User`s manual

Page 24 · 4200-DR Series LaserSource User’s Manual

It is the first of these two drawbacks that is of the biggest concern. In order to

protect against this fault, the instrument always monitors the local voltage at the

connector as well as the remote voltage, and if the difference is too great, a

warning message is displayed. Once the warning is displayed, it will not be

displayed again until the output is turned off and back on. The warning can be

disabled in the menu by setting the Vf Sense Warn to No.

Using remote voltage sense is very simple. First connect pin 4 of the Output DB-

9 to the laser cathode, and pin 8 to the laser anode, then in the menu, set Vf

Sense to Remote. Indicated voltage will now be the remote diode voltage.

When using remote voltage sense, any Cable R setting is ignored.

Using the ‘Cable R’ Setting

The Cable R setting allows you to calculate the voltage at the laser by

subtracting the voltage loss through the cable and connectors. This is done by

measuring or calculating the cable resistance and entering the value, in ohms,

into the menu. The instrument will then use the V = I * R formula to calculate the

voltage loss in the cable and subtract that from the actual measured voltage,

displaying the result as Vf on the display.

Common values for Cable R range from 0.0300 Ω to 0.0900 Ω, but can be

significantly higher if you have long runs, many connector interfaces, or small

gauge wire.

While it is possible to use a DMM to measure the resistance of the cable,

because resistance is so small, you will not typically get proper readings (most

DMMs are not designed to properly measure small resistances). A better

approach is to use the instrument to drive current through the system and then

measure the voltage loss to determine resistance. There are two ways to best

calculate the voltage loss:

Measure the Voltage at the Laser

If you have an accurate DMM and can measure the voltage across the laser (or

accurately know the voltage of the laser at a specific current) and voltage across

the pins at the Output connector, then the difference between the measured

voltages is the voltage loss in the cable. Use this formula to calculate resistance:

AMPS

laserOutput

RCable





Note that the current is expressed in amps, not milliamps, so divide the set point

by 1000 before using it in this equation.