When you search for a double throw single pole switch diagram, you are usually looking for one of two things: a manual toggle switch schematic, or the wiring diagram for a Single Pole Double Throw (SPDT) electromechanical relay. In industrial controls, home automation, and heavy-duty DIY builds, the SPDT relay is the undisputed workhorse. It allows a low-power control circuit to safely switch a high-power load, while providing the flexibility to route power to one of two different paths depending on whether the coil is energized.
Unlike a simple Single Pole Single Throw (SPST) switch that just turns a circuit on or off, an SPDT relay has three contact terminals: Common (COM), Normally Open (NO), and Normally Closed (NC). This article breaks down exactly how to wire, size, and test these components on the bench and in the panel.
Decoding the Double Throw Single Pole Switch Diagram
The most critical concept in any relay schematic is the physical and electrical separation between the coil side (control) and the contact side (load). They share no electrical connection; they are coupled only by a magnetic field.
The Coil Side (Control Circuit)
The coil is an electromagnet. When you apply the rated voltage across the coil terminals (typically labeled A1 and A2 per IEC standards, or pins 13 and 14 on older octal bases), current flows through the copper windings, generating a magnetic field that pulls the armature. For a 12V DC coil on a standard Omron G2R-1-S relay, you will draw roughly 43mA. This side is wired using 18 AWG or 22 AWG control wire, routed through your PLC outputs, microcontrollers (via a transistor driver), or simple pushbuttons.
Whenever you wire a DC coil, you must install a flyback diode (like a 1N4007) in reverse parallel across A1 and A2. When the control circuit opens, the collapsing magnetic field induces a massive reverse voltage spike that will fry your driving transistor or microcontroller GPIO. Wire the diode's cathode (the silver stripe) to the positive terminal (A1) and the anode to the negative terminal (A2). AC coils do not require this, as they rely on internal shading rings and the AC zero-crossing to manage the collapse.
The Contact Side (Load Circuit)
The contact side handles the heavy lifting. The IEC standard pinout for an SPDT relay's contacts is:
- 11 (Common / COM): The movable wiper arm attached to the armature. This is where your load's hot/positive wire connects.
- 12 (Normally Closed / NC): Connected to COM when the coil is de-energized. Power flows here by default.
- 14 (Normally Open / NO): Connected to COM only when the coil is energized.
Rating Table and Load Selection Decision Path
A common mistake is reading the "10A" printed on the relay housing and assuming it can switch any 10A load. Relay contacts are rated differently depending on the physics of the load. When a relay opens under an inductive load, the magnetic field of the load collapses, drawing an arc across the separating contacts. This arc erodes the metal and can weld the contacts shut.
| Specification | Value / Rating | What It Means in Practice |
|---|---|---|
| Coil Voltage | 12V DC, 24V DC, 120V AC, 240V AC | Must match your control circuit exactly. ±10% tolerance. |
| Resistive Contact Rating | 10A @ 250V AC / 30V DC | Heaters, incandescent bulbs. No inrush or arc blowout needed. |
| Inductive Breaking Capacity | 3A @ 250V AC (cos φ = 0.4) | Solenoids, contactor coils, transformers. Severe arcing on break. |
| Motor HP Rating | 1/4 HP @ 120V AC; 1/2 HP @ 240V AC | Locked-rotor inrush current is 6x the running current. |
Which Rating Column Governs This Load?
The governing rule is simple: always use the lowest applicable rating column for your specific load type. If you are switching a 240V AC compressor motor, you do not look at the 10A resistive column. You look at the Motor HP column. A 1/2 HP motor at 240V draws about 4.9A (per NFPA 70 / NEC tables), but the locked-rotor inrush can spike to 30A for a fraction of a second. If your relay isn't explicitly rated for motor duty, you must use the relay to trigger a heavy-duty contactor instead.
Load Decision Tree
| Load Type | Governing Column | Required Suppression / Derating |
|---|---|---|
| Resistive (Heaters, LED drivers) | Resistive (10A) | None. Wire at 100% rated capacity. |
| Inductive (Solenoids, AC coils) | Inductive (3A) | Add an RC snubber across the load to quench the AC arc. |
| Capacitive (Switching power supplies) | Resistive (Derate 50%) | Inrush current charges caps. Add an NTC thermistor in series. |
| Motor (Compressors, fans) | Motor HP | If HP rating is missing, use a contactor. Do not switch directly. |
How to Test It Dead and Live
When troubleshooting a panel, you need to know if the relay coil is intact and if the contacts are actually making a solid connection. Here is the exact bench and field procedure.
Dead Testing (Power Off, Isolated)
- Verify De-energized: Use a non-contact voltage tester, then verify with a multimeter on AC/DC voltage mode across A1/A2 and 11/14. Read must be 0V.
- Test the Coil: Set your DMM to resistance (Ω). Place probes on A1 and A2. A healthy 12V DC coil will read between 150Ω and 400Ω. A 120V AC coil will read in the kΩ range (e.g., 4kΩ to 8kΩ). If it reads OL (Open Line), the internal copper wire is broken. If it reads 0.0Ω, the coil is shorted internally.
- Test the Contacts: Set DMM to continuity (the beep setting). Place probes on 11 (COM) and 12 (NC). It should beep (read < 1 ohm). Move the probe from 12 to 14 (NO). It should read OL. Now, manually press the relay's test armature (if equipped) or apply a temporary 9V battery to the coil. The continuity should swap: 11-12 opens, 11-14 beeps.
Live Testing (Energized and Under Load)
- Check Coil Voltage: Set DMM to AC or DC voltage. Measure directly across A1 and A2 while the control signal is active. If you read nominal voltage (e.g., 118V on a 120V AC coil) but the relay doesn't pull in, the mechanical armature is jammed or the coil is open.
- Check Contact Voltage Drop: This is the ultimate test of contact health. With the load running and current flowing through 11 to 14, set your DMM to millivolts (mV) DC or AC. Place one probe on terminal 11 and the other on terminal 14. A healthy contact will show a voltage drop of less than 50mV. If you read 200mV or higher, the contacts are pitted, carbonized, or welding together, creating a dangerous heat source.
When to Repair vs. Replace
In modern electrical practice, you almost never repair an electromechanical relay; you replace it. A standard DIN-rail SPDT relay like a Finder 40.52 or Schneider RXM costs between $5 and $12. The labor cost to dismantle, sand contacts, and re-tension springs far exceeds the part cost, and sanded contacts lose their silver-alloy plating, leading to rapid subsequent failure.
Replace immediately if:
- The plastic housing shows heat discoloration (browning) near the contact terminals.
- The coil reads open (OL) on a multimeter.
- You measure a voltage drop > 100mV across closed contacts under load.
- The relay audibly buzzes or chatters continuously (indicates a failing AC shading ring or low coil voltage).
The only time "repair" is acceptable is in high-voltage, legacy industrial contactors (not standard panel relays) where contact pads can be professionally filed and arc chutes replaced. For anything under 30A, swap the component.
Frequently Asked Questions
How do I read a double throw single pole switch diagram for a 120V AC load?
In a standard 120V AC SPDT diagram, your hot wire (black) connects to the Common (11) terminal. The device you want powered when the system is off connects to the NC (12) terminal. The device you want powered when the system is triggered connects to the NO (14) terminal. The neutral wires for both devices tie together on a wire nut or terminal block, completely bypassing the relay. The coil (A1/A2) is wired to your 120V control switch or smart relay output.
Why does my SPDT relay coil burn out when switching DC loads?
DC arcs are notoriously difficult to extinguish because the voltage never crosses zero like AC does. If you are using a relay rated for 10A AC to switch a 10A DC load, the arc will sustain, melt the contacts, and potentially weld them shut. Furthermore, if you fail to install a flyback diode across a DC coil, the inductive kickback from the coil itself will degrade the internal winding insulation over time, eventually causing a short that burns out the coil. Always check the DC specific rating (usually much lower, like 3A at 30V DC) and use arc suppression.
Can I use an SPDT relay to reverse a DC motor direction?
No, a single SPDT relay cannot reverse a DC motor. To reverse polarity, you need a Double Pole Double Throw (DPDT) relay, or two SPDT relays wired in an H-bridge configuration. In an H-bridge setup, you wire the motor across the two NO/NC junction points, and route the positive and negative supply rails to the two Common terminals. Energizing one relay sends current left-to-right; energizing the other sends it right-to-left. Ensure you implement a mechanical or electrical interlock so both relays cannot energize simultaneously, which would create a dead short across your power supply.
