We are designing an electric space heater to operate from 120VDC. Two heating el

Question

We are designing an electric space heater to operate from 120VDC. Two heating elements with resistance R1 and R2 are to beused that can be operated in parallel, seperately, or inseries. The highest power is to be 1280W, and the lowestpower is to be 240W. Draw a cicuit diagram. A) What are the values needed for R1 and R2? B) What intermediate power settings are available? c) What are the limits on the current in the circuit? We are designing an electric space heater to operate from 120VDC. Two heating elements with resistance R1 and R2 are to beused that can be operated in parallel, seperately, or inseries. The highest power is to be 1280W, and the lowestpower is to be 240W. Draw a cicuit diagram. A) What are the values needed for R1 and R2? B) What intermediate power settings are available? c) What are the limits on the current in the circuit?

Explanation / Answer

Quite clearly for given net resistance R and voltage V and for a DCsupply power = V2/R hence for maximum power of 1280 watt R = 120*120 /1280 = 90/8 =45/4 similarly the minimum power of 240 W implies R = 120*120 / 240 =60 hence assuming R1 + R2 = 60 and 45/4 = R1R2/R1+R2 or R1(60-R1)/60=45/4 or 60R1 - R12 = 675 or -60R1 +R12 +675 or R1 = 15 or 45 hence a) the value of R1 and R2 are 15 and 45 b) we can also have only R1 and R2 thus 120*120/45 and 120*120/15are also possible which is 320 W and 960 W are possible c) the maximum current will be for 120/min R = 120/45/4 = 480/45 =32/3 A

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