How do Relays work?

You've seen them in your fuse box either hidden beneath the dashboard or located under the bonnet. But what do relays do, how do they work, and how can you use them to improve the electrical systems on your classic car? Let us explain...

An automotive relay is an electro-mechanical device consisting of a coil, an armature, and a set of contacts. When a small current passes through the coil, it generates a magnetic field which pulls the armature towards it, causing the contacts to open or close. This switching action enables or interrupts the flow of current to a specific circuit, in turn controlling selected electrical components in a vehicle. They can be utilised for convenience, but also for safety systems too. 

There are different types of automotive relays, each designed for specific purposes and configurations. Before plugging a random relay into your fuse board you should ascertain that the voltage is correct and that the current rating is at least the maximum current that will flow through it. Some relays also have time delays or additional circuit protection, such as a fuse built in. When comparing two relays, turn it upside down and double-check the number of pins, their position and the numbers assigned to them.  

The four most common types of relay for automotive applications are:

• Single Pole Single Throw (SPST) relay has a single set of contacts that are either open or closed, meaning it has only one path for current to flow. When the relay is energized, the contacts close, and when it is de-energized, the contacts open. This type of relay is typically used for simple on/off switching applications such as the headlights or a radiator fan. 
• Single Pole Double Throw (SPDT) relay has a single set of common contacts that can connect to either of two other contacts, allowing it to switch between two different circuits. It has one normally open (NO) contact and one normally closed (NC) contact. You might find these operating your electric fuel pump, or powering an electric window motor both up and down. 
•  Double Pole Single Throw (DPST) relay has two sets of contacts that are simultaneously operated by the same electromagnet. When the relay is energized, both sets of contacts change their position together. These are commonly used to control the transition between main and high beams on your headlamps. 
• Double Pole Double Throw (DPDT) relay has two sets of contacts that can each connect to one of two other contacts, allowing it to switch between two different circuits. It has two normally open (NO) contacts and two normally closed (NC) contacts. This type would be used for a component like a wiper motor, where there are multiple speed options and an intermittent setting. 

You may come across the term Solid State Relay on your travels. Whilst these perform the same function as above, they do so without any moving parts. Internally, they utilise semiconductors and because of this are marketed to be more reliable and longer lasting than their magnetically operated predecessor. 

There are a few common situations where relays can be installed to make improvements to the electrical efficiency of your classic car. The similarity between most of them is needing to create the shortest possible journey between your power source, either the battery or alternator and the component itself. 

Whilst a lot is said about the physical condition of classic car bodywork and corrosion, it is common for vehicle wiring to also deteriorate over time, becoming brittle or broken, even without the interference of 'have a go' DIY mechanics adding electrical accessories along the way. The less distance a current must travel through these aged cables the better in our experience. 

Of course, using a relay isn't a get-out-of-jail-free card. If you have inherent wiring issues, you should still get those sorted too!

One example of utilising a relay to upgrade a vehicle's wiring is with the headlamps. By taking a direct feed from the battery or the alternator, and using the original wiring from the headlamp switch to simply trigger the relay as required, you can pass almost all of the battery power to the bulbs themselves. Making the bulbs brighter and extending the life of your headlamp switch too. 

Check out our blog on how to make your headlamps brighter here. 

Another prime example of where a relay can make night and day difference is by controlling the operation of the starter motor. We sell a lot of starter motors, and almost all complaints of them being faulty comes down to poor wiring, rather than any sort of part failure. Just as the headlamps can be fed with 12v direct from the battery, so your starter motor can be wired the same. With the starter motor needing maximum voltage, the use of a relay makes perfect sense, leaving your ignition switch to simply control when it clicks on and off. 

Find our hard start relay kits online here.

A popular relay use for those with campervan conversions is in 'split charging'. This process allows current to flow from the main vehicle battery, whilst the campervan is in motion, to charge and maintain a secondary leisure battery. This helps remove the worry that you might run the vehicle battery flat whilst away for the weekend using lights, charging phones or a 12v fridge. 

Find our split charge relay kits online here. 

Hopefully this explains a little more about the role of those small black boxes in the fuse box, and how you can use this clever component to make some improvements to your classic car too. 

Andy