Vref = Imax * 8 * Rs
DRV8825 The equation for calculating
Vref for this board is
Vref = Imax /2 when having a 0.1 Ω sensing resistor.
But first we need to find out the proper value of the current sensing resistor
Rs as it’s exact value is dependent of the A4988 board manufacturer. By looking at the number marked on top of
R5 you may use a tool such as this one to find out it’s exact value in Ohms. If you dont find
R5 search for
Original Pololu boards have a 0.05 Ω resistor while most generic brands use 0.1 Ω.
Let’s take the 0.1 Ω sensing resistor value as an example, just replace
Rs on the formula like so
Vref = Imax * 8 * 0.1.
The step stick is such a a small board, this is both it’s strongest and weakest point. The small form factor makes it an smart choise for projects where overall bulk size matters but also makes it run hot as chilli. Some vendors provide you a stick-in heatsink, the performance improvement using it is rather questionable so your best friend is de-rating the motor current.
If you have a motor which max current is spec at 2A, you should ran it at around 70% to 80% of that, this will not only make your step stick happier but also the motor will run cooler.
Let’s take a 1.5A motor which we want to run at 80% of its rating:
Imax = 1.5 * 0.8 <=> Imax = 1.2A
Let’s also take a generic step stick having the sensing resistor (
Rs) value of 0.1 Ω, take the initial formula
Vref = Imax * 8 * Rs and replace
Vref = 1.2A * 8 * 0.1 <=> Vref = 0.96
Using a multimeter measure the voltage across the top of the potentiometer head and any ground point on the board.
Adjust the pot until you measure, in this example, 0.96V. Now you have your step stick properly adjusted for your motor’s current.
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