engineering

LED circuit

LED circuit is a engineering topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand LED circuit rather than just read about it. In short: In electronics, an LED circuit or LED driver is an electrical circuit used to power a light-emitting diode (LED). The circuit must provide sufficient current to light the LED at the required brightness, but must limit the current to prevent damaging the LED.

Key takeaways

LED circuit belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect LED circuit to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of LED circuit from memory before moving on to harder problems.

Reference excerpt

In electronics, an LED circuit or LED driver is an electrical circuit used to power a light-emitting diode (LED). The circuit must provide sufficient current to light the LED at the required brightness, but must limit the current to prevent damaging the LED. The voltage drop across a lit LED is approximately constant over a wide range of operating current; therefore, a small increase in applied voltage greatly increases the current. Datasheets may specify this drop as a "forward voltage" ( V f {\displaystyle V_{f}} ) at a particular operating current. Very simple circuits are used for low-power indicator LEDs. More complex, current source circuits are required when driving high-power LEDs for illumination to achieve correct current regulation.

Basic circuit The simplest circuit to drive an LED is through a series resistor. It is commonly used for indicators and digital displays in many consumer appliances. However, this circuit is not energy-efficient, because energy is dissipated in the resistor as heat. The LED's V f {\displaystyle V_{f}} depends on its material. Ohm's law and Kirchhoff's circuit laws are used to calculate the appropriate resistor value, by subtracting the LED's V f {\displaystyle V_{f}} from the supply voltage and dividing by the desired operating current. With a sufficiently high supply voltage, multiple LEDs in series can be powered with one resistor. If the supply voltage is close or equal to the LED's V f {\displaystyle V_{f}} , then no reasonable value for the resistor can be calculated, so some other method of current limiting is used.

Power source considerations The voltage versus current characteristics of an LED is similar to any diode. Current is approximately an exponential function of voltage according to the Shockley diode equation, and a small voltage change may result in a large change in current. If the voltage is below or equal to the threshold no current flows and the result is an unlit LED. If the voltage is too high, the current will exceed the maximum rating, overheating and potentially destroying the LED. LED drivers are designed to handle fluctuation load, providing enough current to achieve the required brightness while not allowing damaging levels of current to flow. Drivers may be constant current (CC) or constant voltage (CV). In CC drivers, the voltage changes while the current stays the same. CC drivers are used when the electrical load of the LED circuit is either unknown or fluctuates, for example, a lighting circuit where a variable number of LED lamp fixtures may be installed. As an LED heats up, its voltage drop decreases (band gap decrease). This can encourage the current to increase.

MOSFET drivers

An active constant current source is commonly used for high power LEDs, stabilizing light output over a wide range of input voltages which might increase the useful life of batteries. Active constant current is typically regulated using a depletion-mode MOSFET (metal–oxide–semiconductor field-effect transistor), which is the simplest current limiter. Low drop-out (LDO) constant current regulators also allow the total LED voltage to be a higher fraction of the power supply voltage. Switched-mode power supplies (e.g. buck, boost, and buck-boost converters) are used in LED flashlights and household LED lamps. Power MOSFETs are typically used for switching LED drivers, which is an efficient solution to drive high-brightness LEDs. Power integrated circuit (IC) chips are widely used to drive the MOSFETs directly, without the need for additional circuitry.

Series resistor Series resistors are a simple way to stabilize the LED current, but energy is wasted in the resistor. Miniature indicator LEDs are normally driven from low voltage DC via a current-limiting resistor. Currents of 2 mA, 10 mA and 20 mA are common. Sub-mA indicators may be made by driving ultra-bright LEDs at very low current. Efficiency tends to reduce at low currents, but indicators running on 100 μA are still practical. In coin cell powered keyring-type LED lights, the resistance of the cell itself is usually the only current limiting device. LEDs with built-in series resistors are available. These may save printed circuit board space, and are especially useful when building prototypes or populating a PCB in a way other than its designers intended. However, the resistor value is set at the time of manufacture, removing one of the key methods of setting the LED's intensity.

The value for the series resistance may be obtained from Ohm's law, considering that the supply voltage is offset by the diode's V f {\displaystyle V_{f}} , which varies little over the range of useful currents:

… excerpt ends here. Continue reading the full article.

Illustrations

LED circuit: Schematic with a voltage power source (with voltage V), LED, resistor (with resistance R), and current flow (I). — Schematic with a voltage power source (with voltage V), LED, resistor (with resistance R), and current flow (I).

LED circuit: Schematic of LEDs in series — Schematic of LEDs in series

LED circuit: Schematic of LEDs in parallel — Schematic of LEDs in parallel

Worked examples

Example 1 — a first encounter with LED circuit

Start with the simplest possible case. Write down what LED circuit claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to LED circuit before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about LED circuit ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of LED circuit

In research: LED circuit appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses LED circuit in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: LED circuit is common in secondary-school and first-year university syllabi. It links to neighbouring topics American inventions, Analog circuits, Light-emitting diodes, so understanding it makes those chapters shorter.
In everyday life: Look for LED circuit outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study LED circuit in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what LED circuit means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain LED circuit out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is LED circuit in simple terms?

Why does LED circuit matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study LED circuit?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on LED circuit.