this is greater in carbon film than in
metal film types. That is why the latter
are preferred for critical analog
circuitry such as audio preamplifiers.
Resistor noise hardly matters in digital
applications.
For more information about noise,
see Joe Geller's excellent resistor noise
measurement project (JCan) in the
July 2007 Nuts & Volts. After building
the JCan kit, I could easily measure
more noise in carbon composition
types compared to metal film resistors
of equal value when current was
applied. There was no difference with
zero current — just the thermal noise
"background." The noise difference
between carbon film and metal film
resistors under power was smaller, but
detectable.
The temperature coefficient tells
you how much a resistor's value can
change with temperature in units of
parts per million (PPM) per °C or °K.
Common axial lead (through hole) 1/4
watt metal film resistors are rated 50
or 100 PPM/°C, but their carbon
film cousins are 350 to 700 PPM/°C.
Sensitivity to temperature is a
consideration in many sensor and
measurement applications, but it does
not matter in digital circuits.
Assuming the resistor offerings at
Mouser Electronics ( www.mouser.
com) are typical of the industry, the
vast majority of 1/4 watt axial units are
available only at one percent tolerance for metal film, and five percent
for carbon film. Even if I wanted one, I
could not buy a 5% metal film or a 1%
carbon film resistor. Therefore, choice
of type is related to precision for
practical purposes. The cost difference
is negligible for small quantities; you
are mainly paying for the labor to
count them out and bag them.
Clark Huckaby
via email
[#10144 - October 2014]
Chinese Capacitor Problem
I have a Viewsonic model
VG2230WM LCD monitor that has
quit. I read that these monitors have
the "Chinese capacitor problem" but I
can't figure out what exactly that
means and what to do to fix it. Do I just
replace all the caps on the PCB?
#1 Often, a monitor goes dark
because the backlight power supply
fails. Look for the power module
which has the pink and blue wire
backlight voltage connectors. Usually,
there are two or four connectors for
the backlight power. The capacitors
on this module often fail due to heat
over a period of several years, and
they sometimes have bulging tops
making them easy to spot. Replace all
of the electrolytic capacitors on the
power module; there may be five or
six of them. In many cases, this will
restore operation of the monitor.
Bill Seabrook, WETA-TV
via email
#2 I have repaired several flat
screen monitors: one of them was the
VG2230WM. I was able to find the
power supply online and replace it
after getting it open. Another one was
fixed by buying the capacitor kit
online and replacing each of them.
The hardest part of any flat screen
repair is getting them open without
damage. Good luck.
Bob Smith
Prescott, AZ
Photoresistor Switcher
I would like to know if I have
correctly connected the photoresistors
(CdS photocells) shown in the
provided diagram (printed last month)
in order to turn OFF during the day
and to turn ON during the night the
two LED circuits attached to them. (If
not, please indicate by a new diagram.)
Also, I would like to know:
a) If any photoresistor would
work.
b) What would the optimum
dark/light resistance values be for such
a photoresistor?
c) How would I calculate the values (any formula?) from the transistor
side (2N2222) that would best fit this
Send all questions and answers by email to forum@nutsvolts.com
or via the online form at www.nutsvolts.com/tech-forum
READER-TO-READER TECHFORUM
80 November 2014
Figure 2 Figure 1
