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LED Strip Light Internal Schematic and Voltage Information

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LED Strip Light Internal Schematic and Voltage Information


This article goes over the inner circuitry and workings of an LED strip light. This information is for engineering discussion purposes and is not necessary for typical users interested in regular use of LED strips.

 

Back to Fundamentals - LED Chip Voltage


The specified voltage of an LED strip - e.g. 12V or 24V - is primarily determined by:

1) The specified voltage of the LEDs and components used, and

2) The configuration of the LEDs on the LED strip.

LEDs are typically 3-volt devices. What this means is that if a 3-volt differential is applied between the positive and negative ends of an LED, it will light up.

What happens when you have multiple LEDs in a string, one after the other (series)? In this case, the individual LED voltages are summed together.

Therefore, 3 LEDs in series will require a forward voltage of 9 volts (3 volts x 3 LEDs), and 6 LEDs in series will require a forward voltage of 18 volts (3 volts x 6 LEDs).


In addition to the LEDs, one or more current limiting resistors is also necessary to ensure that the LED strip does not go into overcurrent mode. The resistor is also placed in series with the LEDs, and its resistance value is calculated such that it will draw approximately 3 volts as well.

So, 3 LEDs in series requires 9 volts for the LEDs and 3 volts for the resistor, bringing us to 12 volts.

6 LEDs in series requires 18 volts for the LEDs and 3 volts per resistor (x2), bringing us to 24 volts.


 

These are the "building blocks" for each group of LEDs on an LED strip. The way it is laid out on an LED strip can be visualized in our graphic below:

 

What happens to LEDs in parallel? The voltage remains the same but the current is split equally among each of the parallel circuits. Therefore, if you have 3 parallel groups that each draws 50 mA at 24 volts, the total power draw is 150 mA, also at 24 volts.

These two examples of 3 LEDs and 6 LEDs show how a typical 12 and 24 volt LED strip is configured. Because the LED strips utilize 3 volt LED devices, and are configured to have multiple parallel strings of 3 or 6 LEDs.

 

Do you have to supply exactly the voltage specified?


You may be wondering if 12 volts means exactly 12.0 volts or if 11.9 volts would still work? The good news is that there is quite a bit of leeway in the power supplied to an LED strip.

Below is a chart from an LED datasheet showing how much current will pass through an LED, depending on the voltage.

You will see that at 3.0V, for example, this particular LED will draw about 120 mA. If we decrease the voltage to 2.9V, the LED will draw a bit less, only about 80 mA. If we increase the voltage to 3.1V, the LED will draw more, about 160 mA.

Because in a 12V LED strip there are 3 LEDs and a resistor in series, supplying 11V instead of 12V is a bit like reducing the voltage for each LED by 0.25V.

Will the LEDs still run at 2.75V? If we reference the chart above, it appears that the current draw will drop from 120 mA per LED to about 40 mA.

While that's a pretty significant drop, the LEDs will run just fine, albeit at a much lower brightness level.

What if we supplied just 10V to a 12V LED strip? In this case, we are reducing the voltage per LED by 0.5V each. If we reference the chart, at 2.5V, the LEDs will barely draw any current.

You'll likely see a very dim LED strip at this voltage level.

All voltages less than the LED strip rating are safe, as you will always be drawing less current and therefore avoiding any possibility for damage or overheating. But what about voltage levels over 12V?

Let's take a look at supplying 12.8V to a 12V LED strip. This increases voltage per LED by 0.20V.

Our LED is now driven at 3.2V, at which the chart shows a current draw of 200 mA.

It just so happens that 200 mA is the manufacturers' maximum current rating. Any higher and you risk damaging the LED.

And keep in mind that each LED will have a different rating, and inherent variation in manufacturing can affect the actual voltage ranges that are acceptable for a particular LED strip.

We've shown that for a 12V LED strip, it can go from dark to overdriven in a narrow range between 10V and 12.8V.

While it is possible to supply a voltage that is slightly different from the rated voltage, you will have to be careful and precise to ensure that you do not cause any damage to the LEDs.

 

What about dimming an LED strip?


One way to dim an LED strip is to adjust the input voltage to below is rated level, as we saw above. In reality, however, power electronics are not very good at reducing the voltage output in this way.

The preferred method is to use what is called PWM (pulse width modulation) - where the LEDs are turned on an off at a rapid rate. By adjusting the ratio of time on vs time off (duty cycle), the apparent brightness of an LED strip's light output can be adjusted.

For a 12V LED strip, this means that it is always receiving either the full 12V or 0V, depending on which part of the PWM cycle we are at.

Similarly, we also know that an LED will draw the same amount of current when it is in its "on" state, regardless of its duty cycle. This is an added advantage for LED strip lights whose color temperature consistence must remain constant even when its brightness is altered.

 

Bottom Line


One of the significant advantages of LED strip light products is simple yet versatile they are given compatibility with simple constant voltage power supply devices.

It can sometimes be useful to understand the inner workings of such devices as it can help us understand some of the more nuanced aspects of its performance, such as dimming and voltage input changes.