7/24/2023 0 Comments Burnin boardThe LD is connected at JP1, the pwer supply at JP2.ġ x MAX407 (dual single supply CMOS low offset opamp) - can be used different, having in mind mentioned before (the PCB is designed for DIP package).The input range of the opamp must include both voltages - the ground rail and the reference. If you have stability problems, you can play with the value of R1 and to try to reach the stable operation. This in the schematics is the function of R1. 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. 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. Some opamps are compensated for similar loads, but a plenty of standard opamps will oscillate. The opamp must be able to drive such kind of big capacitive load, without losing its stability. This capacitance appears as capacitive load for the opamp. For some devices it can reach some dozens of nanofarades. 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. R2 and C1 are used to protect the laser diode from some voltage spikes. In my case the maximum power generated by R5 is 2.5W - 5V*0.5A. The power generated by the R5 must be also properly dissipated. Because high current can flow through the NMOS transistor, it must be enough strong to sustain it. If higher current is needed, either the reference voltage should be increased, either the value of R5 shall be reduced. I have used 5V Zenner diode and 10 Ohm R5 resistor - the maximum possible generated current is 500 mA. The generated current is Igen=Vin/R5, where Vin is the voltage drop over R5 and also the input voltage of the second opamp. This voltage appears at the current defining resistor R5. 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. In this way on its middle terminal the voltage can vary between 0 and the reference voltage. The buffer is loaded with trimmer potentiometer connected to ground. This voltage is filtered by the use of the capacitor C2 and applied to the input of the opamp connected as buffer. A 5 V reference voltage is created by the Zenner diode D2 and the resistor R4.
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