The Circuit Design of Automotive Relays

Automotive relays are switches that turn electrical circuits on and off. They come in different shapes, sizes, and can be found in cars, gas valves, alarm systems, and more. Relays are typically used to enable a low amperage circuit to switch a high amperage circuit on or off. Relays can also be used to switch multiple things at the same time with the use of one output. When a single output is connected to multiple relays, you are able to simultaneously open continuity and/or close continuity. In this blog, we will be covering how automotive relays work and their circuit design to better understand these helpful devices.

Automotive relays typically come as either Single-Pole, Single-Throw (SPST) or Single-Pole, Double-Throw (SPDT) and draw currents less than 200 milliamps. Single-Pole, Single-Throw (SPST) relays are often called make or break relays, and their open or closed position is dependent on their design and is controlled by whether the relay is at rest or energized. Generally, it is switched manually by utilizing a push or toggle switch. Single-Pole, Double-Throw (SPDT) are sometimes referred to as changeover relays. These relays alternate closing and opening two circuits. An example of SPDT relays can be seen in the operation of vehicle headlights in the fact that they switch between circuits to activate high beam or low beam lighting.

Moreover, automotive relays consist of normally closed (NC) or normally open (NO) contacts that can handle up to 30 amps. If you were to look inside a relay, you would see an electromagnetic coil, contacts , and a spring. The spring is responsible for holding the contacts in position until the current travels through the coil and generates a magnetic field. In order to understand the complexity of automotive relays, the next section will cover the automotive relays circuit design components.

The Snubber

From the moment the power is disconnected from the relay and the magnetic field collapses across the coil , a voltage spike in the reverse direction of normal current flow occurs. Due to the fact that automotive relays are typically controlled by electronic circuits that are sensitive to sudden changes in voltage, this reverse voltage must be dissipated. This can be done with the help of a resistor or diode, sometimes called a snubber. A diode or resistor can be positioned across the automotive relay’s coil.

Diode Snubbers

A reverse biased direction does not allow current to travel through a diode, as such, when voltage direction is normal, no current flows through the diode. When the reverse voltage occurs, the diode becomes forward biased and excess voltage is allowed to pass through the entire circuit to the other end of the coil. This current travels around in the diode and coil circuit until the voltage is completely dissipated.

Resistor Snubbers

Resistor snubbers allow a certain amount of current to flow through the resistor during a voltage spike. In cars, resistor snubbers minimize the arcing across the switch contact points in the distributor via a small capacitor called a condenser. The proper functioning of a condenser is crucial to extending the points’ service life. When it comes to the service life of an automotive relay, they are generally defined by the number of times they will close a circuit before burning out. Luckily, relays have become standardized and manufactured to be less expensive, so replacing a faulty or damaged relay can be done with ease.

If you find yourself in need of relays, relay components, diodes, or alternatively, aviation, NSN, and electronic parts for other various applications, rely on ASAP Buying for all your operational needs. With an expansive inventory of over 3 billion new, used, obsolete, and hard-to-find parts, you are bound to find what you need when you source from us. Moreover, you never have to worry about the quality of our products as they have undergone a series of quality control tests, verifications, and assessments prior to being shipped out to our customers. We operate with AS9120B, ISO 9001:2015, and FAA AC 00-56B certification and accreditation, ensuring you always receive superior quality items. Kickoff the procurement process today by filling out and submitting an RFQ form as provided below. In as little as 15 minutes, a dedicated representative will reach out to you with a quote for your comparisons. Give us a call or email us at and we thank you for choosing ASAP Buying as your go-to sourcing solution.


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