Doesn't using resistors serial to LEDs all the time waste a lot of energy?

Question

I was wondering for a long time why in schemes which use LEDs for lighting it's very common to put a resistor to go with the LED, and finally it seems like the answer in this question explains why. (It's the easiest way to control the current through he LED to prevent LED from burning up.)

But still, isn't this a big problem? Don't those resistors waste a lot of power and isn't there really any other practical solution?

Is there a reasonable calculation that could provide some numbers to show just how much power is lost to heat from resistors in a typical lighting application? It seems according to the answers that the power loss is likely so small that it doesn't matter. How can one show this using the real numbers?

Answer 1

You wanted a calculation. Here is the basic form of the calculation.

A typical red LED has a forward voltage drop of 1.8 V, and a maximum continuous current of around 20 mA.

Now what's our voltage? Lets say we want to use a 3 V source.

So we will have a voltage drop of 3.0 V - 1.8 V = 1.2 V over our resistor. The current through the resistor will be 20 mA, so our power is 1.2 V * 20 mA = 24 mW. That is not really a lot of power, although it is a significant fraction of power consumption of the LED. The LED itself uses 1.8V * 20mA = 36 mW.

Answer 2

Yes, it wastes power, but most of the time it's not enough power to matter.

In cases where effeciency matters, you use other more complicated means. For example, take a look at the schematic for my KnurdLight example project. This is battery operated and just about all the power is going into the LEDs. In this case I used a boost converter that directly regulated LED current instead of a normal power supply that regulates voltage. There is no series resistor to make a fixed voltage supply look at least partially like a current source because the power supply is a current source in the first place. R6 is in series with the LED string, but is only 30 Ω and is for sensing the current so that the boost converter can regulate it.

Answer 3

Why resistors?

The reason we use resistors to set LED current is that an LED is a diode, and like most diodes, it just looks like a voltage drop when forward biased. There is very little to control current if hooked up to a voltage source; the V/I graph's slope is so steep that a 0.1 V change in diode voltage could mean a 10X change in current. Thus a direct connection to a supply without a workable current limiting mechanism will likely destroy the LED. So we put a resistor in there to make the slope shallow enough to control the current.

Typically, you figure out how much current you want in the LED based on some brightness measurement from the data sheet, or buy one and guess. For typical indicator LEDs, I start with 2 mA for normal or 0.5 mA for high-efficiency LEDs, and usually have to reduce the current further.

Once you pick a current, you take that, the voltage of your source (VS), and the forward voltage of your LED at your current (VF, try to get this from the graph in the data sheet rather than the table, which typically is characterized at 10 mA or more), and plug them into the following equation to get your resistance:

R = (VS - VF) / I

Derivation: Given that the voltage drop across the resistor is VR = I * R (Ohm's Law), that the current in the loop is constant (Kirchoff's Current Law), and that the source voltage is equal to VF + VR (Kirchoff's Voltage Law):

VS = VF + VR = VF + I * R; VS - VF = I * R; R = (VS - VF) / I

High Power LEDs

For applications where the power waste is a problem, such as in large-scale lighting applications, you don't use a resistor but instead use a current regulator to set the LED's current.

These current regulators work like switching voltage regulators, except instead of dividing down the output voltage and comparing to a reference and adjusting the output, they use a current-sensing element (current-sense transformer or low-value resistor) to generate the voltage that is compared to the reference. This can get you lots of efficiency, depending by switching element loss and switching frequency. (Higher frequencies react faster and use smaller components but are less efficient.)

Answer 4

When an LED is driven with a resistor, it's necessary that the supply voltage be higher than the forward drop of the LED; the current drawn from the supply will be equal to the current through the LED. The percentage of supply power that goes to the LED will correspond to the ratio of the LED forward voltage to the supply voltage.

There are other ways of driving LEDs which will work with supply voltages below the forward drop of the LED, or which will draw less current from the supply than they put through the LED. Such techniques may e.g. reduce by half the current drawn from a 5-volt supply to feed 20mA through a 2-volt LED, but the circuitry required will almost certainly be more expensive than a resistor. In many situations, even when running from batteries, the power consumed by an LED will represent a tiny fraction of overall energy usage; even if one could reduce LED-related power consumption by 99% using only $0.05 worth of extra circuitry, the savings wouldn't be worth the cost when compared with simply using a resistor and accepting the sub-optimal efficiency.

Answer 5

Yes, it wastes power. On the other hand, in volume production a resistor will cost a fraction of a penny (US$0.01 for our international people). When the cost/benefit/difficulty analysis is done a simple resistor starts to look really nice.

Answer 6

The wasted power is often very small (tens of milliwatts) if you're driving the LED with 5 V or an equally small voltage.

Sure, it is a problem in systems where you have limited-capacity batteries, but then other schemes (like LED drivers using PWM) are used.

Answer 7

Yes, and no. When current passes through the resistor, it generates heat and therefore wastes energy. However, if you took the resistor out (and therefore drove the LED at a higher voltage) you'd be driving more current through the circuit and thus actually burning more power than with the resister in place.

Remember that with constant voltage, current is inversely proportional to resistance. The more resistance you inline into the circuit, the less current you pass, and therefore the less power you consume. So while the resistor itself plays a part in generating heat in the circuit, its presence there actually means that less heat will be generated overall.

Answer 8

I make mathematical test. I use 12V source and I connect 3 Led diodes with resistors. That one diode with Resistor have 12V, next diode with Resistor I hook up also to 12V, and last one also. At source I have 60mA. Resistor have voltage drop 9V and power consumption was in total 540mW Regardless to P = V * I I get something 720mW total in source.

But when I hook up diodes in string and I add Resistor total power consumption was only 240mW in source. I use diodes 3V 20mA.

It Sims that better is use voltage source as lower as is possible, to have power consumption only there where we want. Or use string of led diodes for higher voltage. That's why we have in computer so many outputs with different voltages from transformer.

Or another idea. We have 9V source, and I hook up one diode 3V and I need use Resistor to droop voltage. Total power will be 180mW Where diode will take only 60mW But when I hook up in string 3diodes then I have still 180mW But when I hook up 3 diodes but each one is separate connected to this same source, then I will have 540mW of used power.

Looks that better is using string rather connecting each one to source.

Answer 9

There is no way to avoid the power loss with only a passive or linear active DC circuit. The reason is that the efficiency is determined by two things:

1. The supply voltage
2. The LED current

It doesn't matter what you put in between the LED and the supply. It could be a resistor, some diodes, a linear regulator, or a transistor-based current source. If the LED needs 10 mA for the desired brightness, and you have a 5 V supply, you're burning 50 mW total. Period.

With a fixed supply voltage and a fixed LED current, your only option for increasing efficiency is to put multiple LEDs in series. If you have a 5 V supply, and your LEDs' voltage drop is 2 V at 10 mA, you can put two in series. This comes with a limitation -- you will not be able to switch the LEDs independently.

If you have control over the power supply, there are a couple other things you can do. If your supply voltages are derived from an AC source, you can add a winding to the transformer to create a low-voltage LED supply. If you only have a DC supply, you can use a switching converter to generate a lower voltage. However, neither of these are terribly practical. If you're running off of AC (mains) power, you're probably not concerned about the efficiency of a few indicator lights. And high-efficiency switching regulators are expensive and trouble-prone.

LEDs typically consume only a small fraction of the total current of a system. It's rarely worth the trouble or expense to add a separate power supply just for them.

Answer 10

Don't think of a resistor as a power sink that diverts electricity (current). Some power is lost in the form of heat, yes, but not much (in general). Using the water analogy, just think of the resistor as making the hose through which the current flows smaller. Given the same initial force (voltage), the amount of electricity that can flow (current) is reduced. this reduces the force available at the output end of the hose (this is known as voltage drop).

Answer 11

If you carefully select the power supply so that the voltage drop across the current limiting resistor is small, then power loss across the current limiting resistor can be almost negligible.

Answer 12

Yes, if you have an LED that requires 1.5 V and you're powering it with a 9 V battery; then the resistor would actually use 5 times the power the LED is using.

The only thing you could do to increase efficiency is either add more 1.5 V LEDs until you get low enough a current, or use an actual computer circuit.

But the power drawn from a high-resistance LED circuit like this is so low that it doesn't really matter. It definitely sounds like a waste, but first of all, you should just get the proper voltage supply like a 1.5 V battery (in this scenario), and second, if you were using LEDs for a practical purpose like lighting something up in your house, you would probably either just use a computer circuit or get an LED with a higher resistance.