I'm trying to understand how current (amps) flows through circuits, and if the max available amps is always flowing.
For example, take the following:
Say you have a battery, and a circuit that connects the positive and negative, and a switch & led as part of that circuit.
Does the max amps that the battery can provide always flow through that circuit? Or does it only provide what the led would draw? I'm confused as to how that works.
Best Answer
Ohm's law states that voltage=current x resistance, or re-arranging with simple algebra, current=voltage / resistance (resistance is actually impedance, but trying to keep things simple). Therefore, your current can be calculated if you know the voltage and the resistance in the circuit. So, if you have 1 volt battery (just for example purposes), and you hook it up to a 1 ohm resistor, you will get 1 amp through the circuit. If you hook the same resistance up to a 2 volt battery, you will get 2 amps flowing. If you hook the 1 volt battery to a 0.5 ohm resistor, you will get 2 amps flowing.
In your question, an LED is a voltage drop device (it's a diode). It has a forward voltage drop and a max current. If you hook it up to a 9V battery with no resistor in the circuit, the LED is essentially a zero ohm resistor and you have a current of 9/0 amps which approaches infinity [there is some resistance in the wires and the LED due to temperature etc, but it is so small that it is negligible]. You must have enough resistance in the circuit to limit the current to the LEDs max per the datasheet. Note, in a series circuit like this, the same current will flow through the resistor as the LED, so you have to watch the power rating of the resistor as well.
So, in short, the source will supply the current [really the power, but for our purposes, I will say the current] that the load needs based on its resistance. No resistance = a short circuit (hook the positive and negative of the battery together and as much current as the battery can put out will flow and things will get hot) and infinite resistance = no current or an open circuit.