occurs as current flows through them. Figure 7
shows a voltage divider with the proportion of
the individual resistor’s value versus the total
voltage that drops across it. Figure 8 makes this
distinction far more apparent with a nine volt
source and nine kilohms of resistance for a one
volt drop per kilohm of resistance. You may
place resistors in parallel as demonstrated in
Figure 9. Here, their total resistance is less
than that of any individual resistor’s value. This
arrangement allows greater current flow since it
splits the overall current through three distinct
paths; one path per resistor.
resistors and their
You can think of Figure 10 as a varistor
(to be discussed) on steroids. These protect
critical devices — e.g., a receiving or transmitting
antenna — by sidetracking or absorbing
electrical energy from lightning strikes. They do
so by routing the current directly to ground.
Secondly, light dependent resistors (LDRs)
inversely change resistances with the intensity
of the light falling upon them (Figure 11). These
are also called photocells. In strong light, their
resistance can be as low as a few ohms. In the
absence of light, it can be in the tens of
megohms. Its base material is predominately
either cadmium sulfide or lead sulfide.
Interestingly, LDRs are sensitive to different
colors of light. Therefore, their datasheets often
express resistances at UV, IR, and different
visible colors. I have seen homemade photo
hobbyist color temperature meters based on
LDRs. To see an LDR used to determine your
Stratum Corneum Hydration and Skin Erythema,
The degree of skin blood flow and stratum
corneum (SC) hydration in response to clothing
are associated in a practical aspect. This is the perception
of fabric comfort the textile industry uses.
The drawback to LDRs is they are fairly slow to
varying light levels (typical two to three second response
times). If you require a faster optoelectronic device,
consider using either a photodiode or a phototransistor.
Before delving into on varistors, I suggest reading the
November ‘08 N&V “Developing Perspectives” by
Figure 11. A light
dependent resistor's transfer
function or how it responds to
various levels of incident light
Figure 10. A lightning
arrestor with an internal
resistive insert that is — in effect
— a varistor on steroids.
Bryan Bergeron. This gives an interesting example of how
he used varistors in a practical and useful way.
The Secret Life of Pots © 1999 R.G. Keen. All rights reserved.
Variable resistors are two-electrode zinc oxide
semiconductor devices with a distinctively voltage-dependent nonlinear resistance and a symmetrical V/I
characteristic curve (Figure 12). This resistance drops
with increased applied voltage and vice-versa, making
them ideal to protect sensitive equipment from power
or lightning strikes. They shunt this energy to ground
and are great surge arresters in power transmission
July 2009 51