What is a Class C (two-quadrant) chopper?
A Class C chopper is a half-bridge leg — an upper and a lower controlled switch, each with an anti-parallel diode — driven with complementary gate signals. It merges the Class A motoring path (upper switch, lower diode) with the Class B braking path (lower switch, upper diode), so the load current can flow in either direction while the average output voltage stays positive. That makes it a two-quadrant converter covering Quadrant I (forward motoring) and Quadrant II (regenerative braking) with no change to the circuit — the diodes simply carry the current whichever way it flows.
Output voltage & current
Here D is the duty ratio of the upper switch. When the motor back-EMF E is below Vo the current is positive (motoring); when E exceeds Vo the current reverses and the machine brakes regeneratively — try both by moving the back-EMF slider. Because the leg is always in conduction (one device or its diode), the current is continuous and Vo = D·Vs holds across both quadrants. This simulator integrates the real R-L-E load and compares the measured average to D·Vs.
Advanced options in this simulator
- Signed back-EMF: raise the back-EMF above the output voltage to flip from motoring to braking and watch the operating point cross from Quadrant I to Quadrant II.
- Duty & frequency: control the average voltage and the current ripple.
- Device model: switch and diode drops with on-resistance; the accuracy check stays locked to Vo = D·Vs.
- Four-quadrant map, ripple spectrum & export: live operating point, FFT of voltage/current and CSV/report/PNG export.
The five chopper classes
| Class | Quadrants | Average output | Use |
|---|---|---|---|
| Class A | I (V+, I+) | Vo = D·Vs | Forward motoring (step-down) |
| Class B | II (V+, I−) | Vo = (1−D)·Vs | Regenerative braking |
| Class C | I & II | Vo = D·Vs | Motoring + braking |
| Class D | I & IV | Vo = (2D−1)·Vs | ±V, one current direction |
| Class E | I–IV | Vo = (2D−1)·Vs | Full four-quadrant drive |
Explore the others: Class A, Class B, Class D and Class E chopper simulators. For the full theory see the Class C chopper tutorial.
Applications
DC drives that must both motor and brake in one direction — battery vehicles, forklifts, conveyors and hoists — where a single leg gives smooth transitions between driving and regenerative braking.
Frequently asked questions
What is the average output voltage of a Class C chopper?
Vo = D·Vs. The current (Vo − E)/R can be positive (motoring) or negative (braking) depending on the back-EMF.
Why is it a two-quadrant chopper?
The voltage stays positive while the current can be either sign, so the operating point covers Quadrants I and II.
How do I make it brake?
Raise the motor back-EMF above the average output voltage — the current reverses and the operating point moves into Quadrant II.
What devices does it use?
Two controlled switches (IGBT/MOSFET) with anti-parallel diodes forming a half-bridge leg.