Application Note AN-LDTC04 Rev. A

TEST LOAD BASICS

January, 2014

Page 1

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Using a test load verifies that the rest of the equipment

is set up correctly and in working order prior to attaching

the laser diode. It also allows you to become familiar with

trimpot rotation and sensitivity, to correctly set up wiring

and monitoring, and to troubleshoot any problems that

may arise.

The test load processes that follow verify the current

output in Constant Current mode and the limits are set up

and can be monitored. When setting limits, you are able to

see when the limits are reached and can adjust the set up

of the controller accordingly. In Constant Power mode, the

test load allows you to set up and monitor the simulated

photodiode current.

The following equipment is necessary when setting up a

test load:

•

Laser diode driver or driver with evaluation board

•

Multimeter, set to read voltage

•

Oscilloscope (optional, needed only if testing analog

modulation input)

•

Signal generator (optional, needed only if testing

analog modulation input)

•

Test load, for examples see Figure 1 for Constant

Current and Figure 2 and Figure 3 for Constant

Power

•

5 V Power supply

•

Connection cables or wires

TABLE OF CONTENTS

Why Use a Test Load for Laser Diode Set Up?

page 1

About Test Loads in Constant Current

page 2

Set Up a Test Load in Constant Current Mode

page 2

About Test Loads in Constant Power

page 3

Set Up a Test Load in Constant Power Mode

page 3

About Temperature Controllers and Test Loads

page 4

Set Up a Temperature Controller & Test Load

page 4

INTRODUCTION

What is a test load and why do I need to use one? How and

when should I use one, and how do I know it’s working?

These are questions our technical support staff are asked

regularly, along with how to set up a test load and which load

is best for each Wavelength product series. This Application

Note will cover that information, as well as what equipment

you need, when to use Constant Current or Constant Power

modes, and how to prepare your equipment for the actual

load. For the purposes of this paper, we will address using

test loads with only temperature controllers and laser diode

drivers.

A test load is a tool for testing a device or system and

measuring a response without using the application load

and, ideally, the test load should closely mimic the eventual

load. Using a test load is a good way to get familiar with a

controller, test how it works, and make sure it operates as

expected before the final set up. For example, laser diodes

can be expensive and are usually not readily available, so

destroying one during set up can be both costly and time

consuming. To avoid this hazard, the system can be set up

with a test load and tried before attaching the laser diode.

A test load can also help show, through monitoring, what

happens when limits are exceeded. Additionally, if you are

having a problem while using a system, a test load can help

troubleshoot any problems.

NOTE:

While this Application Note will address questions

particular to using a test load, it is not meant to replace the

product datasheets, which contain all of the equipment,

instructions, calculations, and troubleshooting information

needed to successfully use the products.

WHY USE A TEST LOAD FOR LASER DIODE

SET UP?