if you want to destroy an led fitting quickly there's no faster way than
connecting a series fitting in parallel across an led driver
in this video we're going to try and help you to avoid making this costly
mistake for many years installing an extra low
voltage light meant feeding it by a transformer
and for you younger viewers it was quite common to just install
one big transformer and then run several spots off it
a couple of simple rules of thumb were applied where you made sure that the
voltage rating of the transformer and the lamps matched
the total power rating of the lamps added together didn't exceed the total
va rating of the transformer and just trying to keep the elv cables
roughly the same length to each lamp to prevent uneven dimming and it was a
fairly straightforward job for quite a while it was then common to
connect one lamp to one transformer now however we've been met by the led
revolution in lighting and at first glance it all seems to map
across fairly simply the incandescent lamps have been replaced by
leds the transformer has become the driver just make sure the voltage is the
same and that you don't overload the driver
simple almost there's one major difference and
it's one that can catch out the unwary electrician
let's head inside to discuss it so led drivers come in two types there's a
constant voltage driver and a constant current driver if you've got a group of
leds in a strip or a tape or a ribbon or a rope then
you'll probably be fitting a constant voltage type
but when you've got a group of fittings with high powered leds that can have a
number of fittings added you're likely to be using a constant
current driver what difference does it make
a really critical one and one that if you don't understand could end up
costing you in time and replacement materials
how well the first thing to know is that leds are not like the traditional loads
that we're used to connecting they're semi-conductors now that doesn't mean
that they're not a very good conductor it means that their electrical qualities
change depending on the electricity they're connected to which i can
demonstrate for you on this led fitting that colling would
have sent to us i'm going to connect it up to this benchtop power supply so this
is going to inject dc electricity into the fitting
behind me here and i'm going to control how much
current flows through it now watch what happens to the current
and the light output as i reduce the voltage
just a little bit so i'm just going to turn the voltage down just a touch so
that's just going to go down by a fraction of an amount and look
what's happening to the light output and look what's
happening to the current that flows you can see that
once we drop below in this case just 10 of the original voltage so we've gone
just beyond 10 percent drop in voltage there is now no current flowing through
the fitting whatsoever and the fitting has turned off so you
see a small voltage change affects the current flow
massively if this was a resistive load we'd expect the current and the light
output to drop in direct proportion to the voltage
what's actually happening is the subatomic particles in the led materials
are arranging themselves differently but that's a very different video now
because the current flowing through the leds is so critical to the operation the
driver like the one i've got here actually operates a bit like the ballast
in a fluorescent light and manages the current through the led of
course this can only be guaranteed if you connect the leds in the driver up
correctly set them up wrong and the current may be
too high reducing the life of the led and if you go way too high with the
current then you'll destroy it even a small increase in current through
them above the rated value can seriously shorten their operating life
and even damage them pretty quickly now let's say you get a
set of lights that needs a constant current supply like these lovely spike
lights here from Collingwood it's so important to decent
manufacturers to try and help you not destroy your shiny new fittings
that collingwood have put warnings on the fittings in three different places
that this must be connected to a constant current driver they've put one
here on the head of the fitting they've put
one on this little removable label and even one on the leads here is a
last-ditch effort to stop you connecting them up wrong
and destroying them so how should they be
connected well the clue is in the description
constant current driver if you're at college or
you can cast your mind back that far you may remember the kindly teacher who
taught your science unit desperately trying to get you to
remember that the current is constant in a series circuit and there it is
finally after all these years we found the application for those
seemingly pointless science lessons that your well-meaning lecturer
tried to get across to you if you've got led fittings that need a
constant current driver then they need to be connected in
series let's just demonstrate the principle using this benchtop power
supply that we've got here now this power supply can be used in one
of two ways one way is to give a constant voltage
and the other is to deliver a constant current
now these fittings require a constant current of 350 milliamps common ratings
include 700 and 1050 milliamps as well colin would have another type of spike
light here that can operate on either 700 milliamps or 350 milliamps
the light output from them changes according to the current level that you
supply if you run them at the higher current
level of 700 milliamps they will obviously give off more light
but also use more power and the drivers have a maximum power rating that they
can deliver just like a transformer so if you're
putting together a system of led lights think about the current that you're
connecting them to how much power each fitting will dissipate and what the
total power will be that you're connecting to the driver
obviously that total cannot exceed the maximum power that the driver can
deliver most drivers for leds also have a lower
value of power that they will operate on so if you connect a 0.5 watt fitting to
a 1 to five watt driver it won't work properly that's why we
occasionally run into problems when dimming leds
because the power goes too low anyway back to our demonstration here in the
importance of connecting in series i've set up the power supply to deliver
the 350 milliamps current required by the lamp
and if i just dial that voltage back up to where we were at the start
about three volts you can see the lamp is illuminated and the current delivered
by the power supply is 350 milliamps so let's add in another fitting but
remember it needs connecting in series now this is where as electricians who
are used to connecting up pretty much everything in parallel
we need to take a deep breath and just follow the directions
now the bit where it gets a little bit weird is that
we need to connect opposite colors together
so we need to connect the black conductor
out of the first fitting so that's this conductor here i'm just going to
take this label off so that that's not getting in my way when i make these
connections now so we need to take the black
conductor out of the first fitting and we're going to put that
into this little connector block here that's part of
the outdoor connection colin would provide so we'll just put
that into there and that black conductor out of the
first fitting needs to be connected to the positive or
the red conductor of the next fitting so that now needs to
go into the red there like that so we'll
pop that into that connection there so you can
see we've got the negative from the first fitting the
black conductor is now connected to the positive from the second fitting
which is the red conductor there so hopefully
you can see that quite clearly so if we did that in a 230 volt light
fitting we'd be messing up the polarity for the rest of the circuit
but not here so we're going to power this back up again so we'll reconnect
the leads so you can see if we turn it back on the
current is still set to 350 milliamps you can see that's not changed but we've
told the power supply to keep the current constant
so because we've connected an additional load in series we've increased the
circuit resistance meaning that the power supply is having
to work harder to push the same amount of current
through the circuit and that means that the voltage has increased as you can see
here so just to prove the point we'll add a
third light in series so again we're going to take the black
conductor or the negative from fitting number two
and we're going to connect that to the red conductor
or the positive from fitting number three so we'll just pop that in there
like that tighten that up so we've got a good
connection and then of course we're going to
connect the black or negative from fitting number
three we're going to hook up there so you can see that actually
the input colours at the start and finish
remain the same so red is still connected to red black is still
connected to black it's only at these kind of intermediate
connections where we've got the black conductor or the negative
connected to the red conductor or the positive
of the next fitting so let's just power it up again so if we turn that on
you can see that the current is staying the same 350 milliamps but the voltage
has increased to compensate for the increased resistance
and this is basically what a constant current driver is doing like this one
it just adjusts the voltage it puts out to keep the same amount of current
flowing through the led fittings it's just obviously bundled into a much
smaller package of course this will only be true up to a
point and there is a limit on the number of lights that you can connect this is
indicated as normal by the power rating of the
driver now the problem can occur in one of two
ways the first is if we connect a constant current led
fitting to a constant voltage driver the driver will deliver a particular
voltage and attempt to push too much current into the led hence it will be
very bright for a very short period of time
seriously reducing its operating life the second problem is that if we connect
up several fittings in parallel to a constant current driver
then the constant current will be shared between the fittings and cause the
fittings to flash or not to work at all so that's the
principle behind how a constant current driver works
now it's all well and good having this all laid out nice and neatly here on the
bench where we can clearly see the connections that need to be made
to make this into a series circuit but what about if we want to install this
into a real-world setting well the principle stays the same
however the actual connections can become a little bit mind bending
especially if the fittings are spaced some distance apart
so in the next video on this subject we'll have a look at a real installation
and how to make those connections properly as well as looking at some
really nice connection solutions and outdoor ip rated drivers that
collingwood have produced so all that remains in
this video is to say thank you very much for watching