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If you are squeamish about electricity or don’t care about electricity you may want to skip this one. But we’re going to talk about some things at are pretty neat, central to the world of electronics and downright cool if you’d like to stick around.
Recent blogs have talked about resistors, capacitors and inductors which, along with diodes of various descriptions, are the building blocks of electronic circuits. In those blogs, the behavior of the devices was described when they were used in conjunction with a direct current voltage source. In this blog we explore what happens when the voltage source is alternating current instead of direct current.
Note – the illustrations below show “steady state” examples. The unique conditions that occur when alternating current or voltage is first applied or disconnected are not addressed as they, although interesting, are not relevant to the topic at this time.
Resistors are “passive” devices. They are nice, simple things – you flow current through them and there is a voltage drop created due to their resistance to the flow of electricity. They behave perfectly to Ohm’s law, Voltage = Amperage times Resistance (E = IR). The current flow follows the voltage exactly. Resistors do not store energy. As long as you use RMS values for voltage and amperage measurements, they perform exactly the same with DC as they do with AC. The energy they consume is dissipated as heat. Resistors are used primarily as voltage dividers and current limiters in electronic circuits. A toaster is just a big resistor.
Reactive Components –
Capacitors and inductors are “reactive” components which react to change. Unlike resistors, capacitors and inductors store and release energy based on changes in applied voltage or current and do not follow ohm’s law.
In the case of capacitors, electrons are stored on conductive plates as voltage is increased. They are released when the applied voltage is decreased or removed and a load is applied (a “load” is a conductive path for electricity). An ideal capacitor infinite internal resistance since the plates are insulated from one another so no heat is generated by this process. In fact, capacitors do have some internal resistance so some heat is inevitably generated but is generally a very small amount.
Capacitors store electrons much like a battery. While a battery stores electrons by creating a chemical change within the battery, a capacitor stores electrons directly on conductive plates and can do so very rapidly compared to a battery. Stored electrons are released when the applied voltage is reduced or removed and a load is connected to a charged capacitor. Note in the above illustration with an alternating voltage source applied, that when the voltage is zero, the current is the highest (or lowest). Electrical guys would say that current leads voltage. The average overall energy consumption is zero through the charge and discharge cycle. Capacitors are mainly used to sustain constant voltages in circuits powered by varying voltage as in the case of alternating current. Capacitors can also be used to multiply voltages when properly configured to store voltage and then release it in a series configuration (like several batteries connected in series in a flashlight produce more voltage than a single battery).
In the case of inductors, energy is stored as a magnetic field created by the flow of current through the inductor. Unlike an ideal capacitor which has infinite resistance, an ideal inductor has NO internal resistance therefore there is no heat generated. In fact, inductors do have some resistance so do dissipate a small amount of heat.
Inductors store energy in a magnetic field. As the stored magnetic field collapses due to a change in applied current, energy is released in the form of current in an attempt to resist the change. In this case, current lags voltage. The inductor actually returns energy to the source to provide an average energy consumption of zero (at least in the case of sinusoidal excitation). This is a little hard to envision (at least for me) but if you think about a transformer, it is really an inductor with a second coil of wire tapping off the magnetic field created by the primary coil of the device. With no load on the secondary coil, the transformer is just an inductor and does not (theoretically) consume any power from the power line until a load is connected to the second coil to tap off the magnetic field.
The significance of the above is that the phase angle (alignment between the voltage and current over time) of the electrical flow is changed by these reactive devices. The change in phase angle invalidates the use of ohm’s law under alternating current conditions throwing circuit analysis into an entirely new realm of mathematics involving “imaginary numbers” to accommodate the return flow of power to the source. Some of you may have heard of the terms forward power and reflected power. This is where that starts and can be a significant consideration when evaluating performance of high frequency electronic equipment including ultrasonic devices.
– JF –