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New England Radio Discussion Society: “Electronics for Amateur Radio operators”
“Getting down to nuts and volts”
Phase Two, PPT3 October 2016
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REVIEW: Coils are called inductors.
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REVIEW: the RL series circuit(R is due to the DC resistance of the wire in the coil)
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REVIEW: the inductance of a coil:
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“ELI the ICE man” for inductors (L)
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REVIEW: Inductive reactance is directly proportional to frequency:
XL = 2 Π f L, and
XL is measured in ohms
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REVIEW: XL increases with frequency
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Enter the capacitor! *
(1) Capacitors store electrical charge.
(2) Like inductors, capacitors store energy --- and return that energy back to the circuits they’re connected to.
Neither inductors nor capacitors dissipate power.
* condenser or condensor
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We know inductors store electromagnetic energy. For their part, capacitors store
electrostatic energy.
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While current generates a magnetic field around a coil, voltage generates an electric field between a capacitor’s plates.
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A huge capacitor with a capacity of one Farad stores one Coulomb of charge if one volt is applied across the cap’s plates.
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NOTE: There’s virtually no internal DC resistance to a capacitor!
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The value of a capacitor in part depends on the material between the plates. It’s called the dielectric, and has a property called the dielectric constant (K).
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Air-dielectric variable capacitors
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Typical transmitting variable capacitors
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Fixed capacitors
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Electrolytic capacitors
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Capacitance is not usually measured in Farads. Caps are typically measured in microfarads (uF), nanofarads (nF), or picofarads (pF). In addition to K, the value of a cap is a function of its physical construction.
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(1) Caps in parallel add to total capacitance value.
(2) The voltage across parallel caps is the same.
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Like resistors in parallel, the total capacitance of a series string is lower than the lowest value of capacitor.
Ct = 1.33 uF
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If there’s a resistor in series with the cap, as the capacitor charges up in a DC circuit, it reaches 63% of its total charge in one time constant, or TC. It charges to 63% of the remaining voltage in the next TC.
T = RC
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In AC circuits, caps exhibit capacitive
reactance, stated as Xc
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(1) AC is continuously reversing polarity. So, a cap is continuously charging and discharging, and charge and discharge currents are always flowing around a circuit.
(2) If the AC frequency is high, there is little time for the cap to charge and discharge. The reactance has little time to build up.
(3) If a large cap is in a circuit, more time will be required to charge the capacitor. More current will be flowing for a longer period of time. The average current will be large, so the cap’s reactance will be low.
In a nutshell
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Unlike XL, capacitive reactance is indirectly proportional to
frequency:
XC = 1 / 2 Π f C
Pi = 3.14159
Like XL for inductors, XC is also measured in ohms
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XC vs. frequency
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“ELI the ICE man” for capacitors
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ELI the ICE man’s vectors
A vector is a graphical tool describing magnitude and direction.
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The Pythagorean right-triangle theorem
So, C is equal to the square root of A2 + B2
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Impedance (Z) is the vector sum of R and reactance (X), derived from the Pythagorean theorem.
C = √A2 + B2
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Simple LC series circuit with sense resistor
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Here’s a vector diagram, where Xl and Xc vectors are equal and opposite
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When XL = XC
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Series reactances cancel, resulting in a low impedance (it’s
only resistive), and current peaks up
This is called series resonance.
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That’s it for now.
73
More info on vectors next time.