Why Does AC Capacitor Block DC but Pass AC?

Capacitors are electrical components found in many applications; they store energy on their plates, separated from each by a dielectric insulator. The insulator prevents the charges from passing from one plate to another, creating an electric field inside the capacitor.

When capacitors are used in a circuit fed by an alternating current, the current flows through the circuit normally; however, when a direct current replaces the alternating current, the flow of the current stops.

Why Does AC Capacitor Block DC but Pass AC?

The current passing through a capacitor is blocked once the capacitor reaches a steady state; in the case of DC, the current flows in one direction, so the current keeps flowing until the capacitor is fully charged. The voltage across the capacitor becomes equal and opposite to that of the power supply.

The voltage difference is the main reason the current flows; it flows from the high voltage to the low voltage, so when the voltage difference between the capacitor and the power supply becomes zero, the current stops flowing. The capacitor is said to be in a steady state.

Whereas in the case of AC, the current flows in two opposite directions, and the voltage drops to zero and reverses. This process can happen several times in a second, with the number of changes equal to the power supply frequency. Therefore, the capacitor never reaches a steady state and keeps discharging and charging in both directions.

Only non-polar capacitors can pass AC; meanwhile, polar capacitors like electrolytic and tantalum can’t pass AC because they have fixed polarity, so when the alternating current changes its direction, it will stop flowing.

How Does a Capacitor Block Direct Current (DC)?

When a capacitor is connected to an electrical circuit before the direct current starts flowing, it will have no voltage or net charge. This is because the capacitor consists of two plates separated by a dielectric insulator; therefore, electric current can’t flow from one plate to the other.

After the current starts flowing from the battery, the electrons will start flowing to the plate of the capacitor. As a result, the electrons or the negative charge will start accumulating on the plate they are connected to. After reaching a certain level, the negative charge will start affecting the neighboring plate.

The negative charge can’t pass through the dielectric insulator to the neighboring plate but can polarize or induce it. The electrons accumulated start repelling the electrons on the neighboring, so they become positively charged and flow from the plate to the battery.

The circuit is now in a transient state; this state continues until no more electrons can be accumulated on the plate. As a result, the capacitor is fully charged, opposite the battery’s voltage, and consequently, the current stops flowing in the circuit, and the state changes from transient to steady.

If the circuit has no resistors, the capacitor will be charged so quickly that it appears that it has blocked the DC from the beginning. The opposition in charges on the capacitor plates creates an electric field; however, the charges can pass through the dielectric insulator.

How Does a Capacitor Pass Alternating Current (AC)?

The alternating current has the same charging process as the direct current; before the current starts flowing, the capacitor will have no net charge or voltage. This is because the dielectric insulator between the capacitor plates prevents the electric current from flowing from one plate to another.

When you power on the circuit and the current starts flowing, the electrons start flowing to the nearest capacitor plate. Upon reaching the plate of the capacitor, the negative charges start accumulating till they reach a certain level and start inducing the neighboring plate electrons.

The dielectric insulator prevents the accumulated electrons from passing to the next plate, so they start repelling the neighboring electrons turning them into positively charged particles. The transient state continues until the capacitor is nearly fully charged; at this moment, the input voltage drops.

Therefore, instead of charging, the capacitor starts discharging, and the current flows in the opposite direction. The current keeps flowing from the capacitor until the input voltage reaches zero volts, and the capacitor becomes fully discharged. The input voltage is then reversed again and starts increasing.

The charging process of the capacitor starts again, and the capacitor keeps accumulating negative charges until it’s nearly fully charged. So, the current never settles and keeps flowing through the current, making the capacitor never reach the steady state, so it can’t block the AC.


Why are Some Capacitors Rated DC?

There are many different capacitors each type has its specific ratings; for example, there are DC capacitors, “VDC,” and AC capacitors, “VAC.” Even if capacitors block the flow of direct current “DC,” the rated DC capacitors are used in DC electrical circuits but can’t be used in AC circuits.

A capacitor’s rated value, whether an AC or DC capacitor, is the maximum voltage the capacitor can tolerate. This value should not be mistaken for the value of the charging capacity; most capacitors used in DC circuits are polarized; meanwhile, the ones used in AC circuits are non-polarized.

What Is the Difference Between AC and DC Capacitors?

DC Capacitor  AC Capacitor
Polarized Polarized Capacitors like Tantalum and Electrolytic work perfectly in DC Circuits. Polarized Capacitors can’t be placed in AC circuits because of their fixed polarity, so when the current reverses, it will not flow through.
Non-polarized Non-Polarized Capacitors can’t be placed in DC circuits because of their non-polarity. Non-polarized capacitors work perfectly in AC circuits because of their non-polarity, so current can reverse and keep flowing through.
Transient State  Present Present
Steady State Present Absent
Current Blocked  After the capacitor reaches the steady state, the current can no longer keep flowing. The capacitor in AC circuits can not reach a steady state because the current keeps changing its direction.
  • Power conditioning
  • Filters
  • Rectifiers (AC to DC conversion)
  • Coupling Capacitor and Decoupling Capacitor etc.
  • Split-phase induction motors
  • Transformerless power supply
  • Power Factor Correction and Improvement etc.

What Happens if You Connect a Capacitor Without Connecting a Resistor?

The result of connecting a capacitor without a resistor depends on the type of current in the circuit. If you connect the capacitor without a resistor to a direct current, the capacitor will be charged very quickly if the current is blocked from the beginning. 

The presence of a resistor in the electrical circuit of a capacitor limits the current reaching the capacitor, so it slows down its charging process. Meanwhile, if you connect the capacitor without a resistor to an alternating current, the current will keep flowing normally as the capacitor never reaches a steady state.

As the current changes its direction before the capacitor becomes fully charged and the capacitor starts discharging until the input voltage rises again. So, the presence of a resistor in an AC circuit doesn’t affect a capacitor, and it usually works.


To summarize, when a capacitor is connected to a DC circuit, the negative charges keep flowing and accumulating on one of the capacitor plates. After reaching a certain accumulation level, the other plate’s electrons turn into positively charged particles and start flowing to the battery.

When there is no accumulation of negative charges, the capacitor is fully charged and reaches a steady state, and as a result, the current stops flowing in the circuit. In AC circuits, the case is different because the current has no fixed direction, so when the capacitor is nearly fully charged, the input voltage drops.

The current changes its direction and starts flowing from the capacitor to the battery until it is fully discharged and the voltage rises again. As a result, the current never settles, and the capacitor doesn’t reach a steady state, so the current keeps flowing.

Related Readings:

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Why Is Electric Current a Fundamental Quantity? Here Is Why

How To Determine If Current Is AC or DC – 3 Steps

How To Tell Positive and Negative Terminals of a Capacitor?

How To Reduce Current Without Affecting the Voltage?

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