resistors, voltage, and you

Doc

I used this calculator to figure out that with a 5v source, 3.3v forward voltage, 20mA desired, and 4 LEDs in parallel (the parallel calc is near the bottom of the page), I should be using a 22ohm resistor. So I went and got one (well, a bunch) and installed it. I used a multimeter to measure the voltage of the circuit after the resistor and it still reads 4.9v, just like it did before. The LEDs are not yet in place - it's just two wires, one with a resistor attached. If I'm reading Ohm's law correctly, I think the voltage should drop to what it's supposed to be when I start drawing amps, is that correct?

shadydentist

What... what exactly are you measuring? You can't really measure a voltage until its properly connected.

Doc

I see, I was pretty sure I was failing hard at measuring. I'll trust in my design and hook it all up.

Terrendos

Yeah, the voltage drop occurs across a circuit. If you only have the resistor in the circuit, the voltage drop across it will be almost equal to the source voltage. If you hook up 1 LED in series with the resistor, you should get the desired voltage drop across it, since the votage drop across objects in parallel is the same.

Doc

Bingo, thanks!