- AC marked capacitors can be used on DC.
- DC marked capacitors can’t be used on AC.
Because, the AC voltages shows the RMS value where the peak value of AC is 1.414 times greater than DC.
Applications of Capacitors in DC
- Filters
- Rectifiers (AC to DC conversion)
- Power conditioning
- Coupling Capacitor and Decoupling Capacitor etc.
Applications of Capacitors in AC
- Transformer less power supply
- Split phase induction motors
- Power Factor Correction and Improvement etc.
FAQ
Why does a capacitor appear to pass AC signals while blocking DC signals?
Short Answer. A capacitor blocks DC signals because once charged, no further current can flow. It passes AC signals because the alternating current causes the capacitor to continuously charge and discharge, allowing the current to flow through it
Why does AC current only pass through capacitor?
As a direct current is constant, the magnetic field produced would not move and the field lines would not intersect the secondary wire resulting in no induction of EMF or current. Therefore only an alternating current works in a transformer
Can a capacitor used as a filter block DC but pass AC?
Consider a capacitor connected to a dc source directly in series. It will get charged and become open circuited. A capacitor in series will act as a high-pass filter; it will block DC and pass AC. A capacitor in parallel will act as a low-pass filter; it will block AC and pass DC
Is a capacitor blocks DC but passes AC state True or false?
Therefore, given statement is False. A capacitor blocks AC but passes DC. A Thevenin ac equivalent circuit always consists of an equivalent ac voltage source and an equivalent capacitance. A high-speed fighter jet is supersonic
Why capacitor block DC but allow AC?
A capacitor permits the passage of alternating current (AC) but blocks direct current (DC) because it stores energy in the form of an electric field between its plates. When an AC voltage is applied, it charges and discharges in a cyclical manner, facilitating the current flow
Why DC Cannot flow through a capacitor?
Because the capacitor’s electrode plates are separated by an insulator (air or a dielectric), no DC current can flow unless the insulation disintegrates. In other words, a capacitor blocks DC current
Can an AC run without a capacitor?
Most of the motors in your air conditioner can’t run without a good capacitor. Like I said, they support these motors. They help the motor start and run efficiently. Some people have gone out to their air conditioner and noticed the fan wasn’t spinning on their AC as it should be
Why is DC not used in home?
DC current is not used in homes because of the following reasons: First of all, DC is difficult to generate because primarily it is generated by batteries and electrochemical of photovoltaic cells. Power loss during transmission is more in direct current as compared to alternating current
Why does the capacitor block the DC signal at steady state?
Why does capacitor block dc signal at steady state? Explanation: Frequency of dc signal is zero. So, Capacitive reactance XC=1/2πfc becomes infinite and capacitor behaves as open circuit for dc signal. Hence, capacitor block dc signal
Why is DC blocking capacitor used?
While blocking DC flow, capacitors can couple AC signals, allowing them to pass through the capacitor unchanged (if attenuation is reasonably low). DC-blocking capacitors can remove noise generated by switched-mode power supplies (SMPS) or high-frequency signals that couple to the power net
