Switches and Relays
If you are buying new components shop around until you find the switch gear etc you like the look of. There are a lot of different companies marketing different styles and colours. Look at other kit cars and see what they use, and ask where they source them from. Remember that switches must be rocker type and not toggle unless they are ‘behind’ the steering wheel. Check the certification regulations.
Switches and relays by convention are drawn in the ‘power off’ position. A normal switch or relay that you turn on to make something happen is termed ‘normally open’ ie the contact is open (unmade). Some switches and relays are normally closed ie the contact is closed (made). You turn them ‘off’ to stop something that will happen automatically otherwise. An oil pressure switch is like this - with no oil pressure the switch is turned on, therefore the contacts are ‘closed’ to make a circuit so the warning light comes on when the engine is stopped. As soon as there is oil pressure the switch opens, breaking the circuit so the warning light goes off.
When you buy your switch gear make sure you find out what its rated capacity is, expressed in Amps. If your circuit totals too many Amps for the switch capacity you will have to use a relay. Most switches will handle about 15 Amps - but make sure you check. Mini Relays are made by a variety of companies and come in a variety of types which generally look the same - a big trap for the uninitiated. The trouble is that they all have the same size and shape pins in the same location - but they are connected to different parts internally.
Mini Relays will generally handle 30 Amps and are also useful where you want an electrically operated switch, say as a cut-out. If you are unsure about a switch’s capacity use a relay. Mini Relays come in two basic types - switching and change over. A switching relay merely turns a circuit on when the operating solenoid is powered up (normally open) while a change over relay will put power down one circuit with no power on the operating solenoid (the normally closed leg), and power down the other circuit when there is power on the operating solenoid (normally open). A change over relay can be used as a switching relay if you don’t connect anything to the normally closed leg. A change over relay can also be used to turn something on when the power to the solenoid is turned off, or conversely turn something off when the power to the solenoid is tuned on. This can be useful when making interlocked circuits, say for an oil pressure cut out that shuts the engine down if you lose oil pressure.
Care must be taken to ensure that all relays have no power going to the operating solenoid coils when the ignition is turned off as they will drain a battery flat over a week. Additionally some switching relays have the same terminal layout as change over relays, however, the extra connection is just a double of the switched terminal. Put it in where a change over is required and you’ll wonder why both circuits are going. Also some relays have the solenoid terminals where the switched terminals are, which also causes problems. Check the physical layout of the terminals. Relays should also be mounted with the terminals down to prevent them filling with water should any drops land on them - condensation can cause droplets of water in the most unlikely of places.
