What are Relays? How do they work? How can we use them in our vehicle or other electrical
circuits? I'm going to show you now.
Hi YouTube, my name’s Geoff the VegOilGuy and today I'm going to be looking at Relays
and there use in vehicle electrical circuits. This was a subscriber requested video and
I always do my best to help out my subscribers. A couple of guys contacted me separately for
advice. Both I think had a good idea what they wanted to do and how to do it,
but it’s always nice to get a second opinion. One subscriber wanted to fit courtesy lights
under each door of his car. Another wanted to fit a third braking light.
The answer to both of them is this – use a relay.
Splice into the parent circuit and use the live feed to prime and trigger the relay.
Wire a separate fused line through the output terminals of the relay to power the new device.
This safely powers the new device and circumvents any danger of over-powering the original circuit
and possibly blowing a fuse. If all of this makes perfect sense to you,
then in honesty this video probably isn't for you.
If I've left you scratching your head a bit, then don’t worry. I'm about to take you
through the basics of Relays, explain what they are, how they work and what we can use
them for. Then I’ll address the issues raised by my
subscribers, explain the answer fully and even provide demo a test circuit to prove
it all works. To begin, if you've got absolutely no idea
about auto electrics, then please look at my Fuse It video. This will give you all the
information you need to follow along. It might also be worthwhile looking at my Splicing
video, as this will show you how to safely make electrical connections with a soldering
iron – and that’s the only way I recommend of making electrical connection.
So, what is a Relay? A relay is simply an electronic switch. They
come in different sizes and shapes and usually have 4 or more metal lugs or terminals on
them. But what’s going on inside them? Here’s a really simple circuit. There’s
a battery, a bulb and a switch. It’s easy to see how when the switch is closed the bulb
lights up. The problem here is, of course, that someone
has got to close the switch. What a Relay does is electronically close a switch for
you. Originally Relays were tiny electromagnets.
When you placed power into them, they created a magnetic field which either attracted or
repelled a metallic strip that formed the arm of a switch. Turn the electromagnet on,
the switch closes. Turn it off and the switch opens.
I'm not sure if all relays are electromagnets these days, but the principal remains the
same. They cause a switch to open or close. Historically the standard electronic symbol
for a Relay was something like this and you can certainly get the idea of a magnet there
from the coil. For this purpose, I'm going to draw my Relays
a little differently. This is the electromagnet and this is the
switch. The large round points represent terminals or pins on relay.
To energise the electromagnet, we need to connect power to these terminals. This causes
the switch to close. So if we wire bulb and a battery to these switched terminals of the
relay, it’s easy to see how the relay can be used to switch the bulb on and off.
Tell you what, I’ll put a Master Switch in there so you can see this working. There
you go. There’s nothing special or fancy about the master switch. We’ll just call
it that for clarity’s sake. So this is a relay in its most basic form,
a Four Pin Normally Open Relay… four Pins for four terminals and normally open because
the switch is open until power is applied to it. Close the master switch and the relay
is energised and the relay switch is closed, lighting the bulb. Open the master switch
and off everything goes. And this is the relay that’s most often
needed. This is certainly the one that my subscribers would need, a Four Pin Normally
Open Relay. But there are different types of relay. The
first obvious variation on this is the Four Pin Normally Closed Relay. At first glance
there’s no obvious difference, but the difference lies with the internal switch. When the Relay
is not energised, when it’s not powered, in a Normally Closed Relay the internal switch
is closed. If you energise it, the Relay switch opens. Sometimes it’s useful to have a circuit
that on all the time and you only want to interrupt that occasionally, and that’s
where a Normally Closed Relay would be of use.
The next variation is a five pin relay. As the name suggests, with these there’s simply
another terminal involved. However this extra terminal gives us a relay that has both a
Normally Open and Normally Closed points. This is extremely useful obviously. Personally
I tend to only buy five pin Relays as there’s virtually no price difference and a 5 pin
can do everything a 4 pin can and a bit more. For instance, let’s use a 5 pin relay to
light the same sort of Bulb circuit we saw with the 4 pin relay.
You can see it here. Close the master switch and the bulb is lit. Open the master switch
and off the bulb goes. However, the clever bit with a 5 pin Relay is that it can control
two circuits. Let’s add another bulb across the fifth
pin. Now see what happens. With the master switch
closed, circuit A is powered and bulb A is lit. But when the master switch is open, the
Normally Closed element of the Relay comes into play and circuit B is now powered.
The one thing that’s missing from my diagram here is numbers. Let me add them now.
To the best of my knowledge this is a fairly universal number system for Relay. Who came
up with the numbers I don’t know, but knowledge of these numbers is very helpful. Each pin
on a Relay is normally numbered and by knowing what each number represents, as shown here,
it’s easy to figure out how to wire them up.
And that’s pretty much all I'm going to say on the subject in this video. There are
other variations on Relays I believe, but nothing we need to be concerned with here.
And there’s also a large array of Relays you can buy, but the differences in these
are usually down to what power is needed to activate them and what amount of power they
can actually draw or handle. That is the real reason behind a Relay. That’s
their real use. That’s why you average vehicle is littered with them. It’s all about the
Power. Generally it takes very little current to
energise a Relay, and where there’s very little current, the average human can safely
handle things. For instance a switch on your dashboard. You don’t want massive, metal
melting power surging through it… you just want a tiny bit. But at the other end of the
circuit, where you need to handle real juice, that’s where the Relay comes in. It can
handle all the dangerous power that would fry us.
If you’re at all unsure what sort of Relay you need to buy, pretty much any Vehicle related
relay will work for you. These usually power at 12 volts, the standard vehicle voltage,
and can carry current of varying rates, 10 amps, 30 amps, etc. Probably 10 amps will
do most projects I would think. But for your project, check your needs first. What’s
the most power you need to pull? What the voltage of your vehicle? With that knowledge,
you’ll easily find a Relay. So with the basic explanation out of the way,
let’s come back to my subscribers requirements… One wanted to power courtesy lights.
One wanted to fit a third braking light. Both wondered if they could simply splice
into the existing circuit and add more lights, etc, and as a general rule I would say no.
It’s a bad practice. Sure you might get away with it, but if you draw too much power
you might blow a fuse and find yourself with brake lights or worse.
The sensible suggestion in both cases is to create a new, separate, independently fused
circuit. A piggyback fuse holder would be an ideal way of getting the necessary power
you need (see my video on the subject if you’re unsure).
But how do we get this new circuit to work alongside the one that’s already there?
How do we get the new brake light to come on when the old ones do? How do we get the
new courtesy lights to come on when the old one does? Simple… we use a Relay.
If we splice into the original or parent circuit, let’s say the brake lights, we can use this
live spliced cable to energise a Relay. The Relay uses so little current there’s virtually
no danger of overpowering the original circuit, so everything’s safe that way. The Piggyback
supply can then be fed through the output terminals of the Relay and used to power the
new brake light without concerns of overtaxing the original circuit or fuse.
So with all the theory out of the way, let’s look at a real demo. Now this set up is in
my shed on a scrap bit of board, but it could just as easily be in my car.
Let’s just look at what we've got here. Well there’s a main power supply, there’s
a switch, there’s a fuse holder and fuse and there’s a light bulb. It could be a
head light, a brake light, a courtesy light, or even an air con unit.
Click the switch, the light comes on, great. But we want to add another light and I'm going
to be using these. These are LEDs on a strip and in honesty they could be used for courtesy
lights or brake lights as they come in a variety of colours. They've got an adhesive backing,
you can cut off as many as you need with just a pair of scissors and you can buy these handy
little connectors to power them up. They also work great at 12 volts.
So here’s my circuit… pause the video if you need to see it longer…
… and here’s the actual application. Everything’s on the board wired up and ready. You can see
the original light and now there’s a small section of LED strip and of course the relay.
Click the switch and the original light comes on but nothing else. Why? Because there’s
no power yet. It’s still only the original fuse.
So let’s plug the Piggyback Fuse Holder
and slot back in the original fuse. And now everything works. The original circuit
is still fused and completely intact. The new circuit has a separate fuse, a separate
supply and is powered through the Relay. And the relay is energised by a splice from the
original, parent circuit, the blue wire you can see coming up from the white connector
block. So let’s just test everything… we’ll
remove the new red fuse… Sure enough the LED doesn't work but the original
light does. Even though the Relay would be energised, because we've removed the red fuse,
there’s no power to the LEDs. With the red fuse back in and the green fuse
removed, what now? Well nothing. There may be power to the red fuse, but the Relay is
not being activated by the original circuit any more. It can’t be.
Put everything back, and everything works again. Great.
Now it’s time for a few handy Relay tips: Relays often come with holders like this.
This makes wiring them up much easier… just a bit of splicing needed.
If you’re not using a holder, you’ll either need to solder wires to each terms – which
makes removal difficult – or you’ll need Spade Connectors. If you use Spade Connectors:
Make sure they’re a nice tight fit. Squeeze with pliers if necessary. You don’t want
them coming off. Ideally purchase ones that are fully insulated,
NOT like these. If you have to use these, then cover them well with insulation tape.
If the come loose, you could easily have a live wire flapping around and that’s never
good. Before installation, clean every terminal
with fine sand paper. This aids conductivity and ensures a good connection.
Once cleaned, lightly brush the terminals with petroleum jelly. You can get proper contact
gel and that’s your choice. I've found that petroleum jelly is just as good at preventing
oxidisation, etc, from dirtying the terminals, leading to poor connections and electrical
failures. And that’s a finished video folks.
I hope you found it helpful and I hope everything was nice and clear.
Please remember, I'm no expert, just a keen amateur, so it’s always worth checking out
some other videos on the subject. This was a subscriber request and I was happy to oblige.
So if you have video request, do get in touch. If I can help, I certainly will.
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So that’s it for now folks, and thanks very much for watching.