Slide 1'&$%ECE/CE 3720: Embedded System DesignChris J. MyersLecture 18: Transistors, Relays, and MotorsSlide 2'&$%Design Rules for NPN Transistors in Saturated Mode(See Figure 8.56)1. Normally, Vc> Ve2. Input current flows from base to emitter, output currentflows from collector to emitter.3. Have maximum values for: Ib, Ic, Vce, and Ic· Vce.4. Act like current amplifiers: Ic= hfe· Ib.5. Activate if Vb> Ve+ Vbe(SAT ), where Vbe(sat)is >0.6V.1Slide 3'&$%Design Rules for PNP Transistors in Saturated Mode(See Figure 8.57)1. Normally, Ve> Vc2. Input current flows from emitter to base, output currentflows from emitter to collector.3. Have maximum values for: Ib, Ic, Vce, and Ic· Vce.4. Act like current amplifiers: Ic= hfe· Ib.5. Activate if Vb< Ve− Vbe(SAT ), where Vbe(sat)is >0.6V.Slide 4'&$%Typical Transistors(See Table 8.9)2Slide 5'&$%Introduction to Relays• A relay is a device that responds to a small current orvoltage change by activating a switches or other devices.• Used to remotely switch signals or power.• Input control usually electrically isolated from output.• Input signal determines whether switch is open or closed.Slide 6'&$%Various Types of Relays(See Figures 8.58 and 8.59)3Slide 7'&$%Types of Relays• Classic general-purpose relay has an EM coil and canswitch power.• The read relay has an EM coil and can switch low levelDC electronic signals.• The solid-state relay (SSR) has an input-triggeredsemiconductor power switch.• The bilateral switch uses CMOS, FET, or biFETtransistors (technically not a relay but behaves similarly).Slide 8'&$%Drawing of an EM Relay(See Figure 8.61)4Slide 9'&$%Electromagnetic Relay Basics• Input circuit is an EM coil with an Iron Core.• Output switch includes two sets of silver or silver-alloycontacts (called poles).• One set is fixed to the relay frame, and other is locatedat end of leaf spring poles connected to the armature.• Contacts held in “normally closed” position by thearmature return spring.• When input circuit energizes EM coil, a “pull-in” force isapplied to the armature and “normally closed” contactsbreak while “normally open” contacts are made.Slide 10'&$%Various Relay Configurations(See Figures 8.62 and Table 8.11)5Slide 11'&$%Relay Parameters(See Table 8.10)Slide 12'&$%Reed Relays(See Figure 8.63)6Slide 13'&$%Solenoids(See Figure 8.64)Slide 14'&$%Pulse-Width Modulated DC Motors• DC motor also has frame that remains motionless and anarmature that moves in this case in a circular manner.• When current flows through EM coil, magnetic forcecreated that causes rotation of the shaft.• Brushes positioned between frame and armature used toalternate the current direction through the coil so that aDC current generates a continuous rotation of the shaft.• When current removed, shaft is free to rotate.• Pulse-width modulated DC motor activated with fixedmagnitude current but duty cycle varied to control speed.7Slide 15'&$%Interfacing EM Relays, Solenoids, and DC Motors• Interface circuit must provide sufficient current andvoltage to activate the device.• In off state, input current should be zero.• Due to inductive nature of the coil, huge backelectromotive force (EMF) when coil current is turned off.• Due to high speed transistor switch, there is a large di/dtwhen the coil is deactivated (activation also but smaller).• Voltages can range from 50 to 200V.• To protect the driver electronics, a snubber diode isadded to suppress the back EMF.Slide 16'&$%Relay Interfaces(See Figures 8.66, 8.67, and 8.68)8Slide 17'&$%Isolated Interfaces(See Figures 8.69, 8.70, and 8.71)Slide 18'&$%H-Bridge(See Figures 8.72, 8.73, and 8.74)9Slide 19'&$%Solid State Relays• Developed to solve limited life expectancy and contactbounce problems since they have no moving parts.• Also, faster, insensitive to vibrations, reduced EMI,quieter, and no contact arcing.• Optocoupler provides isolation between the input circuit(pseudocoil) and the triac (pseudocontact).• Signal from phototransistor triggers the output triac sothat it switches the load current.• Zero-voltage detector triggers triac only when AC voltageis zero, reducing surge currents when triac is switched.• Once triggered, triac conducts until next zero crossing.Slide 20'&$%Solid State Relays(See Figures 8.75 and 8.77)10Slide 21'&$%Stepper Motors• Very popular due to inherent digital interface.• Easy to control both position and velocity in anopen-loop fashion.• Though more expensive then ordinary DC motors, systemcost is reduced as they require no feedback sensors.• Used in disk drives and printers.• Can also be used as shaft encoders to measure bothposition and speed.Slide 22'&$%Simple Stepper Motor Interface(See Figure 8.79 and 8.80, and Table 8.13)11Slide 23'&$%Slip• A slip is when computer issues a sequence change, butthe motor does not move.• Occurs if load on shaft exceeds available torque of motor.• Can also occur if computer changes output too fast.• If initial shaft angle known and motor never slips,computer can control shaft angle and speed withoutposition sensor.Slide 24'&$%Helper Functions to Control Stepper Motorconst struct State{ unsigned char Out; /* Output for this state */const struct State *Next[2];}; /* Next state */#typedef struct State StateType;#typedef StateType * StatePtr;unsigned char POS; /* b/w 0 and 199, shaft angle */#define clockwise 0 /* Next index*/#define counterclockwise 1 /* Next index*/StateType fsm[4]={ {10,{&fsm[1],&fsm[3]}},{ 9,{&fsm[2],&fsm[0]}},{ 5,{&fsm[3],&fsm[1]}},{ 6,{&fsm[0],&fsm[2]}} };StatePtr Pt; /* Current State */12Slide 25'&$%Helper Functions to Control Stepper Motorvoid CW(void){Pt=Pt->Next[clockwise];PORTB=Pt->Out;if(POS++==200) POS=0;}void CCW(void){Pt=Pt->Next[counterclockwise];PORTB=Pt->Out;if(POS==0)POS=199;else POS--; }void Init(void){POS=0;Pt=&fsm[0];}Slide 26'&$%Linked List to Control Stepper Motor;Linked list stored in EEPROMS10: dc.b 10 ;Output patterndc.w S9 ;Next if CWdc.w S6 ;Next if CCWS9: dc.b 9dc.w S10dc.w S5S5: dc.b 5dc.w S9dc.w S6S6: dc.b 6dc.w S5dc.w S10;Global variables stored in RAMPOS: ds 1 ;0<=POS<=199PT: ds 2 ;to current state13Slide 27'&$%Helper Functions to Control Stepper MotorInit:clr POSldx #S‘0stx PTrtsSlide 28'&$%Helper Functions to Control Stepper Motor;Move 1.8 degrees clockwiseCW: ldx PT ;current stateldx 1,X ;next clockwisestx PT ;update pointerldaa ,X ;output patternstaa PORTB ;set phase controlldaa POS ;update