Tuesday, September 15, 2009

Electronics and HVAC/R Systems #2

In electronics # 1 we discussed the basic composition and operation of diodes and rectifiers and how they are used in an electronic circuit. I will not discuss other variations of diodes because these can be studied on various other web sites. My intent with this series is to eventually get you to get a basic understanding of how DDC controls work and their importance in our use of these controls.

You are not expected to become electronic technician, but you are expected to be able to look at an electronic circuit and understand how the particular circuit is expected to work. So to push on, I would like to discuss some other important devices called the transistor, thermister and silicon-controlled rectifier.

Transistor

There are two types of standard transistors, NPN and PNP, with different circuit symbols. The letters refer to the layers of semiconductor material used to make the transistor. Most transistors used today are NPN because this is the easiest type to make from silicon. This page is mostly about NPN transistors and if you are new to electronics it is best to start by learning how to use these first.

The leads are labeled base (B), collector (C) and emitter (E).





The operation of a transistor is difficult to explain and understand in terms of its internal structure. For us it’s not necessary to understand the internal operation, but we need to know that a transistor can amplify current such as in fig..1 and can be used as a switch.



Above we have two transistors in series known as a Darlington pair where the second transistor is of a type known as 'PNP'. The first transistor is an 'NPN' type of transistor. As the potentiometer is adjusted to provide more voltage to the base, the collector current will increase to the point that will drive the motor in a variable speed control fashion.

Thermister


With increasing temperature, the device is called a positive temperature coefficient (PTC) thermister. If the resistance decreases with increasing temperature, then the device is called a negative temperature coefficient(NTC) thermister.
Resistors thermister is a type of resistor whose resistance varies with temperature. Thermister can be classified into two types, the resistance increases that are not thermister are designed so that their resistance remains nearly constant over a wide temperature range.

The thermister is a temperature dependant resistor. It comes in all shapes, sizes and colors. The left hand symbol denotes a positive temperature coefficient thermister. Its resistance rises as the temperature increases. The other symbol denotes a negative temperature coefficient thermister. Its resistance falls as the temperature increases.



Thermisters are often used in a Wheatstone bridge circuit as shown above. If the bridge is balanced at a low temperature then the output from the amplifier is zero. If the ambient temperature rises then since both thermister change in value by the same amount the bridge will remain balanced. If Th-1 is used as a temperature sensor then any changes in temperature affecting its resistance will unbalance the bridge and give an output from the amplifier.

Thermisters are used as resistance thermometers in low-temperature measurements of the order of 10K. Thermisters can be used as inrush-current limiting devices in power supply circuits such as in starting circuits of single phase refrigeration compressors. Thermisters are also commonly used in modern digital thermostats, and to monitor the temperature of battery packs while charging. Thermisters can be small enough to be mounted in places that are inaccessible to other temperature sensing devices.

Silicon-Controlled Rectifier


The Thyristor is also known as the silicon controlled rectifier (S.C.R.). It has the same characteristics as the diode, current flowing from cathode to anode, when the anode is positive with respect to the cathode. However, it will only do this when the gate is also positive with respect to the cathode. In the circuit, with the switch open as shown, no current flows.

When the switch is closed, the diode begins to conduct and current flows from cathode to anode. There is a problem. If the switch is now opened, current continues to flow. Conduction can be stopped by removing the cathode/anode voltage.

Another method of stopping current flow is to reverse the polarity of the cathode/anode voltage. If the thruster is used with an ac supply, then it will conduct on the positive half cycles and automatically switch off during the negative half cycles. The resistor in series with the gate connection limits the gate current to a safe value.

The SCR symbol resembles a diode except for the addition of the gate. The gate is also known as the control in some areas. Current can be controlled through the SCR by introducing a small current at the gate. This triggers a large current flow within the device even though current at the gate is removed. These SCR devices are useful in controlling large amounts of power, such as power needed to operate a shaded pole or universal motor.

The SCR acts as a power rectifier except current only flows for portions of the normal forward conduction time; depending on when the gate is triggered.For example, if the gate is triggered at the beginning of the forward cycle, 1/2 power is obtained to the motor or load, but if the gate is triggered at half cycle, 1/4 power is achieved. (See Figure 16A)


So now we have three more electronic components we should be familiar with in understanding very basic but important components.

To get a working perspective on some of these components please go to this web site:
Falstad.com

copyright(c)2009
Roger J. Desrosiers

About the Author: Roger is a contributing faculty member of HVACReducation.net He has over 40 years experience in Air Conditioning and Refrigeration. He is also a member of R.S.E.S., CM, The Association of Energy Engineers, Certified Energy Manager, ASHRAE, Certified Pipe Fitter United Association and is 608 Universal Certified.