Question

How to block AC and pass DC?

Answer 1

To block AC and pass DC in a circuit, you typically use a **capacitor** or a **series inductor**, as both components have different behaviors with AC and DC signals.

Here’s how each works:

### 1. **Using a Capacitor:**
   - **Capacitors block DC after the initial charging phase**. When a DC voltage is applied across a capacitor, it will charge up to the applied voltage and then act as an open circuit (blocking DC). However, **capacitance offers a path to AC signals** because an AC signal will continuously change voltage, causing the capacitor to keep charging and discharging, allowing the AC to pass through.
   - **How it works**: When DC is applied, the capacitor charges to the DC voltage level and then stops the current flow (blocks DC). For AC, because the voltage changes, the capacitor charges and discharges in sync with the AC, allowing it to pass through.

### 2. **Using a Series Inductor:**
   - **Inductors block AC signals at high frequencies** and pass DC. At DC, an inductor behaves like a short circuit (essentially zero resistance), so it lets DC pass easily. For AC, the inductor offers resistance that increases with the frequency of the signal, effectively blocking higher-frequency AC signals.
   - **How it works**: The inductor resists changes in current, and for DC, the current flow doesn’t change once steady, so the inductor allows DC to pass through. For AC, the inductor's reactance increases with frequency, blocking AC signals.

### In Practice:
- **Capacitor in series with the signal**: Blocks DC and passes AC.
- **Inductor in series with the signal**: Blocks high-frequency AC and passes DC.

You can use either component depending on the frequency range of the AC signal you’re dealing with and the specific application you have in mind.