Last week in How to Electronically Modify Your Car, Part 6 we looked at a special way in which pots can be used to alter the output of sensors used in electronic car systems. It’s a very tricky technique that allows you to achieve lots of effective outcomes at nearly zero cost. This week we’re also looking at simple and cheap electronic components that are again often overlooked as ways of achieving excellent car modifications – relays.
What’s a Relay?
A relay is just a small switch whose movement is caused by the action of an electromagnet inside the box. When power is applied to the relay’s coil, the electromagnet comes alive and pulls across the switching contacts.
The simplest relay is a single pole, single throw (SPST) design. This designation refers to the switching part of the relay where when it's activated, one wire (a "single pole") can be connected only one way (a "single throw"). Just like an on/off switch, when you power up the relay's coil, the connection is made; when you un-power the coil, the connection is broken.
In this diagram the relay's coil is yellow. Near to the coil you can see a switch, which is open. This is called a Normally Open contact - it's open when there's no power being applied to the relay. When power is applied to the relay’s coil, the single contact closes. This is a Single Pole Single Throw relay - SPST.
SPST relays have four terminals - two are to power the coil and the other two are the connections for the internal switch. Look at the diagram and identify which terminals are which. As you can see, there is no electrical connection between the pair of contacts for the coil and the pair of contacts for the switching part of the relay.
In the above example, the relay’s contacts had to take only a very small current – just enough to run the LED. Therefore, a quite light duty relay could be used (more on relay ratings in a moment). But many relays used in cars are heavy duty designs.
In fact, the most common application for a SPST relay is to use a small electrical current to control a large electrical current.
For example, a radiator fan might be triggered by a temperature switch. The temp switch is capable of flowing only 2 amps, but the radiator fan at switch-on takes 15 amps (and then settles back to 8 amps continuous).
If you wire the radiator fan to the switch like this, after a few weeks the temp switch will fail – its contacts are being hugely overloaded.
The solution is to add a SPST relay that is wired into the circuit like this. Now the temp switch only has to pass enough current to turn on the relay’s coil – a much easier job than directly running the fan!
Neither the relay coil nor the switching part of the relay has a polarity – both can be connected either way around to 12V and Ground. As we said earlier, relays are very hard to blow-up!
On automotive SPST relays, the pins are given standardised numbers. The coil connections are 85 and 86, while the two connections for the internal switch are 30 and 87. However, most general purpose relays don’t have any numbers on the pins – instead the functions of the pins are shown on a little diagram on the body of the relay.
But wouldn't it be good if we had two contacts inside the relay – one that was opened at the same time as another one was closed? That's what happens in the Single Pole, Double Throw (SPDT) design. (Can’t think of much use for that type of relay? There is – and I’ll show you in a moment.)
This is what a SPDT relay looks like inside. When the relay is energised, one contact is opened and the other one (the Normally Open contact) is closed. We still have only a single pole to be switched, but now it can be connected two ways - a double throw design. As you can see, it has both Normally Open (NO) and Normally Closed (NC) contacts. (Some people call this a changeover relay.)
A SPDT relay allows you to control two devices, switching one off as the other is switched on. SPDT automotive relays use the following codes for their pins: the coil connections are again 85 and 86, the normally closed output is 87a, the normally open output is 87 and the input is 30.
A Double Pole, Double Throw relay allows you to switch two different circuits simultaneously. The 'Double Pole' bit just means that it has two separate inputs that can be switched - and we now know the 'double throw' means that one contact gets opened as the other is closed. With this type of relay you can:
These relays are less common in automotive aftermarket use and so don’t have coded numbers for the pins.
Using a relay is made a lot simpler if you follow these steps.
Draw a circuit diagram. The first step is to draw a simple circuit diagram showing where the wires go. Which wires go to the relay coil, which to the Normally Open and Normally Closed contacts of the relay?
Decide what type of relay is needed. If just one connection needs to be switched on and off, you’ll use a SPST design. If two connections need to be switched, a DPST or (more commonly) a DPDT design will be the one to use. A changeover (where one device is switched off and the other switched on) can use a SPDT or a DPDT design.
Work out the functions of each pin. If it’s a standard automotive relay, read the numbers. If it’s a general purpose relay, look for the diagram on the relay body. If neither of these apply, by careful use of a short-circuit protected power supply and a multimeter, you can work out the functions of each pin. (Unless you use too high a test voltage, you can’t damage the relay!)
Wire the relay coil first. If you wire the relay’s coil first, you’ll be able to check that the relay is working by listening to its click.
In addition to its contact configuration (SPST, DPDT, etc) there are at least three other specifications that are important.
This refers to the voltage which the relay is designed to have its coil triggered by. A nominally 12V relay is fine on car voltages, even though they can extend as high as 13.8V. However, you shouldn’t use a 5V coil relay on a 12V system.
This is the amount of current the relay coil will draw when energised. This can be expressed directly in milliamps, or indirectly as a coil resistance. A very sensitive relay might have a coil resistance of 360 ohms. 13.8 volts divided by 360 ohms gives a coil current of 0.038 amps, or 38 milliamps. In other words, the switch that you’re using to operate the relay has to handle just 38 milliamps. That is a very low value of required current.
A typical automotive relay is more likely to have a coil resistance of 80 ohms, giving a coil current flow of 170 milliamps. (13.8/80 = 0.17 amps). That’s still low – most switches will handle this without problems.
This spec refers to the max current that a relay’s contacts can handle. To avoid arcing, you should use a factor of safety where the max current of your switched circuit is less than the relay’s spec.
Automotive relays are available with current ratings like 25, 30 and even 60 amps. Be careful when checking max current specs that the listing is for the DC at or above the voltage you’ll be using – ie, in cars, 13.8V. For example, a relay rated at 10 amps at 240V AC is not the same as one rated at 10 amps at 12V DC.
Relays can be utilised in nearly every electrical or electronic car modification. Get your head around their use and you’ll never regret having spent the time to find out how they work.
Next week we’ll look at using an off-the-shelf electronic module.