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- Electronics Design -

-Electronics Design Information-

Electronics reduced to it's simplest form is- the flow of electrons are either on or off, or they vary between on and off. These two methods of controlling electricity are known as digital and analog electronics. A light switch turns a light on and off (digital) and a light dimmer turns a light to any point from off to on (analog). A computer is a digital device that contains may on / off switches.

The flow of electricity is either direct current (DC) or alternating current (AC). A battery is an example of DC, and the power in your house is AC.  AC has advantages in power distribution. DC has power losses in the long transmission lines that AC can reduce.  More on this later, first lets talk about the common parts used to control electricity.

This is very important - complicated circuits are made from many simple sub-circuits wired together to perform complicated functions.

CircuitMaker for students is a schematic capture and simulation program and is included free on the Pilot Pro plans DVD or you can find it on the web. The program is to large to fit on my domain.

Common formulas:

E=Volts (electromotive force), R=Ohms (resistance), I=amps (current), P=Watts (power)

Common components used in electronics:
Resistors (Ohms) -
oppose current by producing a voltage drop, converts Voltage to a current.

Capacitor (Farads) - stores electrons, blocks DC and passes AC
 
Coil (Henry)- passes DC and blocks AC
 
Transformer - changes voltage to a higher or lower value and changes the current inversely


Transistor - turns electron flow off and on, or varies the flow from off to on.

Diode - blocks the flow of electrons in one direction. The arrow points in the positive direction. A- anode +, K- cathode -.

LED - light admitting diode

Always read the data sheets on components for more information.

Resistors

Values can be changed by add components in parallel or series. The smaller resistor in the parallel circuit dominates the total value. The larger resistor in the series circuit dominates the total value.

Resistor used as a current sense.


A resistor limits current and drops Voltage and can be used as a current to Voltage converter using Ohms law - I=V/R.

The color code value for resistors:

color 
Value
  For memorization Tolerance
Black
0
Bad

Brown 1
Boys
+- 1%
Red 2
Rape
+- 2%
Orange
3
Our
+- 0.05%
Yellow
4
Young

Green
5
Girls
+- 0.5%
Blue
6
But
+- 0.25%
Violet
7
Violet
+- 0.1%
Gray
8
Gives

White
9
Willingly

Gold


+- 5%
Silver


+- 10%
None


+- 20%

5% resistors - 1st band (value), 2nd band (value), 3rd band (multiplier), 4th band (tolerance)
1% resistors -
1st band (value), 2nd band (value), 3rd band (value), 4th band (multiplier), 5th band (tolerance)
Wattage is proportionally to physical size

Standard Resistor values


Capacitors


Time constants
Time constants: For resistor/capacitor -- Vt = E (1-e * (-t/RC)).  For resistor/inductor (time for current to reach full value) -- It  = E/R * (1-e * (t*R/L)).  Where R is in ohms, C is in farads, L is in Henries, t is in seconds, and e is in the natural log base 2.71828.

One time constant (R*C) is equal to 63.2% of the final value.  After the second time constant an additional 63.2% of the remaining charge or current will be reached (63%, 86%, 95%, 98%, 99%).  The same is true for discharge.  Three to five time constants are need to get close to full charge or current.
RC time constant



Capacitor values
cap marking = size in pF
105 = 1,000,000pF or 1000nF or 1uF
104 = 100,000pF or 100nF or 0.1uF
103 = 10,000pF or 10nF or 0.01uF
102 = 1,000pF or 1nF or 0.001uF
101 = 100pF or 0.1nF or 0.0001uF
pF = picoFarad, nF = nanoFarad, uF = microFarad

Driving an LED

LEDs or light admitting diodes drop about 2 Volts before they conduct. Check the specification sheet for Vf (forward Voltage) and max current. This LED is rated at 1.7 Volts before it will start to turn on and can use 0.04 amps of current. We need to have a resistor that will limit current to no more than 40 ma or the LED will burn out and fail. Ohms law says R=E/I, we need to subtract the LED Voltage drop from the calculation so R=(9-1.7)/0.04 = 182.5. Round this up to a standard resistor value you get 200 Ohms. If the LED specs are not known then try using Vf = 2V and LED I = 0.02 amps as ball park ratings.

Changing Voltage levels
Wrong-

Right- Voltage divider

Vout = Vin*(R2/(R1+R2)) = 10*(10k/(10k+10k)) = 10*0.5 = 5V
The down sides are this supply is current limited and Vout changes as Vin changes.

Voltage reference

D1=5.1V Zener diode, R1 = (Vin min - Vz) / (Imax) = (10-5.1) / (0.01) = 490 round up to 499 standard value.
Zener diodes have a reverse voltage breakdown point. This Zener Voltage point depends on the diode part number.
Standard resistor values can be found in a Digikey or Mouser catalog.

Voltage Regulator

D1=5.6V Zener diodes are designed to be hooked up backwards because they start to conduct at a specific reverse Voltage. The resistor limits current through the diode and transistor base. The transistor drops about 0.4V through the base - emitter junction. The transistor is wired as a Voltage follower or common collector. With the transistor added the output can supply more current. This transistor can supply about 200ma of current.

Integrator

The
integrator is used for a signal delay or low pass filter. If the RC time constant is a 10th (1/10) the period of the signal then it has a high amplitude. If the RC time constant is 10 time the period of the signal then it acts like a first order filter.

The signal period is 1/1000Hz = 0.001, the RC time constant is 30000*0.0000000047 = 0.000141. The RC time constant is set at 1/10 that of the signal period.

Here the RC time constant (R = 100K, C = 1uF) is set at 10 times that of the signal period. the RC time constant is 100000*0.0000001 = 0.01. This looks like a low pass filter.

Differentiator

The differentiator is used as a trigger or high pass filter.
If the RC time constant is a 10th (1/10) the period of the signal then it has a high amplitude. If the RC time constant is 10 time the period of the signal then it acts like a first order filter.

The signal period is 1/1000Hz = 0.001, the RC time constant is 30000*0.0000000047 = 0.000141. The RC time constant is set at 1/10 that of the signal period.

Here the RC time constant (R = 100K, C = 1uF) is set at 10 times that of the signal period. the RC time constant is 100000*0.0000001 = 0.01. This looks like a high pass filter.

The integrator and differentiator circuits above are made from a RC Voltage divider circuit. The capacitor is like a variable resistor and it's value is base on the frequency passing thought it. This is called capacitive reactance. The formula for capacitive reactance is Xc = 1/(2*Pi*frequency(Hz)*Capacitance(F)).

Reactance for caps and inductors

*Voltage and current phase rule of thumb:
Voltage leads current in an inductor by 90 degrees-
Current leads Voltage in a capacitor by 90 degrees-

Triangle to sine converter


Diodes are one way valves. The diodes start to turn on at about 0.7V (forward Voltage drop). When the diodes conducts the resister drops some Voltage and limits current. Silicon diodes drop about 0.7V, germanium diodes drop about 0.2V. The resistor also limits current through the diodes. To much current will destroy the diodes.

Diode Curve trace


Here is the curve trace of a diode. This shows the Voltage / current response of a forward biased diode. The current through the diode is monitored as a DC sweep of the Voltage from 0.25V to 1.5V.

Curve tracer


OPEN                                      SHORT                               RESISTOR                          DIODE


ZENER                                  CAPACITOR                       EMITTER BASE                 EMITTER COLLECTOR

Sinusoidal Voltage and current

Effective Voltage = 0.707 * Peak [Also know as Root Mean Square (RMS)] - do this in the reverse order (square, mean, root) to get an RMS number.

Half cycle average = 0.637 * Peak

Peak Voltage = 1.414 * effective

Effective Voltage = 1.11 * average

Transistor configuration (bipolar)

letter       Meaning                        For memorization
B|E|C     -common                        -Bob eats carols
P|V|I      -gain                               -Pretty veal in
A|B|G    -alpha, beta, gamma       -a big garage
L|M|H    -input impedance           -liking mostly her
H|M|L    -output impedance         -hot moist loins
-----------------------------------------------------------------
NPN - negative, positive, negative - (Not Pointing iN)
PNP - positive, negative, positive

* Bipolar transistors are current controlled devices. FET transistors or Voltage controlled.
* A transistor is said to be in its active mode if it is operating somewhere between fully on (saturated) and fully off (cutoff).
* Base current tends to regulate collector current. By regulate, it is meant that no more collector current may exist than what is allowed by the base current.
* The ratio between collector current and base current is called "Beta" (β) or "hfe". hfe varies and is not a good parameter.
* β ratios are different for every transistor, and they tend to change for different operating conditions.
* The Vbe (Voltage across the base and emitter diode junction) is about 0.6 to 0.8V.
*Vcc is collector Voltage, Vee is emitter Voltage referring to NPN type

Transistor calculations

The transistor above has a hfe of 100 and we want 20mA and 5V at the collector.
hfe = Icollector / Ibase and Ib = Ic / hfe, Vbe = 0.7V
Ib = 20mA / 100 = 0.2mA
+12V - IbRb - 0.7V = 0 so Rb = (V - Vbe) / I or Rb = (12V - 0.7V) / 0.0002A = 56,500 Ohms
AND
+12V - IcRc - 5V = 0 so Rc = (V - Vce) / I or Rc =  (12V - 5V) /  0.02A = 350 Ohms

Transistor as a switch

The Gnd out Vc is ~ (about) 0.05 to 0.2V at saturation.
Common emitter circuit for a 200ma load and hfe = 100.
Ibase = Icollector / hfe (0.002 = 0.200 / 100)
R2 = V1
- Base Emitter V drop / Ibase (5650 = 12 - 0.7 / 0.002) Round R2 down to the standard 5.6k, 5% value due to the drop in gain or hfe.
This switch is an inverter. When the base is high the output is low or sinks current.
As the the transistor goes into saturation (full on) the hfe rating drops off (gain decreases).
Protect the collector from going lower than the base with a reverse diode to ground at the collector, as in AC loads. Or a diode reversed biased across an inductor load.

Emitter follower


The emitter follower or Voltage follower output follows the input minus the Vbe Voltage drop of about 0.7V. The advantage of this circuit is you can buffer the driver signal from the load. This circuit has current gain and can source low impedance loads.
ΔIe = ΔVb / R1
ΔIb = 1 / (hfe +1) * ΔIe = ΔVb / R(hfe + 1)
Rin = (hfe + 1) * R1 + R2

Transistor as an amplifier

-More on the way-