Loading...
 
Print

LaserDiodePowerSupplies

This text is available elsewhere on the web from multiple locations. Once section quotes a public domain contribtuion I did years ago, so I grabbed a copy here that I could archive. Some of the schematic diagrams required a fixed space font to look right, some of the other formatting interacts in wierd ways with the Wiki rendering logic, include very w-i-d-e text lines. I'll get around to fixing up the format or doing proper schematics eventually.

Diode Laser Power Supplies

Laser diode drive requirements:
----------

The following must be achieved to properly drive a laser diode and not ruin
it in short order:

  • Absolute current limiting. This includes immunity to power line transients as well as those that may occur during power-on and power-off cycling. The parameters of many electronic components like ICs are rarely specified during periods of changing input power. Special laser diode drive chips are available which meet these requirements but a common op-amp may not be suitable without extreme care in circuit design - if at all.

  • Current regulation. Efficiency and optical power output of a laser diode
goes up with decreasing temperature. This means that without optical
feedback, a laser diode switched on and adjusted at room temperature will
have reduced output once it warms up. Conversely, if the current is set up
after the laser diode has warmed up, it will likely blow out the next time
it is switched on at room temperature if there is no optical feedback
based regulation.

  • essay ideas. For college students and professional writers.

Note that the damage from improper drive is not only due to thermal effects
(though overheating is also possible) but due to exceeding the maximum optical
power density (E/M field gradients?) at one of the end facets (mirrors) - and
thus the nearly instantaneous nature of the risk.

The optical output of a laser diode also declines as it heats up. This is
reversible as long as no actual thermal damage has taken place. However,
facet damage due to exceeding the optical output specifications is permanent.
The result may be an expensive LED or (possibly greatly) reduced laser
emission.

I accidentally blew one visible laser diode by neglecting to monitor the
current but it wasn't the sudden effect some people describe - the current
really had to be cranked up well beyond the point where the brightness of the
laser beam stopped increasing. It did indeed turn into a poor excuse for an
LED. One data point and you can conclude the world :-).

Another one was blown by assuming that a particular driver circuit would work
over a range of input voltages when in fact it was supposed to be powered from
a regulated source. At first the degradation in brightness appeared to be
reversible. However, what was probably happening was that damage to the laser
diode was occurring as soon as the brightness appeared to level off. The
natural tendency was then to back off and approach this same point again. Not
quite as bright? Crank up the current. Finally, once it is much too late,
the realization sets in that it will *never* be quite as bright as it was
originally - ever again. This one still lases but at about 1/10th of its
former brightness.

If you then try to power this damaged laser diode with a driver circuit using
optical feedback, further instantaneous damage will occur as the driver
attempts to maintain the normal optical output - which is now impossible to
achieve and only succeeds in totally frying the device as it increases the
current in a futile attempt to compensate.

Also see the section: "How sensitive are laser diodes, really?".

Laser pointers and diode laser modules - the low stress approach:
------------------------

Where what you really want is a working visible diode laser, a commercial
laser pointer or diode laser module may be the best option. Both of these
include the driver circuit and will run off of unregulated low voltage DC.
While the cost may be somewhat higher than that of a bare laser diode, the
much reduced risk of blowout and built-in optics may be well worth the added
cost. It doesn't take too many fried laser diodes to make up this cost
difference!

Believe me, it can get to be really frustrating very quickly blowing expensive
laser diodes especially if you don't really know why they failed. This will
be particularly true where the specifications of the laser diode and/or driver
circuit are not entirely known - as is often the case. Helium neon lasers are
much more forgiving!

Buy one that accepts an unregulated input voltage. Otherwise, you can still
have problems even if you run the device from a regulated power supply. All
laser pointers and most (but not all) modules will be of this type. However,
if you get a deal that is too good to be true, corners may have been cut. A
proper drive circuit will be more than a resistor and a couple of capacitors!

To confirm that the driver is regulating, start with an input near the bottom
of the claimed voltage range and increase it slowly. The brightness of your
laser diode should be rock solid. If it continues to increase even within the
supposedly acceptable range of input voltage, something is wrong with either
the laser diode (it is incompatible with the driver or damaged) or driver (it
actually requires a regulated input or is incorrectly set up for the laser
diode you are using). Stop right here and rectify the situation before you
blow (yet another) laser diode!

See the chapter: "Laser Parts Sources" for a number of suppliers of both diode
laser pointers and diode laser modules.

If you still aren't convinced that someone else should deal with laser diode
drive design issues, the remainder of this chapter provides suggestions for
integrated drive chips, sample circuits, and complete power supply schematics.
But don't complain that you haven't been warned of the sensitive nature of
laser diodes.

Laser diode drive chips:
-----------

Many semiconductor manufacturers offer laser driver chips. Some of these
support high bit rate modulation in addition to providing the constant current
optically stabilized power supply. Other types of chips including linear
and switching regulators can be easily adapted to laser diode applications
in many cases:

Note: Free samples of ICs like laser diode drivers may be available for
the asking even if you won't be buying a million parts in the future.
Manufacturers often provide some means of requesting free samples at their
web sites. Just be honest about your needs - they consider it good PR and
you might just tell a friend or colleague who WILL buy a million parts!

  • Maxim (http://www.maxim-ic.com/).

The MAX3261 (1.2 Gbps) and MAX3263 (155 Mbps) laser driver driver chips
are two examples of their highly integrated solutions.

  • Linear Technology (http://www.linear-tech.com/).

App Note AN52 (and probably others) includes a sample circuit using their
one of their chips (not necessary dedicated laser drivers) for powering
laser diodes. In AN52, the LT1110 Micropower DC-DC converter is used as
the current regulator for operating from a 1.5 V battery.

  • Both Sharp and Mitsubishi manufacture IC's for driving laser diodes. Most
will maintain constant power. Some require two voltages, others just one.
These circuits will drive the common cathode lasers, or the Sharp "P" or the
Mitsubishi "R" configuration which has the laser's cathode connected the the
anode of the photo diode. The Sharp IR3C07 is a good for CW or analog
modulation, and the IR3C08 or IR3C09 will allow digital modulation to 10
MHz. These parts are quite inexpensive.

  • Analog Devices (http://www.analog.com/) has several laser diode drivers
including the AD9660 and AD9661 both of which provide for full current
control using the photodiode for feedback and permit high speed modulation
between two power levels.

  • Burr-Brown? (http://www.burr-brown.com/):

(From: Steve White (stevew@hitl.washington.edu)).

We are using the OPA 2662 (Burr-Brown) for this. It is an OTA with 370MHz BW,
59mA/ns SR, and can source/sink 75mA of current per channel (two channels per
chip which may be paralleled quite easily). The part provides the emitter of
the current source to an external pin (programming side of an internal
current mirror), so that a single resistor sets the voltage-current transfer
characteristic. Watch out for the dependence of the harmonic distortion specs
upon the supplied current and frequency though...if this will be a problem
for your particular application that is (didn't matter much for mine).

Comments on some commercial drivers and detectors:
-----------------

(From John Sojak (trax@allways.net)).

As far as modulation is concerned, the Analog Devices driver is hard to beat
for three bucks. Couple that with a 555 and a battle proven LM317 front end
and cry 'BINGO'. Maxim used PECL inputs ... arrgh! I don't need to spit photon
packets at 150 mhz! Linear Tech IR receiver looks good, although the $7.00
price tag + a handful of linear doesn't really appeal to me. Too bad you can't
get inside the epoxy covered die in the Sharp TV/VCR consumer IR receiver
modules (apx $1.50/100 pcs). Not everyone in the world wants to decode bursts
of 40 kHz back into data!

Oh, by the way - an Optek BP812 Optologic sensor performs quite well at at 760
nm. It's an active device available in either totem pole or open collector
outputs. The applications guy at Optek says the device won't work at 760 but
looking at the response curve, I disagree. It's response is only down about
10% in the reds! Most silicon photo stuff is down about 60-75% at 760ish
nm. From what I have seen, the device is very usable at 760 nm. Useful part
for red diodes and HeNe? stuff.

Visible laser diode power supplies (reverse engineered from commercial units:
----------------------------

These circuits were traced from commercial devices using visible laser diods.
Laser drivers (1) to (3) were from CW laser lights used for positioning in
medical applications while (4) was from a UPC bar code scanner. Errors may
have been made in the transcription. The type and specifications for the
laser diode assembly (LD and PD) are unknown.

The available output power of these devices was probably limited to about 1 mW
but the circuits should be suitable for the typical 3 to 5 mW maximum power
visible laser diode (assuming the same polarity of LD and PD or with suitable
modifications for different polarity units).

Of the 4 designs presented below, I would probably recommend "Laser diode
power supply 2" as a simple but solid circuit for general use. It doesn't
require any special chips or other hard to obtain parts. However, I would
add a reverse polarity protection diode (e.g., 1N4002) in series with the
positive input of the power supply. In fact, funny that you should ask :-).
An enhanced version of this design including a printed circuit board (PCB)
layout is presented in the section: "Sam's laser diode driver SG-LD1".

Testing laser diode driver circuits:
---------------

If you do build these or any other circuits for driving a laser diode, test
them first with a combination of visible (or IR) LEDs and one or more silicon
diodes (to simulate the approximate expected voltage drop) and a discrete
photodiode to verify current limited operation. To accommodate the higher
current of laser diodes compared to LEDs, use several identical LEDs in
parallel with small balancing resistors to assure equal current sharing:


COM o----+---+---+---+---+
| | | | _|_
LEDs _\_/_ _\_/_ _\_/_ _\_/_ ---> /_\ Photodiode
| | | | |
/ / / / +----o PD
5 \ 5 \ 5 \ 5 \
/ / / /
\ \ \ \
1N4002 | | | |
LD o---|<|--+---+---+-----+


Note that the sensitivity of this photodiode to the LED emission will vary
considerably depending on its position and orientation. Tape the photodiode
and one of the LEDs together (sort of like a homemade opto-isolator) to
stabilize and maximize the response.

Using this 'laser diode simulator', it will really only be possible to confirm
that the laser driver current regulator is functional, not to actually set it
up for your laser diode.

Once the circuit has been debugged, power down, and carefully install the
laser diode. Double check all connections!

Use the guidelines below in both cases (written assuming an actual laser diode
is being used):

  • Set the power adjustment of the laser driver to minimum (usually maximum
resistance).

  • If available, use a power supply with both voltage and current limit
adjustments. Then, you can start with the voltage set to 0 and the current
limit set just above the expected laser threshold current (plus the current
drawn by the rest of the circuit - test with no laser diode in place). This
can always be increased later.

  • Attach a voltmeter between the photodiode (PD) terminal and ground. This
will effectively monitor relative optical power output.

If you have a (separate) current meter, put it in series with the power
supply as well (or provide another means of measuring current).

CAUTION: Use clip leads. Leave the meters in place - do not attempt to
change connections while the circuit is powered as this could result in a
momentary current spike which may damage the laser diode.

  • Increase input voltage gradually. Once the laser diode starts lasing, the
PD voltage should climb. The circuit should regulate when the PD voltage
approaches the reference: 2.5 minus .7 V in circuits (1)-(3) or .5 Vcc for
circuit (4). Then, the PD voltage and supply current should level off. If
something doesn't behave as expected, shut down and determine why.

  • Once you are confident that the circuit is operating properly with the laser
diode installed, the output power can be increased modestly. But, without a
laser power meter, DO THIS AT YOUR OWN RISK!

- For visible laser diodes, if you have a laser pointer or other visible
diode laser module OF THE SAME WAVELENGTH, A-B brightness comparisons can
be made if the beams are the same diameter. Otherwise, don't push your
luck unless you have a bucketload of laser diodes you can afford to blow!

- For IR laser diodes, visible light eyeballs won't work. The tiny red dot
that may be visible from an IR laser diode cannot be used as an accurate
indication of power output.

Laser diodes are generally NOT very forgiving. However, if you take your time
and make sure you understand exactly what is happening at every step along the
way, you and your laser diode will survive to light another day!

Laser diode power supply 1:
----------

This one runs off a (wall adapter) power supply from about 6 to 9 V.


D1
Vcc o---|>|---+----+-----+-+--+
1N4001 | | | | |
Rev. Prot. | | Power Adjust | _|_ |
| / R3 10K (2) | PD /_\ LD _\_/_
| R2 \ +----+ | | |
| 560 / | | +---|-
---+
\ +-/\/\+---+ C4
.1 uF
+--
--+ /

+_|_ | | | C2 (1)| \ R4
C1 --- | | E / \ 100 pF| / 3.9
10 uF - | +---|--' Q1 '-----+ |
| |R | BC328-25 (5) | |
| +---+ | (PNP) | |/ Q2 (5)
| | _|_. | +---| BD139
| VR1 +-'/_\ | | |\ (NPN)
| LM431 | | C3 +_|_ E|
| 2.5 V | | 10 uF --- |
| (3) | |X - | |
R1 3.9 | | |Y | |
GND o--/\/\/\-+----+-+------+-----+

Note the heavy capacitive filtering in this circuit. Changes would be needed
to enable this circuit to be modulated at any reasonable rate.

Notes:

1. Capacitor C4 value estimated.

2. Potentiometer R3 measured at 6K.

3. LM431 shunt regulator set up as 2.5 V reference. A 2.5 V zener or even a
visible LED could also be used.

4. Supply current measured at 150 mA (includes power on LED not shown).

5. Transistor types do not appear to be critical.

A circuit similar to this can be found in the Optical Department of the
Electronic Circuits Cookbook Archive under laser.gif and laser.txt.

Derek Weston (derekw@alphalink.com.au) has constructed an IrDA tranceiver
based loosely on this design and a Crystal Semiconductor Corporation CS8130
IR transceiver IC. A complete description of this project may be found at:
http://alphalink.com.au/~derekw/upntcvr.htm.

Laser diode power supply 2:
----------

This one, from the same manufacturer as the one described in the section:
"Laser diode power supply 1", seems to be an improved design including a
soft-start (ramp-up) circuit and an inductor in series with the laser diode.
Otherwise, it is virtually identical and runs off of a 6 to 9 V DC source.

Since both units were from the same company, I assume that these refinements
were added as a result of reliability problems with the previous design - in
fact, I have recently discovered that the unit from which I traced that
schematic is not as bright as it should be!

Interestingly, there does not appear to be any reverse polarity protection on
the input - I don't know why that would have been removed! C1 and Q1, at
least, would likely let their smoke out if the power supply was connected
backwards.

2SC517 (NPN) (6)
Vcc o---+. Q1 .-+---+-------+-+--+----+
| _\___/_E | | | | | |
| | | | | _|_ | \ R5
R1 \ | | | | PD /_\ LD _\_/_ / 1K
3.3K / | | / | | | \
\ | | R2 \ | | | |
| | | 390 / R3 +---|-
-+--+
\ +-/\/\--+-----+ C4 (2)
+-----+ 2.2K 10 pF +
+--
--+ )

| +_|_ C2 | | | C3 (1) | ) L1
| --- 33 uF | | R4 E / \ 47 pF | ) (3)
| - | +---|/\/\--' Q2 '-----+ +
| | |R | 220 BC328-25 (6) | |
C1 +_|_ | +---+ \ (PNP) | |/ Q3 (6)
1 uF - | | _|_. /<-+ R6 +-| BD139
- | | VR1 +-'/_\ \ | 10K | |\ (NPN)
| | LM431 | | | Power Adjist C5 +_|_ E|
| | 2.5 V | ++ (4) 10 uF - |
| | (5) | |X - | |
| | | |Y | |
Gnd o-----+--+---+---+--------+---+

Note the heavy capacitive filtering in this circuit. Changes would be needed
to enable this circuit to be modulated at any reasonable rate.

Notes:

1. Capacitor C3 was marked n47 and very small, probably .47 nF (470 pF).

2. Capacitor C4 was marked 10n and very small, probably 10 nF (.01 uF).

3. Inductor marked Red-Black-Black-Silver?, probably 20 uH.

4. Potentiometer R6 setting not measured.

5. LM431 shunt regulator set up as 2.5 V reference. A 2.5 V zener or even a
visible LED could also be used.

6. Transistor types do not appear to be critical.

Laser diode power supply 3:
----------

This one runs off of a (wall adapter) power supply from about 10 to 15 V
(12 V nominal).

It was apparently designed by someone who was totally obsessed with protecting
the laser diode from all outside influences - as one should be but there are
limits :-). This one goes to extremes as there are 5 levels of protection:

1. Input C-L-C? filter.
2. Soft start circuit (slow voltage ramp up).
3. LM7810 voltage regulator.
4. LT1054 DC-DC voltage converter.
5. Optical power based current source.

The first part of the circuit consists of the input filter, soft start circuit,
voltage regulator, and DC-DC voltage converter. Its output should be s super
clean, filtered, despiked, regulated, smoothed, massaged source of -10 V ;-).

MPSA13
L1 D1 C E I +----+ O -10 V out
+12 o+--CCCC+|>|+. Q1 .-+| LM7810 |+---+ o
| |1N4002 | _\___/_ | +----+ | | C5 |
| | R4 / | | C| | | +--+
| | 10K \ | | | | 8| 7 6 5| 180 |
| | / | | | | +-++--++-+ uF |
+_|_ C10 +_|_ C11 | | +_|_ C8 | C7 _|_ | |16 V|
- 2.2 - 2.2 +---+ - .22 | .1 --- | LT1054 | +_|_
- | uF - | uF | | - | uF | uF | | | ---
| | +_|_ _|_ | | | +-++--++-+ - |
| | C9 - - C6 | | | 1 2| 3| 4| C3 |
| | 4.7 - | | .047 | +--+-----+|--
+
L2 uF uF C4 + - .01 uF
Gnd o+--CCCC+-----+--+---+ 180 uF +-(+----+
16 V
It was not possible to determine the values of L1 and L2 other than to measure
their DC resistance - 4.3 ohms. The LT1054 (Linear Technology) is a 'Switched
Capacitor Voltage Converter with Regulator' running at a 25 kHz switching
frequency. A full datasheet is available at http://www.linear-tech.com/.
The input to the LM7810 ramps up with a time constant of about 50 ms (R4
charging C9). This is regulated by the LM7810.
The LT1054 takes the regulated 10 V input and creates a regulated -10 V output.
There is no obvious reason for using this part except the desire to isolate
the laser diode as completely as possible from outside influences. Like the
use of an Uninterruptible Power Source (UPS) to protect computer equipment from
power surges, a DC-DC converter will similarly isolate the laser diode circuit
from any noise or spikes on its input.
The second part of the circuit is virtually identical to that described in
the section: "Laser diode power supply 1":
Gnd o------+----+-----+-+--+
Power Adjust __
/ R2 20K PD /_\ LD _\_/_
R1 \ +----+
470 / +----
---+

| \ +-/\/\+---+ C2 |
| | | | /
+_|_ | | | \ Rx
C1 --- | | E / \ C /
10 uF - | +---|--' Q1 '-----+ |
| |R | PN2907 | C|
| | \ (PNP) | |/ Q2
| _|_. / R3 +---| PN2222
| VR1 '/_\ \ 1K | |\ (NPN)
| LM385 | / C1 +_|_ E|
| Z2.5 | | 10 uF --- |
| | |X 16 V - | |
| | |Y | |
-10 V o-----+----+-+------+-----+

Note the heavy capacitive filtering in this circuit. Changes would be needed
to enable this circuit to be modulated at any reasonable rate.

I suspect that there are additional components inside the laser diode assembly
itself (like the hypothetical Rx, probably a few ohms) but could not identify
anything since it is totally potted.

Laser diode power supply 4:
----------

This more sophisticated (or at least more complicated) driver board uses a dual
op-amp (LM358) chip instead of discrete parts to control a transistor current
source. Due to the relative complexity of this design, and the fact that it
is entirely constructed of itty-bitty surface mount parts, errors or omissions
with respect to both transcription and interpretation are quite possible!

The schematic for the driver is available in PostScript? and GIF format. The
Postscript versions have been compressed PKZIP (DOS/Win3.1/Win95) and GZIP
(Unix).

  1. lddrive.zip
  2. lddrive.gz
  3. lddrive.gif

The feedback loop consists of the photodiode (PD, part of D1), a non-inverting
buffer (U2A), the inverting amp/low pass filter (U2B, R9, R11, C2, bandwidth
of about 1 kHz), and emitter following current source (Q1, R13, R14, with a
sensitivity of 36 mA/V) driving the laser diode (LD, part of D1).

Separate DC inputs are shown for the laser diode/photodiode itself (Vcc1) and
the other circuitry (Vcc2). Vcc1 must be a regulated supply as there is no
on-board voltage reference. It appears as though Vcc1 and Vcc2 should be set
equal to one-another though there may have been (external) power sequencing in
the original application. If Vcc1 is less than Vcc2 by more than a volt or
so, the laser diode will be turned off. The input voltage range can be from 5
to 12 VDC though I would recommend running on 5 VDC if possible since this
will minimize power consumption and heat dissipation in the current driver
transistor and other circuitry. This is adequate for laser diodes with an
operating current of up to about 80 mA. For laser diodes with an operating
current greater than this, a slightly higher voltage will be required.

The set-point is at about 1/2 Vcc1 so that the laser diode optical output will
be controlled to maintain photodiode current at: I(PD) = .5 Vcc1 / (R6
R7).
Use this to determine the setting for R7 (SBT, Select By Test, Power Adjust)
for the photodiode in your particular laser diode. Or replace R7 by a low
noise variable resistor and use a laser power meter to set the operating
current. (Hint: Start with the minimum current - maximum resistance).
Optical output will be linear with respect to Vcc1 and inversely proportional
to R6
R7 as long as the laser diode is capable of producing the output power
(and thus photodiode current) determined by the equation, above. Beyond the
upper limit, the laser diode will likely be damaged instantly! Don't push
your luck too far :-).

For example, with Vcc1 = Vcc2 = 5 VDC, maximum laser diode current will be
limited to about 90 mA. With R7 (SBT) equal to 5.9K, photodiode current will
be .5 mA. For some laser diodes, this is approximately the value for 1 mW of
optical beam power BUT YOURS MAY BE TOTALLY DIFFERENT!

If you then increase Vcc1 = Vcc2 to 10 V or halve the parallel combination of
R6
R7, the output power will double or the laser diode will die in a futile
attempt to achieve the impossible.
A cutoff circuit is provided to disable current to the laser diode as long
as Vcc2 is more than about 1 V greater than Vcc1 or from an external input
logic signal (ground J1-2 to disable). This consists of Q2, Q3, and their
associated resistors. When Q2 is biased on, it turns on Q3 which shorts out
the input to the main current driver, Q1.
The comparator (U1, LM311) would appear to output a signal based on photodiode
current being above a threshold but its true purpose and function is not at
all clear (or there is a mistake in the schematic).
As noted above, there is NO on-board voltage or current reference. Thus, Vcc1
must be a well regulated DC supply with low ripple and noise and NO power-on
overshoot (especially if the laser diode is being run close to its optical
power limit). However, this isn't quite as critical as driving the laser
diode directly since optical output power (photodiode current) and not laser
diode current is the controlled parameter. A power supply using an LM317 or
7805 type IC regulator with a large high quality filter capacitor on its
output (e.g., 100 uF, 16 V, tantalum, in parallel with a .01 uF ceramic)
should be adequate.
Although the original version of this board uses surface mount devices, common
through-hole equivalents are available for all parts and these are labeled on
the schematic. Note: A heat sink is essential for (Q1) where Vcc1 is greater
than 5 VDC - this part gets warm.
Characteristics of some typical laser diodes:
----------------
Having analyzed the circuit in the section: "Laser diode power supply 4", I
then proceeded to try out a variety of typical visible laser diodes. For
all the undamaged laser diodes that I tested, leaving SBT open resulted in
safe feedback regulated operation at Vcc1 = Vcc2 = 7 V. But, depending on
the particular sample's photodiode sensitivity, optical output power varied
widely.
While testing, I used a regulated power supply with adjustable current limit.
The voltage was set at 7 V and the current limit knob was used to ramp up the
input to the driver while monitoring laser diode current and/or feedback
voltage from the photodiode. This approach may have prevented damage to a
laser diode on more than one occasion.
Sample SBT LD Current LD Power Output
--------------------
1 (49) Open 79 mA .3 mW
39K 80 mA .5 mW
12K 82 mA 1.2 mW
2 (H81) Open 104 mA 1.5 mW
3 (H74) Open 80 mA 2.0 mW
4 (21)* Open >150 mA .3 mW
5 (696) Open 67 mA .2 mW
39K 69 mA .4 mW
12K 70 mA 1.0 mW
5.6K 72 mA 2.0 mW
3.3K 74 mA 3.0 mW
2.2K 89 mA 4.0 mW
6 (H32) Open 51 mA .2 mW
39K 52 mA .4 mW
12K 56 mA 1.0 mW
5.6K 60 mA 2.0 mW
3.3K 70 mA 3.0 mW
7 (D) Open 40 mA .6 mW
39K 43 mA 1.0 mW
12K 47 mA 2.0 mW
8.2K 50 mA 3.0 mW
8 (K)* Open 61 mA .1 mW
39K 66 mA .2 mW
12K 83 mA .5 mW
9 (E)* Open >150 mA 0.0 mW
The numbers in () do not mean anything - they were found marked on each sample
and are only used to identify them uniquely.
Laser output power was estimated to seven significant digits based on the
perceived brightness using my Mark-I eyeballs (with AutoCal(tm) option) :-).
The resistance of SBT (R7) is listed. However, the actual photodiode load
is R7
R6 (33.2K) and thus the photodiode current is (Vcc1/2) = 3.5/(R7||R6)
when optical feedback is successful in maintaining regulation. Since the
photodiode current should be proportional to optical power, you will probably
find that my high mileage eyeballs suffer from some slight non-linearity as
well ;-).

I do not have specifications for any of these laser diodes. However, they
are typical of the 660 to 670 nm types capable of 3 to 5 mW maximum output
power found in readily available diode laser modules and laser pointers.

Samples 1 through 6 were all in a large (9 mm diameter) package while samples
7 through 9 were in a small (6 mm diameter) package. As you will note, for
these types of laser diodes, power output does not really correlate with
package size. Each was mounted along with a collimating lens (adjustable in
some cases) in an aluminum block or cylinder (variety of styles) which also
acts as a heat sink.

I suspect that samples 2 and 3 were of similar construction but that this
differed from that of samples 1 and 4. Note how sensitive sample 1 is to
slight increases in current - dramatic evidence of the risks involved in
running these without optical feedback. Samples 7 through 9 also appeared to
be similar but I only had one fully operational unit of this type to test so
no detailed comparison could be made.

I do not know whether the higher current for sample 2 is due to prior damage
or just a normal variation in laser diode power sensitivity.

Samples 4, 8, and 9 (*) had been damaged to varying degrees previously due to
running with excessive current. These disasters occurred prior to analyzing
the behavior of this laser driver circuit. Sample 9 was absolutely positively
beyond a shadow of a doubt totally dead laser-wise behaving like a poor excuse
for a visible LED in a cool-looking fancy package :-).

In the case of samples 5 and 6, I continued to decrease SBT until a distinct
jump in laser diode current was required to maintain the voltage across SBT
(and thus beam power). For example, with sample 5, the jump from 74 mA to 89
mA may have indicated that losses were building and damage or total failure
would have resulted if pushed any further. However, at that point, no changes
in laser diode behavior had occurred and all lower power levels ran at the
same drive current as before. Note: I do not know if this is a valid approach
for checking the limits of a laser diode but it may work for some types.

All of the other (undamaged) laser diodes tested could probably have been
pushed to higher output power but without knowing their precise specifications
and only using my Mark-I eyeballs for a laser power meter, I chickened out.
However, there was definitely headroom above the power levels listed above.

Laser diode modulation:
----------

None of these designs can be modulated at any reasonable rate without
modifications to reduce the heavy filter capacitance at multiple locations.
However, in principle, this should be straightforward. Since both the
following affect the optical feedback, attempt at your own risk.

The following applies to Laser diode power supplies 1 through 3. Similar
modifications could be made to #4 but this is left as an exercise for the
student!

A bi-level modulation scheme could be easily implemented by connecting a
general purpose NPN transistor across an additional resistor (at point XY).
Then, full power will be achieved with the transistor turned on and reduced
power with it turned off. Select a value for R2 that will still maintain
the current above the lasing threshold - 1K is just a start.

X
o
|
+-----+
|C | Typical transistors: 2N2222, 2N3904.
R1 |/ /
TTL Input o-/\/\-| Q1 \ R2
1K |\ / 1K
|E |
Y o----+-----+

Here is another circuit which should achieve somewhat linear control of laser
power since optical power output should be proportional to photodiode current.
Resistor values shown are just a start - you will need to determine these for
your specific laser diode and operating point.

R
o
|
\ R1 X
/ 10K o
\ |
C1 10 uF | |/ C
o--)|--+--| Q1
- + | |\ E
Line level | 2N3904 |
audio / /
R2 \ R3 \
o 10K / 1K /
| | |
Y o-----+----+----+


Sam's laser diode driver SG-LD1:
-----------

SG-LD1 is an enhanced version of the design described in the section: "Laser
diode power supply 2" with the addition of bilevel (digital) modulation as
described in the section: "Laser diode modulation". It should be capable of
driving most typical small laser diodes including those found in CD players
and CDROM and other optical drives, and visible laser diodes similar to those
found in laser pointers, bar code scanners, medical positioning laser lights,
and other similar devices.

This design assumes a laser diode assembly where the laser diode anode and
photodiode cathode are common (this seems to be the arrangement used most).
If the opposite is true with your device (laser diode cathode and photodiode
anode are common), reversing the direction of polarized components and power
supply input, and changing NPN transistors to PNPs and vice-versa will permit
the same PCB layout to be used. However, if your laser diode assembly has
both anodes or cathodes in common, this circuit is not suitable unless an
external photodiode is used for the optical feedback.

Disclaimer: The cicuit is currently under development so there may still be
errors in the schematic and/or PCB artwork. I will not be responsible for
any damage to your pocketbook or ego if for some reason your laser diodes
do not survive.

The complete schematic is available in PostScript? and GIF format. The
Postscript versions have been compressed PKZIP (DOS/Win3.1/Win95) and GZIP
(Unix).

  1. sgld1sch.zip
  2. sgld1sch.gz
  3. sgld1sch.gif

In some cases, the part values listed should be considered as suggestions as
many modifications are possible depending on your particular laser diode
specifications and application needs. Transistors with heat sinks for Q2 and
Q4 are advised if operating continuously near the upper end of the input
voltage range (say above 10 V) and/or at laser diode currents of 100 mA or
higher.

A printed circuit board layout is also available. The entire single sided
circuit board is 1.7" x 1.15" and includes modulation and enable inputs. It
will run on an unregulated power supply of around 6 to 12 VDC.

The layout may be viewed as a GIF file (draft quality):

  1. sgld1pcb.gif

A complete PCB artwork package for SG-LD1 may be downloaded in standard (full
resolution 1:1) Gerber PCB format (zipped):

  1. sgld1grb.zip

The Gerber files include the solder side copper, soldermask, top silkscreen,
optional component side pads, and drill control artwork. The original printed
circuit board CAD files and netlist (in Tango PCB format) are provided so that
the circuit layout can be modified or imported to another system if desired.
The text file 'sgld1.doc' describes the file contents in more detail.

Simple laser diode power supply:
-----------

(From: Brian Mork (mork@usa.net)).

Best circuit I've found:


In +---+ Out 18 ohm*
(+) o---+-| LM317 |---/\/\/\--+-+----o LD anode
| +---+ | |
_|_ | Adjust | _|_ |
22 uF - +-------+ --- 1 uF _\_/_
| | |
| |
(-) o---+-----------+----o LD cathode


  • Note: Resistor value depends on your specific laser diode current
requirements. Discussion below assumes a laser diode with a 72 to
100 mA drive range --- sam.

Power is 5.5 to 9 VDC. I use a 9 volt battery.

Watch the pin arrangement on the LM317. On the LM317L (the TO-92 plastic
transistor type case) and the LM317T (TO-220 7805-type case), the pins are,
left to right, Adjust-Output-Input?.

For the resistor, I use a small carbon 10 ohm in series with a precision
10-turn 20 ohm adjustable. The combo was empirically set to about 17 ohms.

On initial power on, use three garden variety diodes stacked in series
instead of the laser diode. Put a current meter in series with the diode
stack and adjust the precision resistor for 50-60 mA. Disconnect power and
replace the diode stack with the laser diode. Connect up power again, still
watching on the current meter. The diode will probably initially glow
dimly. I use a diode that lases at about 72 mA, and has a max rating of 100
mA. I use about 85 mA for normal ops.

Turn up the current, never exceeding your diode's max limit. The dim glow
will increase in intensity, but at some point, a distinctive step in
intensity will occur. Your diode is lasing. Remove the current meter as
desired. Enjoy!

Constant current supply for high power laser diodes:
-------------------

(From: Winfield Hill (hill@rowland.org)).

The schematic in the section: "Simple laser diode power supply" is the
standard circuit for making a constant current source from an LM317 or LM338
(e.g. see The Art of Electronics, fig 6.38). The problem with this circuit is
that for large currents (the only currents for which it has good accuracy, and
is a serious part saver) it's hard to make the current variable.

For example, for a 3.5 A current source, the resistor value is 0.357 ohms,
and if you then want a 3.1 A current you've got to unsolder it and replace
it with a 0.403 ohm resistor. Bummer.

One option would be to put a low value pot across the sense resistor and
connect its tap to the voltage regulator common/adjust terminal. This will
work reasonably well for a modest current range - perhaps up to 2:1 as shown
below - but runs into difficulties where a wide range of control is desired.


In +---+ Out R1 1.01 ohm
Vin o--| LM317 |-+-----/\/\+--o 1.25 to 2.5 A current source
+---+ | |
| Adj. +-/\/\-/\/\-+
| R2
R3
| 100 ohms | 100 ohms
+----------+

The reason is that this arrangement can only *increase* the current from the
nominal I = 1.25V/R. So, for example, to get a 10:1 range, the voltage across
the sense resistor would be 12.5 V for the 10x current! In general this is not
attractive for the high current condition because not only have you required
a higher supply voltage, at the maximum current, but the power dissipation in
the sense resistor is also quite high (more like HUGE --- sam).

Let me offer the following simple circuit, which I just created and haven't
tried but 'oughta work' as a solution to this problem.

By contrast, this circuit can only *decrease* the current from the 1.25V/R
value, but it easily handles a 10:1 range (or even much more) and the voltage
across the sense resistor is never more than 1.25V, allowing low supply voltage
(e.g. 5 V) and keeping the dissipation low.


In +---+ Out R1 .25 ohms
Vin o--| LM338 |---/\/\/+---o 0 to 5 A current source
+---+ |
| Adj. +----+
| cw | |
| 1K
/ _|_,
+------>\ '/_\ LM385-1.2
/ |
| |
+----+
|
+--------+
| I = 0.5 to 1.5 mA sink |
+--------+
_|_
-

The 1K pot selects a portion of the floating 1.23 V reference voltage, and
tricks the LM317 or LM338 into correspondingly reducing the voltage across
the 0.25 ohm current-sense resistor. The pot is conventional and may be
panel mounted. It should be possible to nearly shut off the LM338 (a
minimum quiescent current will still flow). The current sink, I, which
powers the floating 1.23 V reference, is not critical and may be a simple
current mirror (sorry to see the TL011 gone!), or even a resistor to
ground or any available negative voltage, depending upon the desired
current-source voltage-compliance range. That's it!

--------------------------


Created by brian. Last Modification: Tuesday 16 of February, 2010 11:54:02 EST by SimonEngard.