5-Pin Relay Connection Guide: Car Cut-Off Test, Button Setup, Pins 85, 86, 30, 87a, Bulb Wiring

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How should I wire a 5-pin relay and button so it actually cuts the car circuit instead of leaving the bulb on all the time?

With a normal 5-pin relay, 85/86 are only the coil, and 30 is switched between 87a and 87: with no power on 85/86, 30 is connected to 87a; when the coil is powered, 30 switches to 87. To use it as a cut-off, you must break the wire you want to interrupt and route that wire through 30 and 87a or 30 and 87, depending on whether the circuit should be closed by default or when energized [#16769140][#20158951] A momentary pushbutton by itself will not latch a standard relay, so if you want one press to toggle the pump/ignition on and off, you need a bistable/impulse relay or an ON/OFF latch type relay, not an ordinary car relay [#16769406][#16769538] The coil should be driven from the button/ignition side, and a fuse of about 5A–7.5A is suggested on the + feed to 85/86 [#16769219] If you add a flyback diode across the coil, connect its cathode to + and its anode to - [#20161884][#20162079]

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#31 20162102 26 Aug 2022 16:51

kp. kp.

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Post #31
20162102 26 Aug 2022 16:51

Domen4 wrote:

kp. wrote:
it is indifferent whether we put + on the output 85 or 86 on the coil?

It is not indifferent in the case of the mounted diode, which, as a colleague mentioned - must always be connected in the same way.

No, of course, here I asked about the connection of the polarity itself, whether it is indifferent, in the next stage the diode must be adjusted with the cathode to the plus as I understand it.
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Topic summary

✨ The discussion revolves around the proper connection of a 5-pin relay for a car cut-off system, specifically addressing issues with wiring a button to control the relay. The user initially connected the button to pins 85 and 86, with pin 30 connected to the battery positive and pin 87a to a light bulb, but the bulb remained on continuously. Responses clarified that pins 30 and 87 (or 87a) should be used to interrupt the circuit of the component being cut off, such as a fuel pump or ignition. It was emphasized that the relay's coil (pins 85 and 86) must be powered to switch between pins 87 and 87a. Suggestions included using a bistable relay for the desired functionality and ensuring proper grounding. The importance of a diode across the relay coil to prevent high voltage spikes was also discussed. The conversation highlighted the need for correct wiring and the potential use of a fuse for safety.
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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, post #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, post #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]

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