The Core Decision: Sizing the Electrical Board Fuse and Relay
When designing or repairing a printed circuit board (PCB) that switches mains or high-current DC loads, the relay and the fuse must be sized as a coordinated system. The relay handles the daily switching, while the electrical board fuse protects the PCB traces and relay contacts from catastrophic short circuits.
The direct answer: For a standard 120V AC, 10A resistive load on a PCB, use a 12A fast-acting electrical board fuse (like the Littelfuse 395 series) paired with a 16A-rated electromechanical relay (like the Omron G5Q-14). The fuse must be rated slightly above the steady-state load but below the thermal limit of your PCB traces, while the relay contacts must exceed the maximum steady-state current.
Treating these components as isolated choices leads to melted traces or nuisance blowing. Below is the exact decision framework for selecting, wiring, and testing this electromechanical pairing.
Rating Table: Coil, Contact, and Breaking Capacity
Relay datasheets list multiple ratings, but they are not created equal. The coil voltage dictates your control circuit, the contact rating defines steady-state thermal limits, and the breaking capacity defines the maximum fault current the relay can safely interrupt without welding shut. Here is how common board-mounted components stack up:
| Component / Model | Coil Voltage (Control) | Contact Rating (Resistive) | Breaking Capacity / Interrupt Rating |
|---|---|---|---|
| Omron G5Q-14 (Relay) | 5V DC (125Ω coil) | 10A @ 250V AC | 100A @ 250V AC (cos φ=1) |
| Song Chuan 833F (Relay) | 12V DC (400Ω coil) | 16A @ 250V AC | 150A @ 250V AC |
| Littelfuse 395 Series (Fuse) | N/A | N/A (Current limiting) | 10,000A @ 125V AC (Fast-Acting) |
| Littelfuse 452 Series (Fuse) | N/A | N/A (Time-Lag) | 500A @ 125V AC (Slow-Blow) |
Wiring the Board: Coil Side vs. Contact Side
Proper PCB layout requires strict physical and electrical separation between the low-voltage control side and the high-voltage load side. According to IPC-2221 standards, maintaining adequate creepage and clearance distances prevents high-voltage arcs from jumping to your low-voltage logic.
The Coil Side (Control Circuit)
The coil is an inductor. When your driving transistor (e.g., a 2N2222 or ULN2003) turns off, the collapsing magnetic field generates a massive reverse voltage spike (back-EMF) that can easily exceed 100V, instantly destroying your driving silicon.
- Flyback Protection: You must place a flyback diode (like a 1N4148 for fast switching or 1N4007 for general purpose) in reverse bias across the coil pins. The cathode (stripe) points toward the positive coil supply.
- Trace Width: Coil current is low (typically 30mA to 80mA). Standard 10-mil traces are more than sufficient.
The Contact Side (Load Circuit)
This is where the electrical board fuse and the relay contacts live in series.
- Trace Width & Copper Weight: For a 10A load, use at least 2oz copper with a 50-mil trace width, or pour a ground/power plane.
- Fuse Placement: Place the electrical board fuse as close to the power entry point as possible, before the relay contacts. If a short occurs downstream, the fuse clears the fault before the relay contacts are forced to interrupt it, preventing contact welding.
Load Type Decision Path: Resistive, Inductive, and Motor
Which rating column governs your specific load? Use this decision tree to select the right relay and electrical board fuse combination.
| Load Type | Governing Rating Column | Selection Rule & Derating |
|---|---|---|
| Resistive (Heaters, Incandescent) |
Nominal Contact Rating (AC/DC) | Size fuse at 125% of steady-state load. Size relay at 125% of steady-state load. |
| Inductive (Solenoids, Contactors) |
Breaking Capacity & Derated Current | Derate relay contact rating by 50%. Use a slow-blow fuse to handle 10x inrush current without nuisance tripping. |
| Motor (Compressors, Fans) |
Locked Rotor Amps (LRA) & FLA | Requires specific 'Motor Rated' relays (e.g., Omron G7L). Size fuse to motor FLA, ensuring it survives LRA startup. |
The Concrete Pick for Inductive Loads
Scenario: You are switching a 120V AC solenoid valve that draws 2A steady-state but has a 10A inrush.
The Pick: Use the Omron G5Q-1A (rated 10A/250VAC, which derates safely to 5A inductive). Protect it with a 5A slow-blow electrical board fuse (Littelfuse 452 series). The slow-blow fuse's time-delay curve will ignore the 10A inrush (which lasts only milliseconds) but will still clear a dead short in under 5ms, saving your PCB traces.
Testing and Diagnostics: Dead vs. Live Checks
When a board fails to switch a load, you need a systematic approach to isolate the fault. Always start with dead checks before applying power.
Dead Checks (Power Off)
- Test the Fuse: Set your multimeter to continuity or resistance. A good electrical board fuse will read < 1 ohm. An open reading (OL) means it has blown.
- Test the Relay Coil: Measure resistance across the coil pins. A 5V Omron G5Q coil should read exactly 125Ω (±10%). An OL reading means the internal coil wire is broken.
- Test the Contacts: Measure across the Common (COM) and Normally Open (NO) pins. It should read OL. Measure COM to Normally Closed (NC); it should read < 1 ohm. If NO reads < 1 ohm without power, the contacts are welded shut from a past overcurrent event.
Live Checks (Power On)
- Coil Voltage: Energize the circuit. Measure DC voltage across the coil pins. It must be within ±10% of the nominal coil voltage (e.g., 4.5V to 5.5V for a 5V relay). A lower voltage indicates a weak driver transistor or excessive voltage drop in the control traces.
- Contact Voltage Drop: With the relay energized and the load running, measure the AC voltage drop directly across the COM and NO pins. A healthy relay will show < 50mV. If you read several volts, the internal contacts are pitted or carbonized and the relay must be replaced.
Repair vs. Replace: When to Rework the PCB
When an electrical board fuse blows, it is doing its job. But why it blew dictates your next move. Furthermore, you must understand the difference between a fuse and a thermal breaker in this context.
Never swap a fast-acting board fuse for a mini thermal breaker without checking the I²t let-through energy curve. Fuses and breakers are not interchangeable. A fast-acting 10A fuse might clear a 1000A short circuit in 1ms, limiting the thermal energy (I²t) to roughly 15 A²s. A 10A thermal breaker might take 20ms to trip under the same fault, letting through 200 A²s of energy—more than enough to vaporize a 2oz copper PCB trace before the breaker opens. For PCB-level protection, high-interrupting-capacity fuses are mandatory.
When to Repair (Rework the Board)
- Clean Fuse Blow: The fuse is open, but the PCB is clean. Replace the fuse and the relay (as a precaution, since the relay contacts may have suffered arc damage during the fault).
- Open Coil: The relay coil reads OL, but the board is pristine. Desolder the relay using a temperature-controlled station (e.g., Hakko FX-888D set to 350°C) and a solder sucker. Install a new relay and flyback diode.
When to Replace the Entire Board
- Scorched FR4 or Carbon Tracking: If the fiberglass substrate is brown/black, or you see carbon tracking across the clearance gap between the coil and contact traces, the board's dielectric strength is permanently compromised. Scrap it.
- Lifted Pads: If the high-current pads for the relay contacts have lifted from the substrate due to excessive heat during a fault or poor rework, the mechanical and electrical integrity is gone. Replace the board.
The Default Recommendation
If you are prototyping a general-purpose 120V AC switching board and need a reliable baseline, do not overthink it. Use the Omron G5Q-14 (5V DC coil, 10A contacts) driven by an optocoupler for galvanic isolation. Protect the load side with a 10A Littelfuse 395 series fast-acting electrical board fuse. This combination provides excellent I²t protection for standard 2oz PCB traces, handles typical household resistive loads safely, and is widely available from major distributors like DigiKey and Mouser for under $3.00 per assembled set.
