This article was first published in 2008.
Relays have been around since the year dot, but
solid state (ie electronic) relays are much newer devices. They don’t have any
moving parts that can wear out, can be triggered by tiny currents but can in
turn switch enormous currents, and can be tweaked to do quite a few interesting
Let’s take a look.
A relay is simply a remote-triggered switch.
Traditional relays use a set of switching contacts and an electromagnet. When
power is applied to the coil of the electromagnet, the contacts are physically
pulled across, closing the switch.
is a good grounding in relays and their automotive uses.
Traditional relays suffer from two problems. The
first is that the more heavy duty a relay is (that is, the more current it can
switch), the greater the current that’s drawn by the coil. This is because the
switch contacts need to be larger (and so heavier), and the contacts need to be
closed with force. These aspects require a strong electromagnet that draws
The second disadvantage is that traditional relays
wear out. The contacts tend to arc (especially when opening) and the contact
faces get pitted. The more operations they perform, the faster they wear
However, a relay is a cheap and effective device,
one which because of the variety of switch contacts that are available, can also
be very versatile.
An electronic relay uses a big switching
transistor to replace the mechanically moving contacts of a traditional relay.
This means that an electronic relay will never wear out – nothing is moving.
(Incidentally, the switching transistor is known as a MOSFET.)
In addition to never wearing out, an electronic
relay can potentially switch very large currents. When equipped with a suitable
heatsink, the relay shown here can handle 100 amps
continuously and cope with a very short term switch-on current gulp of 240 amps.
Those are huge numbers.
Electronic relays can be triggered with extremely
small currents – for example, 20 milliamps. With traditional relays, only very
light duty units have such low coil currents. The very low operating current has
major advantages that we’ll come to in a moment.
Finally, an electronic relay can be pulsed very
fast. Nope, we’re not talking about just switching on and off a few times a
second, but instead being pulsed hundreds of times per second. In applications
like motor speed control and light dimming, and injector/solenoid operation,
this has major positives.
The Jaycar SY4086 solid state relay costs about
AUD$40. (Oh yes – that’s a disadvantage; electronic relays cost more than
mechanical relays!) It’s made by Hongfa Relay and its data sheet can be
downloaded by following the link at the end of this story.
The relay has four connections. The ones marked
‘input’ are connected to power (anything from 3 to 32 volts DC) to switch the
relay on. Note that these connections are polarity conscious – positive must go
to positive and negative to the car’s chassis or negative terminal of the
The other end of the relay is the output – you can
treat these terminals as the two connections of a switch (although note that the
terminals are again polarised.)
Here is the relay wiring. In this diagram the
relay is operated by a switch. Close the switch and power is applied to the left
hand end of the relay – what in a mechanical relay would be the coil
connections. This switches the relay on, so allowing current to flow to the load
(the load could be a lamp, pump, motor, etc).
Here’s the same diagram but in this case the one
power supply feeds the relay the juice needed to switch it on and also powers
Finally, the electronic relay can be switched on
and off by another electronic module – if the module outputs any voltage between
3-32V DC, it can be used to switch the electronic relay.
The most obvious use of an electronic relay is to
replace a traditional relay. However, in most automotive cases this isn’t worth
doing. To replace existing relays, much rewiring will be need to be done and
unless the existing relays are being used in an application that is causing them
problems, no real benefits will occur.
However, there are some cases where it would still
be a positive. For example, in a car using really high powered driving lights,
and where the lights are switched on and off a lot, traditional relays may have
a short life. The electronic relay would then be a good upgrade.
A better use is to use the electronic relay where
only a small switch-on current is available.
For example, sensitive pressure switches,
micro-switches and many temperature switches are rated for very low currents.
But because of the low current draw of the electronic relay, these switches can
be used without problems.
Existing switches such as radiator temperature
switches and oil pressure switches can also be used without problems.
Pretty well any warning light or LED can be used
to additionally trigger the relay – the relay’s input connections are simply
wired in parallel with the LED or warning light (use a multimeter to check the
polarity of the power feed to the light or LED.)
If an electronic module has a pulsed output but
cannot handle the load current, the electronic relay can be used to increase the
capability of the module. For example, the
The Nitrous Fuel Controller is a very cheap kit that
can be used to:
However, the output transistor of this module is
limited to 10 amps. That’s a fair bit but not enough current to speed control
big motors like radiator fans or fuel pumps, or happily pulse multiple car horns
(eg as an alarm indication). But by using this module to trigger the electronic
relay, each of these uses becomes possible.
However, note that there’s a trick to it. Rather
than outputting 12V when the Nitrous Fuel Controller’s output is switched on,
this module connects to ground when switched on. Therefore, the module
connects to the ground line of the relay input rather than the positive side of
(The new range of pre-built electronic modules
that we’ll be covering soon will use the electronic relay in the same way –
allowing the control of very large currents.)
If more than a tiny current is available to trigger the relay, it’s very easy to add an extended ‘on’ time to the relay’s operation. This could
be useful if for example an intercooler water spray is being operated by a
pressbutton switch – you push the switch and release it and the spray continues
for (say) 10 seconds.
To achieve the extended ‘on’ time, all that’s
required is a capacitor wired across the input side of the relay. A 4700uF, 25V
cap, for example, extends the ‘on’ time for about 7 seconds. To increase the
delay, increase the capacitance. These capacitors are polarised, so make sure
that the negative lead of the cap (shown by a line of minus symbols on its body) goes to the
negative connection of the relay input and the positive side goes to the
positive relay input. Cost of such a capacitor is only about $2.
one other thing to keep in mind when using an electronic relay.
inductive loads (eg those with coils like solenoids, injectors, motors and
horns) a big voltage spike is produced when the power is turned off. To protect
the relay, a diode (eg 1N4004) should be wired across the load, band on the
diode towards the negative side of the relay output.
When working at very
high current levels, the
electronic relay covered in this story requires heatsinking.
this graph (taken from the data sheet) shows, at up to 40 degrees C ambient the
relay can handle 25 amps continuously. At higher ambient temps (like found in an
engine bay, for example!), the relay is de-rated.
high currents need to be handled, bolt the relay to a large heatsink (using
thermal grease between the heatsink and the metal back of the relay) and check
that the relay does not exceed 40 degrees C in operation.
In many cases traditional relays are still the
best pick for the job. These applications include those where multiple switching
contacts are required, cost is a factor and switched currents are small. But
where large currents need to be handled, only very small currents are available
to energise the relay, or where delayed ‘on’ times or high speed pulsing are
required, the electronic relay is unbeatable.