Understanding the lu 5 r relay pin configuration is the first step to reliably switching high-current loads in automotive, solar, and industrial control panels. The LU 5 R is a 5-pin Single Pole Double Throw (SPDT) electromechanical relay. The 'R' suffix specifically denotes an internal suppression resistor across the coil, a critical detail that changes how you handle flyback protection compared to standard diode-equipped relays. This guide breaks down the exact pinout, contact ratings, and testing procedures you need to wire it correctly and avoid frying your control logic.
LU 5 R Relay Pin Configuration and Electrical Ratings
The LU 5 R follows the standard DIN 72552 5-pin layout common in ISO mini relays. The pins are divided into two isolated circuits: the low-current electromagnetic coil and the high-current switching contacts.
- Pin 85 & Pin 86 (Coil): These energize the electromagnet. Because this model features an internal suppression resistor, polarity does not matter for basic operation, though the resistor will dissipate heat regardless of current direction.
- Pin 30 (Common / COM): The moving contact arm. This is where your main power feed or load return connects.
- Pin 87 (Normally Open / NO): Connects to Pin 30 only when the coil is energized. Used for the primary switched load.
- Pin 87a (Normally Closed / NC): Connects to Pin 30 when the coil is de-energized. Often used for fail-safe circuits or indicator lights.
| Parameter | Standard 12V Coil | 12V Coil (R-Suffix / Resistor) |
|---|---|---|
| Coil Voltage (Nominal) | 12V DC | 12V DC |
| Coil Resistance | 75 Ω ± 10% | ~69 Ω (75Ω coil || ~1000Ω resistor) |
| Nominal Coil Power | 1.92 W | 2.15 W (includes resistor dissipation) |
| Resistive Contact Rating | 30A @ 14V DC | 30A @ 14V DC |
| Inductive / Motor Rating | 15A @ 14V DC (L/R=7ms) | 15A @ 14V DC (L/R=7ms) |
| Breaking Capacity | 400W DC / 2500VA AC | 400W DC / 2500VA AC |
| Dielectric Strength | 500V AC (Coil to Contact) | 500V AC (Coil to Contact) |
Which rating column governs this load? The 30A rating strictly applies to purely resistive loads like heating elements or steady-state incandescent lighting. If you are switching a DC motor, a solenoid valve, or a contactor coil, the inrush current and inductive kickback dictate that the 15A inductive/motor column governs your design. Exceeding the inductive rating will cause the AgSnO2 contacts to pit, arc, and eventually weld shut.
Coil vs. Contact Side Wiring and DC Flyback Protection
Wiring the LU 5 R requires strict separation of the control side (coil) and the load side (contacts). The coil side draws roughly 160mA at 12V and can be driven by a microcontroller GPIO via a logic-level MOSFET or a BJT like the 2N2222. The contact side handles the heavy current and requires appropriately sized wire (e.g., 10 AWG for continuous 20A-30A loads) and crimped spade terminals.
When interrupting DC current, an inductor (the coil) generates a massive reverse voltage spike (back-EMF) as its magnetic field collapses. In standard relays, an external flyback diode is required to clamp this spike. However, the 'R' suffix on the LU 5 R means it contains an internal suppression resistor.
| Suppression Type | Clamp Voltage | Release Time | Polarity Sensitive? |
|---|---|---|---|
| External Flyback Diode | ~0.7V | Slow (3-5x normal drop-out) | Yes (Cathode to +) |
| Internal Resistor (R-Suffix) | 50V - 100V | Fast (Normal drop-out) | No |
For overcurrent protection on the contact side, use a time-delay fuse or a D-curve breaker to accommodate motor inrush without nuisance tripping; never treat a fast-acting glass fuse and a thermal breaker as interchangeable for inductive loads, as their time-current curves respond entirely differently to momentary inrush spikes.
Load Selection Decision Path by Application
Selecting the right relay requires matching the load's electrical behavior to the relay's contact material and breaking capacity. Use the decision tree below to determine if the LU 5 R is suitable for your specific application.
| Load Type | Inrush Multiplier | Governing Rating Column | Required Derating | Application Examples |
|---|---|---|---|---|
| Resistive | 1x | 30A Resistive | 0% (Use full 30A) | Nichrome heaters, power resistors |
| Inductive (Solenoid) | 5x - 10x | 15A Inductive | 50% (Max 15A) | Fuel injectors, pneumatic valves |
| Motor (DC) | 10x - 15x | 15A Motor | 60% (Max 12A continuous) | Winches, fuel pumps, radiator fans |
| Tungsten / Halogen | 10x - 15x | 15A Inductive | 50% (Max 15A) | Off-road light bars, halogen work lamps |
If your calculated steady-state current exceeds the derated values in the rightmost column, the LU 5 R is undersized. Step up to a heavy-duty 40A/60A contactor or use a solid-state relay (SSR) with an appropriate heatsink. For deeper theory on contact arcing and material degradation, refer to the All About Circuits relay chapter or the SparkFun Relay Tutorial.
Testing Dead and Live, and When to Replace
Troubleshooting a suspected faulty relay requires both unpowered (dead) and powered (live) measurements. Never rely solely on the audible 'click' as proof of healthy contacts; the armature can snap shut while the contact faces remain insulated by carbon tracking.
Dead Testing (Multimeter in Ohms / Continuity):
- Coil Check (Pins 85 to 86): Expect a reading of roughly 69 Ω to 75 Ω. If it reads OL (Open Loop), the coil wire is broken internally. If it reads near 0 Ω, the coil is shorted.
- NC Contact Check (Pins 30 to 87a): With the coil de-energized, expect less than 0.5 Ω. A higher reading indicates oxidized or pitted contacts.
- NO Contact Check (Pins 30 to 87): Should read OL (Open Loop). Any continuity here means the contacts have welded together—a critical failure mode that requires immediate replacement.
Live Testing (Multimeter in DC Volts):
- Energize the coil with 12V DC and apply your actual load to Pin 30 and Pin 87.
- Place your multimeter probes directly on the metal spades of Pin 30 and Pin 87.
- Measure the voltage drop across the closed contacts. A healthy LU 5 R relay under a 20A load will exhibit a voltage drop of less than 0.1V. If the drop exceeds 0.2V, the internal resistance is generating excess heat (P = I²R), and the contacts are degrading.
When to Repair vs. Replace:
Electromechanical 5-pin relays are sealed, non-serviceable components. Never attempt to open the plastic housing to file or sand the contacts. Filing removes the microscopic AgSnO2 (Silver Tin Oxide) anti-welding plating, exposing the softer brass or copper underneath, which will weld shut on the very next high-current switching cycle. If the coil is open, the contacts are welded, or the live voltage drop exceeds 0.2V, discard the relay and install a new unit. Repairing a $5 relay risks thousands of dollars in downstream electrical fires or destroyed control modules.
