High power LEDs:
The recent availability of
high-power LEDs provide an alternative to Lasers for high
intensity light sources that can be easily modulated.
Because the only practical means of modulating most light
sources is through intensity (or "amplitude") modulation,
these modulation schemes invariably the varying of the
current through the LED to achieve the desired modulation.
As with any electrical device, there are some practical
current limits that must be observed in order to avoid
destruction of the device. One of these is excess
voltage (from static discharge, for example) but most
high-power LEDs are rather well protected in this regard.
Excess current, however is another story!
The Luxeon III LEDs are rated for a (nominal) 3 watts of
dissipation, but some of the devices in the series (the Red,
Red-Orange, and Amber) are actually rated for continuous
operation at a bit over 4 watts - somewhat higher power than
the other colors in the Luxeon III product line.
The Luxeon I, III, and IV products have
been phased out and have only limited availability as "new-old stock",
but many other high-power LEDs are available from a wide variety of
manufacturers. In the discussion below note that the current
values are specific to the Luxeon III series and should be scaled
appropriately for the LED that you intend to use.
For the remainder of this discussion, we will be
assuming the use of the Red, Red-Orange, and Amber
Luxeon III devices with the higher power capability.
Given an "infinite" heat sink, the absolute maximum
continuous current permitted for these devices is 1.544 amps
with a peak pulse current of 2.2 amps although the
parameters of that pulse (e.g. width and duty cycle) are not
Through testing, I have determined that a properly
heatsinked Luxeon III will probably tolerate
continuous operation at 2.5 amps for a short period (30
seconds or so) without damage Other experience has
shown that at a current of somewhere between 3 and 8 amps,
the bond wire(s) on the negative lead of the Luxeon will
fuse (open) and render the LED inoperable.
LM317-based current limiter circuit
Click on image for a larger version.
accidentally made several Luxeon III's "inoperable" I
decided that all optical transmitters will contain circuitry
to limit the current to a value that will be safe enough to
protect the LED during short-term accidents.
A few notes concerning use with Luxeon LEDs:
- Red, Red-Orange, and Amber Luxeon I LEDs have a
much lower peak current rating - about 550 mA. At this
lower current, R1 would need to be above 2.5 ohms and R2/R3
would be adjusted accordingly. If a precise value of
R1 (around 2.2 ohms) can be selected, R2 and R3 can be
eliminated and the "adjust" terminal can be connected
directly to the "load" side of R1. This lower
current/voltage also means that the total drop of this
circuit will be in the 3.5-4 volt range at the limiting
- Philips is apparently phasing out the Luxeon I, III,
and V lines in favor of the lower-power Luxeon Rebel
devices. Since I have not used those other
devices, the techniques described here may not directly
apply unless the maximum current is appropriately
adjusted. For the time being, however, the Luxeon III
devices are still available from various sources. The
Luxeon Rebel devices have a peak current of 700 milliamps,
so a 1.8 ohm resistor in place of R1 (with R2 omitted and
the "ADJ" terminal connected to the "load" side of R1) would
Current limiting circuitry:
- With "ultra high-power" LEDs - that is, those with maximum
peak currents in the 10's of amps - the following methods
may not directly apply as the LM317 and similar 3-terminal
regulators simply can't handle the current! In those
cases, current-limited supplies, a slightly more elaborate
overcurrent protection circuit and plain, ordinary care and
common sense are your best defenses against accidentally
"killing" an LED!
simplest current limiter is a series resistor.
While simple, it's somewhat costly in terms of power
dissipation and its effectiveness (both in preventing
damage and in allowing normal operation) is somewhat
dictated by the available supply voltage: Too high
a value and the LED cannot be driven to full
current. Too low a value and the LED is not
adequately protected. If the supply voltage is
lower or higher than expected, the effects are similar.
A solution to this problem is the use of a common
3-terminal voltage regulator, the LM317. This
device is commonly used to regulate voltages, using a
simple resistive divider to set the output voltage, but
it can also be used as a fairly precise current source.
Even though the "official" rating of the LM317 in the
"K" (metal TO-3) or "T" (TO-220 "tabbed) packages is 1.5
amps, it is perfectly capable of operating at somewhat
higher currents than this. One of the features of
the the LM317 is its built-in current limiting and a
quick peek at the data sheet will reveal that its
internal ("room-temperature") current limit is typically
2.2 amps - but it could be anywhere from 1.5 to 3.5
amps. In testing dozen or so devices from
different manufacturers made over the past two decades
or so I observed that they typically exhibited an
absolute current limit in the 2.25-2.75 amp area and
none limited at less than 2 amps when the device was at
It is important to note that this "maximum current
limit" is only valid when the device is at room
temperature. As the device warms up the "thermal
protection" begins to take effect, reducing the maximum
available current with increasing temperature. I
also observed that at "touchable" temperatures (e.g.
temperatures at which one could touch the device for 5
seconds or so before deciding that it was too hot) that
the reduction in the maximum current was minimal:
It wasn't until the device got well above about 150
degrees F (about 65 C) that it really started to limit
Figure 1 shows such a circuit. This
is a modification of the typical LM317-based current
source in that R2 and R3 have been added, but its
operation is the same:
- As the current increases, a voltage appears across R1.
- When the voltage on the "Adjust" terminal is about 1.245
volts below that of the "Output" terminal, the current is
Normally, the adjust terminal is connected directly to the
"load" side of R1: When the voltage drop across R1 exceeds
about 1.245 volts, the current reduction occurs. In
testing, I built such a circuit, using only R1, but I found that
in order to set the limiting current within the 2.25-2.5 amp
range, I needed a resistor of about 0.6 ohms - not a common
While it would have been possible to synthesize such a
resistance by paralleling resistors or just using a piece of
known-length and gauge of small magnet wire, I decided to
construct a circuit that used more common resistor values and Figure
is the result. In this case, R1 is set for a
slightly higher resistance to effect a higher voltage drop while
R2 and R3 divide that voltage such that at 2.25-2.5 amps, the
Adjust-to-Output differential is in the 1.245 volt range.
Two examples of Luxeon III emitters with current
limiters. Note that these pictures do not include
the bypass capacitors C1 and C2.
Click on either image for a larger version.
Connected in parallel with this circuit is bypass
capacitance. If the LED is modulated with waveforms that
contain high frequency components (like those in a PWM circuit,
high frequency subcarrier, or video modulation)
capacitance (C1 and C2)
allows high frequency components to pass
around the LM317 while still offering protection to the LED from
a DC current fault. The use of two capacitors is
recommended as the 0.1uF capacitor will be transparent to the
highest frequency components, while the larger electrolytic will
better-pass the lower frequency components. It is
recommended that 105C (high temperature)
electrolytics (such as the low-ESR types designed for switching
be used - particularly if the capacitor is
exposed to the heat from the LM317.
Total voltage drop of the circuit in Figure 1
amps is around 4.5-5.0 volts. This, in series with a
Luxeon III LED would imply that there is a total voltage drop of
as much as 7.75-8.25 volts - but this is still enough headroom
for many circuits that operate from a 12 volt source - but some
circuits may need to be modified. As mentioned before, if
R2/R3 are omitted and the Adjust terminal is connected to the
load side of R1, the value of R1 can be reduced to an
experimentally-derived value in the 0.6 ohm area, reducing the
voltage drop at 2.5 amps by a volt or so.
Constructing the circuit:
As can be seen from the pictures in Figure 2
circuit can be constructed in a number of ways. It is
essential that the LM317 have at least some heat sinking.
In normal operation, the average LED current of a Luxeon III
will be about 1 amp, peaking to 2 amps or so at 100% positive
modulation. Under these conditions, the LM317 will have to
dissipate about 2 watts of heat while R1 will be dissipating
around 1 watt.
Even though this is a fairly small amount of heat, it is
important that this circuit NOT
on the same heat sink as the Luxeon as a quick check of the
Luxeon's specifications will show you that is it warms up, its
efficiency drops, so it is best not to add another device that
will warm it up! As shown in these pictures, the LM317 is
mounted separately from the Luxeon's heat sink - preferably off
to the side so that convection will not be likely to cause one
heat sink to heat the other.
It worth noting that it is also a good idea not
heat sink the LM317. A good example of this is shown in
the bottom picture of Figure 2
where the LM317 is simply
soldered to a piece of copper circuit board material. This
provides adequate heat sinking for normal operation where the
LM317 is dissipating about 2 watts. If a fault
develops and, say, a full 12 volts is placed across the LED
drive, this will cause the current to exceed the 2.25-2.5 amp
threshold and when this happens the voltage drop across the
LM317 necessarily increases and this will dramatically increase
the amount of heat being dissipated by the LM317. In this
state, the current will start to drop as the LM317 gets hot,
soon reducing the current to well below the 2.25-2.5 amp
threshold. It is under these conditions that a very large
heat sink is less-desirable as it would prevent the LM317 from
heating up and reducing the operating current.
As mentioned before, there must be enough voltage headroom in
the power supply voltage and modulator circuit to accommodate
the added voltage drop of this protection circuit. In the
modulators that I have built, power MOSFETs are used along with
1 ohm current-sense resistors, adding as much as 2.5 volts of
additional drop - but these run will drive the LED to over 2
amps with an 11.5 volt supply - the voltage of a
mostly-discharged lead-acid battery.
Under normal conditions this circuit is completely transparent
in that other than an addition I*R voltage drop across the sense
resistor, there are no effects at all. When the current
exceeds the preset threshold, however, the LED drive is clipped
at that level and could cause some distortion. A quick
look at the schematic will also reveal that I have placed it in
the "high" side of the LED supply. This was done in the
event that the "low" side of the LED (which, in my case, goes to
gets shorted to ground: A high-side current
limiter will still protect the LED in this case.
Note that because this circuit sits inline, there is no real
voltage limit to it aside the voltage differential that would
occur under fault conditions when the LM317 is
current-limiting. What this means is that if you were to
run a string, say, 5 Luxeons in series from a 24-36 volt
supply, you could: This circuit would happily protect them
Other types of circuits:
Close-up view of a red Luxeon III emitter. Note
the fine-gauge bond wire that connects to the conductors
on top of the die: It is this wire that
fuses if too much current is run into the LED.
Click on the image for a larger version.
When I was deciding how to protect the Luxeon I was
considering other types of circuits such as comparators
driving MOSFETs or other transistor-based limiter.
While these would work - and some have the advantage of
having lower voltage drops - none of them were as simple
as the LM317-based circuit shown here and some of them
require their own supply voltage to operate. This
circuit has the advantage of being cheap, simple, and it
need only be placed in series with the current flow, not
needing a ground or voltage reference.
Why not a fuse?
There is also the obvious question as to why not use a
fuse? If you look closely at a Luxeon III emitter
(see Figure 3) you'll note that it already
has a fuse disguised as a bond wire connecting one of
the terminals to the center of the emitter die. If
you use an external fuse, it must be one of a
lower rating and faster than that bond
wire! Clearly, this rules out slow-blow fuses as
well as thermistor-type PTC fuses. It is also
worth noting that there is quite a difference between
the "hold" current of a fuse and the current at which it
will blow quickly, and a current at which the fuse will
fail over a longer period of time - not to mention a
fairly wide range of variability of fuses of the same
ratings - even those from the same manufacturer.
Finally, if one uses fuses, it is imperative that spares
be kept onhand.
Despite the voltage drop which is something that can be
worked around in most circuits, usually through a minor
change, the described protection circuit works very well
to keep peace of mind - and it could prevent the sudden,
expensive end of testing should some sort of fault or
accident occur that might open up the LED.
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