A chopper drive is a method of providing a constant current source to a device such as a stepper motor. It is called such because the circuit rapidly toggles, or "chops", the power flowing through the motor coil once a certain current has been reached.

Roughly speaking, a chopper drive operates in the following manner:

1. Power is applied to the circuit when the coil needs to be energized. This circuit uses a sense resistor to measure the current being generated across the coil. The output of this resistor is a voltage proportional to the current being measured.
2. This voltage is then fed into an analog comparator, where it is measured against a set reference voltage. The reference voltage will be set by a pair of resistors to be analogous to whatever current we want to drive the coil at.
3. Once the sense voltage exceeds the reference voltage, the drive circuit cuts power to the coil; this drops the measured current, and thus the voltage being output by the sense resistor.
4. After a brief period of time, the circuit re-energizes the coil, and the process repeats from step 2. This loop continues as long as the coil should be energized.

This behavior creates a current waveform that looks something like this:

```   |     /|/|/|/|/|/|                  /|/|/|/|/|/|  <-- circuit starts
C |    /           |                 /           |       "chopping" the
U |   |            |                |            |       power to maintain
R |   |            |                |            |       the desired current
R |   |            |                |            |
E |  /             |               /             |
N |  |             |               |             |
T | /              |              /              |
| |              |              |              |
|/               |______________/              |__________ . . .
+---------------------------------------------------------- - - -
ON              OFF             ON            OFF
T I M E

```

(The off edges are of course not actually perfectly vertical, but those are the limits of ASCII art.)

Prior to the creation of the chopper drive, the two accepted methods of powering a stepper motor were a constant voltage source, which results in far reduced torque due to the slow speed at which it takes the coil current to increase; or by forcing a constant current with a power resistor across the terminals, which although providing an excellent current source for the motor, wastes a huge amount of energy in the form of heat being radiated from the resistor. The chopper drive allows the motor to generate a good amount of torque at higher speeds while still being efficient in terms of power consumption.