Resistor - Colour Codes

Resistors

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Res is tor s T he r es is tor 's function is to r educe the flow of electr ic cur r ent. T his s ymbol is us ed to indicate a r es is tor in a cir cuit diagr am, known as a s chematic. Res is tance value is des ignated in units called the " Ohm." A 1000 Ohm r es is tor is typically s hown as 1K - Ohm ( k ilo Ohm ), and 1000 K - Ohms is wr itten as 1M- Ohm ( megohm ). T her e ar e two clas s es of r es is tor s ; fix ed r es is tor s and the var iable r es is tor s . T hey ar e als o clas s ified accor ding to the mater ial fr om which they ar e made. T he typical r es is tor is made of either car bon film or metal film. T her e ar e other types as well, but thes e ar e the mos t common. T he r es is tance value of the r es is tor is not the only thing to cons ider when s electing a r es is tor for us e in a cir cuit. T he " toler ance" and the electr ic power r atings of the r es is tor ar e als o impor tant. T he toler ance of a r es is tor denotes how clos e it is to the actual r ated r es is tence value. For ex ample, a ± 5% toler ance would indicate a r es is tor that is within ± 5% of the s pecified r es is tance value. T he power r ating indicates how much power the r es is tor can s afely toler ate. Jus t like you wouldn't us e a 6 volt flas hlight lamp to r eplace a bur ned out light in your hous e, you wouldn't us e a 1/8 watt r es is tor when you s hould be us ing a 1/2 watt r es is tor . T he max imum r ated power of the r es is tor is s pecified in Watts . Power is calculated us ing the s quar e of the cur r ent ( I 2 ) x the r es is tance value ( R ) of the r es is tor . I f the max imum r ating of the r es is tor is ex ceeded, it will become ex tr emely hot, and even bur n. Res is tor s in electr onic cir cuits ar e typicaly r ated 1/8W, 1/4W, and 1/2W. 1/8W is almos t always us ed in s ignal cir cuit applications . When power ing a light emitting diode, a compar atively lar ge cur r ent flows thr ough the r es is tor , s o you need to cons ider the power r ating of the r es is tor you choos e.

Rating electr ic power For ex ample, to power a 5V cir cuit us ing a 12V s upply, a thr ee- ter minal voltage r egulator is us ually us ed. However , if you tr y to dr op the voltage fr om 12V to 5V us ing only a r es is tor , then you need to calculate the power r ating of the r es is tor as well as the r es is tance value. At this time, the cur r ent cons umed by the 5V cir cuit needs to be known. Her e ar e a few ways to find out how much cur r ent the cir cuit demands . As s emble the cir cuit and meas ur e the actual cur r ent us ed with a multi- meter . Check the component's cur r ent us e agains t a s tandar d table. As s ume the cur r ent cons umed is 100 mA (milliamps ) in the following ex ample. 7V mus t be dr opped with the r es is tor . T he r es is tance value of the r es is tor becomes 7V / 0.1A = 70(ohm). T he cons umption of electr ic power for this r es is tor becomes 0.1A x 0.1A x 70 ohm = 0.7W. Gener ally, it's s afe to choos e a r es is tor which has a power r ating of about twice the power cons umption needed.

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Resistors

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Res is tance value As for the s tandar d r es is tance value, the values us ed can be divided like a logar ithm. ( S ee the logar ithm table ) For ex ample, in the cas e of E3, T he values [ 1] , [ 2.2] , [ 4.7] and [ 10] ar e us ed. T hey divide 10 into thr ee, lik e a logar ithm. I n the cas e of E 6 : [ 1] , [ 1.5] , [ 2.2], [ 3.3] , [ 4.7] , [ 6.8] , [ 10] . I n the cas e of E 12 : [ 1] , [ 1.2] , [ 1.5], [ 1.8] , [ 2.2] , [ 2.7] , [ 3.3] , [3.9] , [ 4.7] , [ 5.6] , [6.8] , [ 8.2] , [ 10] . I t is becaus e of this that the r es is tance value is s een at a glance to be a dis cr ete value. T he r es is tance value is dis played us ing the color code( the color ed bar s /the color ed s tr ipes ), becaus e the aver age r es is tor is too s mall to have the value pr inted on it with number s . You had better lear n the color code, becaus e almos t all r es is tor s of 1/2W or les s us e the color code to dis play the r es is tance value.

Fix ed Res is tor s A fix ed r es is tor is one in which the value of its r es is tance cannot change.

Car bon f i lm r es i s t or s T his is the mos t gener al pur pos e, cheap r es is tor . Us ually the toler ance of the r es is tance value is ± 5% . Power r atings of 1/8W, 1/4W and 1/2W ar e fr equently us ed. Car bon film r es is tor s have a dis advantage; they tend to be electr ically nois y. Metal film r es is tor s ar e r ecommended for us e in analog cir cuits . However , I have never ex per ienced any pr oblems with this nois e. T he phys ical s iz e of the differ ent r es is tor s ar e as follows .

R ou gh s iz e R at i n g pow er T h ickn es s L en gt h (W ) ( mm) ( mm) Fr om the top of the photogr aph 1/8W 1/4W 1/2W

1/8

2

3

1/4

2

6

1/2

3

9

T his r es is tor is called a S ingle- I n- Line(S I L ) r es is tor networ k . I t is made with many r es is tor s

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of the s ame value, all in one package. One s ide of each r es is tor is connected with one s ide of all the other r es is tor s ins ide. One ex ample of its us e would be to contr ol the cur r ent in a cir cuit power ing many light emitting diodes (LE Ds ). I n the photogr aph on the left, 8 r es is tor s ar e hous ed in the package. E ach of the leads on the pack age is one r es is tor . T he ninth lead on the left s ide is the common lead. T he face value of the r es is tance is pr inted. ( I t depends on the s upplier . ) S ome r es is tor networ ks have a " 4S " pr inted on the top of the r es is tor networ k . T he 4S indicates that the package contains 4 independent r es is tor s that ar e not wir ed together ins ide. T he hous ing has eight leads ins tead of nine. T he inter nal wir ing of thes e typical r es is tor networ ks has been illus tr ated below. T he s iz e (black par t) of the r es is tor networ k which I have is as follows : For the type with 9 leads , the thick nes s is 1.8 mm, the height 5mm, and the width 23 mm. For the types with 8 component leads , the thick nes s is 1.8 mm, the height 5 mm, and the width 20 mm.

Met al f i lm r es i s t or s Metal film r es is tor s ar e us ed when a higher toler ance (mor e accur ate value) is needed. T hey ar e much mor e accur ate in value than car bon film r es is tor s . T hey have about ± 0.05% toler ance. T hey have about ± 0.05% toler ance. I don't us e any high toler ance r es is tor s in my cir cuits . Res is tor s that ar e about ± 1% ar e mor e than s ufficient. Ni- Cr (Nichr ome) s eems to be us ed for the mater ial of r es is tor . T he metal film r es is tor is us ed for br idge cir cuits , filter cir cuits , and low- nois e analog s ignal cir cuits .

R ou gh s iz e R at i n g pow er T h ickn es s L en gt h (W ) ( mm) ( mm) Fr om the top of the photogr aph 1/8W (toler ance ± 1% ) 1/4W (toler ance ± 1% ) 1W (toler ance ± 5% ) 2W (toler ance ± 5% )

1/8

2

3

1/4

2

6

1

3.5

12

2

5

15

Var iable Res is tor s

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T her e ar e two gener al ways in which var iable r es is tor s ar e us ed. One is the var iable r es is tor which value is eas ily changed, like the volume adj us tment of Radio. T he other is s emi- fix ed r es is tor that is not meant to be adj us ted by anyone but a technician. I t is us ed to adj us t the oper ating condition of the cir cuit by the technician. S emi- fix ed r es is tor s ar e us ed to compens ate for the inaccur acies of the r es is tor s , and to fine- tune a cir cuit. T he r otation angle of the var iable r es is tor is us ually about 300 degr ees . S ome var iable r es is tor s mus t be tur ned many times to us e the whole r ange of r es is tance they offer . T his allows for ver y pr ecis e adj us tments of their value. T hes e ar e called " Potentiometer s " or " T r immer Potentiometer s ." I n the photogr aph to the left, the var iable r es is tor typically us ed for volume contr ols can be s een on the far r ight. I ts value is ver y eas y to adj us t. T he four r es is tor s at the center of the photogr aph ar e the s emifix ed type. T hes e ones ar e mounted on the pr inted cir cuit boar d. T he two r es is tor s on the left ar e the tr immer potentiometer s . T his s ymbol

is us ed to indicate a var iable r es is tor in a cir cuit diagr am.

T her e ar e thr ee ways in which a var iable r es is tor 's value can change accor ding to the r otation angle of its ax is . When type " A" r otates clockwis e, at fir s t, the r es is tance value changes s lowly and then in the s econd half of its ax is , it changes ver y quickly. T he " A" type var iable r es is tor is typically us ed for the volume contr ol of a r adio, for ex ample. I t is well s uited to adj us t a low s ound s ubtly. I t s uits the char acter is tics of the ear . T he ear hear s low s ound changes well, but is n't as s ens itive to s mall changes in loud s ounds . A lar ger change is needed as the volume is incr eas ed. T hes e " A" type var iable r es is tor s ar e s ometimes called " audio taper " potentiometer s . As for type " B " , the r otation of the ax is and the change of the r es is tance value ar e dir ectly r elated. T he r ate of change is the s ame, or linear , thr oughout the s weep of the ax is . T his type s uits a r es is tance value adj us tment in a cir cuit, a balance cir cuit and s o on. T hey ar e s ometimes called " linear taper " potentiometer s . T ype " C" changes ex actly the oppos ite way to type " A" . I n the ear ly s tages of the r otation of the ax is , the r es is tance value changes r apidly, and in the s econd half, the change occur s mor e s lowly. T his type is n't too much us ed. I t is a s pecial us e. As for the var iable r es is tor , mos t ar e type " A" or type " B " .

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CDS E lements S ome components can change r es is tance value by changes in the amount of light hitting them. One type is the Cadmium S ulfide Photocell. (Cd) T he mor e light that hits it, the s maller its r es is tance value becomes . T her e ar e many types of thes e devices . T hey var y accor ding to light s ens itivity, s iz e, r es is tance value etc. Pictur ed at the left is a typical CDS photocell. I ts diameter is 8 mm, 4 mm high, with a cylinder for m. When br ight light is hitting it, the value is about 200 ohms , and when in the dar k, the r es is tance value is about 2M ohms . T his device is us ing for the head lamp illumination confir mation device of the car , for ex ample.

Other Res is tor s T her e is another type of r es is tor other than the car bon- film type and the metal film r es is tor s . I t is the wir ewound r es is tor . A wir ewound r es is tor is made of metal r es is tance wir e, and becaus e of this , they can be manufactur ed to pr ecis e values . Als o, high- wattage r es is tor s can be made by us ing a thick wir e mater ial. Wir ewound r es is tor s cannot be us ed for highfr equency cir cuits . Coils ar e us ed in high fr equency cir cuits . S ince a wir ewound r es is tor is a wir e wr apped ar ound an ins ulator , it is als o a coil, in a manner of s peaking. Us ing one could change the behavior of the cir cuit. S till another type of r es is tor is the Cer amic r es is tor . T hes e ar e wir ewound r es is tor s in a cer amic cas e, s tr engthened with a s pecial cement. T hey have ver y high power r atings , fr om 1 or 2 watts to doz ens of watts . T hes e r es is tor s can become ex tr emely hot when us ed for high power applications , and this mus t be taken into account when des igning the cir cuit. T hes e devices can eas ily get hot enough to bur n you if you touch one. T he photogr aph on the left is of wir ewound r es is tor s . T he upper one is 10W and is the length of 45 mm, 13 mm thick nes s . T he lower one is 50W and is the length of 75 mm, 29 mm thicknes s . T he upper one is has metal fittings attached. T hes e devices ar e ins ulated with a cer amic coating.

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T he photogr aph on above is a cer amic (or cement) r es is tor of 5W and is the height of 9 mm, 9 mm depth, 22 mm width.

T her mis tor ( T her mally s ens itive r es is tor )

T he r es is tance value of the ther mis tor changes accor ding to temper atur e. T his par t is us ed as a temper atur e s ens or .

T her e ar e mainly thr ee types of ther mis tor . N T C( N egat i ve T em per at u r e Coef f ici en t T h er m is t or ) : With this type, the r es is tance value decr eas es continuous ly as the temper atur e r is es . P T C( P os it i ve T em per at u r e Coef f ici en t T h er m is t or ) : With this type, the r es is tance value incr eas es s uddenly when the temper atur e r is es above a s pecific point. CT R ( Cr it i cal T em per at u r e R es i s t er T h er m i s t or ) : With this type, the r es is tance value decr eas es s uddenly when the temper atur e r is es above a s pecific point. T he NT C type is us ed for the temper atur e contr ol. T he r elation between the temper atur e and the r es is tance value of the NT C type can be calculated us ing the following for mula.

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R : T he r es is tance value at the temper atur e T T : T he temper atur e [K ] R 0 : T he r es is tance value at the r efer ence temper atur e T 0 T 0 : T he r efer ence temper atur e [ K ] B : T he coefficient As the r efer ence temper atur e, typically, 25° C is us ed. T he unit with the temper atur e is the abs olute temper atur e(Value of which 0 was - 273° C) in K (K elvin). 25° C ar e the 298 k elvins .

Res is tor color code

Ex ample 1 (B r own= 1),(B lack = 0),(Or ange= 3) 10 x 10 3 = 10k ohm T oler ance(Gold) = ± 5%

Ex ample 2 (Yellow= 4),(Violet= 7),(B lack= 0), (Red= 2) 470 x 10 2 = 47k ohm T oler ance(B r own) = ± 1%

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Col or

Valu e

Mu lt i pl ier

T oler an ce (% )

B lack

0

0

-

B r ow n

1

1

±1

R ed

2

2

±2

Or an ge

3

3

± 0.05

Y el low

4

4

-

Gr een

5

5

± 0.5

B lu e

6

6

± 0.25

Vi olet

7

7

± 0.1

Gr ay

8

8

-

W h it e

9

9

-

Gol d

-

-1

±5

S il ver

-

-2

± 10

N on e

-

-

± 20

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