Applications/Processing Guide
HOW TO USE THIS SECTION
This Applications/Processing Guide is intended to provide
you with points to consider for designing circuits, selecting
trimmers and arranging board layouts, to achieve maximum
performance and long life for your circuits and systems. We
have also included information on steps your manufacturing
engineers can take to preserve circuit reliability.
For example, are you aware that the trimmers and other
mechanical components on your boards may face a more
extreme environment during boardwashing on your own produc-
tion line, than they ever will in use? For those trimmers that may
need to be reset, are you remembering to select and mount the
trimmers to provide easy accessibility?
In this section, you'll find dozens of pointers, reminders
and useful facts that will help you be more knowledgeable and
successful in using trimmers.
TRIMMER BASICS
In its most common form, a trimmer is simply a device
containing a resistive element, and a wiper, or adjustable tap,
contacting the element. The wiper can be mechanically moved
to vary the amount of voltage or resistance in the circuit. The
resistive element is usually laid out in linear or a circular
configuration:
The Resistive Element
Trimmers for commercial applications typically have a
resistive element made of carbon or cermet (a combination of
CERamic and METal), or of resistance wire wound on an insulat-
ed copper mandrel.
The main advantages of wirewound trimmers are their low
temperature coefficient, higher power dissipation, lower noise,
tighter resistance tolerance, and, when used as a variable resis-
tor, the excellent current-carrying capacity through the wiper due
to the lower contact resistance. Also, their long-term resistance
stability with time and temperature is slightly better than cermet.
Cermet trimmers provide a wider resistance range (10 ohms
to 5 megohms, versus a maximum of 50K ohms for wirewound).
Also, the wiper output can be set closer to the desired value
since the resistive element presents a continuous contact
surface for the wiper, as opposed to the discrete turns (resolu-
tion) of the wirewound. Other advantages with cermet are the
lower reactance in high-frequency applications, the smaller sizes
available, and the generally lower price than wirewound types.
ADJUSTMENT SCREW
RESISTIVE ELEMENT
ADJUSTMENT ROTOR
WIPER
ADJUSTMENT SCREW
WIPER
RESISTIVE
ELEMENT
WIPER
RESISTIVE ELEMENT
83
Specifications are subject to change without notice.
BOURNS [ BOURNS ELECTRONIC SOLUTIONS ]
