FAQ
TL;DR: A 12 V/40 A relay coil draws about 150 mA [Bosch-Automotive, 2022], and “85-86 are the pins from the coil that must have power” [Elektroda, marqqv, post #16769140]; wire 30 to supply, toggle load between 87 and 87a, and fit a flyback diode. Use a 5 A fuse and correct polarity to avoid transistor burnout. Why it matters: Mis-wiring leaves circuits live, drains batteries, or fries control electronics.
Quick Facts
• Coil current: 120–180 mA @ 12 V, 400–500 Ω resistance [Bosch-Automotive, 2022]
• Default state: 30→87a closed, 87 open until coil powers [Elektroda, abart64, post #20158951]
• Recommended coil fuse: 5–7.5 A inline on + feed [Elektroda, marqqv, post #16769219]
• Contact rating: 40 A continuous, 14 V DC, <0.1 V drop @30 A [Bosch-Automotive, 2022]
• Flyback diode cuts switch arcing up to 10× [ON Semi, AN-282]
How do pins 30, 87, and 87a behave in a 5-pin automotive relay?
Pin 30 is the common wiper. Without coil power, 30 is internally linked to 87a (NC). When the coil on 85-86 energises, 30 switches to 87 (NO) and 87a opens [Elektroda, abart64, post #20158951]
Why did my bulb stay on even after adding the button?
You fed battery + into 87a instead of 30, so the circuit never opened. Connect battery + to 30, bulb + to 87, bulb – to battery –, then send 12 V to 85 and ground 86 through the button [Elektroda, marqqv, post #16769140]
Can I interrupt the positive lead instead of the ground?
Yes. The relay contacts are floating, so you may switch either +12 V or ground. Use the same 30-87/87a arrangement and size the wire and fuse for the load current [Elektroda, kamilo1409, post #16769163]
Where exactly should I place the fuse and what value?
Install a 5–7.5 A blade fuse in the +12 V line feeding pin 85 or 86. This protects the coil, switch, and wiring from shorts [Elektroda, marqqv, post #16769219]
What does the flyback diode do and how do I wire it?
The diode clamps the coil’s reverse EMF when power is removed, protecting transistors and switch contacts. Solder a 1N4148 or 1N4007 directly across 85-86; cathode to +12 V, anode to ground [Elektroda, jta, #17047827]. "High-voltage spikes can exceed 100 V without the diode" [ON Semi, AN-282].
What happens if I reverse 85 and 86 after adding the diode?
The diode then sits forward-biased across the supply, creating a short. The fuse pops or the wiring overheats; the coil never energises. Always observe polarity when a diode is present [Elektroda, Domen4, post #20162079]
How much current does the coil draw and can a microcontroller drive it directly?
A standard 12 V/40 A relay coil draws 150 mA (typical) [Bosch-Automotive, 2022]. Most MCU I/O pins source ≤20 mA, so use an NPN transistor (e.g., TIP120) and 1 kΩ base resistor to switch the coil [Elektroda, jta, #17047827].
Why won’t a momentary push-button latch the relay for my immobiliser?
A normal SPDT relay resets when you release the button. You need a latching (bistable) relay or an electronic latch so one pulse toggles state [Elektroda, marqqv, post #16769406]
Will keeping the relay energised drain my battery?
Yes. 150 mA continuous draw equals 1.8 Ah in 12 hours, enough to flatten a small battery overnight. Design the circuit so the relay releases when the engine stops [Elektroda, marqqv, post #16769201]
How do I wire a 3-step fuel-pump cut-off using this relay?
- Cut the existing pump +12 V feed and connect the supply side to pin 30. 2. Connect the pump side to pin 87a (engine off = no fuel). 3. Power 85 from ignition +12 V via a 5 A fuse and ground 86. The pump only sees power when the ignition key energises the relay. Test for heat and solder all joints [Elektroda, marqqv, post #16769158]
