An SPST (Single Pole, Single Throw) relay is the workhorse of electromechanical switching. It acts as a remote-controlled, electrically isolated switch, allowing a low-power control circuit to safely command a high-power load. When you look at a relay SPST diagram, you are looking at two completely separate circuits sharing a single magnetic core: the coil (control) and the contact (load).
Whether you are switching a 120VAC space heater with an ESP32 or controlling a 12VDC water pump with an Arduino, misreading the schematic or ignoring the load-type derating curves will result in welded contacts, fried microcontrollers, or a melted socket. This guide breaks down the schematic, the rating tables, and the exact decision path to select and test the right SPST relay for your bench or jobsite.
Decoding the Relay SPST Diagram: Coil vs. Contact Wiring
A standard SPST relay schematic splits cleanly down the middle. On the left (or top) is the coil. On the right (or bottom) is the switch. They share no electrical connection, only magnetic coupling.
The Coil Side (Control Circuit)
In IEC 60947 standard diagrams, the coil pins are typically labeled A1 and A2 (or 13 and 14 on older industrial relays). This is where you apply your control voltage (e.g., 5V, 12V, 24V DC, or 120V AC). For AC coils, polarity does not matter. For DC coils, A1 is usually positive and A2 is negative, though the coil itself will energize in either direction.
The Contact Side (Load Circuit)
An SPST relay has one pole (the moving armature) and one throw. It comes in two flavors:
- SPST-NO (Form A): Normally Open. The circuit is open until the coil is energized. Pins are typically labeled COM (Common, pin 11) and NO (Normally Open, pin 14).
- SPST-NC (Form B): Normally Closed. The circuit is closed until the coil is energized. Pins are COM (pin 11) and NC (Normally Closed, pin 12).
For a complete deep-dive into the physics of the armature and spring return mechanisms, the All About Circuits relay textbook chapter provides excellent cross-sectional diagrams of the internal air gaps.
SPST Relay Rating Table: Which Column Governs Your Load?
The most common mistake hobbyists make is looking only at the bold '10A' printed on the relay casing. Relay manufacturers print multiple ratings based on the load's electrical characteristics. Here is how to read the datasheet rating table.
| Specification | Typical 10A SPST Value | What It Actually Means |
|---|---|---|
| Coil Voltage | 12V DC / 120V AC | The exact voltage required to pull the armature. Must be within ±10% of nominal. |
| Resistive Contact Rating | 10A @ 250V AC | Governs heaters, incandescent bulbs, and purely resistive loads where inrush = steady state. |
| Inductive Contact Rating | 3A @ 250V AC (cos φ = 0.4) | Governs solenoids, transformers, and contactor coils. The arc on break-open is severe. |
| Motor / HP Rating | 1/2 HP @ 120V AC | Governs compressors and pumps. Accounts for Locked Rotor Amps (LRA) which can be 6x steady state. |
| DC Breaking Capacity | 10A @ 30V DC | DC arcs do not have a zero-crossing to self-extinguish. A 250VAC relay might only safely break 30VDC. |
Which column governs your load? You must identify the load's inrush profile. If you are switching an AC motor that draws 2A running, but has an LRA (inrush) of 12A, the '10A Resistive' column is useless. You must use the 'Motor/HP' column or derate the resistive rating by at least 60%. For comprehensive derating curves based on ambient temperature and switching frequency, refer to the Omron Relay Technical Guide.
Load-Type Decision Path: Sizing Your SPST Relay
Stop guessing. Use this decision tree to select the exact relay part number based on your load type. These are industry-standard, readily available parts as of 2026.
| Load Type | Sizing Rule | Concrete Part Pick (Default) | Approx. Cost |
|---|---|---|---|
| Resistive (Heaters, LEDs, Incandescent) |
Relay Rating ≥ 1.25 × Steady State Current | Omron G2R-1-E DC12 (Slim 10A SPST-NO, PCB mount) |
$4.50 |
| Inductive (Solenoids, Valves, Contactors) |
Relay Rating ≥ 3 × Steady State Current. Must have high break capacity. | Finder 38.51.7.012.0000 (Includes integrated flyback diode & LED indicator) |
$9.00 |
| Motor (Pumps, Compressors, Fans) |
Relay Rating ≥ 6 × Steady State Current (LRA). Must be HP-rated. | Omron G7L-1A-T DC12 (Heavy duty 30A SPST-NO, quick-connect tabs) |
$7.50 |
Bench Testing: How to Verify a Relay Dead and Live
Before soldering or wiring a relay into a panel, verify it on the bench. You need a standard digital multimeter (DMM).
Dead Testing (De-energized)
- Test the Coil: Set your DMM to Ohms (Ω). Place probes on A1 and A2. A healthy 12V DC coil (like the Omron G2R) will read between 100Ω and 400Ω. If it reads 'OL' (infinite), the internal copper wire is broken. If it reads near 0Ω, the coil is shorted. Both mean the relay is trash.
- Test the Contacts (SPST-NO): Set DMM to Continuity or Ohms. Place probes on COM and NO. It must read 'OL' (open). Place probes on COM and NC (if it's an SPDT relay acting as SPST). It should read < 1.0Ω.
Live Testing (Energized)
- Energize the Coil: Apply the rated DC voltage to A1/A2. You should hear a distinct, sharp 'click'. (If it buzzes or chatters on AC, the shading ring on the armature core is cracked).
- Verify Contact Closure: With the coil energized, measure COM to NO. It should now read < 1.0Ω.
- The Voltage Drop Test (Under Load): This is the ultimate test of contact health. Wire a real load (e.g., 5A) through the contacts. Set your DMM to DC Millivolts (mV). Place the probes directly on the relay's COM and NO metal pins (not the PCB traces). A healthy relay will show a voltage drop of < 20mV. If you read > 50mV across a closed contact carrying 5A, that is 250mW of heat dissipated directly at the pin. The contacts are pitted, oxidized, or carbon-fouled.
For more advanced diagnostic procedures, including insulation resistance testing with a Megger, Macromatic's relay testing guide provides excellent field-technician workflows.
Repair vs. Replace: When a Welded Contact Means Trash
A common question on the bench is whether you can open a relay and file down pitted or welded contacts. The default recommendation is always to replace.
Electromechanical relays are factory-sealed for a reason. The contacts are not just bare copper; they are plated with specific alloys like silver-tin oxide (AgSnO2) or silver-cadmium oxide. These oxides are specifically engineered to resist arc-welding and maintain low contact resistance under high inrush currents. If you open the casing and file the contacts with sandpaper or a file, you strip away this critical oxide layer, exposing the softer base metal. The next time the relay breaks an inductive load, the arc will melt the bare metal, and the contacts will weld shut permanently—a massive fire hazard.
When to strictly replace:
- Welded Contacts: The relay fails to drop out when power is removed from the coil. The armature spring is overpowered by melted contact material.
- Coil Degradation: The coil resistance has drifted more than 15% from its nominal spec, indicating thermal degradation of the internal wire enamel.
- Erratic Continuity: The contact resistance fluctuates above 100mΩ, indicating severe carbon tracking from thousands of arc cycles.
Industrial relays like the Omron G2R or Finder 38 series cost between $4 and $12. The labor cost to diagnose a failing relay, combined with the catastrophic risk of a welded contact failing to drop out a motor or heater, makes repair entirely uneconomical. When a relay fails, desolder it, throw it in the e-waste bin, and drop in a fresh, correctly sized replacement.
