If you need a default workhorse for 90% of Arduino, ESP32, and Raspberry Pi switching tasks, buy the Songle SRD-05VDC-SL-C (rated 10A @ 250VAC / 30VDC). It is cheap, widely available, and handles standard resistive loads easily. If you are switching low-current logic, audio signals, or need high-reliability bifurcated contacts, step up to the Omron G5V-2-5VDC (2A @ 250VAC). Stop guessing based on the physical size of the relay; the internal contact metallurgy and coil current draw dictate whether your circuit will survive its first switching cycle.
Decoding the Datasheet: Which Rating Column Governs Your Load?
The most common point of failure in DIY and light industrial control panels is a melted relay contact. This happens when a builder looks exclusively at the 'Resistive' rating column while switching an inductive or motor load. Which rating column governs this load? The answer depends entirely on the inrush current and arc energy of your specific device.
Resistive loads (heaters, incandescent bulbs) draw steady current. Inductive loads (solenoids, transformers) and motor loads store energy in magnetic fields. When a relay contact opens an inductive circuit, the collapsing magnetic field sustains an electrical arc across the separating contacts, vaporizing the metal over time. Furthermore, AC motors draw Locked Rotor Amps (LRA) that can be 600% of their running current during startup.
| Parameter | Songle SRD-05VDC-SL-C | Omron G5V-2 (5VDC) | Panasonic JW2SN-DC5V |
|---|---|---|---|
| Coil Voltage / Current | 5VDC / ~70mA | 5VDC / ~50mA | 5VDC / ~106mA |
| Contact Rating (Resistive) | 10A @ 250VAC / 30VDC | 2A @ 250VAC / 2A @ 30VDC | 8A @ 250VAC / 8A @ 30VDC |
| Contact Rating (Inductive/Motor) | ~1.5A (Derated by 85%) | 0.5A (Derated by 75%) | 3A (Derated by 60%) |
| Max Breaking Capacity | 2500VA | 62.5VA | 2000VA |
| Expected Mechanical Life | 10 Million ops | 50 Million ops | 50 Million ops |
As noted in All About Circuits' relay fundamentals, you must heavily derate the resistive maximum when dealing with reactive components. If your Songle relay is rated for 10A resistive, do not put a 10A motor on it. Treat the inductive limit as roughly 15% to 30% of the printed resistive maximum unless the datasheet explicitly publishes a higher motor rating.
Coil vs. Contact: Wiring the 5VDC Relay Correctly
A relay is effectively two separate circuits sharing a magnetic bridge. Treating the coil side and the contact side as a single electrical entity will destroy your microcontroller.
The Coil Side (Low Voltage DC Control)
The coil is an inductor. When you apply 5V, it draws a steady current (e.g., 70mA for the Songle). An ESP32 GPIO pin is rated for an absolute maximum of 40mA, with a recommended continuous limit of 12mA. Never drive a 5V relay coil directly from a microcontroller pin. You must use a driver transistor. A logic-level N-channel MOSFET like the IRLZ44N or a standard BJT like the 2N2222 (with a 1kΩ base resistor) will safely isolate the GPIO from the coil current.
When the transistor turns off, the magnetic field in the relay coil collapses, inducing a high-voltage reverse spike (inductive kickback) that can easily exceed 100V. This spike will punch through your driving transistor and fry your ESP32/Arduino. You must wire a standard rectifier diode (like a 1N4007) in reverse-bias across the coil pins: the cathode (striped end) connects to the 5V side, and the anode connects to the transistor side. See Texas Instruments' application note on inductive kickback for the exact physics of this failure mode.
The Contact Side (High Voltage / High Current Load)
The contact side consists of three terminals: COM (Common), NO (Normally Open), and NC (Normally Closed).
- COM: The moving armature. Connect your incoming hot/load voltage here.
- NO: Connects to COM only when the coil is energized. Use this for failsafe-off applications (e.g., heaters, lights).
- NC: Connects to COM when the coil is dead. Use this for failsafe-on applications (e.g., emergency cooling fans, brakes).
Load-Type Decision Tree: Picking the Exact Part Number
Use this decision path to terminate your component selection process. Do not over-engineer a logic switch, and do not under-spec a motor contactor.
| Load Type & Current | Decision Criteria | Concrete Part Pick |
|---|---|---|
| Low Current (< 200mA) Audio, Logic, Sensors |
Requires bifurcated (split) contacts to prevent oxidation from ruining low-voltage continuity. Standard contacts fail here due to 'dry circuit' oxidation. | Omron G5V-2-5VDC |
| Resistive (1A - 10A) Heaters, LED Drivers, Pumps |
High steady-state current, low inrush. Needs robust silver-alloy contacts and high thermal dissipation. | Songle SRD-05VDC-SL-C |
| Inductive / Motor (1A - 5A) Solenoids, Compressors |
High inrush (LRA) and severe arcing on open. Requires high breaking capacity and wider contact gaps. | Panasonic JW2SN-DC5V |
| Heavy Motor (> 5A / > 1 HP) HVAC, Large Pumps |
A 5V PCB relay cannot safely break this arc. The 5V relay must only be used to switch the coil of a heavy-duty contactor. | Use Songle to drive an Eaton C25DND220 Contactor |
Bench Testing: How to Verify a 5VDC Relay Dead and Live
Before soldering a relay into a permanent enclosure, or when troubleshooting a suspected failure, run these exact multimeter checks.
1. The 'Dead' Test (Power Removed)
Set your multimeter to the Ohms (Ω) range.
- Coil Integrity: Probe the two coil pins. A healthy 5V Songle coil will read between 65Ω and 75Ω. If it reads OL (Open Loop), the internal fine-gauge wire has snapped; the relay is dead. If it reads near 0Ω, the coil is shorted.
- Contact Isolation (NO): Probe COM and NO. It must read OL. Any finite resistance means the contacts are welded shut or contaminated with conductive debris.
- Contact Continuity (NC): Probe COM and NC. It must read < 1.0Ω. If it reads higher, the NC contacts are pitted or oxidized.
2. The 'Live' Test (Under Load)
Apply 5V to the coil via your driver circuit. You should hear a sharp mechanical click.
- Continuity Shift: NO to COM should now read < 1.0Ω. NC to COM should read OL.
- Voltage Drop Test (Crucial): With the relay switching an actual load (e.g., a 5A heater), set your multimeter to DC or AC Volts (depending on the load). Place the probes directly on the relay's COM and NO metal pins. A healthy relay will drop less than 50mV. If you measure > 200mV across the closed contacts, the internal contacts are heavily pitted, generating excess heat (P = I²R). It is failing and must be replaced.
Repair vs. Replace: When to Toss a Pitted Relay
Electromechanical relays are consumable wear items. The mechanical spring and the contact plating have a finite lifespan. A common beginner mistake is attempting to 'repair' a relay by prying off the plastic cover and filing down pitted or carbon-scored contacts with sandpaper.
Many high-quality relays (like the Omron and Panasonic models listed above) are sealed and filled with an inert gas (like nitrogen or a specific air-mixture) to suppress arcing and prevent contact oxidation. Prying the case open ruins the gas seal. Filing the contacts removes the thin plating of silver-tin-oxide or gold, exposing the base brass, which will rapidly oxidize and fail on the very next switching cycle.
Replace the relay immediately if:
- Coil resistance reads OL or < 10Ω.
- Contact voltage drop under rated load exceeds 100mV.
- The relay fails to release (drop out) when coil power is removed, indicating mechanical binding or melted contact welding.
- You hear a continuous 'buzz' or 'chatter' from the armature when energized (common in AC coil relays if the shading ring is cracked, but rare in 5VDC relays unless the coil voltage is sagging below 3.75V).
