We all know that almost every industrial company uses laser diodes, and many procurement personnel are not clear about how to connect the three pins after purchasing laser diodes. In addition, many technologies are not clear about how to distinguish between PD and LD. So today, the editor will introduce the connection of the three pins of QSI laser diodes and how to distinguish between PD and LD.

A three pin laser diode (LD+PD tube) is different from a regular two pin LED. In addition to the laser emitting diode (LD), it also comes with a monitoring photodiode (PD) to detect the output light power and facilitate feedback control of the driving circuit.

1、 The function of the three pins

General situation:

1. LD positive electrode (LD+)

2. LD negative electrode (LD −, usually connected to the casing)

3. PD (photodiode pin)

Some models have LD common negative electrode, while others have LD common positive electrode, please refer to the datasheet.

2、 How to distinguish between LD and PD pins

1. Check the packaging information

The official QSI datasheet will have pin definitions, which is the most reliable.

2. Measure with a multimeter

Set the multimeter to diode mode.

Find the two pins of the laser diode (LD): there will be a conduction voltage (about 1.5V~2V) in the forward direction, but it cannot be measured for a long time to avoid burning out.

The only one left is the PD foot.

3. Use lighting to test PD

Using a flashlight or desk lamp to illuminate the common terminal between the PD and the casing/LD, and measuring it with a multimeter in DC voltage mode, a weak voltage (tens to hundreds of millivolts) will appear, which is PD.

3、 Typical connection method

LD section:

Positive pole → constant current drive circuit output positive terminal

Negative pole (or shell) → constant current drive circuit output negative terminal

PD section:

PD output connected to the detection input of the driving circuit

Stabilize laser power through sampling resistors or feedback circuits

Use tips:

Laser diodes are very fragile and cannot be directly powered on. A constant current source should be used, and ESD protection should be taken into account.

If you only want to light up for experimentation, the PD foot can be suspended, only the LD two feet are needed, but current limiting must be added.