My E-Mail lewis@sullivan-county.com
Phone: 276-669-0565
You can call me at work at 276-669-9957. This is third shift.

Our textbook will be Electrical Studies for Trades by Stephen L. Herman. (3rd Edition)

Visit my electronics website at http://www.bristolwatch.com/

This page and links are located at http://www.sullivan-county.com/ele/

Note: pdf files require Acrobat reader.

Schedule for 2nd half of semester:

See pages 238-243 in your text book.

Half-wave rectification

A half wave rectifier is a special case of a clipper. In half wave rectification, either the positive or negative half of the AC wave is passed easily, while the other half is blocked, depending on the polarity of the rectifier. Because only one half of the input waveform reaches the output, it is very inefficient if used for power transfer. Half-wave rectification can be achieved with a single diode in a one phase supply.

Full-wave rectification

Full-wave rectification converts both polarities of the input waveform to DC(direct current), and is more efficient. However, in a circuit with a non-center tapped transformer, four diodes are required instead of the one needed for half-wave rectification. This is due to each output polarity requiring two rectifiers each, for example, one for when AC terminal 'X' is positive and one for when AC terminal 'Y' is positive. The other DC output requires exactly the same, resulting in four individual junctions. Four rectifiers arranged this way are called a diode bridge or bridge rectifier.

3 phase rectifier

For more on rectifiers see http://en.wikipedia.org/wiki/Rectifier

Zero crossing

In alternating current, the zero crossing is the instantaneous point at which there is no voltage present. In a sine wave or other simple waveform, this normally occurs twice during each cycle. The zero crossing is important for systems which send digital data over AC circuits, such as X10 home automation control systems, and Digital Command Control type systems for Lionel and other AC model trains. The zero crossing of the mains voltage is the key to determining the proper switching time (phase angle) in both leading edge and trailing edge dimmers.

Autotransformer see pages 329-331 in textbook

Autotransformer

An autotransformer is an electrical transformer with only one winding. The winding has at least three electrical connection points called taps. The voltage source and the load are each connected to two taps. One tap is usually a common connection that is also connected to the source. Each tap corresponds to a different source or load voltage. In an autotransformer a portion of the same winding effectively acts as part of both the primary and secondary winding.

Operation: The ratio of secondary to primary voltages is equal to the ratio of the number of turns of the winding they connect to. For example, connecting at the middle and bottom of the autotransformer will halve the voltage. Depending on the application, that portion of the winding used solely in the higher-voltage (lower current) portion may be wound with wire of a smaller gauge, though the entire winding is directly connected.

Click here for another picture of an autotransformer.

For more information see http://en.wikipedia.org/wiki/Autotransformer

Triacs

A triac is a solid state AC switch. A small current on the gate terminal can switch very large currents. By putting a variable resister into the gate circuit, it will control the speed of a motor or intensity of a light bulb. In the sample circuits below one can use both a variable resister and switch in series to cut a lamp on/off and control the brightness. In hatching eggs, we can use light bulbs or heating elements, switch them on/off with thermostats and/or switches. The only problem is the 110/220 is present on the thermostat or switch creating a possible shock hazard. These can't be connected to a computer or solid state controls without some form of electrical isolation. I'll address that below.

Refer to page 398-401 in your textbook for lamp dimmers.

400 PIV triac printout click here For 220 volts use 600 PIV to be safe.

Basic circuit

Better

Best response with a diac.

FFor more info on Diacs see http://en.wikipedia.org/wiki/DIAC and
http://www.hobbyprojects.com/thyristor_triac_and_diac.html

Commercial lamp dimmer in 220 volt countries. BR100 is a diac.

Triacs and Inductive loads (motors) pdf file
More on Triacs and SCRs pdf file

At 120 volts R = 1000 ohms, at 240 R = 2000 ohms.

A reed switch

Thermostats

Many thermostats consist of two differing metal strips bonded together. Different metals expand in different amounts when heated thus causing the bi-metallic strip to bend, either opening or closing an electrical contact. See How a thermostat works

Another type of thermostat used in home heating systems consists of a bi-metallic spring attached to a glass tube filled with mercury connected to a pivot and arm. As the spring bends the mercury will roll to one end of the tube or other making or breaking electrical contact depending on temperature. An on/off switch is connected in series. Click here to see an outside view, click here to see an inside view. These type thermostats could be used to trip a triac gate on/off with a proper series resistor. These are low power devices and won't carry much current. They normally operate at about 24 volts. If used with a 220 volt triac circuit, 220 volts will be present on the thermostat.

moc30xx series opto-isolator
Click here for more details
Click here for spec sheet pdf file

Opto-Isolators

An opto-isolator is a solid state device designed to provide electrical isolation between input and output. The input consists of a light emitting diode (LED) in a six or eight pin dip (IC) package depending on type. The output can be a photo transistor, photo diac, etc. There is no electrical contact between input and output. When the LED is turned on, the diac, etc. will conduct from the light emitted from the diode thus turning on the triac like a switch. The MOC30xx series is made to connect to triacs, the MOC301x types for 110 volts, and the MOC302x types for 240 volts. (see illustration below.)

Also see Phase controlling a power triac using the MOC3023 pdf file

Comparator Equivalent

Single Supply Comparator Equivalent schematic

  An integrated circuit "Voltage Comparator" is equivalent to an Operational Amplifier, Such as the LM358 or LM324, with two NPN transistors added to the output of each amplifier. (Refer to the above schematic.) This arrangement produces an "Open Collector" output for each of the four comparators in an LM339 chip. Each output can sink 15 Milli amps and can withstand voltages of up to 50 Volts.

  The output is switched ON or OFF depending on the relative voltages at the PLUS and MINUS inputs of the comparator, see the rules below. The inputs are quite sensitive and a difference of only a few millivolt's between the two will cause the output to turn on or off.

  The LM339 (has 4 comparators in a single 16 pin dip) and LM311 comparator chips can operate from a single or dual power supply of up to 32 volts maximum.

See Using Voltage Comparators

See Basic Op Amps

The above circuit is a comparator using a 741 op-amp, but it has to have an external driver transistor. It uses a thermistor., but could also use a photocell or even a resistive humidity sensor. All of these devices change (increase)resistance with temperature, light, or humidity respectively. In this case we have a radiator cooling fan. We can leave out the relay (Re1) and D1 and use the diode section of the above MOC30xx opto-isolator and use this low voltage solid state circuit to safely control 110 or 220 volts AC. Be sure to observe diode polarity! The above circuit could be set to say 105 degrees to cut on a cooling fan to cool the eggs on a hot day.

The only problem we need an a circuit to cut on a heat source if the temperature is under 99 degrees. In other words we want the heater triac on if the temperature goes below 99 degrees with the fan off, but the fan on and heater off at about 105 degrees. The way to solve problem is a second comparator circuit, but connect pin 2 of the MOC30xx to ground, connect Rin to +, then connect pin 1 and the bottom of Rin to the transistor collector or the comparator output. When the comparator switches off, current will flow through Rin and the LED, turning on the heater.

See Resistive Humidity Sensors and Photocell circuit that could be used with a thermistor.

Another thermistor circuit. and another temperature control circuit.

Using the IBM Printer Port

Thermistor

A thermistor is a type of resistor with resistance varying according to its temperature. The word is a combination of thermal and resistor. Samuel Ruben invented the thermistor in 1930. This differs from a mechanical thermostat that uses metals expansion/contraction to break a contact. If the resistance increases, we say it has a positive coefficient. If decreases, a negative coefficient. They are note used alone, but with other electronics. Above is a typical thermistor and its symbol.

Photocell

  In this circuit, when the light falling on the photocell (PC 1) is blocked, its resistance will increase and the voltage across PC 1 will rise. When the voltage rises above 1/2 of the supply voltage the output of the comparator will turn ON and the LED will be lit.

Basic Photocell Detector

  Due to wide variations in CdS photocells it is usually best to install the cell and then measure its resistance under normal lighting conditions. A resistor with a value that is approximately 3 to 5 times the measured resistance of the cell is then selected for R1. For example; If the cell resistance is measured at 400 ohms then a 1200 to 2200 ohms resistor would be used.

  Increasing the value of R1 will cause the sensitivity of the sensor to decrease. This may be necessary when the light falling on the cell is not very strong or shadows can affect the photocell.

  This circuit can be adapted for use in dark areas by placing a small light above the photocell. Note that one could use a thermistor instead of a photocell, or even a resistive humidity sensor. See Resistive Humidity Sensors

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