How a flashlight works,
Case of the OLIGHT S1R Baton II
1.Aim
The
aim of this document is to understand how a flashlight works and the
interactions between the different constituents, taking the
particular case of the OLIGHT. S1R Baton 2.
Many
of you are wondering about thermal, battery, "Turbo" mode,
etc ... And I will try to provide some answers.
I chose the S1RII model
because I appreciate this model, I have models in aluminum, copper
and titanium which will allow us to see the difference in thermal.
As
you will have understood, the rest of this document will be rather
technical, and focus on electronics and thermal. I am not proficient
enough in optics and do not have suitable equipment.
2.Electronic
part
Electronics
is my favorite field, so I'll try not to get carried away and be
accessible to anyone who wants to know a little more.
Basically a flashlight is
made up of two main elements, battery and LED, from these will result
an electronics.
We
will ignore the choice of these elements, because it could take a lot
of time, and consider that at the time of design it was the optimum
choice.
We will therefore do a
paragraph on the LED, the battery, the different power topologies,
and the electronics chosen for the S1RII.
2.1.LED
The
LED is the XM-L2 from the manufacturer CREE.
I
have already written an article on this element, it is available
Here.
My
conclusion was as follows:
We
have therefore seen that the current flowing through a power LED
creates a consequent luminous flux. This current also causes a
potential difference (or voltage) at the terminals of the LED which
depends on various parameters such as temperature, the production
batch, etc. This voltage and this current induce a power (Power =
Voltage x Current). This power will cause a rise in temperature which
will depend essentially on the care that the lamp manufacturer will
take in the assembly to ensure the thermal resistance between the
junction of the LED and the external environment as low as possible.
Because the higher the temperature of the junction of the LED, the
more the luminous flux will decrease.
For
the design of the flashlight, the important characteristic to
remember is that of the maximum operating point. For the model I have
on hand I measured 3.45V of LED voltage for a current of 3A at a 25°C
flashlight temperature.
2.2.Battery
This
is the IMR16340 model with reference 16C05-10C, a proprietary battery
from Olight (there is no equivalent in other brands). Indeed on the
standard 16340 we do not find the 2 poles on the same side (see photo
above.)
It
is therefore a lithium ion battery with a capacity of 550mAh. This
roughly means that once charged we will be able to draw a current of
550mA for 1 hour before being totally discharged.
This is a "high drain"
battery and 10C refers to the maximum discharge current, 10 x the
nominal discharge current of 550mA, so 5.5A max.
We
could take a shortcut by saying that if the accumulator can output
550mA for 1 hour, it can output 5.5A for 6 min. Hey no, the
chemistry, the internal resistance and the internal thermal will
severely limit the thing. The most telling is to draw a response
curve of the accumulator I have (which is not new).
From
this fully charged battery (with a voltage of 4.2V) and I will draw a
current of 3A (maximum allowable current in our LED):
We
can see very clearly on this curve the effect of the internal
resistance of the accumulator. As soon as the current is applied the
voltage drops 520mV (12.4%) which gives an internal resistance of
about 170mΩ (0.52V / 3A).
We
also see that in just under a minute and thirty seconds the voltage
reaches 2.8V (I do not recommend going lower) at this time I stop
drawing 3A (the current goes back to 0A).
A 2nd test at a current of
1.67A (corresponds to 600lm for our LED).
The
shape is different, the time is much longer (timebase x5) and I
stopped the current after 443s arbitrarily.What is impressive is that
for a current 1.8x smaller we take 12x longer to reach the
same voltage. This is due to internal resistance and also to internal
heating. In conclusion, the size of the battery and its capacity
in mAh are far from sufficient to anticipate the behavior of an
accumulator, what happens internally is difficult to predict
unless specified by the manufacturer.
If
you are interested in information on specific battery models, I
recommend this
Website.
We
can find there 16340 of 750mAh with a series resistance almost 3x
greater. I'm not talking about the effective duration of Turbo mode
(knowing the safety of the S1RII stops it at 3V) and the risk of
overheating (see more) of the battery.
This
is one of the reasons for using a proprietary battery, although from
a practical point of view I am not in favor. A bad battery can give
the impression of a low-end flashlight or even injure the user.
2.3.Electronic
choice
This
is the most important element after the choice of the LED and the
battery. This is the interface between these two elements and which
will make the final nature of the flashlight. This electronic has an
impact on:
autonomy,
indeed if the electronic losses
are important, it will be the same for the consumption at the level
of the battery
The duration of ″ high
power ″ modes, the S1RII is a very good example and we will see
this aspect right after.
The final temperature,
in fact the losses of the electronics are added to the power sent to
the LED to turn into heat
2.3.1.Specifications
We
will start from the curve of the accumulator and the voltage of the
LED measured for Turbo mode (worst case).
The
red horizontal line represents the LED voltage in turbo mode. We
realize that during 25s (30 to 40s for a new battery) the voltage of
the accumulator is higher than the voltage of the LED and the rest of
the time (70%) lower. Moreover, even if after one
minute and 30 seconds we reach the end of turbo mode, we see that the
output voltage rises high (> 3.8V) and therefore that the
accumulator is far from empty.
For
all other lighting modes, the voltage will be lower:
1000lm
3A 3.45V
600lm
1.65A 3.22V
300lm
0.75A 3V
60lm
0.184A 2.74V
12lm
0.026A 2.6V
We
need to choose a power electronics topologie between the battery and
the LED, before making this choice we will take stock of different
topologies that exist.
2.3.2.Power
Electronic Topologies
The
subject is vast and complex for those who want more information you
will find it Here.
To
summarize here is a short synthesis:
2.3.2.a.Linear
regulator
In
″ conventional ″ electronics the only assembly which allows to
make a voltage adaptation is the linear regulator, one of the basic
schematic of which is as follows:
This
circuit can only reduce the voltage (step-down). The only power
component is the transistor (Top, middle).
To put it simply, the
transistor will absorb the voltage difference between the input
voltage (Uin) and the output voltage (Uout). This voltage difference
multiplied by the current drawn by the output will give the power
lost in the transistor.
It
is therefore easy to understand that the more there is of difference
between the input and output voltage, the more there are losses and
the less the efficiency is good.
To
improve this efficiency, there are switching topologies (see
following paragraphs) which can also provide an output voltage
greater than the input voltage. Their only weak point is reliability
as they use a lot of component (especially for control) and switching
transistors. It is only relative, the MTBF (mean time between 2
failures) remains in the order of several million hours if done well.
2.3.2.b.BUCK (Step-down converter)
This
circuit makes it possible to have an output voltage lower than the
input voltage (like the linear regulator), but with a much higher
efficiency. The schematic of the power part is as follows:
2.3.2.c.BOOST (Step-up
converter)
This
circuit gives an output voltage higher than the input voltage, with
high efficiency. The schematic of the power part is as follows:
2.3.2.d.BUCK-BOOST (Step-ud and Step-down converter)
As
the name suggests, it lowers or raises the input voltage, it is no
more and no less than a BUCK followed by a BOOST. Its major drawback
is that it has a lot of components so it has a lower efficiency than
a BUCK or a BOOST, and its schematic is as follows.
2.3.3.Topologie
choice
So
if we analyze the data, we see that the ″ step-up ″ solution is
not viable because if the accumulator is charged we ″ destroy ″
the LED (too much voltage → too much current → destruction).
If we choose a "step-down"
topology, the turbo mode will be rather short while the battery is
far from being discharged, but it is perfectly suitable and the
autonomy will be very important.
If
we want a longer turbo mode, we must move towards a “step-down /
step-up” solution, but knowing that the space available for the
electronics is low, we will certainly have to make compromises.
Here
is a photo of the electronics of the S1RII, for the size we see in
the photo on the left that it is identical to a 1 cent french coin
(0,64 inch). The diagram I have drawn up is the following I think is
close to reality, but doing reverse-engineering is not easy:
Extremely
simple schematic (in view of the functions implemented) but
effective:
L1,
IC1, R1 and R2 are
a Boost. IC1 integrates control and switching
components.
Q1 is
a MOSFET used as linear
regulator.
IC4
which is a precision amplifier, which regulates the current in the
LED. It measures the current through the SHUNT and arranges for it
to be the image of the setpoint sent by the micro-controller.
To do this, he drives Q1 and if it is
necessary, it
drives the Boost.
IC2
is the micro-controller
which coordinates everything, and is powered by the regulator IC3.
Olight
therefore turned to an unconventional Buck-Boost, a Boost followed by
a linear regulator. The components used are components from very
reputable manufacturers (Texas Instrument, Microchip) or even
high-end (IC4). It only remains to measure the performance.
2.3.4.Electronics
performance
As
in most fields the performance index measurement is done by an
efficiency measurement. In our particular case, the efficiency is
therefore the power sent to the LED divided by the power extracted
from the battery.
On a red background, the area
that the accumulator cannot reach.
The
red numbers in bold represent the operation of the Boost (with a more
than correct result).
The zone on a blue background
is the nominal zone, in which we have the most chance of being with
mixed use (a little of each power) and that we will use the entire
battery.
The output at 12lm may seem
low but the 52% at 4.2V corresponds to 65mW of losses knowing that
the consumption of the micro-controller and the battery status LED
are part of these losses.
This
is why I prefer to also look at losses in electronics, I find that
complementary.
I
find that rather not bad, we have a little high losses when the
accumulator is well charged and that we are not in Boost, but this is
the price to pay for a longer turbo mode, a lack space, and increased
reliability. It seems like a good choice to reach as many users as
possible.
3.Thermal
I wanted to make a little
point on the thermal side, because I think that it can interest
people.
I am sorry but some values
are in °C and not in °F, I have changed all values I can.
On the other hand, we will
see that in the first order that is enough to get an idea, no need
for simulation by finite element.
We
can consider that the losses in the electronics and the power
injected into the LED will turn into heat. To have an idea of what
that represents, we will make approximations then compare to
measurements.
If
we inject energy into a material and we consider that it does not
transmit any to its environment (which is obviously false) we can
make a simple calculation from the heat capacity of this material,
from its weight and the energy injected.
In
order not to make too big mistake, you have to take a rather short
time with a strong energy. We are therefore going to do a test
with the turbo mode for 45 seconds. I measured an average of 11W
supplied by the battery in this case. This gives the following
table:
ΔT
therefore represents the temperature rise of the material if an
energy corresponding to 11W is injected into it for 45 seconds. The
weight is that measured by the mechanics of the 3 lamps in my
possession. You just have to pay attention to the fact that the lamps
are made on an alloy whereas the data in the table are for pure
materials.
We
can see that copper is the material that will have the lowest
temperature rise, then comes aluminum and then titanium which is
relatively close to aluminum.
Since the exchange of the
material with its environment has been neglected, the measurement
will necessarily be lower.
Before
doing a real test we will look at another important characteristic of
these materials, it is thermal conductivity. The stronger it is, the
more the heat "travels" and is distributed well in the
material.
We
can see here that copper still comes first, then aluminum and
titanium are far behind.
In the following test, the
lamps are covered with a tape with a known emissivity to make the
most accurate measurements possible. The test is carried out in an
air-conditioned room at 64°F. each sample is turned on in turbo mode
and a reading is taken every 15 seconds.
We
can therefore see that the temperature in:
copper
remains homogeneous and achieves 88°F
aluminum
is a little less homogeneous and achieves
102°F
the
titanium is not homogeneous at all and its hot spot a achieves
126°F
We also see that with simple
1st order calculations, we were not that far for copper and aluminum,
for titanium its low thermal conductivity has too great an impact.
4.Assembly
For the LED to heat as little
as possible, the assembly between the LED and the box must be as
efficient as possible from a thermal point of view. Everything
happens in the "head" of the S1R2.
To
achieve this The LED is mounted on an IMS, which is postponed to the
housing using a good dose of thermal grease, we can still see the
traces of this grease on the photo below (gray residue).
IMS
details
An IMS means Insulated Metal
Substrate, it is a metal plate on which is fixed an electrical
insulator (very thin and rather good thermal conductor) on which is
fixed a sheet of copper which is etched like a printed circuit
classic.
Most
often in industry the metal plate is made of aluminum but when we
want maximum thermal performance we choose it in copper which is
the case in our S1RII.
If
you have read the article on LED, you must have noticed that the heat
exchange is done by a surface of 0,11inch x 0,19inch. This IMS makes
it possible to make the electrical (printed circuit) and thermal
connection, it increases the exchange surface while minimizing the
thermal rise between it and that of the LED.
To
prevent the electronics from overheating, components L1, IC1 and Q1
are thermally transferred to the housing using a "Gappad".
Its texture resembles Patafix and its thermal conduction is good.
This is more than enough to keep these components at an acceptable
temperature.
So
Olight also took care of the blend for the best possible results.
5.Conclusion
With
its S1R baton II, Olight has chosen to please the greatest number
with a rather long Turbo mode, good autonomy, reliable electronics in
a small footprint, while doing with the constraints governed by the
physics and economic laws.
For
the different materials offered, we have seen the impact on the
thermal, aluminum is good, copper will offer a significant gain and
titanium is not made for that, but it is not for this criterion that
we choose it.
To
make big shortcuts the copper will be heavier but will light more
strongly in Turbo mode (the hotter the LED is internally the less it
lights). We should be able to observe it between copper and titanium
but we should be able to test with the same battery, the same LED and
the same electronics.
We
have also seen that what limits the duration of the turbo is the
accumulator for this model, but for other more powerful it will
surely be the thermal. Indeed, for humans, the burning sensation
is from 131°F and apart from large and
high-end models where effective thermal safety is possible, limiting
the time in turbo mode is the only way to protect the user.