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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.

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  • \$\begingroup\$ When you turn on the tap / faucet does the maximum current that the water utility can provide flow into your sink / basin? No the resistance of the pipework and the variable resistance of the tap control the flow. \$\endgroup\$ – Transistor Sep 10 '16 at 16:40
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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.

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  • \$\begingroup\$ Ah ok, so in real-world applications, resistors would be used to limit amps, so that the device (load) doesn't get more than it needs, and burn up? \$\endgroup\$ – xil3 Sep 10 '16 at 16:31
  • \$\begingroup\$ Exactly. But, voltage ratings of devices can't be exceeded either (determined by the insulation and di-electric of the load), so a 6V motor can't be run from a 12V battery, even with the right current limiting because you could ruin the insulation of the windings. \$\endgroup\$ – nu77p01nt3r Sep 10 '16 at 16:34
  • \$\begingroup\$ Well, resistors are a very inefficient method of limiting current. They are used with LEDs because it's simple and LEDs do not require much current. And LEDs are somewhat special as they have an exponential I-V curve. In general, no extra components are used to limit current, most devices are designed such that they only draw the current they need and no more. Current draw can vary quite significantly with device operation. For example, a DC motor will draw more current when under load. When the load is removed, the motor will generate back-emf internally and its current draw will decrease. \$\endgroup\$ – alex.forencich Sep 10 '16 at 17:19
  • \$\begingroup\$ @nu77p01nt3r a motor's windings are damaged by overheating. If you limit the current, it won't be possible for the windings to overheat. However, you may run in to other issues, such as the motor running at a higher RPM and wearing out the bearings faster than normal. \$\endgroup\$ – alex.forencich Sep 10 '16 at 17:23
  • \$\begingroup\$ "In general, no extra components are used to limit current, most devices are designed such that they only draw the current they need and no more." - I'm confused again. If there is no resistor, then isn't the current limitless - how do the devices only use what they need in that case? \$\endgroup\$ – xil3 Sep 10 '16 at 18:49
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The battery acts like a voltage source and so will try to provide a constant voltage (well, more or less) across the connected load. The load will then draw whatever current it requires. Open circuit? Zero current. LED with current limiting resistor? I = (Vbat - Vled) / R.

One thing to note is that a source can only drive a specific voltage OR a specific current. The load will determine the other. However, usually power supplies are rated something like 5V, 3A. This usually means that it will supply a constant 5V at up to 3A. The load could draw 0.1A, 1A, 2.5A, or even 0A from the supply.

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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 want to give a bit different view of this than the other answers have done. The other answers only apply to certain common, but specific cases. I'll try to give you a quick rundown on how to find the

Every device has its DC (steady-state) current-voltage (I-V) characteristic. If you change the voltage applied to it, the current will change, or vice versa. Even voltage and current sources in the real world are not perfect --- if you change the voltage across them the current output will change.

For example, a resistor's I-V characteristic is

$$I = V/R$$

A diode's I-V characteristic is something like

$$I = I_s \exp\left(\frac{qV}{nk_B T}-1\right)$$

where \$q\$ and \$k_B\$ are known physical constants, \$T\$ is the device temperature, and \$I_s\$ and \$n\$ are characteristics of the particular type of diode.

A (slightly idealized) real voltage source has an I-V characteristic like

$$I = \frac{V_0-V}{r}$$

where \$V_0\$ is the open-circuit voltage and \$r\$ is the internal resistance of the source.

Generally you can find some relationship like this for any type of circuit element, including transistors, vacuum tubes, or whatever.

In addition to the equations describing each device in your circuit, you also need to make some equations describing how the circuit elements are connected to each other. You do this using Kirchoff's Voltage and Current Laws.

Once you have a complete system of equations, you can solve them to find the current in your circuit (or the current in each branch of a complex circuit). In more complex cases, there may not be an analytical solution to the equation system, and you'll need to use some numerical method to find an approximate solution.

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