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MM2EMD L7.Pdf University of Nottingham Electromechanical devices MM2EMD Lecture 7 – Transistors - Switching high voltage things on with a low voltage Dr. Roderick MacKenzie [email protected] Summer 2015 @rcimackenzie Released under Outline of the lecture •No recap of last lecture :) •Transistor basics •Relays (Mechanical transistor) •NPN Bipolar Junction Transistors •PNP Bipolar Junction Transistors •MOSFETs •Push pull pairs to drive MOSFETs •One last thing •Summary 2 Roderick MacKenzie MM2EMD Electromechanical devices Outline of the lecture •No recap of last lecture :) •Transistor basics •Relays (Mechanical transistor) •NPN Bipolar Junction Transistors •PNP Bipolar Junction Transistors •MOSFETs •Push pull pairs to drive MOSFETs •One last thing •Summary 3 Roderick MacKenzie MM2EMD Electromechanical devices Roderick MacKenzie Roderick electrical devices suchas motors. high voltage control electronics • This lecture is making low voltageThis lecture Think back to lecture 1. lecture Think backto Dave Jones Smart Electronic Circuits (low voltage) Circuits voltage) (high S.J. de Waard Simple Simple Electrical MM2EMD Electromechanical devices 4 Think about an AND gate chip And gate Biro •Look at the tiny thin pins which are used to carry current in and out of the chip. •These pins can supply 25 mA @ 5V at the most. 5 Roderick MacKenzie MM2EMD Electromechanical devices But why is this? i •If we take the top off the chip h p o with acid. G ● Look how much small the actual chip is and look at the tiny bond wires (25 mA @ 5V max!!!) 6 Roderick MacKenzie MM2EMD Electromechanical devices Roderick MacKenzie Roderick 500 V • It needsIt Now think about this motor. this about Now think to run.to 10 Amps 10 at CMBJ the current. thick as my finger to carry • It will also need cable as MM2EMD Electromechanical devices 7 S.J. de Waard Using chips to run large voltage/current loads. •Now imagine trying to run this motor which needs 500V @ 10 Amps from this tiny chip chat can only deliver 5V @ 25 mA. 8 Roderick MacKenzie MM2EMD Electromechanical devices Using chips to run large voltage/current loads. •It would not work – the chip would just get hot and melt because it can not deliver enough voltage/current. 9 Roderick MacKenzie MM2EMD Electromechanical devices Voltage/current amplifier •So if we want to use our chip to run a motor, we need some type of voltage/current amplifier component. 500V/10 A 5V/25 mA Amplifier (Notice this is different from a transformer) 10 Roderick MacKenzie MM2EMD Electromechanical devices Outline of the lecture •No recap of last lecture :) •Transistor basics •Relays (Mechanical transistor) •NPN Bipolar Junction Transistors •PNP Bipolar Junction Transistors •MOSFETs •Push pull pairs to drive MOSFETs •One last thing •Summary 11 Roderick MacKenzie MM2EMD Electromechanical devices One such component is the relay •A relay is a an electromechanical switch An electromagnet Switch •And this is how it works..... 12 Roderick MacKenzie MM2EMD Electromechanical devices Relay example Relay + + - - 5 V/25 mA 500V / 10 A •Low voltage side •High voltage side 13 Roderick MacKenzie MM2EMD Electromechanical devices Relay example •When the 5 V supply is turned on current flows in the low voltage side of the relay. This current could come from a chip. Relay + + - - 5 V/25 mA 500V / 10 A 14 Roderick MacKenzie MM2EMD Electromechanical devices Relay example •The electromagnet becomes energized Relay + + - - 5 V/25 mA 500V / 10 A 15 Roderick MacKenzie MM2EMD Electromechanical devices Relay example •The magnetic field pulls the switch shut. Relay + + - - 5 V/25 mA 500V / 10 A 16 Roderick MacKenzie MM2EMD Electromechanical devices Relay example •Current flows in the high voltage circuit. Relay + + - - 5 V/25 mA 500V / 10 A 17 Roderick MacKenzie MM2EMD Electromechanical devices Relay example •And the motor turns Relay + + - - 5 V/25 mA 500V / 10 A 18 Roderick MacKenzie MM2EMD Electromechanical devices Relay example •When the 5V supply is turned off... Relay + + - - 5 V/25 mA 500V / 10 A 19 Roderick MacKenzie MM2EMD Electromechanical devices Relay example •When the 5V supply is turned off... Relay + + - - 5 V/25 mA 500V / 10 A 20 Roderick MacKenzie MM2EMD Electromechanical devices Relay example •The magnetic field disappears and the motor turns off. Relay + + - - 5 V/25 mA 500V / 10 A 21 Roderick MacKenzie MM2EMD Electromechanical devices Relay example •The magnetic field disappears and the motor turns off. Relay + - 5 V/25 mA 500V / 10 A 22 Roderick MacKenzie MM2EMD Electromechanical devices A real relay looks like this: 23 Roderick MacKenzie MM2EMD Electromechanical devices Advantages of using relays for switching things on and off •You get physical isolation of both circuits. Important when you are dealing with very high voltages. •Relays were the primary way to turn high voltages on/off before the advent of the transistor in the 1960s – they not so bad at this. 24 Roderick MacKenzie MM2EMD Electromechanical devices The drawbacks of using relays for switching •You have to keep that coil energized if you want to keep your device on. •This uses a lot of power. •They are mechanical devices and therefore slow. •They are full of copper and cost a lot (a few pounds.) •They are big. 25 Roderick MacKenzie MM2EMD Electromechanical devices Outline of the lecture •No recap of last lecture :) •Transistor basics •Relays (Mechanical transistor) •NPN Bipolar Junction Transistors •PNP Bipolar Junction Transistors •MOSFETs •Push pull pairs to drive MOSFETs •One last thing •Summary 26 Roderick MacKenzie MM2EMD Electromechanical devices Transistors •You can think of a transistor as being just like a relay except they have only one control wire. Relay Transistor Control Input Input Control Control Output Output 27 Roderick MacKenzie MM2EMD Electromechanical devices You can think of a transistor like this.... The Transistor Man Output (Emitter) Input (collector) Control (base) After Horowitz and Hill 28 Roderick MacKenzie MM2EMD Electromechanical devices Transistors – The Transistor Man Output (Emitter) Input (collector) Control After Horowitz and Hill (base) 29 Roderick MacKenzie MM2EMD Electromechanical devices Transistors – The Transistor Man Output (Emitter) Input (collector) Control (base) After Horowitz and Hill30 Roderick MacKenzie MM2EMD Electromechanical devices Transistors Collector Collector Collector Base Base Base Emitter Emitter Emitter 1. No base 2. Small base 3. Big base current – no current – small current – big output current output current output current 31 Roderick MacKenzie MM2EMD Electromechanical devices Transistors •Transistors look like this •Little black beads (although sometimes silver boxes) •They always have three legs. •Invented in June 30th 1948 o t s i s n a r John Bardeen, William T Shockley and Walter Brattain 32 Roderick MacKenzie MM2EMD Electromechanical devices Advantages of Transistors •No moving parts so they won't break – very long lifetime. •Don't need as much control current to operate as a relay. •Cheep as chips! o t s i s n a r T 33 Roderick MacKenzie MM2EMD Electromechanical devices Transistors – Different types of transistors •There are lots of different types of transistors – too many to learn about in this course. •I have decided to teach you about the three of the most useful ones. 34 Roderick MacKenzie MM2EMD Electromechanical devices Transistors – Different types of transistors •There are lots of different types of transistors – too many to learn about in this course. •I have decided to teach you about the three of the most useful ones. 35 Roderick MacKenzie MM2EMD Electromechanical devices Outline of the lecture •No recap of last lecture :) •Transistor basics •Relays (Mechanical transistor) •NPN Bipolar Junction Transistors •PNP Bipolar Junction Transistors •MOSFETs •Push pull pairs to drive MOSFETs •One last thing •Summary 36 Roderick MacKenzie MM2EMD Electromechanical devices Transistors – The NPN Bipolar Junction Transistor (BJT) •It's called an NPN transistor because it's made of three layers of material, one with negative charge, one with positive charge and one with negative charge. Collector (input) Collector - n-type Base Base p-type (control) + - n-type Emitter Emitter (output) 37 Roderick MacKenzie MM2EMD Electromechanical devices Transistors – The NPN Bipolar Junction Transistor (BJT) •This is how you would use one in 20V a circuit....... Light bulb. C B 1V E 0V 38 Roderick MacKenzie MM2EMD Electromechanical devices Transistors – The NPN Bipolar Junction Transistor (BJT) •Put a small current into the 20V Base. C B 1V E 0V 39 Roderick MacKenzie MM2EMD Electromechanical devices Transistors – The NPN Bipolar Junction Transistor (BJT) •The transistor switches on 20V C B 1V E 0V 40 Roderick MacKenzie MM2EMD Electromechanical devices Transistors – The NPN Bipolar Junction Transistor (BJT) •A large current flows between 20V the Collector and Emitter. C B 1V E 0V 41 Roderick MacKenzie MM2EMD Electromechanical devices Transistors – The NPN Bipolar Junction Transistor (BJT) •When you stop feeding the 20V base current, no more current flows between the Collector and the Emitter. C B 1V E 0V 42 Roderick MacKenzie MM2EMD Electromechanical devices Transistors – The NPN Bipolar Junction Transistor (BJT) •And the light 20V bulb will turn off. C B 1V E 0V 43 Roderick MacKenzie MM2EMD Electromechanical devices Transistors – The NPN Bipolar Junction Transistor (BJT) •The base current could come 20V from a chip! •Now we can power any sized device using a decision made form a tiny chip we want. C B E 0V 44 Roderick
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