Mini Relay — R5-1240R 40A 12V 5-Pin N/O Resistor

Q: What's the difference between a mini relay and a micro relay?
Size and base footprint. A mini relay is roughly 26 by 26mm with 6.3mm blade terminals and drops into a mini-ISO 280 Series socket. A micro relay is smaller, around 15 by 15mm with 2.8mm blade terminals, and uses a micro-ISO socket. The electrical function is the same; the two aren't interchangeable because the sockets and terminal sizes don't match.

Q: What does 'normally open' (N/O) mean on a relay?
The 40A switched contact (pin 87) is open, so no current flows, when the coil has no power. Energise the coil (apply 12V across pins 85 and 86) and pin 87 closes to pin 30, letting current flow. Cut power to the coil and the contact opens again. This is how a small switch signal controls a high-current circuit without the switch itself carrying the load.

Q: What's the 87a pin for?
Pin 87a is the normally closed contact. When the coil is unpowered, pin 30 is connected to 87a (not 87). Energise the coil and the connection snaps from 87a to 87. Use 87a to run one circuit when the relay is off and another when it is on, useful for driving-light or high-beam swap logic or any either-or switching.

Q: What does 'resistor protected' actually mean?
A resistor sits across the coil (pins 85 and 86). When the coil switches off, the collapsing magnetic field creates a reverse voltage spike that can feed back into the vehicle's electrical system and damage sensitive ECUs, keyless-entry modules or infotainment. The resistor dissipates that spike as a small amount of heat instead of letting it loose on the wiring loom.

Q: Can I use this on a 24V system?
No. The coil is 12V only. 24V across pins 85 and 86 will overload the coil winding and burn it out within seconds. For 24V trucks and industrial systems use a 24V-coil equivalent, such as the Britax RM4-2435R 24V 35A micro, or request a 24V 40A mini relay specifically.

Q: Will this fit standard relay bases?
Yes. The 26 by 26mm housing and 6.3mm blade pattern matches ISO 7588 mini-ISO 280 Series, which is the de facto standard across automotive, marine and industrial 5-pin relay sockets. If the existing socket accepts a Narva 68038 or Hella 4RD mini relay it will accept this one.

Q: What size fuse should I use?
Fuse the circuit at or below the relay's switched rating. For this 40A relay use a 30A or 40A blade or MIDI fuse upstream of pin 30 (the battery feed). Never fuse above 40A: if the downstream load jams or shorts, the fuse should blow before the relay contacts weld shut.

Q: Can I use this to switch a winch or large inverter?
No. Winches draw 200 to 500A under load and large inverters above 1500W draw over 150A at 12V, both well beyond this relay's 40A ceiling. Use a solenoid or contactor rated for those currents (ANL 300-500A or Class T 300-400A protection to match).

Q: How do I wire the coil?
Pin 86 goes to switched 12V (ignition, dash switch, aux switch). Pin 85 goes to chassis earth. Either orientation works for a resistor-protected coil, but conventionally positive feeds 86 and negative to 85. Pin 30 is the battery feed, pin 87 is the switched output (goes to the load), pin 87a is the alternate output used only if normally-closed logic is needed.

Q: Where should I mount it?
Inside the vehicle in a dash fuse box, under-dash relay bank, or dry area of the engine bay. The housing isn't rated for direct water spray or submersion. If mounted in the engine bay, use a sealed relay base or cover the whole assembly to keep road spray off the terminals.

Q: Does it come with a mounting bracket?
No bracket. It drops into any mini-ISO 5-pin relay socket. Pair it with the Elevate mini relay base below if the circuit doesn't already have one.

Q: What's the difference between this R5-1240R and the 4-pin R4-1280R?
The R4-1280R is an 80A 4-pin SPST (N/O only) relay with a higher current rating and no 87a pin, so it only has one switched state. The R5-1240R here is 40A but adds the 87a normally closed pin, so it can run two circuits off one trigger. Pick the 80A for pure high-current switching (large driving lights, pumps, auxiliaries up to 80A); pick this 40A 5-pin for either-or switching logic (driving lights off and fogs on, high-beam swap, trailer feed select).