Start with the coil data
For a simple DC actuator coil, Ohm’s law provides a useful first estimate: I = V/R, where I is current in amperes, V is applied voltage and R is coil resistance in ohms.
| Equation | Meaning |
| I = V / R | Actuator current |
| P = V x I | Electrical power |
| P = V² / R | Equivalent power form |
Example: one 200-ohm coil at 24 VDC
I = 24/200 = 0.12 A, or 120 mA. The associated resistive power is P = V x I = 2.88 W while that coil is energized.
Example: one 300-ohm coil at 24 VDC
I = 24/300 = 0.08 A, or 80 mA. If only one coil is energized at a time, the supply should be designed for at least that load plus appropriate engineering margin and any electronics used by the switch.
Multiple coils
Do not multiply the single-coil current by the number of switch positions unless the circuit actually energizes those coils simultaneously. An SP6T switch may contain multiple actuators but its normal control scheme may energize only the selected path. Use the schematic to determine the worst-case simultaneous load.
Tolerance and temperature
Coil resistance changes with manufacturing tolerance and temperature, so the calculated current is an estimate rather than a guaranteed maximum. Supply sizing should use the manufacturer’s specified actuator current when available.
Driver selection
The external transistor, relay, MOSFET, I/O module or driver array must safely handle the required coil current and the inductive nature of the load. Suppression components may be required to control the voltage transient produced when coil current is interrupted.
Key takeaway
Use Ohm’s law for estimates, but size the real supply and driver from specified worst-case actuator requirements.