To calculate approximate power dissipated on mosfet you can use this formula:
P = ( Uin - Ud ) · I
Where:
P - power dissipated on mosfet
Uin - Input voltage
I - Current set by a potentiometer, flowing through the diode
Ud - Diode working voltage
Typical working voltage and PSU choice for particular laser diodes:
Diode |
Diode Working Voltage Ud |
Recommended PSU voltage |
Infrared |
1.8 - 2.5 V |
3.3 V - 5 V |
Red |
2 - 3 V |
5 V |
Green |
5.5 - 7 V |
7.5 V |
Blue |
4.5 - 6 V |
5 V - 7.5 V |
To avoid producing additional heat, try to adjust the input voltage close to the diode working voltage. For example, for the green laser diode, 7.5 V PSU is the best typical choice. Very low voltage dropout on the driver's circuit allows you to select proper input voltage depending on the diode working voltage. You can drive your 445 nm M140 diode over 1.3 A using 5 V power supply which is actually around 5.1 - 5.3 V.
Notice that wrong settings such as too high input voltage or lack of proper heatsink will cause high power dissipation on the mosfet making it vulnerable to be burned. Be careful while choosing the power supply.
For high power, infrared diodes consider using low input voltage 3.3 V or 5 V because of large power loss on MOSFET while using 12 V. It can drive 808 nm diodes even powered by 12 V, but MOSFET will need a really big heat sink, like the one on the CPU in the computer.
Recommended heat sink:
Diode example |
Wavelength [nm] |
Power [mW] |
PSU Voltage [V] |
Heatsink |
M140 |
445 |
500 - 2000 |
5 V |
Small or without |
M140 |
445 |
500 - 2000 |
12 V |
Large or medium |
NDB7875/ NDG7475T |
445 / 520 |
2000 - 3000 |
7.5 V |
Medium or small |
NDB7875/NDG7475T |
445 / 520 |
2000 - 3000 |
12 V |
Large or medium |
Mitsubishi/Oclaro |
638 |
500 / 700 |
5 V |
Small |
Mitsubishi/Oclaro |
638 |
500 / 700 |
12 V |
Medium or large |
Infrared |
808 / 940 / 980 |
500 - 1500 |
5 V |
Medium |
Infrared |
808 / 940 / 980 |
3000 - 5000 |
3.3 V |
Medium or large |
Infrared |
808 / 940 / 980 |
3000 - 5000 |
12 V |
Large (like the one on CPU) |