A double throw switch—often designated as Single Pole Double Throw (SPDT) or Double Pole Double Throw (DPDT)—is an electromechanical device where a single common terminal routes current to one of two alternate paths: Normally Open (NO) or Normally Closed (NC). When built as a relay or contactor, a magnetic coil actuates this mechanical movement. In industrial and home automation panels, "double throw" usually refers to these coil-driven Form C contacts, which allow you to switch a load on while simultaneously switching another load off, or reverse motor polarity.
The Anatomy of a Double Throw Electromechanical Switch
To wire and troubleshoot these components effectively, you must separate the device into two distinct circuits: the coil circuit (control side) and the contact circuit (load side).
The Coil Circuit (Control Side)
The coil is an inductor wrapped around an iron core. When you apply the rated voltage (e.g., 24VDC or 120VAC), it generates a magnetic field that pulls the mechanical armature, shifting the common contact from the NC to the NO position. When voltage is removed, a physical spring returns the armature to its resting state.
The Contact Circuit (Load Side)
The contact side handles the actual load current. A standard SPDT (Form C) relay has three load terminals:
- Common (C): The moving blade that connects to either NO or NC.
- Normally Open (NO): Connects to Common only when the coil is energized.
- Normally Closed (NC): Connects to Common when the coil is de-energized (resting state).
Rating Table and Load Selection Decision Path
The most common mistake DIYers and junior technicians make is sizing a relay based on its maximum resistive thermal rating, then watching the contacts weld shut when switching a motor. According to Macromatic's relay application guidelines, you must match the contact material and rating to the specific utilization category.
| Parameter | Resistive Load (AC-1) | Inductive Load (AC-11) | Motor Load (AC-3/AC-4) |
|---|---|---|---|
| Coil Voltage | 24VDC / 120VAC (Independent of load type) | ||
| Contact Rating (Steady State) | 10A @ 250VAC | 3A @ 250VAC | 1/3 HP @ 120VAC |
| Breaking Capacity (Inrush) | 10A | 18A | 30A+ (Locked Rotor) |
| Electrical Life (Operations) | 100,000 | 50,000 | 10,000 |
Which Rating Column Governs This Load?
The governing column is dictated by the inrush current multiplier of your specific load.
- Heaters/Incandescent Bulbs (Resistive): Governed by the Resistive column. Inrush is roughly 1x to 10x steady state (cold filament).
- Solenoids/Transformers (Inductive): Governed by the Inductive column. Inrush is 4x to 6x steady state. The phase shift between voltage and current makes arc extinction difficult.
- Compressors/Fans (Motor): Governed by the Motor (HP or FLA) column. Motors draw 6x to 10x Locked Rotor Amps (LRA) on startup. Never use the 10A resistive rating for a 10A motor.
Note on upstream protection: Unlike fuses, which blow based on thermal mass, magnetic breakers trip instantaneously on short circuits; ensure your upstream breaker's trip curve (e.g., Type C or D for motor inrush) coordinates with the switch's short-circuit withstand rating so the breaker clears the fault before the relay explodes.
Wiring, Testing, and Maintenance
Proper installation and diagnostics require isolating the control logic from the power circuit. Keep coil wiring physically separated from contact wiring inside the panel to prevent inductive noise from resetting sensitive microcontrollers.
How to Test Dead (De-energized)
Lock out and tag out the panel. Verify zero voltage with a multimeter before proceeding.
- Test the Coil: Set your meter to Ohms (Ω). Place probes on A1 and A2. A healthy 24VDC coil typically reads between 400Ω and 800Ω. A reading of 0Ω means a shorted coil; OL (infinite) means a broken internal wire.
- Test the Contacts (Resting): Set meter to Continuity. Place probes on Common and NC. You should read ~0.1Ω (a solid beep). Place probes on Common and NO; it should read OL (open).
- Test the Contacts (Actuated): Manually press the armature with a non-conductive tool. Common-to-NC should now read OL, and Common-to-NO should read ~0.1Ω.
How to Test Live (Energized)
Safety Note: Mains voltage is lethal. Only perform live testing if qualified, using Category III or IV rated meter leads.
- Verify Coil Voltage: Set meter to AC or DC Volts. Measure across A1 and A2 while the control signal is active. It must be within ±10% of the nominal coil rating. A 24VDC coil pulling down to 18V indicates a weak power supply or excessive voltage drop in undersized control wires.
- Measure Contact Voltage Drop: With the load running, measure the voltage across the Common and NO terminals. A healthy contact will drop less than 50mV. If you read 1V or more, the contacts are pitted, oxidized, or carbon-fouled.
When to Repair vs. Replace
Electromechanical switches are consumable components. Replace the unit if:
- The coil smells like ozone or burnt plastic (insulation breakdown).
- The live voltage drop across closed contacts exceeds 100mV.
- The contacts have physically welded together and the armature sticks.
Frequently Asked Questions
What is the difference between a single throw and a double throw switch?
A single throw switch (Form A or SPST) acts like a standard light switch: it simply makes or breaks a single connection. It has two terminals. A double throw switch (Form C or SPDT) acts as a diverter: it has three terminals and routes the common connection between two alternate paths. You use single throw to turn one thing on or off; you use double throw when you need to switch between two different loads, trigger an alarm when a machine stops, or reverse polarity.
Can I use a double throw relay to reverse a DC motor?
Yes, but you need a DPDT (Double Pole Double Throw) relay, not an SPDT. By wiring the DC power supply to the two outer terminals of one pole, and crossing the wires to the outer terminals of the second pole, the common outputs will deliver positive/negative in one state, and negative/positive in the energized state. This is the standard H-bridge mechanical equivalent used in simple winch controls and garage door openers. Ensure the relay has a mechanical interlock or a brief delay in your control logic to prevent short-circuiting the supply if the relay bounces.
Why did my double throw switch contacts weld together?
Contact welding almost always occurs when breaking or making a highly inductive load without arc suppression. When contacts open under an inductive load, the collapsing magnetic field sustains an electrical arc across the gap. This arc melts the silver alloy on the contact faces. If the contacts bounce slightly upon closing (make-bounce), the localized heat can fuse the two faces into a single solid lump of metal. To prevent this, add an RC snubber network across the contacts for AC loads, or ensure you are using a relay specifically rated for the motor's Locked Rotor Amps (LRA).
