Electrical Engineering - Energy and Power - Discussion

Hi friends:.

Here R=120 ohm.

I (max) =25 mA.

{I (max) means current will not go beyond 25mA}.

So power P=I*I*R= 25m*25m*120 = 75mW.

It means that power will not increase beyond 75mW.

And the word rating means that whatever device you are using will breakdown or may get burn. So in the question 75mW is necessory to keep resistor active (must not get burn). So 15mW is wrong. So 75mW rating resistor required. BUT BUT in market u'll not get exact same rating resistor. So you must go beyond the 75mW. And nearly rated resistor is 150 mW. So the correct option is (B) =150mW.

Because no one wants to go for higher rating, because cost increases, so option A and D are also wrong.

Hope you understood. Bye.

@All.

The correct answer is the one where we have to measure Irms (= Ief = effective current) first. Hence, 150 mW.

Other who points out that the answer is 150 mW but NOT calculating Irms first - e.g. just jumped into the P=I^2*R - their answer just so happens to be the right answer (150 mW). You should calculate Irms first.

i^2*r = 75mW....... 4.8W resistor carries max current of 200mA {i = root[4.8/120] = 200mA}. 150mW resistor carries max current of 35mA... 15mW resistor carries max current of 11.2mA... 480mW resistor carries max current of 63.2mA.....

Among those options 150mW resistor has nearer value 35mA max current. so, B is ryt ans.

How could 150mW be correct answer. It is clear from the question that for a power rating beyond 75mW the resistor's characteristics will be altered.

So the maximum rating is 75mW and among options only 15mW is less than this value. So the answer should be 15mW. Anyway a 20W rated doesn't burn out at this value.

P=I^2*R is correct when current passing through it is given but they have given maximum current we known P=VI we known I 25ma. So the answer should be in multiples of 25.

So from the given options 150 is multiple of 25 so option b is correct I feel so the rating should contain maximum current and max voltage.

The resistor is power dissipating element.

Then current take in RMS value,
The problem is given in maximum value
so we convert the current maximum value into RMS value,
current rms=max current/sqr(2).
p=(I^2)R.
after calculation
p=2*75mw=150mw.

The power rating of resistor depends on RMS value.

150mA is the correct answer.

Given current is the rms value, and the rating must be decided on maximum current.
Imax = Irms * sqrt2 = 25mA * sqrt2 = 0.0353,
Hence rating =(Imax)^2 *120 = 150mW.

The answer should be 75 mw because which is not in option. Some of guys have said that it will be 150 mw. How if you really did then I think you must solved after seeing an option?

@All.

The concept of max and rms fundamental is not right bz max current already given
then why you are calculate Imax = Irms * sqrt2 = 25mA * sqrt2 = 0.0353?

Please explain me.

Option B is correct.
Explanation:

Rating current value = Irms value.
Maximum current value = Imax value.
Imax = Irms/sqrt(2).

P = Irms^2*R.
= (sqrt(2)*Imax)^2*R.
= 150mA.