Project Circuit

PC Temperature Alarm :

If your PC overheats, it could damage its expensive components. Here’s a circuit that warns you of your PC getting heated. Today’s computers contain most of the circuitry on just a few chips and reduced power consumption is a byproduct of this LSI and VSLI approach. Some PCs still have power supplies that are capable of supplying around 200W, but few PCs actually consume power to this extent. On the other hand, apart from some portable and small desktop computers that use the latest micro-power components, most PCs still consume significant amount of power and generate certain amount of heat. The temperature inside the average PC starts to rise well above the ambient temperature soon after it is switched on. Some of the larger integrated circuits become quite hot and if the temperature inside the PC rises too high, these devices may not be able to dissipate heat fast enough. This, in turn, could lead to failure of devices and eventually of the PC. Various means to combat overheating are available, ranging from simple temperature alarms to devices like temperature-activated fans to keep the microprocessor cool. Here is a temperature alarm that
activates an audio ‘beeper’ if the temperature inside the PC exceeds a preset threshold. This temperature is useradjustable and can be anywhere between 0°C and 100°C. The unit is in the form of a small PC expansion card, which you simply need to plug into any available slot of the host PC. It is powered from the PC and consumes only about 12 mA. The sensor (LM35) used here provides a substantial amount of on-chip signal conditioning, including amplification, level shifting and phase inversion. As a result, it provides an output of 10 mV per degree centigrade rise in temperature. It caters to a temperature measurement range of 0°C to 100°C, which corresponds to 0V to 1V
of voltage. The voltage-detector stage compares the output voltage of the temperature sensor with the preset reference voltage. The output of the comparator goes high if the output potential from the sensor exceeds the
reference voltage. When this happens, the voltage comparator enables a lowfrequency oscillator, which, in turn, activates the audio oscillator. The output of the audio oscillator is connected to a loudspeaker (LS1), which sounds a simple ‘beep-beep’ alarm. The reference voltage determines the temperature at which the alarm is activated. figure shows the circuit of the PC temperature alarm and Fig. shows the pin configuration of sensor LM35. IC LM35 (IC1) is an easy-to-use temperature sensor. It is basically a three-terminal device (two supply leads plus the output) that operates over a wide supply range of 4 to 20V.

SOLID-STATE DEVICE THEORY

Quantum physics

 

“I think it is safe to say that no one understands quantum mechanics.”

Physicist Richard P. Feynman
To say that the invention of semiconductor devices was a revolution would not be an exaggeration.
Not only was this an impressive technological accomplishment, but it paved the
way for developments that would indelibly alter modern society. Semiconductor devices made
possible miniaturized electronics, including computers, certain types of medical diagnostic and
treatment equipment, and popular telecommunication devices, to name a few applications of
this technology.
But behind this revolution in technology stands an even greater revolution in general science:
the field of quantum physics. Without this leap in understanding the natural world, the
development of semiconductor devices (and more advanced electronic devices still under development)
would never have been possible. Quantum physics is an incredibly complicated realm
of science. This chapter is but a brief overview. When scientists of Feynman’s caliber say that
“no one understands [it],” you can be sure it is a complex subject. Without a basic understanding
of quantum physics, or at least an understanding of the scientific discoveries that led to its
formulation, though, it is impossible to understand how and why semiconductor electronic devices
function. Most introductory electronics textbooks I’ve read try to explain semiconductors
in terms of “classical” physics, resulting in more confusion than comprehension.
Many of us have seen diagrams of atoms that look something like Figure 2.1. 

                     Figure 2.1: Rutherford atom: negative electrons orbit a small positive nucleus.

Tiny particles of matter called protons and neutrons make up the center of the atom; electrons
orbit like planets around a star. The nucleus carries a positive electrical charge, owing to the presence of protons (the neutrons have no electrical charge whatsoever), while the atom’s
balancing negative charge resides in the orbiting electrons. The negative electrons are attracted
to the positive protons just as planets are gravitationally attracted by the Sun, yet the
orbits are stable because of the electrons’ motion. We owe this popular model of the atom to the
work of Ernest Rutherford, who around the year 1911 experimentally determined that atoms’
positive charges were concentrated in a tiny, dense core rather than being spread evenly about
the diameter as was proposed by an earlier researcher, J.J. Thompson.

THYRISTORS

Hysteresis

Thyristors are a class of semiconductor components exhibiting hysteresis, that property whereby
a system fails to return to its original state after some cause of state change has been removed.
A very simple example of hysteresis is the mechanical action of a toggle switch: when the
lever is pushed, it flips to one of two extreme states (positions) and will remain there even
after the source of motion is removed (after you remove your hand from the switch lever). To
illustrate the absence of hysteresis, consider the action of a ”momentary” pushbutton switch,
which returns to its original state after the button is no longer pressed: when the stimulus is
removed (your hand), the system (switch) immediately and fully returns to its prior state with
no ”latching” behavior.
Bipolar, junction field-effect, and insulated gate field-effect transistors are all non-hysteric
devices. That is, these do not inherently ”latch” into a state after being stimulated by a voltage
or current signal. For any given input signal at any given time, a transistor will exhibit a predictable output response as defined by its characteristic curve. Thyristors, on the other
hand, are semiconductor devices that tend to stay ”on” once turned on, and tend to stay ”off ”
once turned off. A momentary event is able to flip these devices into either their on or off
states where these will remain that way on their own, even after the cause of the state change
is taken away. As such, these are useful only as on/off switching devices – much like a toggle
switch – and cannot be used as analog signal amplifiers.
Thyristors are constructed using the same technology as bipolar junction transistors, and
in fact may be analyzed as circuits comprised of transistor pairs. How then, can a hysteric device
(a thyristor) be made from non-hysteric devices (transistors)? The answer to this question
is positive feedback, also known as regenerative feedback. As you should recall, feedback is the
condition where a percentage of the output signal is ”fed back” to the input of an amplifying
device. Negative, or degenerative, feedback results in a diminishing of voltage gain with increases
in stability, linearity, and bandwidth. Positive feedback, on the other hand, results in
a kind of instability where the amplifier’s output tends to ”saturate.” In the case of thyristors,
this saturating tendency equates to the device ”wanting” to stay on once turned on, and off
once turned off.
In this chapter we will explore several different kinds of thyristors, most of which stem from
a single, basic two-transistor core circuit. Before we do that, though, it would be beneficial to
study the technological predecessor to thyristors: gas discharge tubes.