is is possible to drop voltage without knowing a components amperage

Question

I am new to circuits/electrical stuff

Correct me if i am wrong.

if i have an led (2.1V / 0.02A) and if i give it 0.02A it will only take 2.1V or if i give it 2.1V it will take 0.02A

Ohms law (\$I=\frac{V}{R}\$).

If start with 9V i can use ohms law to get the correct power for the led because i know how many amps it requires

My question is:

If i started with 9V. Could i drop the voltage down for a component that takes 5V without knowing how many amps it uses?

Sub-question:

Wouldn't a circuit like this drain a battery or cause the circuit to short out?

Answer 1

No need for a voltage dropper with two resistors. Just assume (a fairly accurate assumption I might add) that, the LED has 2.1 volts across it then, put a resistor in series with it to the 9 volt rail. The value of the resistor sets the current: -

$$I = \dfrac{\text{9 volts - 2.1 volts}}{R} = \dfrac{6.9}{R} = \text{20 mA}$$

Rearrange to find R (it equals 345 ohms).

Wouldn't a circuit like this drain a battery or cause the circuit to short out?

It's extremely sub-optimal.

If i started with 9V. Could i drop the voltage down for a component that takes 5V without knowing how many amps it uses?

You could (and waste a lot of power) or, just use a voltage regulator - that is what they are for.