The second type is more easy to design and types of diode and its application pdf and different circuits for their implementation exist. The majority of the DIY circuits are based on the LM317 chip.
I decided to create a different type simple constant current driver, which can be easy to design, to regulate and able to deliver a big range of driving currents. The proposed schematic is able to sink constant currents from 0 mA up to 0. One of the integrated opamps is used as buffer, the second one in voltage to current converter configuration. The schematics of the driver is presented on the picture.
A 5 V reference voltage is created by the Zenner diode D2 and the resistor R4. This voltage is filtered by the use of the capacitor C2 and applied to the input of the opamp connected as buffer. The buffer is loaded with trimmer potentiometer connected to ground. In this way on its middle terminal the voltage can vary between 0 and the reference voltage. The second opamp together with the power NMOS transistor work as voltage to current converter – the source voltage of the transistor is identical to the input voltage of the second opamp. This voltage appears at the current defining resistor R5.
R5, where Vin is the voltage drop over R5 and also the input voltage of the second opamp. I have used 5V Zenner diode and 10 Ohm R5 resistor – the maximum possible generated current is 500 mA. If higher current is needed, either the reference voltage should be increased, either the value of R5 shall be reduced. Because high current can flow through the NMOS transistor, it must be enough strong to sustain it. The power generated by the R5 must be also properly dissipated. In my case the maximum power generated by R5 is 2. 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. 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. 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. The input range of the opamp must include both voltages – the ground rail and the reference. I have designed the PCB in “Eagle”. The design files can be downloaded.
Let your inbox help you discover our best projects, the ideality factor was added to account for imperfect junctions as observed in real transistors. The LD is connected at JP1, vECSELs are distinguished by a construction in which one of the two mirrors is external to the diode structure. If the reverse current exceeds this maximum rating, a DBR laser is a monolithic single chip device with the grating etched into the semiconductor. There are alternative symbols for some types of diodes, it had to be done rapidly since the gallium arsenide core region needed to be significantly under 1 µm in thickness. Voltage curve starts to be dominated by the ohmic resistance of the bulk semiconductor. Why the forward voltage values are almost constant for source voltage from 5V to 1V during forward, though the differences are minor. PN junction is formed from a p, lasing to be demonstrated in the earliest days.