Thin-film resistors often have a maximum power rating, above which they undergo

Question

Thin-film resistors often have a maximum power rating, above which they undergo catastrophic self-disassembly (i.e. the blow up). What is the maximum allowable current through a 4.7 k Omega,1/4 W resistor? What power rating is needed for a 149 Omega resistor if 450 V are to be applied across it? For assembling the voltage divider shown below, two of each of the following resistors are available: 250 Omega, 500 Omega, and 1.00 k Omega. Which combination of resistors would yield the voltages shown in Figure 1 (i.e. what resistances should be chosen for R_1, R_2, and R_3)? What is the IR drop across R3? How much current would be drawn from the source (i.e. power supply)? How much power is dissipated by the circuit?

Explanation / Answer

1.a) Power = i2R

1/4 = i2 *4700

i2 * =1/(4*4700)

i=0.00729 A

=7.29 mA

b)power = v2/R

=450*450/149 = 1359 W

2a) R1= 250 ohm

R2 = 1 k-ohm + 250 ohm......in series

R3 = 500 ohm + 500 ohm....in series

b) IR drop = 20-2-10 = 8V

C) for registance R3, IR = 8V, R=500+500=1000 ohm

i = 8/1000 A = 8 mA

D) power = v2/R = 20*20 /(250+1000+250+500+500) =400/2500 = 0.16 W

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