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Blog posts of '2013' 'July'

Inductive Reasoning

Q. In my lessons on AC and resonant circuits, it gives the memory aid, ELI the ICEman meaning that the voltage leads the current in an inductor and the current leads the voltage in a capacitor. I understand this. But I do not understand why this is so - could you please explain?

A. A capacitor draws current when the voltage across it changes. In other words, it "reacts" to a change in voltage by drawing a current. This is why it is said to have "reactance". If the voltage across it is a sine wave, the voltage changes fastest where it goes through zero and changes from one polarity to the other. The current is also a sine wave, so it peaks at the points where the voltage sine wave goes through zero. This describes a sine wave that is 90 degrees ahead of the voltage sine wave.

An inductor does just the opposite. It produces a voltage when the current through it changes. This is another way of saying that it reacts to a change in current by producing a voltage. As with the capacitor, the voltage across and current through the inductor are sine waves. As the current sine wave goes through zero, the voltage sine wave peaks. This describes a sine wave that is 90 degrees ahead of the current sine wave.

Both the capacitor and inductor are energy storage devices. The capacitor stores energy in its electric field, where the inductor stores energy in its magnetic field. Electrically, they both react to changes, but the capacitor reacts to voltage changes, where the inductor reacts to current changes. So the two are sort of mirror images of each other.

Remote Possibility

Q. How does a TV remote work? I know they use infrared light but that's about all.

A. TV and other remotes generally use infrared light-emitting diodes to send out their signal. The LEDs are pulsed so that the signal can carry information. Different patterns of pulses are used for different functions.

On the front of the TV, usually in the control area for the tuner, is a phototransistor or photodiode. The light pulses from the remote control and received and converted into electrical pulses. These pulses are amplified and shaped, then sent to a section of the tuner controller that decodes the pulse patterns and performs the selected functions in the TV.

The fact that the light is pulsed helps to greatly increase the range of the remote, reduces battery drain, and helps make the system immune to interference from environmental light sources.

Switching Subjects

Q. I have taken the lesson on SCRs and UJTs. But where I work we have equipment with things called triacs. I've been told that they are like SCRs. What are triacs and how do they work?

A. SCRs only conduct in one direction. When they are reverse-biased they don't conduct. Triacs can conduct in either direction, so they make good AC switches.

A triac has two terminals that carry the load current. They also have two other terminals called the "main terminals". There are used to turn the triac on. They function like the gate of an SCR, except that they can work with either polarity of the AC cycle. Triacs will not conduct until a signal is applied to their main terminals.

Their conduction can thus be timed to start early or late in each half-cycle of the AC. This way, they can be used to control the amount of power delivered to the load. The timing of a pulse applied to the main terminals is simply varied in order to vary the power.