DC Motor Driver Selection Guide: Current, Peak Window, PWM and Protection
A DC motor driver is selected by stall current and peak window, not by rated current. The physics of stall = V/Ra, the thermal reason behind the peak/continuous ratio, PWM frequency, H-bridge, cable and fuse sizing, and an ST42/KS250/PT500 comparison.
A DC motor driver is selected by startup current and by the duration of the peak window, not by rated current. With the rotor stopped, back-EMF is zero and the only thing limiting current is the winding resistance: I = V / Ra. That is why, at startup and during a jam, the motor draws typically 3–10 times its rated current. A driver chosen by looking at rated current alone will go into overcurrent protection on the first start.
Startup and stall current: where the number comes from
The armature circuit of a brushed DC motor is described by V = I·Ra + Eb, where Eb = Ke·ω, that is, back-EMF is proportional to speed. With the rotor stopped, ω = 0, so Eb is also zero and the equation reduces to I = V / Ra. The first milliseconds of a start are electrically identical to a stall.
| Quantity | Typical value | Duration |
|---|---|---|
| Stall / locked rotor current | 3–10 times rated | continuous — the motor burns |
| Startup (inrush) surge | 5–10 times rated | 50–500 ms |
| Limit targeted with a ramp | 1.5–2 times rated | for the duration of the ramp |
Example: a motor with 2.5 A rated current and a stall factor of 5 will try to draw 12.5 A when it jams. The driver must be able to pass that surge, but it does not have to carry it continuously.
What is the difference between "continuous current" and "peak current"?
Continuous current is the current the driver can sustain at maximum junction temperature once it has reached thermal equilibrium. A MOSFET in conduction behaves like a resistor and produces heat equal to P = I²·R; the temperature rise is that power multiplied by the thermal resistance. What limits peak current, however, is not thermal equilibrium but transient thermal impedance — the mass of the die can absorb heat for a short time.
The measurement in the Texas Instruments SLVA505A application note puts numbers on this. The same driver die carries the following currents when only the duration changes:
| Duration | Thermal resistance (θJA) | Current that can be carried |
|---|---|---|
| 0.1 second | 4.3 °C/W | 15.8 A |
| 1 second | 9.2 °C/W | 10.8 A |
| 10 seconds | 13.6 °C/W | 8.9 A |
| Continuous | 30.3 °C/W | 5.9 A |
A die that carries 15.8 A for 0.1 seconds carries only 5.9 A continuously — about a 2.7x difference, entirely due to the thermal time constant. If a product label says "45 A / 5 s", the time there is an inseparable part of the specification.
There is a second limit: overcurrent protection (OCP). Even if the driver's peak capability is sufficient, if the OCP threshold is below the startup surge the system will still cut out. So both the peak current and the protection threshold must be read from the datasheet.
How is PWM frequency selected?
PWM frequency is a three-way trade-off: as frequency goes up, audible whine and current ripple decrease, but switching losses increase and efficiency drops.
- Keeping the switching frequency above 15 kHz largely removes the audible component; above 20 kHz removes it entirely.
- For small low-voltage brushed DC motors the typical range is 10–25 kHz, with the common "sweet spot" at 20–25 kHz.
- The efficiency cost is real: running at 10 kHz can be noticeably more efficient than at 20 kHz.
In an H-bridge, peak-to-peak current ripple is calculated as ΔI = Vs · D · (1 − D) / (L · f); ripple is maximum at D = 0.5 and is inversely proportional to inductance and frequency. For 24 V, 1 mH and 50% duty the calculation gives 6.0 A peak-to-peak at 1 kHz and 0.3 A at 20 kHz. Ripple current produces no torque, only I²R heat in the winding.
H-bridge, shoot-through and braking
An H-bridge connects the motor across supply and ground through four switches. If the upper and lower switch on the same leg conduct together, shoot-through occurs: this current does not pass through the motor, it is a direct supply-to-ground short, and because it is not limited by inductance it can kill the MOSFET within nanoseconds. To prevent it, dead-time is inserted between the two switches.
A motor is an inductive load; if its current is interrupted abruptly it produces a destructive voltage spike per V = L·di/dt. The body diodes of the MOSFETs limit this by opening a freewheeling path. In dynamic braking the motor terminals are shorted through the bridge; the motor behaves like a generator and the kinetic energy turns into heat in the winding resistance. Part of the energy flows back to the supply side — a battery absorbs it, and without one the bus voltage rises. That is why losing the connection between driver and battery during braking (a failed fuse, a main switch opened under load) is the riskiest scenario.
Why does soft start reduce current?
At startup, current scales as I ≈ D · V / Ra; a PWM ramp raises the duty cycle gradually and so lowers the average voltage the motor sees. As the motor speeds up, back-EMF grows and current stays under control even as duty increases.
| Condition | Duty (D) | Average voltage | Startup current |
|---|---|---|---|
| No ramp, full voltage directly | 1.00 | 12.0 V | 40.0 A (8 times rated) |
| Start of ramp | 0.15 | 1.8 V | 6.0 A (1.2 times rated) |
| End of ramp | 1.00 | 12.0 V | rated (~5 A) |
Assumptions for the calculation: 12 V, Ra = 0.3 Ω, rated current 5 A, motor not yet turning. Actual current stays below these values because of back-EMF. The target in ramp design is to limit startup current to 1.5–2 times rated current. A ramp also reduces nuisance fuse blowing, battery voltage sag and mechanical shock.
Cable size and fuse selection
In a DC circuit, voltage drop is calculated over the sum of the outgoing and return conductors: Vdrop = 2 · L · I · ρ / A. Industry practice targets 3% — at 12 V that means a budget of only 0.36 V. For non-critical loads, up to 10% is accepted.
| Conductor size | Approximate ampacity |
|---|---|
| 1.5 mm² (14 AWG) | 15 A |
| 4.0 mm² (12 AWG) | 20 A |
| 6.0 mm² (10 AWG) | 30 A |
| 10 mm² (8 AWG) | 55 A |
| 16 mm² (6 AWG) | 75 A |
The values are for a single conductor in free air; capacity drops for cables run in bundles or in hot environments.
The fuse is selected at 125% of the continuous load and rounded up to the next standard size. The invariable rule: the fuse rating must never exceed the ampacity of the cable it protects. In a DC motor circuit, a slow-blow type is preferred so it can pass the startup surge.
Note: in 12 V systems, for the same power the current is twice that at 24 V and the loss (I²R) is four times. On products supporting both 12 and 24 V, 24 V should be chosen over long distances.
Thermal design and derating
Heat comes from two sources: conduction loss (I²·RDS(on), rising with the square of current) and switching loss (rising roughly linearly with frequency). Datasheet values are typically for a 25 °C ambient; in a hot enclosure or in summer conditions, 10–20% derating should be applied. The thermal shutdown threshold is typically around 150 °C, and repeatedly reaching that threshold wears the driver out over time.
In fully sealed housings such as IP67 there is no convective cooling; heat flows only by conduction into the housing and from there to the mounting surface. That is why mounting sealed drivers in full contact with a metal surface is not only a mechanical but a thermal requirement.
ST42, KS250 and PT500: which job for which?
| Feature | ST42 | KS250 | PT500 |
|---|---|---|---|
| Voltage | 12 / 24 V DC | 12 / 24 V DC | 12–24 V DC |
| Continuous current | entry level | 15 A | 30 A |
| Peak current | — | 45 A / 5 s | 40 A / 15 s |
| Peak / continuous ratio | — | 3.0x | 1.33x |
| Speed adjustment | none | yes | yes (SPEED pot) |
| Ramp | none | yes | yes (RMP pot) |
| Current-limited stop | — | built-in AntiSwitch | separate per direction via A1 / A2 pots |
| Protection rating | — | IP45 | IP67 |
| Inputs | direction (forward-reverse) | IN1–IN5 | 4 digital + 1 analog |
The KS250's 3.0x peak ratio is of the same order as the 2.7x thermal behavior in the TI measurement, and it is meant for loads with a short, violent startup surge. The PT500's 1.33x ratio is lower but its window is 3 times longer — the right profile for work requiring sustained high load such as high inertia, long stroke, or inclines.
The absence of speed and ramp on the ST42 is consistent with no AntiSwitch-type function being listed: without a ramp the startup surge cannot be suppressed, and a current threshold would trigger falsely all the time. Current-based stopping can only be set up reliably on a driver with a ramp.
Quick selection flow
- Measure the motor's rated current or read it from the nameplate.
- Calculate the stall current (I = V / Ra) or assume 3–10 times rated.
- Make sure the driver's continuous current is greater than the motor's rated current even after 10–20% derating.
- Make sure the driver's peak current covers the startup surge, and the peak duration covers the length of the start (typically 50–500 ms).
- If there is a ramp, steps 3 and 4 get easier: the surge drops to 1.5–2 times rated.
- Select conductor size by voltage drop (3%) and the fuse by the 125% rule.
- Determine the IP rating by the environment: IP4X is not enough in a dusty workshop.
Standards
Ratings and duty cycles of rotating machines fall under IEC 60034-1, ratings of adjustable speed drive systems under IEC 61800-1, EMC requirements under IEC/EN 61800-3, and general industrial EMC under EN IEC 61000-6-2 / -6-4. A wired driver with no radio transmitter falls under the EMC Directive 2014/30/EU; the moment an RF receiver is added to the product, the assessment moves under the Radio Equipment Directive (RED 2014/53/EU).