This article was first published in 2005.
In any electrical system, relays are amongst the most useful of components.
They let you control heavy loads with light-weight switches, allow one device to
be switched off at the same time as another is switched on, are very hard to
blow-up, and are cheap and widely available. That’s a damn good list of
So how do you use them?
What’s a Relay?
A relay is just a small switch whose movement is caused by the action of an
electromagnet inside. When power is applied to the relay’s coil, the
electromagnet comes alive and pulls across the switching contacts.
Single Pole, Single Throw
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 unpower 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. 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 side of the relay.
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.
The most common use 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
switch would fail – its contacts are being hugely overloaded.
The solution is to add a SPST relay that is wired into the circuit like this.
Neither the relay coil nor the switching part of the relay have polarities –
both can be connected either way around to 12V and Ground. As we said earlier, a
relay is very hard to blow-up!
Single Pole, Double Throw
But wouldn't it be good if we had a contact that was broken at the same time
as the switch was made? That's what happens in the Single Pole, Double Throw
design. Another contact has been added that is Normally Closed. When the relay
is energised, this contact is broken 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. This type of relay is
therefore called a Single Pole, Double Throw relay. As you can see, it has both
Normally Open (NO) and Normally Closed (NC) contacts. Some people call this a
A SPDT relay allows you to control two devices, switching one off as the
other is switched on. A recent example of where I needed to use a relay in this
way is in a fuel system that needed to be switched between two different fuel
pressures. To raise the fuel pressure, a solenoid valve had to be turned off and
at the same time, a fuel pump needed to be switched on. Both devices draw a fair
amount of current so a heavy duty automotive relay was used.
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.
The circuit diagram for the fuel system relay looks like this. Power is
normally supplied to the solenoid through the Normally Closed (NC) relay
contact, energising the solenoid. But when the relay’s coil is activated (by
closing the High/Low Fuel Pressure switch), the relay pulls the contact across,
switching off the solenoid and switching on the fuel pump. The High/Low Fuel
Pressure switch has to handle only enough current to switch the relay’s coil, so
this can be a light-duty switch (eg a boost pressure switch or a microswitch).
Double Pole, Double Throw
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 what the 'double throw' stuff means. With
this type of relay you can:
- turn on two completely independent circuits
- turn one off and one on
- turn off two completely independent circuits
These relays are less common in automotive aftermarket use and so don’t have
coded numbers for the pins.
So what use is a DPDT relay, then? Again, I’ll use an example from a recent
car modification I have been working on. What was needed was the on-demand
disconnection of two oxygen sensor input signals to the ECU. The two signal
wires from the oxy sensors to the ECU needed to be kept completely separate, so
they couldn’t be joined together and a SPST relay used. Instead a DPDT relay was
used. (It didn’t actually have to be a double throw design, but DPDT relays are
more common than SPDT designs.)
Using a Relay
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
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.
It’s easy to think of relays as being suitable for just simple car
modifications, but that’s not always the case.
This circuit shows the use of two relays that deactivate traction control
without affecting ABS or stability control. The system works by connecting the
un-driven wheel ABS sensor outputs to the driven wheel ECU inputs, so that the
ECU cannot see a speed difference between the undriven and driven wheels. The
modification is automatically switched off whenever the brakes are applied, or
by a manual on/off switch. This diagram shows only half of the system - the
complete the system just mirror-images the wiring for the other side of the car.
The total cost of the modification was well under AUD$30 – relays are cheap!
For more on this approach, see Modifying Electronic Car Handling Systems, Part 3
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. (But note that a 24V relay, if it’s a sensitive design, will often work
fine on 12V!)
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. But this 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)
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 (even when it first switches on with a current gulp) 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.
Other specs that you might fine listed include life (ie how many millions of
operations the relay will do before failure) and perhaps response time.
Relays can be useful 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 can be used.
A Relay-based Immobiliser
Once you’ve got familiar with relays, you might want to try out this – a
relay-based immobiliser. It works very well and is very hard for a thief to
bypass – see The World's Best DIY Immobiliser