Thursday, September 3, 2009


Why will you eventually need to have knowledge in Electronics? What does Electronics have to do with Refrigeration and Air Conditioning, Heating and Ventilation, Humidity Control? Well, everything really, because as you advance in knowledge in this business you will see that the state of the art of modern control is knowing Electronics. In most modern controls today you will hear the term "Direct Digital Control"(DDC). DDC is a mini computer that can be pre-programmed from the factory, or in many cases can be programmed with a computer interface by a technician sitting in an office. Programming consists of writing algorithms (instructions) so that the computer will make adjustments to hot and chill water valves, dampers, switches, relays, etc. according to all the many sensors that provide input to the computer.

So with this brief introduction I would like to begin a series of introducing you to the basics of some electronic components, and build from there an understanding of the magic and majesty of DDC control.So let’s begin our study with Diodes and Rectifiers.

The diode and rectifier are the simplest of the components and are basically the same device. The word diode defines a two-element component. The term rectifier refers to the component's ability to rectify AC to DC or take an AC current input and make the output DC.

The only differentiation between a diode and rectifier in common usage is in the current rating, and this is not well defined. Usually a component rated at less than one ampere is called a diode. Above this rating the component is called a rectifier. The word diode indicates that the component is frequently used as a small signal device. A rectifier is considered a power device.

There are various types of diodes and rectifiers. When reference to just diode or rectifier is made, one can assume the most common, conventional type is designated. Other types such as zener, compensating, tunnel, and varactor diodes and rectifiers are less frequently used. They are all p-n components. Performance characteristics differ, and the difference in characteristics is basically the result of different types and amounts of impurities added to the semi-conducting materials.

In appliance control systems and systems employing low frequency voltages and currents, conventional and zener (sometimes called voltage reference and breakdown), diodes and rectifiers are more frequently used.

Conventional diodes and rectifiers conduct electricity in one direction much more readily than in the other. This is a desired characteristic for rectifying AC voltage to DC.

Without going into the details of component manufacture, the diodes and rectifiers are composed of p material intimately bonded to n-type material. This provides a solid-state part composed of one material having free electrons bonded to a material having holes or vacancies for electrons.

Electrons can flow from the n material (free electrons) to the p material (holes for electrons to pass through) much more readily than in the reverse direction. The resultant current flows from the p material to the n material. Therefore, the conventional diodes and rectifiers act as good conductors with voltage applied in one direction and very poor conductors, and to some degree like an insulator, when voltage is applied in the other direction. This is shown in Figure 23f29. Therefore, the amount of current that passes through a diode or rectifier is dependent on the direction of the applied voltage, as well as its intensity. (N = negative, P = positive).

The preceding has been a rather elementary explanation of the conventional diode and rectifier under ideal conditions for rectifying AC to DC. This is the application for which the components are most frequently used. However, when you see how current varies with voltages applied both directions (to conduct current from n to p and p to n) on the various types of diodes and rectifiers, you can see that the former explanation is incomplete.

Different types of diodes and characteristics are provided by different concentrations and kinds of impurities in the semiconducting materials. Diodes and rectifiers will even pass a significant number of electrons in reverse (from p to n) depending on the voltage conditions, concentration of impurities, and location of the impurities with respect to the junction of the n and p materials.

The characteristics of a conventional diode and rectifier are shown in Figure 23F20A.

Observe the terms forward and reverse are used with current and bias. Forward bias is the voltage (electrical force) on the component that causes electrons to flow in the forward direction, or from n to p. Reverse bias is the voltage, or electrical pressure, in the opposite direction. So reverse current is the resulting current in this opposite direction.

As the voltage rises in the forward direction (forward bias) the forward current rises readily. Virtually no current flows in the reverse direction until the reverse bias voltage is raised a great deal. A typical application for semiconductor rectifier devices is shown in Figure 23F20B. This is a circuit for converting alternating current and voltage to direct current and voltage.

The transformer shown takes the voltage from the AC source and induces the voltage to the rectifying circuit, composed of two semiconductor rectifiers and a filter circuit. The voltage output of the transformer to the rectifiers is shown as having the same wave shape as the input. The + and - symbols with the wave shapes designate direction of voltage (+ and -) varies with time. The wave shape shown into and out of the transformer is one cycle. Conventional AC power source applies voltage of one cycle every 1/60 of a second, or cycles at the rate of 60 cycles per second (frequency of voltage). Hertz is a term used to designate cycles per second.

The two solid state rectifier components will only conduct current in one direction. They are connected so rectifier A is conducting one half cycle and rectifier B the other half cycle.

For one half cycle of the transformer output to the rectifier circuit, rectifier A has a voltage applied in the forward direction (cathode more negative than anode). During this same half cycle, the voltage applied to rectifier B is in the reverse direction, (cathode more positive than the anode).

With these conditions rectifier A will conduct current and rectifier B will not conduct. During the next half cycle of voltage from the transformer the conditions are reversed, so rectifier B conducts current and rectifier A is blocked and will not conduct. As connected, the current through rectifier A and B meet at point C to pass through the filter circuit.

This current is pulsating DC. It travels in only one direction and it pulses.
The filter circuit is composed of resistors, inductors, and/or capacitors. Using the characteristics of these components, the pulsations can be smoothed out to provide a more constant DC. The amount of pulsation allowed dictates the requirements of the filter circuit.

Other solid state components might even be used with resistors, inductors, and capacitors to aid in reducing pulsations. The smoother the DC output required, the greater the cost of the filter circuit. The output of this circuit could be used to supply DC voltage to a solid state circuit incorporating transistors, etc.

To get a working perspective on these components please go to this web site:Click Here

Roger J. Desrosiers

About the Author: Roger is a contributing faculty member of 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.