Sunday, October 17, 2010

Trial Selangor 2010 ~ paper 2 ( Transistor)

Question 8 :( Selangor trial 2010)

Diagram 8.1 shows an automatic switch circuit that will light up a street lamp during night-time.
Rajah 8.1 menunjukkan litar suis automatik yang menyalakan lampu jalan pada waktu malam.


(a) Name the type of transistor T?
Namakan jenis transistor T?

notes:
~ There are two types of transistors ; the n-p-n transistor and p-n-p transistor.
~ arrow of n-p-n transistor is pointing downward.

(b) State the function of resistance R2?
Nyatakan fungsi perintang R2?



notes :
~ The current which flows in the base circuit is called base current.
~ Resistance R2 is used to protect the transistor from an over supply of the current.




(c) State what will happen to the resistance of the light-dependent resistor (LDR) when the light intensity is low?
Nyatakan apakah yang berlaku kepada rintangan perintang peka cahaya (PPC) apabila keamatan cahaya adalah rendah?

notes :
~ The LDR resistance increases when it placed in darkness. ( about 1 Mega-ohms )


(d) State what will happen to the street lamp when the positions of R1 and LDR are switched.
Explain why.
Nyatakan apa akan berlaku kepada lampujalan apabila R1 dan PPC ditukar kedudukan?
Terangkan mengapa.



notes:
~ The base current is varying because of the condition of the LDR. ( depends on light intensity).

~ A small change in the base current, lb causes a large change in the collector current, lc.
~ Cara senang , rintangan di XY kecil daripada YZ hanya boleh membenarkan arus base,lb mengalir .


(e) Diagram 8.2 shows a transistor circuit.
Rajah 8.2 menunjukkan sebuah litar transistor.



You are given three resistors R3 with resistance 50 ohms, 100 ohms and 200 ohms each.
Calculate the voltage across the base-emitter, VBE for each ofthe resistor.
Anda diberi tiga perintang R3 yang setiapnya berintangan 50 ohm 100 ohm dan 200 ohm.
Hitungkan beza keupayaan merentasi tapak-pengeluar, VBE bagi setiap perintang itu.


(i) VBE for 50 ohms.
VBE untuk 50 ohm.



(ii) VBE for 100 ohms
VBE untuk 100 ohm

(iii) VBE for 200 ohms
VBE untuk 200 ohm



notes:
~ Potential divider






(f) The bulb will light up if the VBE is greater than 0·7 V. Based on your answers in 8(e), choose the most suitable resistor to be used as resistor R3.
Give one reason for your answer.
Mentol akan menyalajika VBE adalah lebih besar dari 0·7 V. Berdasarkan jawapan anda dalam 8(e), pilih perintang yang paling sesuai untuk digunakan sebagai perintang R3.
Berikan satu sebab bagi jawapan anda.



notes:
~ Transistor turn on if
VBE
is larger than minimum value .






Suggested Answers:


a) n-p-n transistor

b) to limit the base current

c) Increases

d)
P1 : street lamp does not light up
P2 : voltage across VBE decreases
P3 : base current decrease, transistor is off


e) 200 ohms
reason : V is greater than 0.7 V and transistor can function.

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