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Why do you need relays?

Table of contents

Reading time 4 minutes

Updated - June 18, 2025

Relays are used to switch consumers (loads) of higher power (W - Watt - P) than the switching contact of the switch or the cable cross-section at the switch would allow.

Everyone knows the rhythmic clicking of the blinker relay in their car. An integrated electronic system ensures the legally prescribed flashing interval and switches the relay on and off. The blinker lights connected to the relay and supplied with voltage via the relay flash at the same rate.

Now indicators don't need a lot of power, they don't flow high currents, but it's a completely different story with driving lamps!

Cable cross section

Ultimately, even smaller Cable cross-sections (cable cross-sections in the DC voltage range can be determined here) between the switch and relay, which reduces costs and benefits the larger cross-sections from the relay to the load. These cables can be kept short, which in turn keeps costs and voltage drops on the cable path low.

If cables are too thin for the required power, they heat up and may become a heating wire. Thin cables have a high ohmic resistance (Ω (Ohm) resistance - R), which results in less tension at the end than at the beginning.
One cable reel with 50 m and a cable cross-section of 3 x 1.5 mm2 supplies a load with 16 amperes (A - amperage I) is still just 210 volts (V - voltage - U). If you use a cable with 3x 2.5 mm instead2you are still at 221 V.

Campers are often confronted with this problem, carrying long cable reels with them and later complaining about the high power consumption or responding circuit breakers, while everything worked perfectly at home - with a short cable ...!
This is because a load draws more current if it receives too little voltage to deliver its power. Accordingly, the power consumption is higher when the voltage is lower than "normal".

Cable cross-sections in the AC voltage range - AC - can be used in this Table be calculated.

Relay types

In vehicle construction, there are usually relays operated with 12 or 24 V direct voltage (DC) that can switch loads of up to around 30 A. Power relays can also handle up to 400 A.

The connections of the relay coils are marked 85 and 86. For relays with a built-in spark suppression diode, 86 is assigned to plus. Relays without a spark quenching diode can be connected with any polarity, but it has proven useful to select the polarity as if a spark quenching diode were installed (in electronics this is referred to as Freewheeling diode). In this way, a relay of any configuration can be used in the same plug-in socket without being damaged by incorrect polarity.

Normally open / Normally closed contact

Relays for switching a load ON have a NO contact, or an NC contact for switching it OFF. The contact of a normally open relay is therefore open in the de-energized state and closes as soon as the relay is supplied with voltage. The NC contact acts in the opposite way.

The switching contacts are labeled 30 (plus) and 87 (consumer).

The spark quenching diode under the relay coil has the task of absorbing and compensating for voltage peaks of several hundred volts that occur in the coil when the power supply to the relay is switched off.
Even if most relays used in automotive engineering do not have such a protective circuit integrated, because the relay coils are quite "robust", it is a wiser decision, if possible, to use relays with a built-in spark suppression diode!

Changer

The changeover relay enables two loads to be switched and therefore has three switching contacts and thus also three connections, 30 (positive), 87 (NO) and 87a (NC).

Special relay

These include flasher or timer relays with integrated electronics.

Practical example

The aim is to be able to switch several consumers on and off centrally via a switch panel.

Retailers offer a wide variety of products, all of which have one thing in common: the cables are very thin and only designed for low currents. We remember: copper is expensive ...

What starts out as a small LED light grows into an entire lighting system faster than expected. This also increases the power requirement and therefore the power to be switched. A case for the use of a relay.

The wiring of these switches is usually bridged: all positive and negative poles are connected to each other. The load connected via the last switch draws its current via all previous bridges. This means that the sum of the currents of all connected loads must not exceed the capacity of the supply line.

The 12 V 20 A usually specified by the manufacturer actually only refers to the standard cable length of a few centimeters. If only ONE meter of supply cable is included, the permissible current is just 2 A, a tenth!

In this respect, the use of switching relays (normally open contacts) is therefore more than advisable.

The FREE contact marked "V" leads to the connection 86 of a relay, the wire marked "-" is connected to terminal 85.
The consumer draws its ground (minus) directly from the battery (or vehicle ground connection), while it receives plus via the switched relay connection 87.

If the associated switch is now actuated, the relay receives positive at contact 86, picks up and switches via its contact (30 - positive) to the opposite contact 87, whereupon the load also receives positive and is therefore switched on. As soon as the switch is switched off again, the relay drops out and disconnects the power supply to the load.

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