Step 1: The Principle of Work of the LD Driver
The power generated by the R5 must be also properly dissipated. In my case the maximum power generated by R5 is 2.5W - 5V*0.5A. I have used 5 W resistor. The resitor R3 is optional. In some cases R1 also. R2 and C1 are used to protect the laser diode from some voltage spikes.
Some words about the used opamp and NMOS transistor:
The power NMOS transistor normally has a big working area, what in most of the cases presumes big input capacitance. For some devices it can reach some dozens of nanofarades. This capacitance appears as capacitive load for the opamp. The opamp must be able to drive such kind of big capacitive load, without losing its stability. Some opamps are compensated for similar loads, but a plenty of standard opamps will oscillate. You have carefully to check in both datasheets ( of the opamp and the NMOS ), what is the gate capacitance of the power NMOS transistor, and is the opamp stable with this load. In some cases, even the opamp is not stable with the specific NMOS transistor as load, the stability can be drastically improved by the "isolating" the load from the opamp output by the use of simple resistor. This in the schematics is the function of R1. If you have stability problems, you can play with the value of R1 and to try to reach the stable operation.
The LD is connected at JP1, the pwer supply at JP2.
Step 2: Parts List
1 x SSS7N60B (Fairchild 600V 7A DMOS NMOS transistor) - I have used this, because I had this one available, it can be used another power NMOS transistor like IFR120...and others
1 x 5V Zenner diode ( can be used other than 5V - the reference voltage divided over R5 value gives the maximum value of the generated current - this voltage can not be higher than the maximum supply voltage of the opamp + (1~2)V )
1 x 680 Ohm resistor
1 x 200 Ohm resistor (can be omitted)
1 x 100 KOhm resistor ( can be 10 KOhm - 200KOhm)
1 x 1 Kohm resistor
1 x 10 Ohm 5 W resistor (can be different, the power dissipation should be recalculated)
1 x 10 KOhm trimmer potentiometer (can be 5KOhm ~ 100KOhm)
1 x 10 uF capacitor
1 x 50 uF capacitor
1 x 100nF ( can be 1uF) capacitor
I have used 3 different types of coolers (radiators) - one to cool the NMOS transistor, the other to to make the housing of the LD
Step 3: PCB Design
I have used the following approach:
The PCB is designed by the use of two conducting layers : bottom and top. If you want to order the PCB in the fab, you can use dual metal technology. I have used the toner transfer DIY method and my PCB is with single metal (only the bottom metal layer). The top metal layer connections are replaced by trivial wire bridges. A PDF file prepared for the toner transfer "lithography" is available also for download.
A picture of the PCB with the transferred toner and retouched with resistant DVD marker is shown on the picture.
The same PCB after etching can be seen also.
Step 4: The Soldered LD Driver PCB
Step 5: Making the Housing for the LD
Step 6: The LD Cooler
At first I made a hole in the middle of the bigger cooler. After that I have shaped it by the use of round file to pass exactly to the LD placed in its original housing.
I have measured the all special potentials ( the reference voltage, the voltage at the input of the second opamp, the voltage at the source of the NMOS transistor - the same as the top terminal of R5, the voltage drop over the LD). I adjusted the supply voltage to be 10V. Remark. Be carefull what is the maximum supply voltage of the opamp!After that I started slowly to turn the potentiometer, increasing the voltage over R5, and increasing the current through the LD. When the trigger current of the LD is reached, it starts to light. You can easy astimate the flowing current through the LD measuring the potential at the top terminal of R5 and dividing it on R5.
The maximum allowed current through the LD must not be exceeded, because your LASER diode can be destroyed.
Instead for driving LASER diodes, the presented driver can be used also for driving of LEDs. The second picture shows the driver loaded with bright white LED, and sinking around 10 mA driving current.
Except for the driving of LDs , LEDs the driver device can be used also for generating a stable with the temperature and supply and constant in the time currents for different sensors, current reference circuits and other electrical and physical experiments. Important for this purpose is to have Zenner diode with stable over the temperature voltage. The chosen opamp have very small offset and parameters temperature drift. R5 must be chosen with small TCR.
Another possible application:
With the time the electronics hobbyists collect a plenty of different diodes. Between them are a lot Zenner diodes. Sometimes it is impossible to read their labels and to know what is their clamping voltage. The presented constant current driver can be used for their measurement and functionality check. The trimmer potentiometer should turned so, that ~ 1mA current is flowing - the voltage at the top terminal of R5 shall be ~10mV. The Zenner diode should be connected on the place of the LD, but in inverse way ( the Cathode connected to the supply wire, the Anode terminal to the wire coming of the NMOS transistor). The voltage drop over the Zenner diode shall be measured. Constantly monitoring the voltage over the Zenner diode, the current shall be increased until the voltage over the Zenner diode becomes stable. This is its clamping voltage. To be able to measure big range of different Zenner diodes, the supply voltage must be set as maximum as possible (Remember the maximum allowed supply voltage of the opamp chip!). If the voltage drop measured on the Zenner diode is ~0.5-0.7V, that means - it is false connected. Its terminals must be swapped.