Lesson 48 - Biasing & Class of AmplificationPower Supplies Filters Amplifiers Oscillators Mixers Logic Circuits Almost any device or type of electrical equipment is made up of a COMBINATION of these circuits. With lesson 48, we will begin to examine these circuits in detail, using both Tubes and Transistors - as you may come across either one in your career. To be honest, most of you unfortunate folks will never have the opportunity to work with tubes. I suggest that if you ever get the chance - take it. It is very rewarding. While there are certain advantages to transistors, there are also advantages to tubes - the least of all is a better, more full understanding of electronics in general. That being said - Let's discuss Power Supplies. The purpose of the power supply is to provide constant power to a circuit for proper operation. Power comes in two basic varieties - AC and DC. You know by now that AC stands for Alternating Current - much like that which comes out of the wall. DC stands for Direct Current, like what comes out of a battery. Equipment which runs off AC may use the raw power from the wall, or it may convert it to a different voltage. You may run into AC-to-DC converters, as well as DC-to-AC converters. There are even power supplies (you might have one in your home) which convert AC to DC, then DC back to AC. Why would you want to do that? Example - an uninterruptable power supply - otherwise known as a UPS. It takes power from the wall, and converts it to DC battery power. Your computer works of AC house power - but you don't want it to die if there is a power outage. So you put a UPS in line with it. If the power fails, your computer keeps working, because it is running off the batteries in the UPS. But the batteries are charged daily by the house power. So we convert AC to DC - then back to AC. So what exactly is a power supply made of? Typically, it is a combination of diodes, capacitors, coils and resistors. There may be more involved - but we'll get to that. Most electronic equipment actually works off DC power. This is because it is easier to control the quiescent biasing of an amplifier, or set the control point of a switch, if you begin with known DC voltages. The problem is that most power sources are AC. So somehow - we have to start with Alternating Current, of whatever voltage, and convert it to Direct Current. We've already discussed this in some minor detail - let's get dirty now, shall we? First - let's look at AC power systems. While I can't go through the specs for EVERY country, I'll go through the specs for the United States as an example, and if you are from another country, you can look up the power codes for your particular country - consider this your homework. For a more detailed version of how this works, you may go to http://www.osha.gov/SLTC/etools/electric_power/illustrated_glossary/index.html But basically, it works like this: Power comes out of the power generation plant at 20,000 volts. It is then transformed with a step up transformer at a transmission station to 345,000 Volts. It is stepped up to this high voltage, because at higher currents, it would require larger diameter wire to carry the current. By running high voltage, lower current - they can use less expensive wire and have lower resistive losses. One key thing to keep in mind when looking up at power lines is - the higher up in the air they are - the higher the voltage they carry. This 345,000 Volts (345KV) lines take the power to subsequent sub-stations, where the voltage is stepped down to suit the customer's needs: 
The power comes into the customer's building as 4000, 480,440,240, 220,208,or 120 Volts. 120 and 240 are typically single phase lines like you have in your home. On the other hand, the other voltages are three phase, either in a Wye or Delta configuration. Don't worry about these terms in detail at this time. The important thing to know is that each of these various powers come into the building being stepped down from a higher voltage by a transformer. Transformers, as we have discussed before, are groups of coils configured so that the magnetic lines of flux from one coil, are shared with another nearby coil. This causes the electric power from one coil to be transferred magnetically to the second coil. If the second coil has more turns or "loops" than the first coil, the voltage is stepped down. If the second coil has fewer turns than the first coil, the voltage is stepped up to a higher voltage. The ratio that the voltage is stepped up or down is the same as the ratio of the number of turns in the first coil to the number of turns in the second coil. Example:  In this example, assume we have a transformer with 5 turns on the input, and 15 turns on the output.
That's a 5:15 ratio - same as a 1:3 ratio.
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