What a choke is and why you'd use one

A choke in QSpice is an inductor placed in series with a power supply or signal line to block high-frequency noise while allowing direct current (DC) to pass through. The term "choke" comes from its function: it chokes off unwanted high-frequency signals. In circuit design, you use a choke to filter out switching noise from power supplies, prevent electromagnetic interference (EMI) from radiating out of your circuit, or isolate different sections of a circuit from each other's noise.

QSpice is a free SPICE simulator that runs on Windows, Mac, and Linux. Creating a choke in QSpice means drawing an inductor symbol on your schematic and assigning it the right inductance value for your frequency range. The inductor itself does the actual work — QSpice simply lets you design the circuit, simulate it, and see whether the choke actually reduces the noise you're trying to eliminate.

Key Takeaways

  • A choke is an inductor placed in series with a power line or signal to block high-frequency noise while passing DC current.
  • In QSpice, you add a choke by placing an inductor component from the library, setting its inductance value in henries, and connecting it in series with the line you want to filter.
  • Choke inductance values range from microhenries (µH) for high-frequency switching supplies to millihenries (mH) for lower-frequency power filtering, depending on the noise frequency you want to block.
  • You can test whether your choke works by running a transient simulation or AC analysis in QSpice and comparing the noise before and after the inductor.

Adding an inductor component to your schematic

Open QSpice and create a new schematic or open an existing one. In the schematic editor, click the Add Component button (usually a resistor symbol with a plus sign) or press R on your keyboard. A component library window will appear. Search for "L" or "inductor" — QSpice labels inductors with the letter L, following standard SPICE naming.

Select the inductor symbol from the list and click to place it on your schematic. Position it in series with the power supply line or signal line where you want to filter noise. In series means the current flows through the inductor before reaching the rest of the circuit — not in parallel across the line. Connect one end of the inductor to the source (power supply or signal input) and the other end to the component or circuit section you're protecting.

Setting the inductance value

Right-click on the inductor symbol you just placed and select Edit or Properties. A dialog box will open showing the component's parameters. Find the field labeled L or Value and enter the inductance in henries. Use standard metric prefixes: µ for microhenries (10⁻⁶ H), m for millihenries (10⁻³ H), or u for microhenries if your keyboard doesn't have the µ symbol.

The inductance value you choose depends on the frequency of the noise you want to block. For a switching power supply running at 100 kHz, a choke of 1 to 10 microhenries is typical. For lower-frequency noise around 50 Hz or 60 Hz (mains frequency), use 1 to 100 millihenries. A rough starting point: multiply the desired cutoff frequency (in Hz) by the series resistance in your circuit, then divide by 2π to estimate inductance. If you're unsure, start with 10 microhenries and adjust after simulation.

Adding series resistance to model real-world behavior

Real inductors have resistance — they are not perfect. To make your simulation match real behavior, add a small series resistance to your choke. Right-click the inductor again and look for a field labeled R, ESR (equivalent series resistance), or Rser. Enter a value in ohms. For a small choke, 0.1 to 1 ohm is reasonable; for larger inductors, 0.01 to 0.1 ohm.

This resistance represents the copper wire's resistance inside the inductor coil. Including it makes your simulation more realistic and helps you see how much voltage drop the choke will cause in your actual circuit. If the voltage drop is too high, you can either choose a choke with lower resistance or increase the wire gauge (which lowers resistance but makes the inductor larger and more expensive).

Connecting the choke in your circuit

Wire the inductor in series by connecting its input terminal to your power source or signal source and its output terminal to the rest of your circuit. If you're filtering a power supply, the choke goes between the supply output and the load. If you're filtering a signal line, it goes between the signal source and the input of the next stage.

For power supply filtering, you often pair the choke with a capacitor to ground on the output side. The capacitor handles high-frequency noise, and the choke blocks it from flowing back toward the supply. This combination is called an LC filter. In QSpice, place a capacitor from the output node of the choke to ground, with a value of 10 to 100 microfarads for power supplies or 0.1 to 10 microfarads for signal lines, depending on your frequency range.

Simulating and testing your choke

Once your choke is in place, run a simulation to see if it actually reduces noise. Click Simulate or press Ctrl+R. Choose Transient analysis if you want to see noise over time, or AC Analysis if you want to see how the choke attenuates different frequencies. Transient analysis is better for seeing switching noise; AC analysis is better for understanding the choke's frequency response.

After the simulation runs, plot the voltage at the input of the choke and the output of the choke on the same graph. The output should show less noise than the input. If the noise reduction is not enough, increase the inductance value and simulate again. If the voltage drop across the choke is too high, decrease the inductance or choose a choke with lower series resistance. Iterate until the noise is acceptable and the voltage drop is within your circuit's tolerance.

Common choke values and when to use them

For switching power supplies at 100 kHz to 1 MHz, use 1 to 47 microhenries. For audio circuits and lower-frequency signals below 20 kHz, use 1 to 100 millihenries. For mains-frequency filtering (50 or 60 Hz), use 100 millihenries to several henries. Larger inductance values block lower frequencies more effectively but also increase size, cost, and DC resistance.

If you are filtering multiple noise sources at different frequencies, you may need multiple chokes in series or a choke paired with resistors and capacitors to form a more complex filter network. QSpice lets you experiment with different topologies quickly without building physical prototypes, so test several configurations and compare their frequency response using AC analysis.

Frequently Asked Questions

What's the difference between a choke and a regular inductor?

There is no electrical difference — a choke is simply an inductor used for filtering noise. The term "choke" describes the function (choking off high-frequency signals) rather than the component itself. In QSpice, you use the same inductor component for both.

Can I use a choke on both AC and DC circuits?

Yes. A choke blocks AC (alternating current) and high-frequency noise while passing DC (direct current) with minimal resistance. This makes it ideal for power supplies that deliver DC voltage but need to reject switching noise. For AC-only circuits, a choke still works but is less commonly used because capacitors are often simpler.

How do I know if my choke inductance is too high or too low?

Run an AC analysis in QSpice and look at the frequency response. If the choke does not attenuate noise at your target frequency, the inductance is too low — increase it. If the DC voltage drop across the choke is too large, the inductance is too high — decrease it. Start with 10 microhenries and adjust based on simulation results.

Do I need to add a resistor in parallel with the choke?

Not usually. A parallel resistor would dissipate energy and reduce the choke's filtering effectiveness. You may add a small resistor in series (as ESR) to model real-world losses, but parallel resistors are rarely needed unless you are designing a damped filter to prevent ringing at the choke's resonant frequency.

What happens if I don't include the choke's series resistance in my simulation?

Your simulation will show lower voltage drop and slightly better filtering than your real circuit will achieve. Including series resistance makes the simulation more accurate and helps you predict real-world performance. For initial design work, you can omit it, but add it before finalizing your design.