aman said:
I have a 15V voltage regulator V which supplies current to a device D.
This device supplies current at 5V.
Is it correct to say that for 300 mA supplied by device D at 5V, the
device D draws 100mA at 15V.
Your question confuses me, so I'm assuming you meant:
I have a 15V power supply which supplies a 5V linear regulator U1.
Linear regulators are series elements (for the most part) and thus pass
exactly the same current as the circuit as they draw from the supply
(there is some leakage for control, but they try to minimize this.)
Thus, for a linear regulator, the current in (300mA) is equal to the
current out (300mA), and the regulator itself dissipates I*V = 300mA *
10V = 3W.
On the other hand, a switch mode power supply will attempt to use other
means to minimize the power consumed by the regulator itself. For a
perfectly efficient regulator (which does not exist) the loss in the
regulator would be 0W. Thus, the total power drawn from the supply would
be 300mA * 5V = 1.5W, and you would thus draw 1.5/15 = 100mA.
However, there is always some inefficiency in the regulator, expressed
as a percentage. Thus, a 90% efficient regulator would dissipate 10% of
the total energy drawn from the supply, and deliver 90% to the load.
Consequently, the power consumed would be
Pload = 0.300 * 5
Preg = Pload / 9 = 167mW
Ptotal = 1.666W
So the average current from the supply would be
I = 1.666/15 = 111mA
Does this make sense by power conservation ? (100ma * 15 = 300ma * 5)
Not unless you neglect power used by the regulator.
--
Regards,
Robert Monsen
"Your Highness, I have no need of this hypothesis."
- Pierre Laplace (1749-1827), to Napoleon,
on why his works on celestial mechanics make no mention of God.