symbol_of_thyristorThe Thyristors is a semiconductor switch either of the pnpn or npnp type, whose bi-stable action depends on regenerative internal feedback. Figure at the left is symbol of thyristor, where A is anoda terminal, C is cathode terminal and G is gate terminal.

The four layer device is usually silicon although germanium has been used. Devices with the two endmost layers only connected to external terminals (anode and cathode) are called four layer diode, those with three layers accessible (anode, cathode, and p gate) are called thyristors. SCR (Silicon Controlled Rectifier) is other name of thyristors.

The structure is best visualized as consisting of two transistors, a pnp and an npn interconnected to form a regenerative feedback pair as shown in figure bellow.

Current gain around the internal feedback loop G (Gate) is hfe1 x hfe2, where hfe1 and hfe2 are the common emitter current gains of the individual sections. If Ico1 is the collector to base leakage current of the npn section and Ico2 is the collector to base leakage of the pnp section, then:
for the pnp section: Ic1 = hfe1 (Ic2 + Ico1) + Ico1
for the npn section: Ic2 = hfe2 (Ic1 + Ico2) + Ico2
and the total anode to cathode current Ia = (Ic1 + Ic2)

from which Ia = [(1 + hfe1) (1 + hfe2) (Ico1 + Ico2)] / [1 – (hfe1) (hfe2)]

With a proper bias applied, ie positive anode to cathode voltage, the structure is said to be in the forward blocking or high impedance 'off ' state. The switch to the low impedance 'on' state is initiated simply by raising the loop gain G to unity. As this occurs the circuit starts to regenerate, each transistor driving its mate to saturation. Once in saturation all junctions assume a forward bias, and the total potential drop across the device approximates to that of a single junction. Anode current is then only limited by the external circuit.

To turn off thyristor in a minimum time it is necessary to apply a reverse voltage and under this condition the holes and electrons in the vicinity of the two end junctions will diffuse in these junction and result in a reverse current in the external circuit. The voltage across the thyristor will remain at about 0.7 V positive as long as an appreciable reverse current flows.

After the been removed, the reverse current will cease and the junction assume a blocking state. The turn-off time is usually of the order of 10-15 μs. The fundamental difference between the transistor and thyristor is that with the former conduction can be stopped at any point in the cycle because the current gain is less than unity. This is not so far the thyristor, conduction only stopping at a current zero.


There's a lot of photo electric devices, such as photo-cell relays, photoelectric switch units, silicon photo-electric cells and silicon blue cell.

Photo-cell relays. Basic component of photo-cell relay is an integral light activated switch. It combines a silicon planar photo diode with integrated circuit on a single substrate to provide a highly sensitive photo electric device. Operation is such that when light of a selected intensity falls upon it, the device switches on and supplies current to an external load. When the light intensity falls below the critical level the load current is turned off. This critical level can be adjusted within wide limits.

The equipment comprises a projector containing a light emitting diode and optical system projecting a beam of light either directly or by reflection onto a photo-cell mounted in a receiver unit. The relay coil is energized when the light beam is made and de-energized when it is broken. Thus the relay contacts can provide a change over operation which can be used to perform some external control function. The control unit can contain additional circuitry such as time delays or LED failure circuits to meet a wide variety of application requirement. System are available for operating over distances from 10-15 mm up to 50 m or more.

Applications include conveyor control, paper breakage alarm, carton sorting and counting, automatic spraying, machinery guarding, door opening, level controls, burglar alarms, edge alignment control and punched card reading.

Photoelectric switch units. Light sensitive switches are used for the economical control of lighting. They consist of a photo-cell which monitors the intensity of the light and automatically switches the lighting on or off. Construction of s typical unit is shown in figure 4.10 and this will switch a resistive load of 3 A at 250 VAC. The unit is based on a cadmium-sulphide cell and it incorporates a 2 minute time delay to prevent 'hunting'. Larger units are available with resistive switching capacities up to 10 A.

Silicon photo-electric cells. These cells are designed to provide large output current even under low illumination intensities. Currents of several milliamperes are obtainable. Structure of a photo-electric cell will be seen to consist of a thin p type layer on n type silicon. Due to its linear photo-voltaic effect there is no need for a bias power source. A linear output can be obtained by selecting a suitable load resistance for a wide range of illuminance. Like the silicon blue cell, described below, it has no directivity of receiving light, so there is no need to adjust the optical axis as is the case with photo-transistors.

Silicon blue cell. The sharps silicon blue cell manufactured by photain controls is claimed to be the world's first photo-electric diode possessing high sensitivity over the entire visible light spectrum. It is more reliable than the selenium or cadmium-sulphide photo-cells and has superior time response. No bias power is required, it has a lower noise level than the other two type and it is non-directional.

Application include illumination meters, exposure meters, optical readouts of film sound tracks, colorimetry, flame spectrometry, photo spectrometry and color or pattern recognition equipment.


The resistivity of any material is the resistance of a piece of material having unit lenght and unit sectional area. The symbol is ρ and the unit is the Ohm meter. The resistivity of a material is not usually constant but depends on the temperature.

Table below shows the resistivity (with its reciprocal, conductivity) of the more usual metals and alloys
Table Resistivity of any material at 20 °C
Material Resistivity (Ohm meter) Conductivity (Siemens per meter)
Silver 1.64 x 10-8 6.10 x 107
Copper (annealed) 1.27 x 10-8 5.8 x 107
Gold 2.4 x 10-8 4.17 x 107
Aluminium (hard) 2.82 x 10-8 3.55 x 107
Tungsten 5.0 x 10-8 2.0 x 107
Zinc 5.95 x 10-8 1.68 x 107
Brass 6.6 x 10-8 1.52 x 107
Nickel 6.9 x 10-8 1.45 x 107
Platinum 11.0 x 10-8 9.09 x 106
Tin 11.15 x 10-8 8.7 x 106
Iron 10.15 x 10-8 9.85 x 106
Steel 19.9 x 10-8 5.03 x 106
German silver 16-40 x 10-8 6.3-2.5 x 106
Platinoid 34.4 x 10-8 2.91 x 106
Manganin 44.0 x 10-8 2.27 x 106
Gas carbon 0.0005 200
Silicon 0.06 16.7
Gutta-percha 2 x 107 5 x 10-8
Glass (soda-lime) 5 x 109 2 x 10-10
Ebonite 2 x 1013 5 x 10-14
Porcelain 2 x 1013 5 x 10-14
Sulphur 4 x 1013 2.5 x 10-14
Mica 9 x 1013 1.1 x 10-14
Paraffin-wax 3 x 1016 3.3 x 10-17


By using a transformer without ct, a bridge rectifier circuit system, and rotary switch, we can create a power supply for electronic equipment with voltage range from 3 V, 4.5 V, 6 V, 7.5 V, 9 V, to 12 V.

Add component such a rocker switch or a slide switch as the main switch, and put one LED as an indicator tool.

Power supply circuit with variable voltage as shown below

Click to enlarge

Components are required:
  1. S1 = Rocker switch
  2. S2 = Rotary switch (2 pole 6 lanes)
  3. T1 = Step-down transformer without ct (220V/12V 500mA)
  4. D1, D2, D3, D4 = Diodes IN4002
  5. D5 = LED
  6. R1 = Resistor 220 Ω
  7. C1 = Elco (electrolytic capacitor) 2200 μF/16V