# resister rating which is connected to an AC power

Discussion in 'General Electronics Chat' started by electronicsunzz, Dec 16, 2009.

1. ### electronicsunzz Thread Starter New Member

Dec 2, 2009
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hi friends this is my question
Suppose if i want to connect a 12 watt bulb in series with a resister to
home power supply(220v AC,50hz) then what value of resister should
I take(in terms of ohms and power rate) to make my bulb glow at high intensity of light.

2. ### KMoffett AAC Fanatic!

Dec 19, 2007
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What's the "voltage" rating on the 12 watt bulb?

Ken

3. ### electronicsunzz Thread Starter New Member

Dec 2, 2009
8
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its nothing written on it expect power (12watts) anyways i just connected to my variable dc battery to measure the voltage, its sustaining up to 20 volts emitting maximum light. u so i think can take it as 20v.

4. ### KMoffett AAC Fanatic!

Dec 19, 2007
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12W/20v=0.6A
(220V-20V)/0.6A=3333Ω
200V*0.6A=120W

So you would need a ~3500Ω, 150 watt resistor. Dissipating 120 watts in heat to light a 20 watt lamp.

Why not just buy a 20W/220V lamp?

Ken

5. ### electronicsunzz Thread Starter New Member

Dec 2, 2009
8
0
how would go with this to know the total current in the circuit [It(total current)=Lp(lamp power)/voltage]
what i say is, in a series connection power quantity is added then how could u take power only at lamp instead total power
i mean [It(total current)=Pt(lamp power+resister power )/voltage]
I remember In my circuit unknown quantities are TOTAL CURRENT,TOTAL RESISTANCE so obviously unknown total power.

6. ### KMoffett AAC Fanatic!

Dec 19, 2007
2,760
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A 12W lamp at 20 V requires 0.6A of current through it.... I=P/V Since you resistor will be in series with the lamp, the current through the resistor and lamp will be the same...0.6A. That's the total current in the circuit. Total power is the lamp power plus the resistor power....20W+120W=140W

Ken

7. ### SgtWookie Expert

Jul 17, 2007
22,201
1,809
You really should use a step-down transformer instead. A transformer will be far more efficient than a current limiting resistor. A current limiting resistor alone will not provide isolation from mains power.

8. ### KMoffett AAC Fanatic!

Dec 19, 2007
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Agreed! It was looking like a homework assignment. I forgot that the OP had ...looked at the bulb!

Ken

9. ### electronicsunzz Thread Starter New Member

Dec 2, 2009
8
0
anyways i have also read a message from sgtwookie saying that "using a transformer in place of resistor is very much much efficient" may i know the advantages of using transformer in place of a rated resistor. what makes the difference??

Dec 19, 2007
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