Basic Electronics

I wrote a course that I presented to college level students back in the 90's. It covers a basic understanding of how Integrated Circuits worked. Since then, the technology has improved significantly, but the basics remain the same. I'm going to try to break the course down into digestible segments for those who'd like to read it.

I plan to break it down into the following posts (Click on the links to take you to the previous posts):

• Hole Movement and Current
• Voltage and Resistance
• Capacitors and Inductors
• Diode and Transistors
• Logic Gates
• Flip-Flops and Memory
• To Be Determined

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Capacitance and Inductance

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Capacitors

• A simple capacitor consists of two plates with an insulating material (dielectric) between them.

• When a voltage source is applied, the two plates will build up a charge equal to that voltage source.
  

• That charge remains after the voltage source is removed
  

• The amount of charge these plates can hold per unit of voltage is called the capacitance

• The unit of measure for Capacitance is the Farad

• Capacitance is usually represented by the letter C

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Capacitance Time Constant (τ)

• To build up the capacitive charge, the plates need electrons.
• Now, we need a universal way of measuring how fast a capacitor can charge up. τ is the amount of time (in seconds) it takes a capacitor to charge up to 63.2% of it’s steady state value.
• It is also the amount of time the capacitor takes to discharge by 63.2%
  
• Why would they use such an unusual number for τ?
• As discussed earlier, if you increase the resistance, you decrease the number of electrons that can get through the path.
• So if you increase the resistance, it will take longer for the capacitor to charge up.
• Without going into the mathematical equation, τ=R*C where R is the total resistance the capacitor sees between it and its voltage source
• Since the charging curve for a capacitor is exponential instead of linear, it works out that 5 τ = the time it takes to charge or discharge the capacitor 99.3%

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Uses for Capacitors - Noise Reduction

• As mentioned, Capacitors take time to build up charge.
• For DC power supplies, you want to keep the output voltage as constant as possible. In electrically noisy environments, fluctuations in the voltage can be caused by electromagnetic interference.
• By putting a capacitor with a large τ across the output, you can dampen voltage “spikes” in your signal.

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Uses for Capacitors - Timing

• τ = R*C for both charging and discharging
• A circuit can be designed that is triggered by a certain voltage level. When the voltage across the capacitor reaches a certain upper level, the circuit flips a switch which discharges the capacitor. When the voltage reaches a certain lower level, the circuit switches again.
• This creates an oscillator with a frequency that can be controlled by adjusting the resistance or capacitance.

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Uses for Capacitors - Memory

-Since a capacitor can hold it’s charge once the voltage supply has been removed, the voltage across the capacitor can be viewed as an on or off state. - When the voltage across the capacitor is high, the state is “1” or ON. - When the voltage is low, the state is “0” or OFF. - This is the basic principle behind DRAM.

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Capacitance - Review

• With the Switch in position A, C charges up.

• With the Switch in position B, C discharges.

• Assume the switch is being throw like this.
  

• If 5τ is very large, the capacitor will never fully charge. (Triangular Waveform)
  

• If 5τ = T, the capacitor fully charge the moment before it discharges. (Saw-Tooth Waveform)
  

• If 5τ is very small, the capacitor charge almost instantly and discharge almost instantly. (Square Wave)
  

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Electro-Magnetism

- As current runs through a wire, a magnetic field is generated around the wire. (This is part of the principle behind an electric motor) - On the other hand, if you introduced a magnetic field to a wire, you can produce current. (This is part of the principle behind a generator) - If you hold the wire in your right hand with the current flowing in the direction your thumb is pointing, the magnetic field (flux) flows in the direction your fingers are pointing. (Right Hand Rule)

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Inductance

- Remembering Newton’s third law of mechanics - “For every action, there is an equal and opposite reaction”, think of a wire that’s been twisted into a coil with two loops. - As the current rounds the first loop, a magnetic flux is created.

• The second loop creates an equal but opposite flux. This induces a current in the second loop that is equal but opposite to the original current.

• The inductance (L) is the coils opposition to a change in current
• If you add more turns to the coil, there will be more resistance to a change in current.
• Inductors do to current what capacitors do to voltage.
• If you have a circuit consisting of a voltage source, a switch, a resistor and an inductor. When you first turn on the switch, the current is zero, so all voltage is across the inductor. As the current through the inductor increases, the voltage drop on the resistor will increase (Ohm’s Law) and the voltage across the inductor will decrease.

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Uses for Inductors - Timing

• τ = L/R for both charging and discharging
• A circuit can be designed that is triggered by a certain voltage level across a resistor. When the voltage across the resistor reaches a certain upper level, the circuit flips a switch which discharges the inductor. When the voltage reaches a certain lower level, the circuit switches again.
• This creates an oscillator with a frequency that can be controlled by adjusting the resistance or inductance.

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Uses for Inductors - Noise Reduction

• As mentioned, Inductors take time to allow current to flow
• For some applications, you may want to control current spikes.
• If you put an inductor with a large  in series at your output, current “spikes” would be dampened

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References

Capacitor
Farad
Inductor
Flux

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Any comments/questions/recommendations?

Let me know if you found this useful/interesting. Do you have any recommendations for formatting?