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Power Supply Rectification, and Control Circuits

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Resistors

Resistors limit the flow of current through the circuit. In electronics they play a key role in manipulating flow to components.

Types of Resistors
Wire wound - High Power
Carbon Composition - Least Expensive
Carbon Film - Easily constructed, greatest accuracy

Please click here for a resistor chart of values. There will be questions on your tests regarding these selections.


The power supplying a resistor should never exceed the wattage or the resistor will become damaged.


Capacitors

Opposes the change of voltage. A capacitor has a dielectric sandwiched in between two metal plates.

The first capacitor shown is a fixed value. The second is a variable value capacitor. The common capacitor types are mica, ceramic, paper, and plastic. The dielectrics that can be used include ceramic, paper, air, wax paper, and oil.

**Please note that the indentation is on the `+` side. This is important because it is the exact opposite when you get to a diode.


Farad

Capacitors are rated in farads. The following are a few of the engineering units.
Micro = u. (.000,001)
Nano = n. (.000,000,001)
Pico = p. (.000,000,000,001)



Charging Voltage and Current for Capacitors

RC charge curve is the time it takes a capacitor to fully charge, or to fully discharge. It takes five full units for a capacitor to charge or discharge. It also has an identical fingerprint in how it achieves this on both levels. The level and rate it charges is the exact way it discharges. See below. The values at each increment are 63.2 %, 86.5%, 95%, 98.2%, and 99.3%.




t = RC (t = time R = resistance C = capacitor)

Exercise
A 100 microfarad capacitor and a 3.6 Ohm resistor are connected across a power source. How long will it take to FULLY charge the capacitor?

Answer:
t = RC
t= (3600
Ohm)(.0001)
t= .36 seconds
Full Charge = 5 Units (5RC) x .36 seconds = 1.8 seconds
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Capacitors Parallel
CT = C1 + C2 + C3

Exercise
Three capacitors rated at 12, 20, and 30 microfarads are connected in parallel. The total capacitance would be?

Answer:
12 + 20 + 30 = 62 microfarads

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Capacitors Series



Exercise
Three capacitors rated at 10, 15, 50 microfarads are connected in series. The total capacitance would be?

Answer:
= 5.35 microfarads
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Capacitive Reactance


Exercise
Two capacitors rated at 20 and 30 microfarads are connected in series to a 60 Hz source. What is the capacitive reactance?

Answer:
= .000012 microfarads
= 221.04Ohm
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Inductors


Inductors oppose changes in current. Lenz's law states that whenever there is an induced voltage or current in a conductor, it opposes the motion or source that causes it.
Look above for the time constant values, but keep in mind that the following values change for inductors. Here are the new formulas.

Inductive reactance


Time is now inversely proportional to resistance and inductance.

Inductors oppose changes in current. Lenz's law states that whenever there is an induced voltage or current in a conductor, it opposes the motion or source that causes it.
Here are a few symbols used to represent inductors in schematic diagrams.





Solid State Electronic Components


Solid state components are made up of semi-conductors. Conductors have 1, 2, or 3 valence electrons but semi-conductors have 4 valence electrons and are usually comprised of silicon and germanium.


Doping

Doping is a process that takes elements like (aluminum, arsenic, and phosphorous) in a high temperature oven with disks of silicon and germanium. It creates an N (Negative) type or a P (Positive) type. They are not useful when they are used by themselves, but when joined in electronics, they become very useful.


Diodes

Diodes can be forward biased. (anode +, cathode -) or they can be reverse biased. (anode - , cathode +). There are free wheeling diodes, light emitting diodes (LED), Photo diodes, and zener diodes (used as voltage regulators). The stripe on the diode represents the Negative (cathode) portion. Below is a symbol of a diode, along with a corresponding picture of a diode.

(Vf) - Forward Voltage Drop (.7) - max voltage in forward bias.
(PIV) - Peak Inverse Voltage - max peak voltage rating. (AC)
(VDC) - Max DC Reverse Voltage - max voltage in reverse
(Tj) - Operating Junction Temperature - max temp for junction of the N and P with the "barrier" If it gets too hot it will start conducting on its own.

A diode will rectify AC to DC. This is where we will introduce half wave, and full wave. If there is only one diode present in the circuit, it will produce results for a half wave sine wave. The output is DC.


The full wave design utilizes four of the diodes and becomes a full wave bridge rectifier. This produces a sine wave that does not skip degrees in the cycle. The output is DC.




Average DC Outputs

Ripple frequency can be best described by the illustrations above to the left. A half wave only sees one wave in one cycle. A full wave sees two waves in one cycle. A three phase full wave sees six waves in one cycle. Below are calculations done with a 60 Hz system. Remember these are AVERAGE values (RMS).

Single Phase Half Wave 120AC x .45 = 54 volts (60hz = 60 ripple frequency)
Single Phase Full Wave 120AC x .90 = 108 volts (60hz = 120 ripple frequency)
Three Phase Full Wave 120AC x 1.35 = 810 volts (60hz = 360 ripple frequency)



SCR (Silicon Controlled Rectifier)

SCRs have three terminals, A (anode), K (cathode), and G (gate). It is basically known as an electronic switch. Since it`s a solid state device there are no moving parts to wear out like you would see in a set of magnetic contacts, so this is a huge advantage for maintenance budgets. An Scr acts like an amplifier so that the small amount of gate current switches a larger amount of power. It is part of the thyristor family of triacs and diacs.




Firing Angle or Conducting Angle

SCRs have an advantage because they can be controlled to manipulate voltage output. If timed correctly, during the sine wave, they can either utilize full voltage output or a portion of the voltage output. The terms for this can be triggering, firing or gating.




Circuit Calculations


Half Wave for 120/16V, capacitor 50mF, resistor 100 Ohms (50W)

Edc = 16 volts x .45 = 7.2 volts
Voltage drop rectifier = 1 diode = .7 volts
E load (resistive load) = 7.2 - .7 = 6.5 volts
I dc (output current) = 6.5 volts x 100 ohms = 65 mA
W load (power consumed) = 6.5 x .065a = .4225 watts
Ripple frequency = 60
E dcp (peak voltage) = 16 - .7 = 15.3 x (1.414 peak) = 21.63 volts


Full Wave for 120/16V, capacitor 50mF, resistor 100 Ohms (50W)

Edc = 16 volts x .90 = 14.4 volts
Voltage drop rectifier = Two diodes = 1.4 volts
E load (resistive load) = 14.4 - 1.4 = 13 volts
I dc (output current) = 13 volts x 100 ohms = .13 a
W load (power consumed) =13volts x .13a = 1.69 watts
Ripple frequency = 120
E dcp (peak voltage) = 16 - 1.4 = 14.6 x (1.414 peak) = 20.64 volts


Filtering

Filtering gives a smoother waveform. The types of filtering are:
1)capacitance (voltage filtering)
2) inductance (current filtering)
3) capacitance / inductance (PI filtering)

The below illustration shows the addition of a capacitor to a circuit, and the corresponding result in the waveform.




SCR Data

(VGT) - Gate Voltage - the gate voltage required to produce gate current.
(IGT) - Gate Current -the current requires to `fire` the SCR into conduction.
(IH) - Holding Current- current need to sustain conduction. Dropping below this level will result in the SCR shutting off.
(PFV) - Peak Forward Voltage- if this level is exceeded the SCR will conduct without a signal.
(PRV) - Peak Reverse Voltage- if this level is exceeded the SCR will be damaged.
(IT) - Maximum Continuous RMS Current Rating- if this current is exceeded the temp of the SCR will damage it.




Triac

A triac is in the thyristor family, and is like an scr except it is bi-directional. The current flows in both directions so that the circuit sees the + and - of the waveform. The voltage drop across a triac is 1-2 volts, (1.4) because it is like two SCRs combined. The Diac is the same as a triac, except there is no gate. A triac is show below.




Transistor

The symbol for a transistor is "Q". B stands for base, C stands for collector, and E stands for emitter.



The
PNP (pointing inward) - if the base is at a lower voltage than the emitter, current flows from the emitter to the collector.

The
NPN (not pointing inward) - if the base is at a higher voltage than the emitter, current flows from the collector to the emitter.