A choke is an inductor you add to a circuit in QSpice to smooth out voltage ripples and reduce noise

In QSpice, a choke is an inductor — a coil of wire that resists changes in electrical current. You create one by placing an inductor component in your circuit and setting its inductance value. The choke sits in series with the power supply line or signal path, and it works by storing energy in a magnetic field, then releasing it to fill in the dips when current tries to drop suddenly. This smoothing action reduces the spiky voltage noise that switching power supplies and high-speed circuits create.

The reason you use a choke instead of just calling it an inductor is context: in power supply design, an inductor used specifically to filter noise is called a choke. The component itself is identical — the name just tells you what job it's doing in the circuit.

Key Takeaways

  • A choke in QSpice is an inductor component placed in series with a power line or signal to smooth voltage ripples.
  • You create one by dragging an inductor from the component library, setting its inductance value in henries or microhenries, and connecting it in the current path.
  • Chokes work best when paired with a capacitor on the output side, forming an LC filter that removes both high and low frequency noise.
  • The inductance value you choose depends on the current you expect and the frequency of the noise you want to filter out.

Placing an inductor component in your schematic

Open your QSpice schematic and go to the component library. Search for or browse to the inductor symbol — it looks like a coil and is usually labeled L or Inductor. Click it to select it, then click on the schematic canvas where you want the choke to go. The inductor will appear as a coil symbol.

Position it in series with the power supply line or signal path you want to filter. In series means the current flows through the inductor before it reaches the rest of the circuit. If you're filtering a power supply output, the choke typically sits between the rectifier or switching stage and the load, with a capacitor to ground on the far side.

Setting the inductance value

Right-click the inductor symbol and select Edit or Properties. A dialog box will open showing the component's parameters. Find the field labeled L or Inductance and enter a value. Use standard units: H for henries, mH for millihenries (thousandths of a henry), or uH for microhenries (millionths of a henry).

For a typical switching power supply choke, values range from 1 microhenry to 100 microhenries, depending on the switching frequency and the current ripple you want to allow. A higher inductance value filters more aggressively but also stores more energy and takes longer to respond to sudden current changes. Start with a value in the middle of your expected range and adjust after you run the simulation.

Some inductor models in QSpice also include a series resistance field labeled R or ESR (equivalent series resistance). This represents the real-world resistance of the wire in the coil. For a simulation, you can leave it at zero initially, but adding a small value like 0.01 ohms makes the simulation more realistic.

Pairing the choke with a capacitor for better filtering

A choke works best as part of an LC filter — an inductor and capacitor working together. Place a capacitor from the output side of the choke to ground. The inductor smooths the current, and the capacitor smooths the voltage. Together they form a low-pass filter that removes both the high-frequency switching noise and the lower-frequency ripple.

The capacitor value depends on the same factors as the inductor: the switching frequency and the ripple you can tolerate. For a 100 kHz switching supply, a capacitor in the range of 10 to 100 microfarads is typical. Use a ceramic or electrolytic capacitor rated for the voltage and current in your circuit.

Running a simulation to check the choke's effect

Set up a transient simulation in QSpice. Run the circuit with the choke in place and observe the voltage at the output node — the point between the choke and the capacitor. Compare it to a simulation without the choke. You should see the voltage ripple decrease, especially at the switching frequency and its harmonics.

If the ripple is still too high, increase the inductance value or the capacitance value and run again. If the circuit is slow to respond to load changes, decrease the inductance. The goal is to find the balance where the output voltage stays smooth without the filter being so large that it slows down the power supply's ability to react to sudden current demands.

Common mistakes when adding a choke

The most common error is placing the choke in parallel with the circuit instead of in series. A choke must be in the current path — the current flows through it, not around it. If you connect it across the power supply like a capacitor, it will not filter anything.

Another mistake is choosing an inductance value that is too large. A very large choke stores a lot of energy and can cause voltage spikes when the circuit suddenly stops drawing current. It can also make the power supply unstable if the choke's resonant frequency with the capacitor matches a frequency in your load circuit.

A third issue is forgetting to include the capacitor. A choke alone will reduce current ripple but will not smooth the voltage well. The capacitor is what actually holds the voltage steady between the peaks of the inductor's current flow.

Frequently Asked Questions

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

There is no electrical difference — a choke is an inductor. The word "choke" is used when the inductor's job is to reduce noise or ripple in a power supply. In QSpice, you use the same inductor component either way.

Can I use a choke on both the input and output of a power supply?

Yes. An input choke filters the noise coming from the AC mains or from a previous stage. An output choke filters the noise going to the load. Many switching supplies use both, especially in high-current applications where ripple must be very low.

How do I know what inductance value to use?

Start with the switching frequency of your power supply. A choke's filtering effect peaks at frequencies above its cutoff frequency, which depends on both the inductance and the capacitance. For a 100 kHz supply with a 10 microfarad capacitor, a 10 microhenry choke is a reasonable starting point. Adjust based on your simulation results.

What happens if the choke value is too high?

A very large choke stores too much energy and can cause voltage spikes when the load suddenly stops drawing current. It also slows the power supply's response time to sudden load changes, which can cause the output voltage to sag. Simulation will show these effects clearly.

Do I need to model the choke's resistance in QSpice?

For a basic simulation, no — you can leave resistance at zero. But real inductors have wire resistance, which dissipates power as heat. Adding a small ESR value like 0.01 to 0.1 ohms makes the simulation more realistic and shows how much power the choke wastes.