This motor with integrated 48 CPR (counts per revolution) quadrature encoder is intended as a replacement low-power (non-HP) motor and encoder for Pololu's 25D mm metal gearmotors. The output shaft has a non-removable pinion gear that works with all of Pololu's 25D mm gearmotor gearboxes. Note that we do not sell the 25D mm gearboxes separately, but if you have a gearmotor with a damaged motor or encoder (or if you want to effectively add an encoder to a version without an encoder), you can transfer the gearbox to this replacement motor.
The motor has a diameter of 24.2 mm (0.95 in) and a length of approximately 43 mm (1.7 in) from the top of the motor can to the bottom of the encoder. The top of the motor has two mounting holes threaded for M3 screws. These mounting holes are 17 mm apart and form a line with the motor shaft at the center. The mounting holes have a depth of approximately 6.5 mm.
A higher-power motor with encoder with identical dimensions and an identical pinion gear is also available.
Pinion Gear Specs
- Metric with module m = 0.4
- Number of teeth: 12
- Face thickness: 2.5 mm
- Pressure angle: 20°
- Gear position: 4.5 mm from top of motor to top of gear
You will typically want to combine this motor with a gearbox to give it a more appropriate combination of torque and speed (without a gearbox, it offers very high speed with very low torque). Pololu's 25D mm line of metal gearmotors consist of this non-HP motor or an HP motor combined with different gearboxes. We do not carry the gearboxes by themselves, so unless you are looking at this as a replacement motor for a compatible gearbox you already have, Pololu strongly recommend you consider getting a preassembled gearmotor with the gear ratio that best suits your project requirements.
|Gear Ratio||No-Load |
@ 6 V
|Stall Torque |
@ 6 V
|Stall Current |
@ 6 V
|1:1||9800 RPM||2 oz-in||6 A||motor without gearbox|
|5600 RPM||1 oz-in||2.2 A||motor without gearbox|
|4.4:1||2220 RPM||8 oz-in||6 A||25Dx48L mm HP|
|1280 RPM||5 oz-in||2.2 A||25Dx48L mm|
|9.7:1||1010 RPM||17 oz-in||6 A||25Dx48L mm HP||25Dx48L mm HP|
|580 RPM||11 oz-in||2.2 A||25Dx48L mm||25Dx48L mm|
|20.4:1||480 RPM||36 oz-in||6 A||25Dx50L mm HP|
|275 RPM||24 oz-in||2.2 A||25Dx50L mm|
|34:1||285 RPM||60 oz-in||6 A||25Dx52L mm HP||25Dx52L mm HP|
|165 RPM||40 oz-in||2.2 A||25Dx52L mm||25Dx52L mm|
|47:1||210 RPM||80 oz-in||6 A||25Dx52L mm HP||25Dx52L mm HP|
|120 RPM||50 oz-in||2.2 A||25Dx52L mm||25Dx52L mm|
|75:1||130 RPM||130 oz-in||6 A||25Dx54L mm HP||25Dx54L mm HP|
|75 RPM||85 oz-in||2.2 A||25Dx54L mm||25Dx54L mm|
|99:1||100 RPM||160 oz-in||6 A||25Dx54L mm HP|
|57 RPM||110 oz-in||2.2 A||25Dx54L mm|
|172:1||57 RPM||260 oz-in||6 A||25Dx56L mm HP|
|33 RPM||170 oz-in||2.2 A||25Dx56L mm||25Dx56L mm|
|227:1||25 RPM||220 oz-in||2.2 A||25Dx56L mm|
|378:1||15 RPM||250 oz-in||2.2 A||25Dx58L mm|
|499:1||12 RPM||300 oz-in||2.2 A||25Dx58L mm|
Note: At some of the higher gear ratios, these motors can generate enough torque to damage themselves. Any torque greater than about 250 oz-in (18 kg-cm) is likely to damage the gearbox, so they recommend that you avoid stalling the 172:1 HP, 227:1, 378:1, and 499:1 versions of these motors at 6 V.
These motors are intended for use at 6 V. In general, these kinds of motors can run at voltages above and below this nominal voltage, so they should comfortably operate in the 3 – 9 V range, though they can begin rotating at voltages as low as 1 V. Higher voltages could start negatively affecting the life of the motor.
|25D mm metal gearmotor with 48 CPR encoder: close-up view of encoder.|
Using the Encoder
A two-channel Hall effect encoder is used to sense the rotation of a magnetic disk on a rear protrusion of the motor shaft. The quadrature encoder provides a resolution of 48 counts per revolution of the motor shaft when counting both edges of both channels. To compute the counts per revolution of the gearbox output, multiply the gear ratio by 48. The motor/encoder has six color-coded, 11" (28 cm) leads terminated by a 1×6 female header with a 0.1″ pitch, as shown in the main product picture. This header works with standard 0.1″ male headers and Pololu's male jumper and precrimped wires. If this header is not convenient for your application, you can pull the crimped wires out of the header or cut the header off. The following table describes the wire functions:
|Red||motor power (connects to one motor terminal)|
|Black||motor power (connects to the other motor terminal)|
|Blue||encoder Vcc (3.5 – 20 V)|
|Yellow||encoder A output|
|White||encoder B output|
The Hall sensor requires an input voltage, Vcc, between 3.5 and 20 V and draws a maximum of 10 mA. The A and B outputs are square waves from 0 V to Vcc approximately 90° out of phase. The frequency of the transitions tells you the speed of the motor, and the order of the transitions tells you the direction. The following oscilloscope capture shows the A and B (yellow and white) encoder outputs using a motor voltage of 6 V and a Hall sensor Vcc of 5 V:
|Encoder A and B outputs for 25D mm (non-HP) metal gearmotor with 48 CPR encoder (motor running at 6 V).|
By counting both the rising and falling edges of both the A and B outputs, it is possible to get 48 counts per revolution of the motor shaft. Using just a single edge of one channel results in 12 counts per revolution of the motor shaft, so the frequency of the A output in the above oscilloscope capture is 12 times the motor rotation frequency.
Selecting the Right Gearmotor
Pololu offer a wide selection of metal gearmotors that offer different combinations of speed and torque. Pololu's metal gearmotor comparison table can help you find the motor that best meets your project’s requirements.
|Some of the Pololu metal gearmotors.|
Documentation and Resources:
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