Elektor_Mag_-_January-February_2021

40 January & February 2021 http://www.elektormagazine.com

current increases further until it finally reaches

its nominal value. These current-limiting resis-

tors are usually bypassed by an (electrome-

chanical) relay after a few seconds so that no

more power is lost in the NTC-resistor.

Finally, Figure 6 shows a Light Dependent

Resistor (LDR). The resistance value of the

LDR decreases when the light that falls on it

becomes brighter. In the past, these resistors

were often used in optical sensors. However,

one of their disadvantages is that they are

relatively slow to react to changes in light

level. These days photo-diodes or photo-tran-

sistors are preferred for such applications.

This brings us to the end of our discussion

on the topic of resistors. Next time we will

continue with capacitors.

200551-04

The magazine article series “Starting Out in Electronics”

is based on the book Basic Electronics for Beginners by

Michael Ebner, published by Elektor.

quantity. The most common ones are briefly

discussed below.

Figure 4 shows the schematic symbol for a

Voltage Dependent Resistor (VDR). Once a

certain voltage has been exceeded, the resis-

tance of a VDR becomes very low. In this

way the component can protect other parts

from over-voltage conditions. VDRs are often

used to protect power transformers against

over-voltage (something that could happen

when the neutral is not connected correctly).

There will then be enough current through the

VDR to blow the mains fuse and thus avoid

further damage. However, the VDR often has

to be replaced after such failures too.

In Figure 5 we see several symbols for

temperature-sensitive resistors. The two

symbols on the left represent an NTC resistor

(Negative Temperature Coefficient); the two

symbols on the right represent a PTC resis-

tor (Positive Temperature Coefficient). The

resistance value of an NTC resistor decreases

as it becomes warmer; with a PTC resistor

this is, of course, the exact opposite. NTC

resistors are sometimes used to limit inrush

currents. In conventional power amplifiers

there are usually heavy mains transformers

with large, discharged smoothing capaci-

tors when powered off. As a result there is

an extremely large incoming current at turn-on

that could blow the mains fuse. An NTC can

help prevent this problem by limiting the initial

inrush current. As the NTC becomes warmer,

as a result of the current flowing through it, the

Linear or logarithmic?
Potentiometers in parts lists often have the
abbreviation log or lin in the specification. This
means logarithmic and linear respectively
and this indicates how the resistance value
varies as a function of the angle of rotation.
Logarithmic potentiometers are mainly used
as volume controls in amplifiers because
our hearing has a more or less logarithmic
characteristic.

In Figure 3 you can see the schematic symbols
for potentiometers: a potentiometer on the
left, in the middle a trimpot, and the equiva-
lent circuit on the right. The value of each of
the resistors depends on the position of the
wiper (the middle connection of the schematic
symbol). As with ordinary resistors, potenti-
ometers also have a maximum power rating.

Special resistors
There are a whole range of resistors whose
value is dependent on a specific physical

Questions or Comments?

Do you have questions or comments

regarding this article? Then email

Elektor at [email protected].

Figure 3. Potentiometers: schematic symbols. Figure 4. Voltage dependent resistor (VDR).

Figure 6. Light dependent resistor (LDR).

Figure 5. NTC- and PTC-resistors.

Contributors

Idea and illustrations: Michel Ebner

Te x t : Eric Bogers

Translation: Arthur de Beun

Editing: Stuart Cording

Layout: Giel Dols

related products

Basic Electronics for Beginners

> Book: http://www.elektor.com/13950

> E-book: http://www.elektor.com/18232