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LDC200C Series
3.1.2 LD and PD polarities
Connect laser and photodiode to the LDC200C Series controller as shown below.
The ground connection of the laser diode (pin 3) can be connected to the anode of the
photodiode (pin 4), or to the cathode of the photodiode (pin 2). This connection should be as
close as possible to the laser diode to avoid measurement errors.
If the selected polarity (key "LD POL"
) and the pin connection of the laser diode do not
match, no current is flowing through the laser diode.
The laser diode is always sourced with respect to ground. Compared to a floating ground
scheme, the advantages of this operation mode are higher safety for the laser diode, better
stability of the laser current and less introduced interferences.
Prior to turning on the laser current, the correct polarity of the laser diode must be selected:
·
Make sure the laser is switched off (key “LASER ON” F8).
·
Select the required polarity by pressing the key "LD POL" (F23).
·
For laser diodes with a built-in monitor diode the common pin of laser diode and
photodiode is the ground pin.
·
The selected polarity of the output is indicated by the LED "AG" (F19) or "CG" (F20).
If the laser diode was connected correctly (see above) but the polarity of the laser diode was
selected wrong with the key “LD POL” (F23), an “OPEN” error will be indicated (LED “OPEN”
F4).
After pressing "LASER ON" (F8) the LED "OPEN" (F4) extinguishes and the output can be
switched on by pressing "LASER ON" (F8) again.
Biasing the photodiode
If the photodiode shall be operated with bias voltage, a battery can be connected in series to
the photodiode.
If the photodiode is of AG polarity, connect the positive terminal of the battery to the cathode of
the photodiode, and the negative terminal of the battery to pin 2.
If the photodiode is of CG polarity, connect the negative terminal of the battery to the anode of
the photodiode, and the positive terminal of the battery to pin 4.
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