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Newport Laser Diode Driver Schematic
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. The generated current is Igen=Vin/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 2.5W - 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.
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.
Newport Model 505 Laser Diode Driver
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