ry

SECTION B· ACTIVITIES
ry

ACTIVIT Y

To identify a diode, an LED, a resistor and a capadtor from a mixed collection of

such items.

~ PARATUS AND MATERIAL REQUIRED

A multimeter, a mixed collection of diode, LED, resistor and capacitor.

l.I_HEORY

Diode. A two terminal device which conducts current when forward biased and not \Yhen
reverse biased. It does not emit light during its conduction.
LED. A light emitting diode is a two terminal device which conducts current when forward
biased and not when reverse biased. It emits a characteristic light during its conduction.
Resistor. A two terminal device which conducts equally in both directions.
Capacitor. A two terminal device which offers infinite resistance to de but has a finite reactance
for ac. When connected across a de source, a multimeter shows a large current initially (for
C > µF) which decreases to zero quickly. This is because the capacitor initially draws a charge.

DIAGRAMS
.....___

Carbon resistor Diodes

1000 µF
H p
LED

~
8 --@)-
Paper

CAPAOTORS
Electrolytic

Fig. 1 Diagrams of carbon resistor, diodes, LED and capacitors.

PROC EDURE

1. Look for the colour bands on the given components. If a component has a set of three colour
bands followed by a silver or gold band, then the component is a resistor.
2. Insert the black and red leads (or probes) into common and positive terminals of the
multimeter. Turn its selector switch to resistance mode - highest range (0-MQ).
3. Touch the two probes to the two ends of each component one by one. Note the direction of
deflection in the multimeter. Interchange the positions of two probes for each component
and again note the direction of deflection.

181
 ◄
Lf\B M..\N U.t\ l PHYSICS ~XII

4. lf the multinwtw shows ,m l'\lUJI detk ction in bi-1th din,-t1un., thm Ule rompl
~I is a ..~~· _
S. If ,1 1nultinwtl'r shQws dcih,'\.i ion in pnc dirt>etil)ll w1tht1 ut ,m,· emi~io
n ol ~ight from the
\.."\)mpone-r\t illhl 111.l de-tlrctit,n in the opr()Si tl' Jm'\.i.lQl"I, thl'n thr (\"lffif"(lnent
15- d '- .
6. lf the multimeter shows detlt>ctit">n in one d in-ti.ion .ilongwith thr t:'mi~io
n oi light from~
'-'omponent and no detlcctio n in the op~itc d irection, then the comp(ln ent
is an L:2 _
7. If the multimeter does not show arn- detlec-t ion on C\1nnN..-ting. its
, P . ~ either way . to ~
component. then the O."'mponcnt is ;-i ~ 1r K t,,r_But if the ~,1po1citanc r ot the capaci
tm i.s ~ ,
tht> multinw ter will show a largt> deflectio n initiall~' wh ich gradual!~· decn>a,s
es tu z~ro.
8. R&."Ord all your obset-Yations in cl tabular fom, .

OBSERV ATIONS

Table Bl : State of conduction of each component

Item code State of conduction of a component Identified component

A Conduct s equally in both di.re<..iions
B Conducts in one di.rection "ithout emi!-Sion of light
C Conduct s in one direction " ith emission of light
D Does not conduct, gi,·es an initial dellectio n which
decays to zero

RES ULT

From the mixed groupin g of compon ents, the compon ents marke-.i .1.
R C and D ha,-e been
identifi ed as resistor, diode, LED and cap acitor respectiwl~·-

PRECAUT IONS
1. While checkin g the conduct ion state of any compon ent, clean i~ leads
prore.rly .
2. Use the selector S\.vitch of the multime ter in resistan a- mode wi.th :l:~:1e:::
~ :-an~e option.
3. While testing any compon ent, an., id t,•l.!Cbi"; the metal end of either
of multime ter probe.
Body resistan ce in parallel with the compon ent resistan ce maY creatt>
Cl..•nfus.it•n abt"'ut the
conduct ion state of the compon ent. ·

1. What is a res istor ? 5. H1.,\ J,, t'1e -::,,:-_iu_-: ,,,n ::: t.1:c>:- ,,f ,w. ,,r,1i.n.1tY ,ii,,:<'
Any material that has some resistanc e is called a ,,m~ ,m LE[' ,~ir:cr ~ ·
resistor. An ordi.n<1r~' di1.1de u.1mh1cts in fo rw,mi bi.15ing
2. What is a Jinec1r res istor? without any emi::-sil,n of li~ht whilt' .m Lff\ l."\"'''•lm-ts
A linear resistor is one which obeys Ohm's law o r for in tor\\":i rd dir\:'("tion with emi::._,;_il'tl 1.' t \(~ht. &,th ~fo
which V-l graph is a straight line passin g through the not conduct in '"''·erse bi,,sil\~-
origin. 6. l \L) \1· li1.'1.':" ,1 c\1r'.lc1ti.,r l'c'h.1, <' k'\ \',wd s ci-: ~
3. What is a non-ohm i c device? A cnpacito r d0es not Ct)1\liuct de. But ., 1.·ap,,cito.r ,,f
A device which does not obey Ohm's la w is ca lled ,, lar~e l"ap;i..:it,m ct' show s ,m initi,,I detlt:,·ti~,n in ~
non-ohm ic device. Semicon ductor diodes, LEDs, etc ; nmmekr w hich dec.,ys 11., zen, ~1u ia.ly . This is du~ I\)
are non-ohm ic devices . chn rgi nb oi the c,,pal"itor.
4. Does an o h mi c resi stor co ndu ct L'qually ll•r bo ih 7. l 1L1,, Lll11.'~ ,1 c'>ll-',1cik1r l'd1.n·,:, t1.,,,·.1 rd.s .,c ~
forwa rd and reve rse bias ing? A c.1pncitor conduct$ ,\C bec-,mse its cap,Kitin'.
I Yes, an ohmic resistor conducts equally when cum:'nt is
passed in one direction and then in opposite direction .
l"l:'ach1na:' ( xc = 2 ~;. C j is finite against ac-.

182
 Section 8 : ACTIVITIES

ACT IVIT Y

/\11-1
U,e of multimeter to see the unidirectional flow of current in case of a diode and an
LED and check whether a given electronic component (e.g., diode) is in working order.

fq ,1 1 1- 11 / i lJ ' , Al ll) MA 1111. IAI l"~FQ U IIU D

A multimete r, a diode, LED, sand paper.

I r1u () I< (
1. To check the unidirectional flow of current through a junction diode/LED. When a
junction diode/LED is forw ard bi ased, a substantial current (l'::l few mA) will flow through it.
When a junction diode/LED is reverse biased, a negligible current (i:::: few µA) will flow
throu gh it.
2. To check whether a diode is in working order. A junction diode offers a low resistance (a
few O to kn) during forw ard biasing and it offers a very high resistance (::::: M Q) during
reverse biasing. Thus the working of a junction diod e can be examined by measurin g its
resistance in forward and reverse biased conditions.

I
·
)/ <;11Vllt<J UIAC,HAM

--t>I- [[[] J
Diode

p-11
R

+ Red p robe

I J
- (·)- -:i:-¥ ;..
K 6 V vnriable
Bl ack probe

d e battery

i'iq . ? To check unidirectional flow of current through a diode.

JJr1 <JU· fJI) Hf

(a) To check unidirectional flow of current through a junction diode/LED
1.. As shown in Fig. 2, connect the junction diode, a resistance box, a 6 V variable de battery
and a plug key K in series. Adjust the battery to minimum voltage.
2. Set multimete r in , ·11 rr1' 11I measuring rn od l' at a suitable range of mA (starting from high
current range). Take out a suitable resistance R from the resistance box so that the current
fl ows within the range chosen. Insert the plug in the key K and note the value of current
flowing in the circuit.
3. Increase the forward bias in steps of 0.2 V. Note the current in each case. Beyond a certain
applied voltage, current increases rapidly with the increase in forward bias.
4. J1, ·v<·1· ;,• the terminals of the junction diode so that its p-end is at lower potential and ,1-end is
at hi.gher p otential. Observe the current in multimete r on µA scale. Negligible current
reading will indicate the unidirectional feature of the diode. Increase in reverse bias will
show negligible change in reverse current.

183
b
 I Al\ M;\ NLIAI PttYSICS- XII

~- Rl'I''•" "' llw di(ldl' "" I.Iii ) lll\d rq wo t ~ll'l'/l I 1_11 4. It will be s~en that LED allows the flo
l''\ H IY I\I ""' " "''"'" ii Is 11,rward bln/lt'1I. I .ED 11tc1 rts glow ing
when the applied \V of
,' \ l'\'('d~ 1.i \ '. lnlti,,llv. II glnwil fo lntly. Its glow becomes brighter and brighter as the :~~:age
is µr,hhmll r inl.'1,-.,~1•,l. age
(MT,, ,·ht'l'" whether the junction dlodr 111 In working order or not
6, ~ -I the mult lnwll'I' in ,,.,,,.,l.mll' nll'nsurin g 11H Hl1•.
; , fou\'h llw l\n, p,, ,lws ,,1 tlw multimeter to the two end terminals of the junction diode
tlw di,ldt' 1\.':-i:-l,111l'l'. Rl'\ll'rs,· the diodr w nncctions . Aga in note the diode resistance • Note
di\ld,, l'\"•i:-1,\lll\.' is 111" ill Lllll' ,-.,:-l' ,md h, ,·,h in the other case or vice versa then the d·. 1f the
· ' Iode i
in Wl1rkin~ lll'd,• r. s
8. Ii tlw di,,,k l"l' Si:-t,lll\'l' is llm1 both during fo rwa rd and reverse biasings, then the diod .
,lh11t 1. ll dll l \' (l. lf thl' diodl' resistance is very hi gh both during fo rward and rev e 15
bi,,sings, thl'n the diode junction is d1 -..u1ntin m1u-.. or opl'n. In both of these situationserse
dhxfo is not in working order. Record the observa tions in a tabular form. lhe
0BSER\ ATIO NS
A. For unidirectional flow of current through the junction diode

Table B2 : When the junction diode is connected in forward bias

Forward bias voltage Forward current

S.No.
(V) (mA}
1.
2.
3.
4.
5.

Table B3 : When the junction diode is connected in reverse bias

Reverse bias voltage Reverse current
S.No.
(V) (~1A)
1.
2.
3.
4.
5.

B. For unidirectional flow of current through the LED

Tdble B4 : When the LED is connected in forward bias

Forward bias voltage Forward cunent

S.No.
(V) (mA)
1.
2.
3.
4.
5.

]84
 Section B : ACTIVITIES

>f Table BS : When th e LED is connected in reverse bias

e
~
S.No. Reverse bias voltage Reverse current
V A
1.
2.
3.
4.
5.

C. For working condition of the junction diode

Table B6 : Measurement of the diode resistance with multimeter

S.No. Biasing condition of the diode Resistance Nature of resistance

n
1. Forward biasing LowNery high
2. Reverse biasin hi h

~ SULT

1. The unidirectional feature of a junction diode and LED has been established, both conduct
only when forward biased and not when reverse biased.
2. The given junction diode is in working order as it offers low resistance in forward biasing
and high resistance in reverse biasing/Ihe given junction diode is not in working order as it
offers low (or high) resistance both during forward and reverse biasing.

i.,__
PRECA U T IO NS
1. For measuring resistance of any component, its leads should be cleaned properly with a
sand paper.
2. For measuring current and resistance, app ropriate selection of function switch and range

I
switch should be .made.
3. The polarity probe leads should be connected to the proper polarities for measurements in de
circuits.
4. While measuring resistance of any component, avoid direct touching of the metallic ends of
the multimeter leads. Body resistance in parallel with the component resistance will affect
the resistance measurement.
5. Each time when we select a resistance scale of different range, we should set the pointer to
zero using 'zero Adj. knob'.

A junction diode/LE0 c;<;>riducts suq-ent ~ils~y, when

forward , biase
r 1
d ~d itioes
1
not
,
conduct
1 '
current in
I I ·11
revf:rse ~i~ihg. ~s p, ,t~e 1rf
<;lifectioryal fea,t,ure of
current flow of a junction diode/LED. '
' ,,, I I I

2. H ow do you check whether a diode is in work4ig

order or not ? · :· 1·
1
, ,

• ' I '
fl ,tci 1kn\ Ii
1
If 1·unction
, , low
diode offers , ,,, , r;;-,1-11 (few
, ,resistance/ 1
11 , I , 11' I/
1 1 1 , {

1
, ••
'i 1

in forward biasing ' and a very ,lµgh resi,s~anpe I 1ir}I' I ' '
' ,I 'I I I

185
 LAB MANUAL PHYSICS-XII

AC TIV ITY

AIM

To study the effect of intensity ,of light (by varying distance of the source) •On
anLDR.

AP r i\RATUS AND MATERIAL REQUIRED

LORI 3 W LED bulb with holder an iron stand for the bulb, a battery
eliminator (12 V) a pl
• ug
I •
key, milliammeter (0-500 mA), voltmeter (0-10 V), a resistance of I

47 n, a half metre SCale

connecting wires and a piece of sand paper.
'
THEOR Y
A light dependent resistor or a photoresistor is a light sensitive device whose resistance
decreases with the
increase in intensity of incident light. It is made from a semiconductor material cadmiu
m sulphide,
CdS [other materials being CdSe, PbS, PbSe, InSb].
The basic structure of an LOR and its symbols are shown in Fig. 3. A snake
like or zigzag pattern
of the light sensitive material is deposited on an insulating substrate
such as ceramic. Such a
pattern provides desired resistance and power rating. Also this zigzag pattern
separates a thin metal
film into two areas for which two low resistance metal contacts are made.
A thin transparent
coating on the top surface provides a window for the incident light.

Transparent coating over entire surface

Electro~ e 1 ,,.--.__ • Electrode 2

(Thin metal film) ::;.,--,1 ,_
::;...,-,"."~ ,,~
,. _~'
c:.;,..~--:'-::'°'~; ~ i,i \ --- Photoconductive
Metal ~ "" , 1~ material over
contact ;_, , .. "']( top surface

~::::.:~-\ .., I . .:::-1p ~ ~ ·

t t .-Wire \! ~
~- (,) f • i,,minal (b)
0

I I
11 Fig . 3 (a) Basic structure (b) Symbol of an LDR.
I
I·
i i; When light of certain minimum frequency falls on the LOR, the absorbe
d photons give bound
I" ii electrons enough energy to jump to the conduction band where
I I Ir they are free to conduct
electricity. This increases the conductance or lowers the resistance of
I \1 the device. A typical L~R
has a high resistance of several MO. in total darkness ·and just
·.I'
I;'
.; 11l :1
a few hundre d ohJI\5 lil
bright light.
11
·\ \1l1
I '!1

186
jiP

Section B : ACTIVITIES

LE RCU IT OIAGRAtvl
Tone
mains

Iron
stand

o---- - - - - 12 V ----- □

Fig. 4 Circuit to study the effect of intensity of light on an LDR.

~ ROCEDUR E

1. Draw the circuit diagram as shown in Fig. 4 and assemble the apparatus on the working
table accordingly.
2. Make neat and tight connections by connecting a 12 V battery eliminator in series with
the
LDR, milliammeter mA, plug key Kand a 47 n resistor. Connect the voltmeter Vin parallel
with the LDR.
3. Keeping the LED lamp switched off, insert the plug in the key K
4. Note the readings of voltmeter and milliammeter. Then calculate the value of LDR resistance
R' .
5. In order to take into account the background illumination, the reference resistance R'
is
added to all further measurements of resistance R of LDR.
6. Clamp a 3 W LED lamp on a rigid -iron stand facing LDR. Adjust the lamp normally above
the LDR at a di~tance of 10 cm with the help of half metre scale.
7. Switch on the LED lamp and note the voltmeter and milliammeter readings.
8. Repeat the actiyity by adjusting the LED lamp at distances of 15 cm, 20 cm, 25 cm, and 30 cm
I
from the LDR. Record the reading of voltmeter and milliammeter in each case, calculate the
resistance of LDR at different distances of the lamp.
L <2_BSERVATI ONS AND CALCULAT ION S

Range of voltmeter :::: 0 to _ _ V

Least count of voltmeter = V
Range of milliamrneter = 0 to _ _ rnA
Least count of milliammeter = mA

Observations for background illumination

Reading of voltmeter =_ _ V
Reading of milliammeter ~ - - mA = _ _ A

LDR resistance for background illumination, R' . -n

=VI =-

187
 LAB MANUAL PHYSICS- XII

'l\,ble D7 : Variation in resistance of LOR with

distance
Observed resistance
Distance of LOR Voltmeter Milliammeter V
S.No. of LOR R0 = - Actual resistance of
from the lamp reading V reading I
(cm) (V) (mA) I LORR = Ro + R'

---
! (n ) (O)
I l. lO
2. 15
3. 20
4. 25
5. 30

~ SUL T
-
1. As distance of LDR from the sour ce
increases, inte nsit y of ligh t decr ease s and
LDR increases. resistance of

~ REC AUT IONS

1. All the conn ectio ns shou ld be neat

and tight.
2. LOR mus t be plac ed norm ally to the
ligh t sour ce so that angl e of inci denc e
rem ains sam e thro ugh out the expe rime of light rays
nt.
3. A suita ble prot ecti ve resistance mus
t be conn ecte d in series with LDR to prev
dam age. ent it from
4. Resistance of LOR for back grou nd
illum inat ion mus t be take n into account.

I:,' I,
'\ tl ,,: I
1. Wha t is an LuR I . ,I·, 11' I\' 'I , '
i? , ,, ·, 1 , .
1·11
A li nt' d ,, ,,il l:i. 11\ 1 1I band s of the phot osen sitiv e mate rial. Thes
1,,.111t I·) a, . , h t ii I''
l'
,, ,1 .. ·'.II,
I
I e electrons
', . 9 i~ff1 \~ ~~~~1111~·
il.
? if1
resis tanc e decre<\se,s1w1t'n tue' mcre l'i~\ 1_ev_i~e . ,w_ osef ' 1
~ 1;
beco me free to cond uct electricity. This
increases the
'ase m mtens1ty o 1
cond ucta nce or decr ease s the resistance
ligh t incid ent up~~ 11~.
·· I 11!1,1 .
• • • 1
)1 !
I 1!

: I 1 111 :•i , ,
I '

5. ' How docs the inten sity of light vary with

, I 11 1
the distance
2. Na me som e mate 6alsi'u s1Jqlf9r 1mak ing\l:IDRs. ' '· , "
front the sour ce ?
II q11'l 1, I \\ i' ,1, ' I '
Sem icon duct or mate rials 'like CdS I
)IIPoS, CdSe, RbSe
• ln~ensity of light is inve rsely proportio
I ' ,' nal to the
and InSb . 11 ' , · ' ' ' ' ' ' 1
' ' I ' '

squa re of ,t he dista nce from the source.

,, '\'

1 ;,, '

3. H ow does the resis tan ce of ,a n LDR I .\

'ci;i.an ge with ,the
'
' I
I ',Ii' . '
\1! I
'
11 '
6: H ow m;e LDRs differe nt from photodio
des aotl
li g ht inten sity ?
1
' phototran$isto rs ?
,

At low ligh t inten sitie s, an LOR has,~ high Ev'e n th'o ugh LDRs are mad e from semic
resistan,c~·:, onductor
As the inten sity of incid ent l~g~t
1
, ma~er[als, yet they ·are · diffe rent from
\incr~c1s~s, t~e 1 photodi~ees
resis tanc e of an l,DR decreasef,.
1
and ;I;'hot otran sisto rs beca use t1'ey are &imP . l ass1v
11 ' 111 devi1c~~ ,J ~ci 1 YP,
~
tio ,noth ave p- njun ction s. , ' ' •'·•I
,4. ,Why does th~ resis tan,ce o f,i] n ~,QR ' . . ii
c\q ::rC<) f,O w ill, the ! l'
7. Narp,e som ,e anpl jcati ons of LDE s .. I

I
I

,, 4itre ase jn i1;it,e ~sHy of jnciq e11t lig~t . , , ,1 ,. 1:i!\li I

I? , .
1

1 ,: · ·''' tfeet
, 1

' ' Al's th~ ~ten sity' 0£1~ncid en t ligh~ incre 11 LO. Rs
I ' ' are u.. :se'd as.lig,,1111'1
,:,,,q,, ht sens ors indi'al~'1m\ ' , \<;1~\,k
5
' '·'. ' s ases, more '¥'d ' ' : 1'' 1 11 l1 l ,.,.\J.,1~~•1J,.,,
..... ,•.
lights, light , inten sity rhetei;sII;
'~re knoc ke d 1off £'rom tne' ·I yai~
1

\ i elec tron
wri . ,, , .
nce
I
. ·t . · ano' ll:l,qrg .¥i\ ! ii·:'\li.'
1

i • , . ,1, CtrCU l s. I 'I I ': i,' I II ' ,I, I, 1' 11'1 ;II.
'i I', ':; I '1 I
! •. •• ,,. '11• ,,, '\, I',, 'ill . I ' ' 1•\111·1
•11'•• 111 1,h,,I.
'1' I• II
! l , 11.1•
!''·.I\ \ \1, 'I\\ II' "I ,1'
I I

<'1 ! il ' ,ii 1 1 1 1 ' •

ii •
''''\'\'''

I, I
'

1 •\' Ii.
' ! , ' : l ,,;
' ', \ ' II '1 (ti l

18 8