Pololu stepper driver board
Release status: working
| Description | Stepper driver for RAMPS, Sanguinololu, Gen7
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| License | Commercial
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- Please note: Pololu stepper driver boards has 0.05 ohm sense resistors instead of StepStick 0.2 ohm.
List of Boards
Pololu driver boards (8+8 pins):
- Pololu A4988 stepper driver - A4988-based; equivalent to A4983-board but offers overcurrent protection. If the boards get too hot, they will interrupt the current until it cools a bit. If the current is too high for the heat sinking, the motors will pulse as the current is interrupted and restored. See http://forums.reprap.org/read.php?4,116813,116832,quote=1 and its video. Try reducing the current until the pulsing stops.
- Pololu A4983 stepper driver - A4983-based; (discontinued).
Pololu driver boards with Voltage Regulators (longer boards - 8+14 pins):
- Pololu A4988 Stepper Motor Driver Carrier with Voltage Regulators - A4988-based; equivalent to A4983-board but offers overcurrent protection.
- Pololu A4983 Stepper Motor Driver Carrier with Voltage Regulators - A4983-based; (discontinued).
As user Nophead has pointed out, that Pololu driver is a nice design, but with one big shortcoming: it will run hot, and is difficult to cool because it's so small. So what I've done is to design the electronics in such a way that they both perform their function and act as a physical duct for the flow from a fan to direct their own cooling. A happy side-effect of this is that the resulting PCBs are very simple, and can be made single-sided without any thin tracks. That is is why RepRap itself can make them. We must walk before we can run... an another way to improve heat dissipation is the use of small heat sink, unfortunately the small adhesive pad are not so adhesive.... so the use of some thermal conductive glue make the thing easier (like this one, in french www.pc-look.com/boutik/63648.html but if you google: Arctic Silver - Arctic Alumina™ Thermal Adhesive you will find it or equivalent)
Alternatives
- StepStick
- SureStepr SD82B - http://www.panucatt.com/product_p/sd82b.htm
Tuning motor current
Per the A4988 datasheet [[1]], the calculation for the maximum trip current is:
I_TripMax= Vref/(8*Rs)
With Pololus, the sensing resistors are Rs=0.05 ohm, so a Vref of 0.4 should produce a maximum current of 0.4/(8*0.05)=1A.
As another example, aiming for 50% temperature rise on 1A rated steppers by using max 0.7A, so rearrange it as:
Vref = I_TripMax * 8 * Rs or
Vref = 0.7 * 8 * 0.05 = 0.280V
With a measured Vref of 0.273V, I should expect 0.6825A, and I measured the current through one coil as 0.486A in full step mode, which should be 0.7071 of the full trip current, or I_TripMax= 0.486/0.7071= 0.687A, which seems close enough.
The Vref signal is accessible as the "VREF" pin on the carriers with voltage regulators, as the through-hole via on the carriers without, and also as the wiper on the trim pot itself on both carriers.
Note : for StepStick, the Rs=0.2 ohm, so you'd get 4 times the Vref at the same current.
The maths in full
For repraps, logic supply voltage (VDD) is 5V. For Pololu driver boards, the trimpot is 100kohm, trimpot resistor R5 is 20kohm, sense resistors R7 and R8 are 0.05ohm
VREF max = (TrimpotMaxR/(TrimpotMaXR+R5)) x VDD = (100,000 / (100,000 + 20,000)) * 5 = 4.16V
ITripMAX (effectively max motor current) = VREF / ( 8 x Sense_resistor) = 4.16 / ( 8 * 0.05 ) = 10.4A
To calculate amps from measured VREF: A = VREF / 0.4
To calculate VREF required for a target current: VREF = A * 0.4
