Choosing a stepper motor power supply confuses even expert engineers. Lack of voltage causes the motor to lose torque when it runs up; too much risks damage to the driver or motor windings.
Calculator
Stepper Motor Voltage Calculator
Estimate coil voltage, electrical time constant, and driver supply voltage from your motor’s nameplate values — with driver-type awareness and rating checks.
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Current, resistance, inductance and voltage are simple in theory but hard to get correct especially when driver ratings and motor insulation restrictions are considered.
Through the use of this post you will learn the electrical calculations that underlies our Stepper Motor Voltage Calculator as well as the meaning of each number and how to accurately read the findings.
Why Voltage Selection Important?
A stepper motor coil is fundamentally a resistor and an inductor in series.
As the step rate increases the driver has less and less time per step to force current into the winding against its own inductance and torque falls off a cliff unless the supply voltage is high enough to push current in faster than the winding resists it.
This is where the electrical time constant, τ = L / R, becomes useful.
It describes how quickly current can rise in the coil after a step.
A larger time constant from higher inductance (or) lower resistance means the driver needs more supply voltage headroom to hit rated current before the next step arrives particularly at higher speeds.
The Rule-of-Thumb Formula and Its Limits
For current-regulated “chopper” drivers – the type used in almost all modern stepper systems, including the DM542, TB6600 and DRV8825 – suggests sizing the supply at roughly 20 to 32 times the square root of the coil inductance in millihenries (mH).
The lower end is a workable minimum values near 25x√L tend to give a good balance of torque and driver heat and the upper end approaches the point where switching losses and voltage stress start working against it.
Formulas
These are the governing equations used by the calculator.
L is coil inductance in millihenries (mH),
R is coil resistance in ohms (Ω) and
I is rated phase current in amps (A).
| Quantity | Formula | Description |
| Motor nameplate voltage | V = I x R | Baseline coil voltage needed to reach rated current at standstill |
| Electrical time constant | τ = L / R | Time for coil current to rise; L in (mH) and R in Ω gives τ directly in (ms) |
| Rule-of-thumb minimum | Vₘᵢₙ = 20 x √L | Lowest supply voltage generally usable with a chopper driver |
| Rule-of-thumb typical | Vₜᵧₚ = 25 x √L | Balanced point between torque & driver heat |
| Rule-of-thumb maximum | Vₘₐₓ = 32 x √L | Upper bound before switching losses / voltage stress dominate |
| Estimated supply (chopper) | Vₑ = Vₜᵧₚ x (1 + h) | h = headroom fraction (e.g. 20% → h = 0.20) |
| Estimated supply (linear) | Vₑ = V x (1 + h) | Applied to nameplate voltage, not the √L rule |
Note
The 20-32 x √L range is an industry rule-of-thumb for current-regulated chopper drivers, not an IEEE/IEC/NEMA standard.
It does not apply to linear or constant-voltage drivers (e.g. L298, ULN2003).
Step-by-Step Calculation Method
Follow these steps in order.
The same sequence is that the calculator performs internally.
Step-1: Collect motor and driver data
- Record rated phase current (I), coil resistance (R) and coil inductance (L) from the motor datasheet.
- Identify the driver topology: current-regulated chopper (or) linear / constant-voltage.
- Note the driver’s maximum voltage rating and if published the motor insulation voltage limit.
Step-2: Calculate Nameplate Voltage
V = I x R
Step-3: Calculate the Electrical Time Constant
τ = L/R
Step-4: Apply the Rule-of-Thumb (Chopper Drivers only)
Apply the square root (√) of inductance in (mH): √L.
Multiply by 20, 25 & 32 to get the minimum, typical & maximum bounds.
For linear drivers skip this step and use nameplate voltage instead.
Step-5: Apply Voltage Headroom
Add the chosen headroom percentage (h) on top of the typical rule-of-thumb value (chopper) (or) the nameplate voltage (linear):
Vₑ = Vₜᵧₚ x (1 + h) (or) Vₑ = V x (1 + h)
Step-6: Compare against Rating Limits
Check the estimated voltage Vₑ against three limits, and flag any that are exceeded rather than silently reducing the number:
- Rule-of-thumb maximum (32 x √L) – chopper drivers only.
- Driver rated maximum voltage.
- Motors maximum / insulation voltage rating.
Solved Example
A NEMA 17 stepper motor is driven by a DM542-style chopper driver.
| Rated current, I | 1.5 A |
| Coil resistance, R | 2.1 Ω |
| Coil inductance, L | 3.2 mH |
| Driver max voltage | 50 V |
| Headroom, h | 20% (0.20) |
Solution
| Step | Calculation | Result |
| Nameplate voltage | V = 1.5 x 2.1 | V = 3.15 V |
| Time constant | τ = 3.2 / 2.1 | τ = 1.52 ms |
| √L | √3.2 | √L = 1.789 |
| Rule-of-thumb range | 20 x 1.789 to 32 x 1.789 | 35.8 V to 57.2 V |
| Typical point | 25 x 1.789 | Vₜᵧₚ = 44.7 V |
| Estimated supply | 44.7 x (1 + 0.20) | Vₑ = 53.7 V |
Result
Estimated driver supply voltage ≈ 53.7 V
Summary
- Nameplate voltage (I x R) is a baseline not a supply-voltage recommendation on its own.
- The electrical time constant (L / R) shows how quickly current can rise – higher values need more voltage headroom at speed.
- The 20-32 x √L rule-of-thumb applies only to current-regulated chopper drivers.
- Linear / constant-voltage drivers should be sized from nameplate voltage plus a modest headroom never from the √L rule.
- Always compare the final estimate against the driver’s maximum voltage & the motor’s insulation rating – flag, don’t clip, any value that exceeds them.
