>
> Hi my electronic fellows:)
>
> I have a very simple question about some basic electrical
calculations.
>
> I need to know how to calculate the amount of watt and the amperes
> drawn if i set up a circuit with two light bulbs with the following
> charateristics in series (I will like to get the calculations for
the
> bulbs individually and for the whole circuit).
>
> Light bulb X: 12V 5 Watts
> Light bulb Z: 12V 10 Watts
>
> The power supply is set to 12 volts.
>
> As i get it. Light bulb X will draw 0,42A and bulb Z will draw 0,83A
> but I'm in doubt wether or not if i should add the two numbers(amps)
> together getting a total of 1,25A and use this number in the
following
> calculations, like this.
>
> P = V * I
>
> 12 * (0,42 + 0,83)
>
> Resulting in a total amount of 15 watts. Is that correct?
>
> Finnaly, how do I calculate the need of a resistor?
>
> Thanks in advance...
>
Well, we really don't do homework.
However, you said the lamps would be in series. You are ok for the
situation where the lamps are in parallel. In series you need to
calculate the R for each lamp P = E^2 / R or R = E^2 / P so 14.4 ohms
for the 10W lamp and 28.8 ohms for the 5W lamp.
Then add the resistances to get 43.2 ohms and the new P = E^2 / R or
3.33 W. FWIW, the current is I = E / R = 0.28A
Now, all of this assumes that the resistance of the lamp is the same
at all currents and this simply isn't true for incadescent lamps.
But, this is homework, not reality.
Why do you need a resistor?
Richard
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