ECE 2210 Transistor Switching Circuit Examples A.Stolp 11/21/06 rev 12/3/14 Ex.1

Transcription

1 Transistor Switching Circuit Examples A.Stolp 11/21/06 rev 12/3/14 Ex Ω B. 0.7 CC. 12 I C = CC. 12 C =. 5 kω B P Q = The little open circles are connections, in this case to unseen power supplies. BB. 5 = BB. 5 BB 0.7. If: 100 = 0.86 ma I C. I C = 86 ma C. CC I C C = 3.4 P. Q C I C P Q = mw BB 0.7. If: 200 = 0.86 ma I C. I C = 172 ma C. CC I C C = 5.2 must be in saturation:. C 0.2 CC C I C I C = 118 ma P. Q C I C P Q = 23.6 mw Since saturation can depend on, You usually assume a small when designing a circuit that should saturate (a switching circuit). If: 50. Ω I C. I C = 172 ma C CC R. C I C C = 3.4 same 200 P Q. C I C P Q = mw Saturation also depends on and CC. What is the largest value that could be and still keep the transistor in saturation? CC = 236 ma = 1.18 ma max = = kω Ex.2 CC. 10 BB. 6 B = = P Q =? E =? I E = R E = 50 Ω B. 6 E. 5.3 R E. 50 Ω I E E R E I E = 106 ma Doesn't depend on I E 1 I E = 0.5 ma ~ = 0.53 ma If is big enough. P. Q CC E I E P Q = W Transistor Switching Circuit Examples, p1

2 Ex.3 Transistor Switching Circuit Examples, p2 If the load must be connected to ground, a PNP transistor is often a better choice. =? CC. 20 Let's assume a a small and saturation and find the necessary. a small : 20 C. CC 0.2 C = 19.8 B =? R. C 15 Ω P Q =? C = =? C I C =? =? 1.32 A = 66 ma R. C 15 Ω B. CC 0.7 B = 19.3 B = 292 Ω P.. Q 0.2 I C P Q = 34 mw Ex.4 CC. 15 Sometimes one transistor can't provide enough amplification. Sometimes you want to "invert" the input (make high off and low on). =?. 500 Ω C1 = B2 =?. 30 Ω I C2 =? C2 =? 2 25 B1 =? BB kω 1 =? 2 =? 1 80 Switch open 1o 0 B1o. 0 B2o. 0.7 CC o 2o = 28.6 ma o 2o o = 28.6 ma I. C2o 2 2o I C2o = 715 ma C2o CC. I C2o = C2o 6.45 Q 2 must be in saturation: C2o. 0.2 CC C2o I C2o I C2o = ma Switch closed Transistor Switching Circuit Examples, p c 1c = 1.08 ma assume Q 1 is in saturation C1c. 0.2 CC C1c I C1c I C1c = 29.6 ma. 1 1c = 86 ma I C1 is controlled by B2c C1c B2c = 0.2 2c 0 I C2c 0 = I RCc When the switch is open, current flows in through the load resistor,, When it is closed, no current flows though the load. This is an example of logical "inversion".

3 Transistor Switching Circuit Examples, p3 Ex.5 Modified from F07 Final CC. 5 A transistor is used to control the current flow through an inductive load (in the dotted box, it could be a relay coil or a DC motor). a) Assume the transistor is in saturation (fully on) and that switch has been closed for a long time. What is the load current? I C =? CC 0.2. = 600 ma b) 80 find the minimum value of S, so that the transistor will be in saturation.. 50 mh Inductive load. 8 Ω R S. 200 Ω. 80 Ω = 7.5 ma S. Smin R S R Smin = 2.8 Use this S for the rest of the problem. c) Does the diode in this circuit ever conduct a significant current? If yes, when and how much? When the switch opens. I Dmax = = 600 ma from part a) d) You got a bad transistor. 60 Find the new I C, and CE and P Q. I C =? I. C I C = 450 ma Now operating in active region CE =? CE. CC I C CE = 1.4 P Q =? P. Q CE I C P Q = 0.63 W = 60 Use this for the rest of the problem. c) Find the minimum value of so that the transistor will be in saturation. Smin 0.7. = 7.5 ma R S I. Cmax I Cmax = 450 ma CC 0.2. min min = 10.7 Ω I Cmax d), can't be changed, so find the maximum value of so that the transistor will be in saturation. = 600 ma from part a) = 10 ma Smin 0.7. max = R S = 10 Ω Transistor Switching Circuit Examples, p3

4 Ex.6 Transistor Switching Circuit Examples, p4 From F05 Final with modifications from F06 Final A transistor is used to control the current flow through an inductive load (in the dotted box, it could be a relay coil or a DC motor). a) 25 Assume the transistor is in the active region, find I sw,, L, EC and P Q. S 0.7. = R S I. L = 465 ma. 200 Ω S. 10 Inductive load R S. 300 Ω mh 18.6 ma= I sw I sw L R. L 10 Ω = 10 Ω. L L = 4.65 _ EC S L EC = 5.35 P. Q EC P Q = W b) Was the transistor actually operating in the active region? yes no (circle one) yes How do you know? (Specifically show a value which is or is not within a correct range.) c) Find the maximum value of, so that the transistor will be in saturation. If saturated: EC. 0.2 S 0.2. = 0.98 A EC = 5.35 > 0.2. = 39.2 ma S 0.7. max = R S = 63 Ω NOT POSSIBLE d) = 200 Ω and can't be changed, find the minimum value of so that the transistor will be in saturation. = 0.98 A min min = 52.7 e) How much power is dissipated by the transistor if it has the you found in part d) P.. Q 0.2 P Q = W f) Does the diode in this circuit ever conduct a significant current? If yes, when and how much? When the switch opens. I Dmax = = 0.98 A from part a) g) The switch is open for a while. What is the load current ( ) now? 0 I Dmax = = 980 ma Transistor Switching Circuit Examples, p4

5 Transistor Switching Circuit Examples, p5 CC1. 8 Ex.7 From F13 Final A transistor is used to control the current flow through an inductive load (in the dotted box, it could be a relay coil or a DC motor). a) In order for current to flow in through the load, the switch should be: i) closed or ii) open (Circle one) b) Assume the switch has been in the position you circled above for a long time. is 1.3A. Find the power dissipated by transistor Q 2 (neglect base current and BE ) switch. 40 Ω Q 1 CC A P Q2 =?. 3 Ω Inductive load CE2. CC2 CE2 = 1.1 L. L 60 mh Q 2 P. Q2 CE2 P Q2 = 1.43 W c) This is an unacceptable power loss, so you would like to determine the minimum 2 needed so that Q 2 will be in saturation. Assume Q 1 is also in saturation. You may assume I E = I C for both traistors. 2min =? = 3 Ω CC = 1.6 A = I C2 E2. CC2 0.2 E2 = 4.8 B2. E2 0.7 B2 = 5.5 C1. B2 0.2 C1 = 5.7 CC1 C1 I C1 2 I C1 2 = 57.5 ma 2min = = Better answer I. 1 1 B2 I C1 = ma 2min = = You replace Q 2 with a new transistor that has a greater than what you just calculated. d) How much power is dissipated by the new transistor Q 2 (neglect base current and BE )? P Q2 =? P.. Q2 0.2 P Q2 = 320 mw e) What is the maximum value of needed to saturate Q 1? 1 = 100 1min I C1 1min = ma B1 B B1 = CC1 B1 max max = 3.13 kω 1min f) Does the diode in this circuit ever conduct a significant current? If yes, when and how much? When the switch closes. I Dmax = = 1.6 A from part c) Transistor Switching Circuit Examples, p5

6 Transistor Switching Circuit Examples, p6 CC. 8 Ex.8 From F12 Final A couple of transistors are used to control the current flow through an inductive load. The switch has been closed, as shown, for a long time. a) You measure the voltage at each collector (referenced to ground) as shown on the drawing. Find the power dissipated by transistor Q 2. C1. 5 C2. 2 CC 2. = 1.5 A P. Q2 C2 P Q2 = 3 W. 10 kω Q Ω Inductive load. 80 mh. 4 Ω b) Find the of transistor Q 2.. R R2 = 4.3 Q 2 2 R2 = 43 ma 2 = c) Find the of transistor Q 1. CC 0.7. I R I R1 = d) Find the minimum for transistor Q 1 to be in saturation. 1min =? If Q 1 is saturated: R2. CC R2 = 7.1 R2 If Q 1 is saturated: = 71 ma 1min 1min = 97.3 I R1 You replace Q 1 with a different transistor so that now: Use this from now on. e) Find the new load current ( ) assuming transistor Q 2 is in the active region. Q 1 is saturated: = 71 ma I. L 2 = A f) Check the assumption that Q 2 is in the active region and recaculate if necessary. I.. R2 2 = CE2.. CC 2 CE2 = CC 0.2. Q 2 is saturated: = 1.95 A g) Does the diode in this circuit ever conduct a significant current? If yes, when and how much? When the switch opens. I Dmax = A from part f) Not possible Transistor Switching Circuit Examples, p6

7 Ex.9 From S13 Final NOT IN HANDOUT A transistor is used to control the current flow through an inductive load (in the dotted box, it could be a relay coil or a DC motor). a) In order for current to flow in through the load, the switch should be: i) closed or ii) open (Circle one) b) Assume the switch has been in the position you circled above for a long time and transistor Q 2 is saturated. Find the power dissipated by transistor Q 2 (neglect base current and BE ). CC 0.2. = P Q2 =? 2.9 A P.. Q2 0.2 P Q2 = 580 mw switch I R1. 1 kω c) Assume 2 is as shown. Find the maximum value of, so that Q 2 will be in saturation. =? Q 1 = 72.5 ma 2 CC 0.7. = 73.1 Ω CC. 6 Inductive load. 50 mh. 2 Ω Q Use this value of for the remainder of the problem d) If 2 were actually half the value shown shown, how much power would be dissipated by transistor Q 2 (neglect base current and BE )? P Q2 =? 2 I. L 2 = 1.45 A CE. CC CE = 3.1 P. Q CE P Q = W Use the value of 2 shown for the remainder of the problem. (not the half-value) e) When the switch is changed from the position you circled in part a), the load current should go to zero. What is the minimum value of 1 needed to saturate Q 1? CC 0.2. I C1 I C1 = 79.3 ma CC 0.7. I C1 1 1 = 5.3 ma 1min 1min = 15 1 f) If 1 were actually half the value you found above, what would be? CC 0.7. = 72.5 ma 1min I. C1 1 I C1 = ma 2 2 I C1 2 = ma I. L 2 2 = A Transistor Switching Circuit Examples, p7