Homework question #1

A 500 Ohm resistive load, that is consuming 30W of power, is connected in series with a protection resistor across a 220V DC source. Determine the resistive value and teh power dissipated in the protection resistor.

 

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hgmjr

 

I used I = sq root of (power/resistance) or sqr(30/500) = 250mA
R=220V/250mA = 880 Ohm.
P is 220V squared/880 = 55Watts

 

cgeorge,

Kirchoff's Voltage Law applies.

 

First draw it up in schematic form

W = I^2 x R
W = E^2 / R
W = E x I


Then work with the equationS (plural) that has both variables in your schematic.

Then you should see clearly what the voltage should be across the unknown resistor.

 

Hints:

Calculate the voltage across the 500R resistor

The resistors are in series so they have the same current