Hi, Just wanted to let you know this site is moving to Learn Basic Electronics The Easy Way Thanks. Soldering Skills     Resistors - Resistor Color Code - Ohms Law Calculator     Simple Projects     Radio Frequency Projects Capacitors, Inductors, Diodes and Schematics Now that you understand the symbol for the battery, resistor and wires. (if you don't click here: All About Resistors) Lets add a capacitor and a coil known as a "inductor". I'm not going into detail here about the electric properties of capacitors and inductors since it involves alternating current theory and that is too big a topic for this webpage. (learn more here) But we can learn the symbols and some basic things about their behavior. A capacitor is made up of two metal surfaces that face each other but are insulated from one another. The basic symbol looks like this If you put it in the circuit above in place of the resistor what would happen? Well, since the two plates are insulated from each other nothing would happen. Actually, if you measured current flow in the circuit you would find a small amount of current would flow for just a brief moment. This is because a capacitor can hold a electric charge. It is stored as an electric field between the two plates and the current was the capacitor charging up. The amount of energy it can hold depends on it's value. Capacitors are measured in Farads or more commonly in micro farads and pico farads. The name comes from Michael Faraday (1791-1867), a very interesting man. Read some about him when you get the chance. Now for the coil or inductor. An inductor is a lenght of wire wound in a coil. Sometimes on a iron form or without depending on the amount of "inductance" desired. The more turns of wire on the coil the higher the inductance. The symbol for an inductor with an air core is . With an iron core Inductance is measured in henrys and again more often in micro or pico henrys. Now what will happen if we put an inductor in place of the resistor in our famous circuit? Watch closely it behaves the opposite of the capacitor. For a very brief moment nothing will happen then current will begin to flow and the only thing limiting the current will be the internal resistance of the coil. The inductor stores energy in a magnetic field. Remember the capacitor stores energy in an electric field. The combination of the capacitor and the inductor make magical things happen. OK, let's add in a diode to the mix so far. A diode is a semiconductor that will allow current to flow in only one direction. A common use is to rectify alternating current into pulsating direct current and then you use a large capacitor to filter or "fill in" the gaps between pulses to get pure direct current as an output. This is an example of a power supply running off of household mains. A diode is rated in reverse voltage and maximum current. The symbol for a diode is The "arrow" is the anode and the line is the cathode. To make the diode conduct it must be forward biased. That means it needs a positive voltage on the anode and a negative voltage on the cathode. There are many types of diodes and one that we "see" everyday is L.E.D. light emitting diode. It's symbol is the little upward arrows indicate light given off. Now if the arrows point the other way like this it's light sensitive and called a photodiode. We need one more thing to tie this all together and that's magnetic fields. When current flows through an inductor a magnetic is formed around the inductor. When the current stops the field collapses back. Also, if you take a magnet and pass it around a coil a current will be induced in that coil or inductor. So a changing current creates a magnetic field and changing magnetic field creates a current. Now, if you take two coils and arange them so that the magnetic field of one "cuts" through the windings of the other you have a transformer. Transformers can have either an air core or iron core. The symbol looks like this if it's air core and this if it's iron core. Iron cores are "usually" used at low frequencies up through the audio range where higher inductance is needed and air cores are used at radio frequency. The exception is with special powdered iron compounds which are used at high rf frequencies. Here's a look at a complete schematic of a low voltage power supply. I put in a few new symbols to keep things exciting! Let's start at the left side with the power plug. The top wire connects to "F1" and as you guessed it's a fuse. From there it goes to SW1 which is a off-on switch shown in the 'on' state and then to the top of T1 the power transformers "primary" winding. The bottom of the winding returns to the power plug to complete the circuit. Remember, we are using AC voltage here so the primary winding of the transformer is 'seeing' a changing current and changing magnetic field. This changing field induces a voltage in the "secondary" winding of T1. The amount of voltage depends on the turns ratio of the windings. If the secondary has fewer turns then the primary you have a "step down" transformer more is a "step up" transformer. In this low voltage power supply we would call for a step down to 12 volts AC. If we used a 10:1 transformer and applied 120 volts AC to the primary we would see 12 volts AC on the secondary. (Tip) Back in the 'old days' this would be called a filament transformer and would be used to power (light up) the filaments of 12 volt vacuum tubes. OK, from the top of T1's secondary a wire connects to the anode of diode D1. It's very important to connect to the anode as we want a positive voltage. If it got connected to the cathode we would get a negative voltage and the supply would not work. It would damage the capacitor and if the capacitor failed as a short then the fuse would blow and hopefully protect the diode and the transformer! So watching polarities is upmost important. The diode conducts only on the positive half cycles of the alternating current so the output of the diode is positive pulses, the top half of the sinewave. Next a wire connects to the inductor L1 and then from L1 to capacitor C1. The combination of L1 and C1 form a low pass filter. Each time a positive pulse arrives at C1 it charges up, then during the time between pulses it discharges supplying current until the next pulse arrives. This filtering action is what gives a nice DC output. Next a wire connects to R1. This resistor is called a dropping resistor and is used to reduce or drop the voltage supplied to D2 the LED diode. The LED is forward biased and will light when the power supply is turned on. If there is no load on the power supply (as in this drawing) then the LED will slowly go out when the supply is switched off due to the capacitor C1 discharging though the LED until it has completly discharged. Depending on the size of C1 the LED could run for four or five seconds after the supply is turned off. Finally, a wire connects from the cathode of D2 (LED) back to the secondary of T1 completing the circuit. Wow, that's a whole bunch of information but now you can read schematics and you understand how a simple power supply operates. Links to other Basic Electronics pages on this site soldering skills    resistors capacitors & inductors    simple projects          intermediate projects    radio frequency projects FREE SPECIAL REPORT The Crystal Radio Receiver Find out the secrets behind a radio receiver that can operate forever without any power source. Also, learn why the "crystal set" demodulates AM broadcast signals with the absolute best fidelity of any known radio! 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