Understanding "Current Draw" in terms of "Ohm's Law"

Question

I'm having trouble understanding Current Draw as it relates to Ohm's Law (V = IR).

I have a power supply rated at 5V and 500mA. I'm trying to power a string of LED lights. According to my datasheets, each LED draws ~50mA of current. If I have a string of 5 LEDs, that would draw 250mA. If I have a string of 10 LEDs, that will draw 500mA of current.

Here's my problem:

According to my understanding of Ohm's law, Current = Voltage / Resistance. As I understand it, when I add additional LEDs to my circuit, each LED should contribute resistance to the circuit. If this is the case, according to Ohm's Law, I would expect that current would DECREASE with each additional LED. In other words, as Resistance increases and as Voltage stays the same... when I divide V by an increasing R, this equation results in a lower I.

Instead, it is INCREASING!

Am I misunderstanding something here?

EDIT:

Yes, I was misunderstanding something there. The LEDs on my LED strip are wired in parallel, not in series. Resistances wired in parallel lower the total resistance through the circuit, so current would INCREASE coming from my power supply.

Additionally, LEDs don't function exactly like a regular resistors. As Voltage increases, current increases non-linearly, so Ohm's Law cannot apply perfectly.

Answer 1

Well first off I would say that an LED is an active device, which means voltage and current are not linearly related like a passive device such as a resistor. The resistance of an LED is not static, it changes according to the device's IV (current-voltage) relationship. That is more of an aside, since Ohm's Law is not as straight forward as it may appear when you are dealing with active devices.

However the explanation to your problem is fairly straight forward. You are adding LEDs in parallel I assume, each LED has a fairly static resistance since current and voltage are stable. I am assuming you are stringing the LED's in parallel. When you add resistance in parallel it decreases the total network's resistance. Look up effective resistance of parallel resistors and you will see.

Answer 2

You have a number of misunderstandings.

Ohm's law applies to resistors. LEDs are non-linear elements and therefore Ohm's law doesn't apply.

Elements in series have the same current flowing through them. If you have 3 LEDs in series and one has 50mA flowing in it, so do the other two. Total current? 50mA. Your source has to have enough voltage to "overcome" the sum of the forward voltages of the LEDs plus some headroom for whatever is regulating or limiting the current.

Elements in parallel have the same voltage across them, and may have different currents flowing through them. The sum of the currents through each element is the total current drawn by the elements from the source. Three parallel LEDs with 50mA flowing in each would draw 150mA from the supply.

You have to control the current through LEDs by using a constant current drive or a resistor in series to set the appropriate operating point.

So the draw will depend on how you design your lighting system. Series, parallel, drive scheme and target current.

Answer 3

A string of LEDs is a bunch of LEDs in series and this means that they share the same current. If the forward volt drop across each LED is (say) 2 volts, then 5 LEDs requires a supply of at least 10 volts by the way.

Answer 4

It can often help to flip it upside down and think of conductance instead of resistance. Conductance has a unit of Siemens or S.

Conductance in Siemens is 1 / resistance in ohms.

  C E = I     Conductance * voltage = current

  E = I / C 

So for instance a 10 ohm resistor has a conductance of 0.1 Siemens. A 5 ohm resistor has a conductance of 0.2 Siemens.

What happens if you parallel them? Their conductance is - hold onto your hat - 0.3 Siemens! In parallel, conductances just add.

Boy, that's easier to work with.