Want to connect a fused circuit from your vehicles fuse box?
Want to power an auxiliary device like a dashcam or power socket?
Want it safe, reliable and with clear and simple instructions?
Then keep watching… Hi YouTube, my name’s Geoff and today I’m
going to be looking at adding a fused circuit directly from the fuse box of your vehicle
to safely power any auxiliary device. I was planning on shooting this video using
my own vehicle, but as you can see my cars fuse box is under the steering wheel and not
very convenient for making a vid. So in yet another YouTube first I’m going
to be using a plasticine fuse board and a few helpful diagrams to make this really simple
to understand. I just want to start off by saying I’ve
seen some YouTube videos out there, some by so-called professionals, telling you to add
a circuit like this – by coiling wire around an existing fuse.
No! Don’t ever do this. It might work short term, but there’s a
real danger of arcing or short-circuiting if you have any loose wire rattling around
in your fuse box. It’s much easier and safer to use a piggyback
fuse like this which provides a secure, reliable connection and a fused outlet.
But before I get on with that, I want to very quickly run through some extremely basic electronics
as this very basic knowledge will help you safely connect circuits in the future.
Now here is the makings of a very simple circuit – a battery and a lightbulb.
To make the lightbulb work we have to connect it to the battery using wires. One to the
positive battery terminal. One to the negative. And then there was light!
So now we can imagine that the electrical current is flowing around this circuit. It’s
going from the positive terminal of the battery, down the red wire, into the lightbulb, down
the blue wire and back into the battery. On a vehicle things aren’t quite the same.
We have a battery and a lightbulb still, but the wiring will be a little different. Ignoring
switches, fuses and relays, etc, to power the lightbulb we need to run a wire from the
positive terminal of the battery to the headlight as we did before. But in vehicles, rather
than run a negative wire, manufacturers use a shortcut. As the vehicle body is made of
metal and conducts electricity, the negative terminal of the battery is simply bolted to
the chassis. So to complete our circuit all we need to do is connect the headlight to
the chassis of the vehicle and it now has a positive and negative supply. That means
the electrical current can flow two and from the battery and that mean the headlight will
work. The process of connecting the negative terminal
to the chassis in vehicles is often called earthing or grounding. It’s usually accompanied
by this symbol. So once you understand this idea, you can make more sense of vehicle electrical
circuits. So let’s look at our simple lightbulb circuit
again but this time we’ll introduce a fuse. The fuse is a necessary safety item that protects
all the electrical devices, so don’t ever try to short these out with wires or tin foil.
Of course the fuse needs a fuse holder and as you can see from this diagram the fuse
is not plugged into the holder so the current can’t flow. But when we plug the fuse into
its holder, we complete the circuit. The bulb is lit and the current flows.
We can see how the current goes IN to the fuse and OUT again. That’s an important
concept to grasp for what follows. PAUSE
There’s an IN and an OUT on the fuse board of your vehicle and you’ll need to find
this. I’d recommend using a multimeter for this purpose, set to Direct Current (DC) Voltage,
in the lower range. I personally have found that circuit testing screwdrivers are unreliable
and sometimes difficult to see, so for me the multimeter is the way to go, and these
days they’re really cheap and worth the investment.
So coming back to our circuit, with the fuse pulled out, we can determine the IN and the
OUT. Sure we know it on this diagram, but in your vehicle it might not be so simple.
Don’t assume – test it first. Place the black probe of the multimeter onto
the chassis of the vehicle, maybe a nearby bolt or screw, remembering of course that
this is the negative connection. The positive probe can be placed against one side of the
fuse holder. You can see here this measures 12 volts as that’s a typical vehicle voltage.
As it registers 12 volts, we know this is the IN terminal we’re looking for. To confirm
this, move the red positive probe to the other terminal of the fuse holder. You can see it
measures zero vaults because right now there’s no current flowing through it as there’s
no fuse in place. If there was, we’d obviously register 12 volts, so remember the fuse MUST
be pulled out. And with no fuse in place we can see this is indeed the OUT we’re looking
for. So how can we apply all this to your vehicle?
Here’s an illustration of a typical fuse board. There’s lots of different coloured
fuses of different ratings, and that doesn’t matter right now. Our first job is to discover
which way the power comes IN to each fuse. So let’s pull a fuse. Here I’ll pick this
bottom green fuse. In your vehicle, you may find it helpful to
use long-nose pliers. Gently grip the fuse as too much pressure will crush it. Then just
pull it upwards. We can now use the positive probe to check
for the IN and the out by inserting it into the hole left by one of the terminals of the
fuse. PAUSE
If one registers 12 volts, that’s our IN. From now on, we need only take our measurements
from this terminal. The other of course will register zero volts.
But what if they both register zero volts. That is possible and highly likely because
some of these fuses will be permanently on – permanently live. Others will only be
on when the vehicle ignition is turned on. It’s important to differentiate between
these Permanent INS and Switched INS as it will have an effect on the device you’re
installing. A dashcam, for example, needs a switched supply. Otherwise it would be operating
in when the vehicle was empty. An intruder alarm, on the other hand, would need a permanent
connection as it would be pointless having an alarm that was only active when you were
driving the vehicle… So have we found a permanent IN here, a permanent
live connection? With the ignition turned completely off, the
red probe registers a voltage of 12 volts. As this ignition is off, this must be a permanent
live. So let’s try another fuse.
PAUSE With another fuse removed and the ignition
completely off, we can measure the voltage on the IN terminal. Here it registers zero.
But if we now turn the ignition on… … we can see that the voltage is now 12
volts, so we’ve found a Switched fuse, perfect for a dashcam or similar item.
It’s very easy to connect to the wrong sort of fuse, so taking your time to determine
which is Permanent and which is Switched is critical for the correct function of your
devices. Typically the inside cover of your fuse box
will have a diagram showing which fuse is for which electrical item in your vehicle.
This should help you cut out the guesswork and let you make an initial educated guess
as to which fuses will be permanent and which will be switched.
Once we’ve determined which fuse we’re going to use, it’s simply a case of replacing
it with the piggyback fuse holder. Some people argue that you shouldn’t use
a piggyback fuse on just any old fuse, especially if it’s for a safety item like airbags.
I’ll leave that decision to you. Personally I think that anyone having that opinion just
doesn’t really understand how a piggyback works. It doesn’t affect the original fuse
in any way. It simply extends the original fuse holder and incorporates a second one.
If you look closely at the piggyback, you can consider the bottom section to be a simple
fuse holder. This is where you’d plug back in the original fuse – in this bottom slot.
It lets current flow through it just how you’d expect a fuse holder would.
If we look at the upper section of the piggyback, we can see this is a little different. For
starters it has a red wire trailing off it. This upper holder shares the first terminal
with the lower holder. This is the section into which the fuse for the NEW circuit needs
to be placed. You can see how the current flows from the first terminal, through the
new fuse, and out through the red wire. It has no affect at all on the lower fuse.
PAUSE It’s very simple to insert the piggyback.
Once the original fuse is inserted to the bottom holder and the new fuse to the top,
it’s just a matter of placing it back in the fuse board where the original fuse came
from. I have seen in some forums arguments about
piggyback holders with folks claiming they can be inserted into the fuse holder the wrong
way round. I tried my own experiments to test this, which I’ll show later on in the video
if you’re interested, but personally I couldn’t agree with this argument. It seems to work
either way round. PAUSE FOR 5
It’s as easy as that. The piggyback holder creates a new, secure, fused circuit.
All that remains is to extend the red wire from the piggyback holder to the new device.
These piggybacks generally come with one of these crimp fittings attached. You simply
insert the wire into the connector… … squeeze it with a crimping tool…
… and the wire is extended. Personally I prefer to cut these off and solder a join,
but either way you now have a live, fused feed which you can attach to your chosen device.
And as you’ve discovered, you can usually get a negative connection nearby on the vehicle
from a handy bolt or screw.
That’s it… you’re all done… please remember to Like, Comment or Subscribe…
But hang on, you say, what about that ‘wrong way’ business? Are these things safe to
use or not? Can you really put them in the wrong way round?
Well I wasn’t sure either. The arguments sounded plausible. The idea of them sharing
the fuses or running one circuit in series or in parallel with the other did sound quite
daunting. So I decided to do my own low tech bench test.
Here you can see a simple circuit. There’s three 1½ volt batteries lashed together to
give me 4½ volts supply. You can see I’ve made a crude fuse holder and after this is
an LED. It’s a pretty fair representation of a vehicle circuit I think you’ll agree.
As you’d imagine, when I added the fuse the LED lit up. Try and remember how bright
it is. So then I created a second circuit, this one
representing the addition of a new device but, importantly, no new fuse holder. To make
this circuit work I needed the piggyback holder with the original and new fuses added. As
you’d expect, both LEDs lit up nicely. But what happens if I flip the piggyback around,
plugging it in the other way round. Well personally I can see no difference at
all. Both LEDs look just as bright both ways. And if I flip the piggyback to it’s previous
position, it’s the same story. But some people will be claiming they can
see a slight difference in the brightness of the LEDs. They can’t actually, it’s
just the camera angle, but to appease these doubters I rewired the circuit, deliberately
placing each LED in parallel. Look how dim they are now. That’s the sort of brightness
drop we should have seen if there really was any power sharing issue with the piggyback.
You can be sure I tested the piggyback properly, with and without the original fuse… with
and without the new fuse, etc. It performed just how it should.
My personal conclusion is that they work well. They’re safe and reliable and MUCH better
than wrapping a wire around a fuse. After all, that’s why they sell them.
So there you have it folks. We’re all done. I hope you found this video helpful and informative
and if you did, please remember to like it. Please feel free to add a comment and if you
subscribe you’ll get notified whenever I produce a new video.
If you have any questions, please do get in touch. I always maintain that I’m no expert,
but I’ll do my best to help. Check out my website www.vegoilcar.co.uk if
you’re interested in the joys of free, environmentally friendly motoring.
Please look out for my other videos on my Channel and thanks for watching.