Electronics and LED Selection
Choosing the best LED to use for your bike light will probably differ
with your needs and/or your budget. Also keep in mind that superbright
LED technology is always advancing. So the LED you choose today might
be surpassed in output/efficiency six months from now. For the year
2005 and most of 2006,
the best choice would be to choose one of Lumiled's Luxeon emitters.
Luxeons come in four basic models:
Luxeon I - one watt, 45 lumens @ 350
mA (mA = milliamps)
Luxeon III - three watt, 80 lumens @ 1000 mA
Luxeon V - five watt, 120 lumens @ 700 mA
Luxeon K2 - up to 140 lumens @ 1500 mA
Sometime around October 2006, Cree began shipping the XLamp 7090 XR-E series LED.
The XR-E (P3 bin) is a rated to produce 80 lumens (typical) @ 350
mA or
about 130
lumens @ 700 mA. When (and if) the Q3 binned version of this LED is
available (early in 2007), it should be able to produce about 160
lumens at 700mA.
Note: Cree announced during the first
part of 2007 that it is safe to drive the XR-E at 1000mA, providing
adequate heat-sinking is used. This can result in about 160 lumens
output from a P3-binnned XR-E and even more from the now widely
available P4-binned XR-E.
Shown below is a close-up photo of a Cree XR-E (P4-bin) emitter mounted on a nice looking star MCPCB
(Metal Core Printed Circuit Board). As you can see soldering to any of the tabs on the star would be much
easier than soldering to the connectors on the emitter and less likely to cause damage to the emitter.
Update 01/06/2007.
I've just ordered, for testing purposes, some U-binned, Seoul
Semiconductor P4 emitters, model W42180. These are supposed to output
between 91 and 118 lumens when driven at 350ma. They can be driven with
up to one amp (3.8v) and are supposed to output anywhere from 200 to
260 lumens, depending on which end of the bin your luck places you.
Therefore, we can build a 3 LED bike light that outputs anywhere from
600 - 780 lumens, depending on our luck. This should be equivalent or
greater in brightness than an HID bike light of the same power (around
12 watts)!
Don't be confused by the fact that Seoul calls their emitter the "P4"
and Cree makes an XR-E emitter that is available in a P4-bin rating.
Different companies have different bining schemes!
Shown below is a Seoul Semiconductor P4 (U-bin) emitter. Notice that Arctic Alumina
Adhesive is used to mount the emitter and to electrically isolate the emitter's
slug (back surface) and solder leads from contact with the aluminum surface onto
which it is mounted.
Stars and Emitters -
As shown above, LEDs can be purchased as just a bare emitter or
with the emitter soldered to a slightly larger star-shaped circuit
board that allows for easier mounting and soldering. The stars also
have a special aluminum backing that lets the heat transfer through
from the emitter to dissopate heat to whatever surface that you mount
it to. The stars can be mounted to your case with screws or fastened
with an adhesive that transfers heat as well, such as Arctic Alumina
Adhesive. For most bike light construction situations, I recommend
using the star. There are some situations where it is better to use the
bare emitter, but we won't get into that (using the bare emitter can
result in better heat dissopation, but it is more difficult to work
with... it's up to you).
Note: most star MCPCBs provide electrical isolation from the metal
heatsink surface that the star is usually mounted to, but there have
been exceptions with some Luxeon Stars. This is not usually true when
using just the bare emitter. For example, the Seoul emitters' heatsink
slug, on the back of the emitter, is electrically connected to the
positive lead of the package.
Lambertian versus
Side-Emitting - Luxeon LEDs are available in two
different light output patterns. Lambertian and side-emitting. The
Lambertian pattern concentrates most of the light out the front of the
LED in sort of a parabolic dispersion pattern, which is ideal for use
with most reflectors and optics. The side emitters are probably not
well-suited for bike lighting as I have not seen any reflectors or
optics available for them that would provide the kind of forward-facing
beam that we need.
LED Binning and the Lottery -
LEDs (both Luxeon and Crees) are graded, like diamonds, into certain
bin groupings. The binning represents several qualities of the LED
which include: lumens output, color, and required foward voltage to
light the diode. Generally, you'll try to select a bin that gives you
the most lumens of white light with the minimal foward voltage. As you
might expect, the better grade LEDs demand a higher price tag.
This chart from Cree shows the different Luminous Output ratings for the different
bins of the LED @350mA.
The graph below, from Cree, shows how driving the LED at higher current
levels will result in greater intensity. For instance a P3-binned Cree XR-E
will average about 77 lumens at 350mA. When driven at 1000mA, the
intensity will be about 220% more than at 350mA. Thus, a P3-binned
XR-E will be jamming at over 160 lumens when driven at 1A, providing
you can keep it cool enough.
LED Drivers
LEDs are rated to
output a specific number of lumens of light at a
given current (typically, 350mA, 700mA, 1000 mA, etc) and a specified
forward voltage (typically around 3.75 volts for the ones we discuss
here). We could drive an LED directly
from a battery if the battery voltage was close to the forward voltage
of the
LED, but we would probably need to add a resistor to limit the amount
of current flowing to the LED, otherwise it might get too much juice
and burn out.. Many inexpensive LED flashlights and bikelights work
this way. So
if we had 2 x 1.5v batteries, in series, and an appropriate resistor,
we could probably get enough
to light our LEDs sufficiently. The preferred method to light an LED is
to use a constant current regulator/driver. It will provide a constant
current to the LED even as the battery's voltage changes (as it
drains)... down to a certain point.
Some drivers offer support for dimming
the LEDs to a lower output than the full rated drive level. The nFlex
and bFlex (taskled.com) offer several fixed maximum drive current
settings as well as 5 levels of dimming within each of those. The 3021
Buckpuck controller overs continuous dimming via a 5k potentiometer
across its control and reference pins. The 3023 Buckpuck controller
offers the convenience of being pre-wired, but typically does not have wires connected for the control and reference pins, so you just get full power... no dimming.
(Note: just to add to the confusion, I have seen some wired 3023
Buckpucks that do offer dimming... just be careful if you are looking
for dimming capability).
Shown below is a DC powered, 1000mA 3021 BuckPuck that offers connectors for
wiring an external potentiometer for dimming . It is a little difficult to work with the pins
coming out of the BuckPuck and you will notice in the photo that this one is mounted to a
perforated circuit board. The 3023 version of the BuckPuck has wire leads coming directly
out of it, and it is easier to work with, but it typically does not offer dimming via the ctrl and ref pins.
Should I Buck or Boost?
When shopping for LED drivers, you will soon discover the terms
"boost" and "buck." Basically a driver with a boost type of circuit
will be able to drive an LED with less battery voltage than that
required to light the LED. So with a boost driver you can drive an LED
that requires 3.5 volts with a 1.5 volt battery. I've got several small
flashlights that work this way. The "buck" type of circuit reduces the
voltage of a higher voltage battery down to the required voltage for
the LED.
Quite a few LED drivers are available to serve our purpose, and they
typically run about $20-$25. You could build your own if you're
electronics savvy, but it's hard to beat the small packages available
that will easily fit into the small body of our bike light. I'll
mention a few here, but for purposes of building our bike light, I'll
choose the one I consider the easist, no brainer LED driver to use, the
3021/3023 Buckpuck by LED Dynamics. I also like the nFlex and bFlex controllers offered by TaskLED.
In our bike light examples we will use two or three LEDs that are wired
in series. Wiring them in series allows each of the LEDs to receive the
same current. The total voltage required to drive them will be the sum
of their forward voltages, plus another volt or so that is soaked up by
the inefficiency of the driver. The illustration below shows a battery,
a constant-current LED driver, and 3 LEDs wired in series so that
each LED receives equal current.
If you've developed a blank look after reading the previous paragraphs,
don't worry too much. You don't have to be an electrical engineer to
figure this stuff out. I'm not an electrical engineer, so I won't go
too deep into electronics theory, but a basic understanding of Ohm's
Law, V= IR (Voltage = Current x Resistance) is often helpful. I will
not teach basic electronics here, but a few examples will be given
where we discuss current draw, voltage, and power. There are plenty of
good web sites that will give you what you need on Ohm's Law. Try this one.
Power - What is all this talk about Watts?
Power, in terms of watts,
is a common term when talking about the output of lights, but really it
is not a measure of light.. Power is simply the product of current and
voltage. The manufacturers of lightbulbs
often market the output of lightbulbs in terms of power, i.e. 60w
light bulb in your home is not as bright as a 100w light bulb. This is
simliar for 1w, 3w, and 5w LEDs, but... really we are interested in the
lumens of light that are produced as output from our light. LEDs are
becoming more and more
efficient in terms of lumens per watt. For instance a Cree XR-E that
uses around 3 watts of power can put out more lumens than a 5 watt
Luxeon V... and this is really cool. We all want lights that use less
energy and output more light! We want more lumens per watt! Hopefully
we'll be seeing LEDs that can output over 100 lumens per watt in a few
years.
If
you want to figure out how many watts your light is using, you can
figure it like this:
P = IV. Power(watts) = Current(amps) x Voltage(volts)
Examples of Power and
Lumens/Watt
Luxeon I: 0.35A @ 3.75 volts = 1.3 watt (approx)
Lumens = 45 (34 lm/watt)
Luxeon III: 1.0A @ 3.9volts = 3.9 watts(approx) Lumens = 80 (20 lm/watt)
Luxeon V: 0.700A @ 6.9 volts = 4.83 watts(approx) Lumens = 120
(25 lm/watt)
Cree XR-E: 0.700A @ 3.75 volts = 2.63 watts(approx) Lumens
= 160 (60 lm/watt)
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