Simple: Using Resistors To Limit LED Current

Basic Calculation

Wiring Multiple LEDs In Series

Note that the LEDs form a chain, cathode (short wire) to anode (long wire), cathode to anode.

It's OK if the LEDs are different sizes, colors, and/or brightness.

This works fine, as long as:

• All of the LEDs can be safely operated at the same current level.
• You have enough voltage for each LED to get the amount it needs (plus a little extra).

The math is still pretty simple. Let's take the example of 2 red (1.7V) and one green (2.1V) LED, with a 9V battery.

The resistor must handle whatever voltage from the battery that is not consumed by the LED. If we have a 9 Volt battery:

9 Volt battery - 1.7Volt_led1 - 1.7Volt_led2 - 2.1Volt_led3 = 3.5 Volts in the resistor

Resistance = Voltage / Current

175 Ohms = 3.5 Volts / .020 Amps

The voltage consumed by the resistor has to go somewhere. It is radiated as heat, and you must select a resistor wattage that can handle that power level.

Power = Current x Voltage

0.07 Watts = .020 Amps x 3.5 Volts

Please note that the published current rating for a LED is a maximum value. It is perfectly safe to use less current (a larger resistor value); the LED will simply be less bright.

Wiring Multiple LEDs In Parallel

Note that all of the LED cathodes (short wire) are hooked together, and all of the anodes (long wire) are hooked together.

To make this parallel wiring scheme work, you calculate the resistor so that you get three times as much current running through the circuit. Then, each of the three LEDs consumes one third of the (triple) current, which is exactly what it needs. Neat. Simple. Wrong.

This scheme is predicated on the assumption that all of the LEDs need exactly the same amount of power. If this assumption is met, or fairly close to being met, this circuit will work.

If one of the three LEDs needs less power, the electricity will take the path of least resistance. That one "easy" LED will light brighter, and the other LEDs won't get enough power, and will be dim. Remember that you calculated the resistor for three times the current of a single LED. If the other LEDs don't take their fair share, the easy LED will get more power than it should.

But why might one LED need less power than the others?

• LEDs of different colors are likely to have very different electrical properties.
• LEDs of different sizes and manufacturers are likely to have different electrical properties.
• LEDs manufactured in different batches may have slightly different in electrical properties.
• As components age, their electrical characteristics change.
• When components heat up, their electrical characteristics may change.

So, your best chance of making LEDs operate in parallel is to do it with few LEDs, all of which are exactly the same model number, from the same batch. Even then, the system may collapse, as the components degrade at different rates over time.

Of course, this kind of thing works often enough that you can often get away with it. But why not buy a couple more resistors and do it the right way?

Don't believe me? Here are some references:

• http://members.misty.com/don/ledd.html
  Do not put LEDs in parallel with each other. Although this usually works, it is not reliable. LEDs become more conductive as they warm up, which may lead to unstable current distribution through paralleled LEDs. LEDs in parallel need their own individual dropping resistors. Series strings can be paralleled if each string has its own dropping resistor.

• http://www.bivar.com/eLetter/driving-la.htm
  You can also connect LEDs in parallel. However, variations in the forward voltage requirements of individual LEDs will result in non-uniform current distribution, and non-uniform current distribution results in non-uniform brightness.

• http://www.kpsec.freeuk.com/components/led.htm
  Avoid connecting LEDs in parallel! Connecting several LEDs in parallel with just one resistor shared between them is generally not a good idea. If the LEDs require slightly different voltages only the lowest voltage LED will light and it may be destroyed by the larger current flowing through it. Although identical LEDs can be successfully connected in parallel with one resistor this rarely offers any useful benefit because resistors are very cheap and the current used is the same as connecting the LEDs individually. If LEDs are in parallel each one should have its own resistor.

• http://www.otherpower.com/otherpower_lighting_leds.html
  It's important that each string has its own resistor.... putting them in parallel with a single resistor is bad practice. We didn't know this when this article was first written....thanks to all the folks that pointed this out!

• http://home.nikocity.de/andymon/hfg/ledlamps.htm
  If you want to operate LEDs in parallel with higher power levels, you must select them for equal voltage/current characteristics! It won't do to buy four LEDs if you want to build a four-LED-Lamp! Buy ten and measure the voltage of each LED at 20mA.

• http://www.help.ip3.com/s.e.design/Connecting-soLip.shtml
  {both sides of an argument on this subject}

Wiring Large Numbers Of LEDs In Series-Parallel

Help With Large Numbers Of LEDs

Fancy: Using A Constant Current Source

Avoid Running LEDs Straight Off AC

This sometimes works.

If you are tempted to do this, I suggest that you carefully read the technical specifications for your LEDs and make sure that you won't be exceeding the peak reverse voltage.

In general, I try to avoid this kind of thing.

Running LEDs Off AC Line Current

What you need:

What you need:

Increased Power Via Pulse Drive

General Theory Of Driving LEDs With Pulses

You can run it all day long at 20 mA.

You can raise the current beyond the maximum continuous current if the LED is only operated in pulses.

Here we show the same LED operated at twice the maximum continuous current, but we are only running the LED half the time.

Now we're running the LED at four times the maximum continuous current, but are only running the LED one quarter of the time.

You can keep increasing the current while reducing the duty cycle, up until you reach the absolute maximum pulse current, which should be documented someplace.

Please note that our manipulations increase the peak current, but since the time during which that current is applied reduces, overall the average current over time is the same.

The maximum continuous current derives from the efficiency of the LED and its ability to shed heat. Because no LED is 100% efficient at turning electricity into light, the wasted electrical energy turns into heat within the LED. If the heat builds up too high, the LED chip will melt. But excess heat can be conducted out of the LED package through the "lead frame" and metal lead wires. The maximum continuous current is the maximum current that you can feed through the LED that it is able to dissipate without heating up to damaging temperatures.

If the LED is not run continuously, it has time to shed some of the heat, and can be run at higher current levels.

So why did I say that pulsing LEDs for higher lower is an urban legend? Clearly you can pulse them and get higher power levels.

The average user of LEDs cares only about how bright the light is. The eye is slow to react, and registers the average brightness of the light. Although it is true that we have increased the instantaneous brightness of the light, average brightness (that the eye sees) is the same.

The only time that you get something by pulsing is when you were going to pulse the LED anyway, perhaps as a carrier signal. If you know the duty cycle of the pulse, you can increase the drive current beyond the specified maximum continuous current.

Practical Complications

• Higher current levels increase the forward voltage.
• Higher current levels produce more heat.

Pushing Hard: Read The Data Sheets

But if you want maximum performance, or intend to push any of the operating parameters, you need detailed data sheets. This is because many of the figures quoted in short data sheets is an average, and the real values vary significantly depending on other conditions.

Note: Getting this information is sometimes difficult. LEDs from many of the hobby vendors come with little or no information. You're lucky to find a couple of lines on the back of a blister pack from Radio Shack.

Most of the following graphs were taken from the data sheet for the Optosource 110147 series, kindly provided by Wolfstone reader Ronald Jansen. These graphs are used only for illustrative purposes, and are unlikely to match your LEDs, unless you happen to use the Optosource 110147 series.

The Effect Of Ambient Temperature

Heat kills LEDs, and the heat can come from outside the LED or inside. The amount of heat generated within the LED depends on the current, so the hotter it is outside, the less current you can use before the total heat cooks the part.

At 80°C, the LED is already so hot that you can't run it at all.

[Optosource, a division of Marl International Limited, www.optosource.com]

This graph shows Ambient Temperature vs. Forward Voltage for three different current levels.

The temperature of the LED chip affects its electrical characteristics.

As the LED gets hotter, the forward voltage decreases. This effect is more pronounced at higher current levels.

[Optosource, a division of Marl International Limited, www.optosource.com]

This graph shows Ambient Temperature vs. Luminosity on a semi-log scale.

This LED puts out less light as it gets hotter.

[Optosource, a division of Marl International Limited, www.optosource.com]

The Effect Of Increased Current

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