Solenoid / Actuator
Actuator
A coil with a movable iron plunger: energise it and the plunger snaps in, giving linear push or pull motion. The muscle behind locks, valves and vending machines.
What it looks like
Solenoid / Actuator
A typical actuator — exact shape, colour and markings vary between manufacturers.
Types & variants
Key specs
Coil voltage
V6/12/24 V DC common — drive at rated voltage.
Force
NPull force, strongest near fully-retracted; falls off with stroke.
Stroke
mmUsable travel distance.
Duty cycle
%Continuous-rated vs intermittent — many burn out if held on.
Laws that govern it
- Ampère's circuital law∮ B · dl = μ₀ × I(enc)
The coil's ampere-turns produce the field that pulls the plunger; halve the turns and you halve the pull for the same current.
- Hopkinson's law — reluctance formulaMMF = Φ × S, S = l / (μ₀μᵣA)
Force depends on the air gap: reluctance is highest when the plunger is out, which is why solenoids are weak at the start of their stroke and slam at the end.
- Lenz's lawthe minus sign in EMF = −N × dΦ/dt
Big inductance means a big turn-off spike, so a flyback diode is mandatory — and it also makes the release slower.
Each law is stated in full — with its diagram, variables and worked meaning — on the electrical laws page.
Markings
Voltage and duty printed on the coil body or label.
Standard values
12 V push/pull solenoids with 5–10 mm stroke dominate hobby use; 12 V solenoid valves for water/air.
How to choose
Force needed at the END of the stroke (not the headline force), voltage to suit your supply, and duty cycle honestly — a door-lock (pulse) part dies if held energised.
Pinout & package
2 wire leads to the coil, not polarised (unless latching). Mounting bracket + plunger with clevis/spring.
Example circuits
- MOSFET + flyback diode driving a 12 V lock solenoid
- Solenoid valve on a plant-watering timer
- Vending machine coin gate
Common failures
Burnt coil from exceeding duty cycle, jammed plunger from grit or misalignment, and killed driver transistors when the flyback diode was omitted.
How to test
Coil resistance: expect tens of ohms (12 V types) — open = burnt, near 0 = shorted turns. Powered briefly at rated voltage it should clack firmly.
Substitutes
A small geared DC motor + cam, or a servo, gives controllable motion where a solenoid's snap is too violent; a relay is a solenoid that switches contacts instead of moving a load.