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.