* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Applications of Semiconductor devices – Switches.
Ground (electricity) wikipedia , lookup
Current source wikipedia , lookup
Electrification wikipedia , lookup
Electrical ballast wikipedia , lookup
History of electric power transmission wikipedia , lookup
Electrical engineering wikipedia , lookup
Stray voltage wikipedia , lookup
Control system wikipedia , lookup
Power engineering wikipedia , lookup
Power inverter wikipedia , lookup
Electronic engineering wikipedia , lookup
Voltage regulator wikipedia , lookup
Light switch wikipedia , lookup
Surge protector wikipedia , lookup
Three-phase electric power wikipedia , lookup
Schmitt trigger wikipedia , lookup
Voltage optimisation wikipedia , lookup
Electrical substation wikipedia , lookup
Alternating current wikipedia , lookup
Resistive opto-isolator wikipedia , lookup
Variable-frequency drive wikipedia , lookup
Crossbar switch wikipedia , lookup
Mains electricity wikipedia , lookup
Pulse-width modulation wikipedia , lookup
Buck converter wikipedia , lookup
Applications of Semiconductor devices – Switches. Electrical and Electronic Principles © University of Wales Newport 2009 This work is licensed under a Creative Commons Attribution 2.0 License. The following presentation is a part of the level 4 module -- Electrical and Electronic Principles. This resources is a part of the 2009/2010 Engineering (foundation degree, BEng and HN) courses from University of Wales Newport (course codes H101, H691, H620, HH37 and 001H). This resource is a part of the core modules for the full time 1 st year undergraduate programme. The BEng & Foundation Degrees and HNC/D in Engineering are designed to meet the needs of employers by placing the emphasis on the theoretical, practical and vocational aspects of engineering within the workplace and beyond. Engineering is becoming more high profile, and therefore more in demand as a skill set, in today’s high-tech world. This course has been designed to provide you with knowledge, skills and practical experience encountered in everyday engineering environments. Contents Mechanical Switches- Relays Switching Using a Thyristor- DC Supply Switching Using a Thyristor- DAC Supply Opto-isolator Credits In addition to the resource below, there are supporting documents which should be used in combination with this resource. Please see: Green D C, Higher Electrical Principles, Longman 1998 Hughes E , Electrical & Electronic, Pearson Education 2002 Hambly A , Electronics 2nd Edition, Pearson Education 2000 Storey N, A Systems Approach, Addison-Wesley, 1998 Applications of Semiconductor Devices- Switches Mechanical Switches - Relays Conventional mechanical switches have the following characteristics: Excellent ON resistance Excellent OFF resistance High drive capability less than 0.01 typical 0 open circuit depending upon switch 100s volts and 10s amps Input output isolation no electrical connection between input and output slow tenths of seconds at best. Speed of switching The last parameter makes mechanical switches of no use in computer circuitry or in power control. Semiconductor switches are an alternative but are limited in some of their parameters: Poor values for: ON resistance OFF resistance High drive capability Input output isolation What is in the favour of such devices is speed of switching. These can be switched at 100 MHz GHz. Applications of Semiconductor Devices- Switches Consider the set up below. Vs = 9V Ic L O A D Rc = 4.5k Vout Rb Vin Ib 2N2222 To determine the quality of the ON/OFF switching we need to use the characteristics and plot a load line. Vbe intersect on the Vce axis is at Vs = 9V intersect on the Ic axis is at Vs/Load = 2mA Applications of Semiconductor Devices- Switches 2N2222 Characteristic 2.5 Ib = 10 A Ib = 8 A Ic (mA) 2 Ib = 6 A 1.5 1 Ib = 4 A 0.5 Ib = 2 A Ib = 0 A 0 0 1 2 3 4 5 6 7 8 9 10 Vce (volts) Let us assume the input can be switched between 0V and 5V For 5V we want 10A When the input = 0V Ib = 0A Vce = 9v Ic = .01mA R = 900k When the input = 5V Ib = 10A Vce = 0.5v Ic = 1.9mA R = 263 This shows that the switch is far from ideal but there is a factor of nearly 3500 between ON and OFF. This device can only switch a couple of 10s of volts at a few milli amps. Larger transistors can do better. There is no electrical isolation between input and output but switching is rapid. FETs can be used in a similar way to switch devices ON and OFF and on the whole have similar characteristics and values. Applications of Semiconductor Devices- Switches Switching using a Thyristor. D.C. Supply Ia L O A D Rb Vin Vs Vout With no input the thyristor will be switched off – Ia will be very close to 0A and all of Vs will appear across the thyristor. If the thyristor is fired by applying a voltage on Vin then Vout drops to very near ground potential and the load will control Ia. If Vin is removed then the thyristor will remain on, keeping the load powered up. Uses – this is a latching device and can be used for applications such as alarms etc. A.C. Supply. If the D.C. is replaced by an A.C. supply then when the thyristor is fired the load will see the positive half cycle. When the input is removed then the thyristor will remain on until the supply drops to 0V – i.e. the end of the positive half cycle. We therefore have a half wave rectifying semiconductor switch. The real power of the thyristor for controlling AC comes from the periodic switching of the device at different points during the positive half cycle. This has the effect of supplying the load with different amounts of the half cycle. See below. Applications of Semiconductor Devices- Switches Complete Half Cycle Thyristor switched on early in the half cycle Thyristor switched on late in the half cycle The sine wave areas on the second two graphs represent the power that would be switched to the load. Control the switching point, and you control the load power. The control of the switching point is achieved by using a phase shift circuit as shown: Vin Vout Vin Vout The amount of phase lag between the two voltages depends upon the values of the resistor and the capacitor. When R is small 0 the phase lag will be small 0 When R is large the phase lag will be large 90 If the input to the R.C. network is derived from the AC driving the thyristor then if the output voltage is used to fire the thyristor then we will have 100% of the positive half wave when the angle is 0 and 50% of the half cycle when the angle is 90. Applications of Semiconductor Devices- Switches See below 90 phase lag. Load voltage Supply voltage Firing voltage This gives us control over only half of the wave. In order to have total control over the full half-wave we must use a centre tap transformer – as shown below: R C L O A D Rs D The centre tap transformer will generate two sine waves 180 of phase with each other. When R is small the firing voltage is in phase with the supply and the thyristor fires as soon as the positive half cycle begins. Load receives maximum power When R is large the firing angle approaches 180 and the thyristor is turned on just as the positive half cycle ends. Load receives zero power. To control the whole of the cycle – positive and negative cycles we use two thyristors back to back. This is called a triac. Symbol Structure ©André Karwath aka Aka’ available via http://en.wikipedia.org/wiki/File:Optoisolator_%28aka%29.jpg under a Creative Commons Attribution-Share Alike 2.5 Generic license. Opto-isolator In electronics, an opto-isolator (or optical isolator, optocoupler or photocoupler) is a device that uses a short optical transmission path to transfer a signal between elements of a circuit, typically a transmitter and a receiver, while keeping them electrically isolated — since the signal goes from an electrical signal to an optical signal back to an electrical signal, electrical contact along the path is broken. A common implementation involves an LED and a light sensor, separated so that light may travel across a barrier but electrical current may not. When an electrical signal is applied to the input of the opto-isolator, its LED lights, its light sensor then activates, and a corresponding electrical signal is generated at the output. Applications of Semiconductor Devices- Switches Unlike a transformer, the opto-isolator allows for DC coupling and generally provides significant protection from serious over voltage conditions in one circuit affecting the other. With a photodiode as the detector, the output current is proportional to the amount of incident light supplied by the emitter. The diode can be used in a photovoltaic mode or a photoconductive mode. In photovoltaic mode, the diode acts like a current source in parallel with a forward-biased diode. The output current and voltage are dependent on the load impedance and light intensity. In photoconductive mode, the diode is connected to a supply voltage, and the magnitude of the current conducted is directly proportional to the intensity of light. Applications of Semiconductor Devices- Switches An opto-isolator can also be constructed using a small incandescent lamp in place of the LED; such a device, because the lamp has a much slower response time than an LED, will filter out noise or half-wave power in the input signal. In so doing, it will also filter out any audio- or higher-frequency signals in the input. It has the further disadvantage, of course, (an overwhelming disadvantage in most applications) that incandescent lamps have finite life spans. Thus, such an unconventional device is of extremely limited usefulness, suitable only for applications such as science projects. The optical path may be air or a dielectric waveguide. The transmitting and receiving elements of an optical isolator may be contained within a single compact module, for mounting, for example, on a circuit board; in this case, the module is often called an optoisolator or opto-isolator. Applications of Semiconductor Devices- Switches The photosensor may be a photocell, phototransistor, or an optically triggered SCR or Triac. Occasionally, this device will in turn operate a power relay or contactor. Applications Among other applications, opto-isolators can help cut down on ground loops and block voltage spikes. One of the requirements of the MIDI (Musical Instrument Digital Interface) standard is that input connections must be opto-isolated. Applications of Semiconductor Devices- Switches Opto-isolators are used to protect hospital patients from accidental electric shock. Patients with IV's in their bodies are particularly susceptible, sometimes succumbing to 'carpet shock.' They are used to isolate low-current control or signal circuitry from transients generated or transmitted by power supply and high-current control circuits. The latter are used within motor and machine control function blocks. The classical computer mouse is a common application, using infrared emitter led's and phototransistors to form optocouplers. They are used to translate the mousewheel velocity into digital motion information. The principle of operation does not require to utilize infrared light, though this frequency range is somehow resistant against interference with visible light. Applications of Semiconductor Devices- Switches