How to choose 2V/3V indicator beads? Guidelines for Forward Voltage, Current Limiting Resistance Calculation, and Low Power Drive
Is the indicator light bead 2V or 3V? This is one of the most frequently asked questions by procurement and engineers. Half of the people asking this question want to confirm if it can be directly driven by batteries or GPIO, while the other half are confused about the current limiting resistor and the burnt out lamp.
This article explains the origin and development of the voltage gate for LED light beads, and includes a quick check table for current limiting resistors.
1、 First, correct a concept: what is the "voltage" of LED
LED is not a resistive load and there is no term for "rated voltage". In the specification bookVF (forward voltage)It refers to the voltage drop naturally generated at both ends of the lamp bead under rated forward current (usually 20mA).
- Driving voltageBelow VFThe light bead is not on or slightly on
- Driving voltageHigher than VFExcess voltage must be consumed by a current limiting resistor, otherwise the current will spiral out of control and surge, causing light decay or even dead lights within seconds to hours
So the true meaning of "2V LED beads" and "3V LED beads" is the LED beads with VF in the 2V or 3V gear.
2、 Different colors naturally result in different voltages
VF is determined by the chip material, and VF varies depending on the color. This is the first lesson of selection:
| Color | Chip system | Typical VF @ 20mA | commonly known as |
|---|---|---|---|
| Red, yellow, orange, and plain green | AlGaInP | 1.8-2.2V | "2V 灯珠" |
| Blue, pure green, white, purple | InGaN | 2.8-3.4V | "3V 灯珠" |
| Infrared 850/940nm | GaAs | 1.2-1.6V | Low voltage LED beads |
Practical inference:
- A 3V button battery (CR2032) or two dry batteries (3V) can directly light up red/yellow light (VF ≈ 2V), but cannot light up blue/white light (VF ≈ 3V, insufficient battery voltage margin)
- Single lithium battery (3.7-4.2V) can drive all colors
- When mixing red light and pure green/blue/white light on the same panel, the current limiting resistors of the two channels must be calculated separately
3、 Current limiting resistor: One speedometer is sufficient for 90% of situations
Formula:R = (VCC - VF) / IF， IF is typically set at 20mA (0.02A), while in low-power scenarios, it is set at 2-5mA.
| power supply voltage | Drive red/yellow light (VF=2.0V) | Drive blue/green/white (VF=3.2V) |
|---|---|---|
| 3.3V | 68 Ω (recommended 75 Ω derating) | 5.1 Ω (the margin is too small, it is recommended to increase the voltage or choose the low voltage mode) |
| 5V | 150Ω | 91 Ω (take 100 Ω) |
| 12V | 510Ω | 430 Ω (take 470 Ω) |
| 24V | 1.1kΩ | 1kΩ |
Two engineering habits:
1. Calculate to move up to the next higher levelIt is better to be slightly darker than overcurrent, as LED is far more sensitive to overcurrent than your intuition
2. Don't ignore the power consumption of 12V/24V system resistors: 24V drives a single lamp, and the dissipation on the resistor is about 0.44W,Select at least 0.5W specificationOtherwise, the resistor will burn itself first
4、 Low power indicator light: Reduce the current to 2mA
Battery powered devices (thermostats, smart door locks, smoke detectors) are sensitive to static power consumption, and a 20mA indicator light can significantly reduce battery life. The approach is not to simply increase the resistance, but to chooseLow current high brightness LED bead：
- The brightness of ordinary LED chips at 2mA is only 1/10 of the rated level, and many low-end LED chips do not light evenly at low currents
- Low power dedicated indicator beads have been optimized with chips and phosphors for 2mA, which can achieve the visual brightness of 5-10mA of ordinary beads with 2mA
Our Hongcheng Optoelectronics low-power indicator light bead series can achieve a luminous intensity of 80-300mcd (depending on color) under 2mA drive. It is widely used by customers for smart door locks, gas meters, and NB IoT devices, and comes in standard red/green/blue/white full-color options.
5、 Pit with multiple parallel lights
It is a common mistake for multiple lamp beads to be directly connected in parallel and share a resistor: the VF of the same model of lamp bead also has ± 0.1V dispersion, and the one with low VF steals most of the current, first turning on, then fading, and then dying. The correct approach isEach independent current limiting resistorOr simply use a constant current driver chip.
VI FAQ
Q: Can 5V be directly connected to a 2V LED without a resistor in series?
No. The 3V pressure difference turns into overcurrent, and the light bulb may turn on instantly, then rapidly decay and die out within a few hours to days.
Q: Why do some light bulbs light up more when they are the same 20mA?
The luminous efficiency (mcd/mA) depends on the quality of the chip and the luminous efficiency of the package, which is why "same current, different brightness" - the selection of brightness should be based onUnder the same IF conditionsCompared to.
Q: How to drive the 220V indicator light for communication?
The interior of industrial 220V panel lights consists of finished components such as lamp beads, rectifiers, voltage reducing resistors/capacitors, rather than bare lamp beads directly connected. The selection of bare lamp beads will still be calculated based on VF and IF, and the power supply design will be completed by the finished product factory.
7、 Summary
Remember three things:VF looks at color (red, yellow, orange, light green ≈ 2V, blue-green, white ≈ 3V); Calculate the resistance according to the formula and take the larger value; Choose 2mA dedicated LED beads instead of hard resistors for low-power scenarios.
Hongcheng Optoelectronics offers a full range of indicator beads with VF division (± 0.05V narrow range optional), and the low-power 2mA series supports free sample testing. Contact information: 139 2571 4318 (same WeChat account), sales@led-hc.com .
