LCD Backlight LEDs: Colors, Series Resistor Calculation and PWM Dimming
Quick answer: A monochrome LCD needs a backlight when its rear polarizer is transmissive (always) or transflective (in dim light); a reflective display has no backlight and is read by ambient light. LED backlights come in white, yellow-green, blue, amber, red and RGB, and the LED color combined with the LC mode sets the look of the product. Electrically, a backlight is a string of LEDs: it needs a current, not a voltage. A series resistor sized as R = (VS − Vf) ÷ If, or a constant-current driver, sets that current, and PWM switching above about 200 Hz dims it without changing the color.
Take Vf and If from the module’s specification. Running a backlight at its maximum current shortens its life, especially in a hot enclosure, so leave margin.
Which LCDs Need A Backlight
| Rear polarizer | Backlight | Typical use | SuccessLCD examples |
|---|---|---|---|
| Reflective | None; a mirror layer reflects ambient light | Battery products used in daylight: meters, calipers, instruments | S3543D yellow-green STN instrument display, S4745D FSTN caliper display |
| Transflective | Optional; reads by ambient light, backlit in the dark | Thermostats, handheld equipment, products used indoors and out | S3420D FSTN segment module with FPC and white backlight |
| Transmissive | Required; nothing is visible without it | Negative-mode displays, indoor equipment that is always powered | S1702D blue negative HTN treadmill display, S3203D blue negative STN refrigerator display |
The trade-offs between these three are covered in transflective LCD displays. For color TFT modules, the backlight is a built-in light guide assembly; its structure is described in the LCD backlight module guide and its brightness in LCD brightness in nits.
Backlight Colors And LC Modes
| Combination | Appearance | Notes |
|---|---|---|
| Positive FSTN or HTN + white backlight | Dark characters on a white background | The most neutral look; FSTN gives the cleanest black and white |
| Positive yellow-green STN + yellow-green backlight | Dark characters on a green-yellow background | The classic character LCD; the LED color matches the STN glass tint |
| Blue negative STN + white backlight | White characters on a blue background | Popular for instruments and audio equipment; transmissive, so unreadable without the backlight |
| Negative FSTN or DFSTN + white or colored backlight | Characters in the backlight color on a black background | DFSTN gives a darker background than plain negative STN |
| Negative mode + RGB backlight | Character color selectable by the host | Used for status indication: green normal, red alarm |
Amber and red backlights are chosen for night-vision friendly equipment and for very bright, warm looks. The modes themselves, including why negative displays need a transmissive stack, are explained in monochrome LCD types.
Edge-Lit And Array Backlights
- Edge-lit: a few LEDs at one edge of an acrylic light guide, with a reflector behind and a diffuser in front. Thin, low current, and the standard construction for character and small graphic modules. Uniformity depends on the light-guide pattern, and the LED edge can look brighter on large panels.
- Array (bottom-lit): a grid of LEDs on a board behind a diffuser. Brighter and more uniform on large displays, but thicker and higher in current. Common on large segment displays and older graphic modules.
- Electroluminescent (EL): a thin panel driven by an AC inverter, found in legacy equipment. Even light, but limited life and an inverter that needs EMC attention; replacements today use LEDs.
Setting The LED Current
The specification gives a forward voltage Vf and a forward current If, usually at 25 °C. White and blue LEDs typically have a Vf of about 2.8 V to 3.6 V each; yellow-green, amber and red about 1.8 V to 2.4 V; a backlight with LEDs in series adds the values, so an array backlight may specify 4 V or more. For a single string from a fixed supply:
- Resistor: R = (VS − Vf) ÷ If. With VS = 5 V, Vf = 3.2 V and If = 20 mA, R = 90 Ω, so 91 Ω from the standard series; the resistor dissipates (5 − 3.2) × 0.02 = 36 mW, so a 0.125 W part is fine.
- Vf spread and temperature: Vf varies between batches and falls as the LED warms, so the current with a resistor rises slightly in a hot enclosure. Choose If below the maximum rating, and check the current at the highest operating temperature.
- Constant-current driver: for currents above about 50 mA, for battery supplies that vary, or for several strings in parallel, a constant-current regulator or LED driver IC keeps the current and brightness stable and makes dimming easier.
- Built-in resistor: some modules include a series resistor on the PCB so the backlight can be connected to 5 V directly. The drawing says whether pins A and K are the bare LED or the resistor-protected input; connecting a bare LED to 5 V destroys it.
- Parallel strings: never parallel LEDs with one resistor; each string needs its own resistor or driver channel, or the string with the lowest Vf takes most of the current.
PWM Dimming
Dimming by lowering the current also shifts the LED color slightly and becomes non-linear at low levels. Switching the backlight with a PWM signal keeps the LED at its rated current while it is on and changes only the on time, so the color stays constant and brightness follows the duty cycle. A small transistor or MOSFET on the cathode side, driven from a microcontroller PWM pin, is enough for a character LCD backlight; LED driver ICs usually have a PWM input.
- Frequency: at least 200 Hz so the eye sees no flicker. If the product will be photographed or filmed, or sits near rotating machinery, use 1 kHz or higher; see LCD camera banding and flicker.
- Minimum brightness: below about 5 percent duty, the first PWM step becomes visible; a logarithmic duty table gives even visual steps.
- EMC: fast edges on a long backlight cable radiate; keep the switch close to the LED and add a small series resistor or filter on the gate if needed.
- Audible noise: PWM between about 100 Hz and 20 kHz can make ceramic capacitors and inductors in the supply sing; move the frequency or add decoupling.
Backlight Life And Heat
LED backlights are usually rated for a time to half brightness, often tens of thousands of hours at 25 °C and rated current. Two things shorten it: current above the rating and temperature inside the enclosure, which is often far higher than the room. A display that is dim after a year in a sealed, sun-exposed housing has normally lost backlight output, not LCD contrast; our review article why an LCD becomes dim in the field explains how to separate the two. Running at 50 to 70 percent of rated current, and dimming when full brightness is not needed, extends life considerably.
What To Specify
- Rear polarizer type: reflective, transflective or transmissive, decided together with the LC mode and the lighting the product is used in.
- Backlight color and whether it must match a brand color or an indicator convention.
- Vf, If and the connection: bare LED pins, built-in resistor, or a separate connector.
- Brightness and uniformity targets if the product is used in daylight; see the brightness guide.
- Dimming method and the lowest brightness required, for example for night use.
- Operating temperature and expected daily on-time, which set the current derating.
LCD Backlight FAQs
Do all LCDs need a backlight?
No. Reflective displays are read by ambient light and have no backlight, which is why they suit battery products. Transflective displays can be read by ambient light and backlit in the dark. Transmissive displays, including all negative-mode displays, show nothing without a backlight.
How do I calculate the resistor for an LCD backlight?
R = (VS − Vf) ÷ If, using the forward voltage and current from the module specification. With a 5 V supply, Vf of 3.2 V and If of 20 mA, R is 90 Ω; use the next standard value, 91 Ω, and check the resistor’s power, here 36 mW.
What PWM frequency should I use to dim an LCD backlight?
At least 200 Hz to avoid visible flicker. Use 1 kHz or more if the display will be filmed or photographed, and avoid frequencies that make supply capacitors audible.
Can I connect an LCD backlight directly to 5 V?
Only if the module has a built-in series resistor, which the drawing will state. A bare LED connected to 5 V draws far more than its rated current and fails quickly. Otherwise add a resistor or a constant-current driver.
Why has my LCD backlight become dim?
LED output falls with hours of operation, faster at high current and high temperature. A display that has dimmed after long service in a hot enclosure has usually lost backlight output; derating the current and dimming when full brightness is not needed extends life.
Choosing a backlight for a monochrome display? Send the LC mode, lighting conditions, brightness target, supply voltage and dimming needs. Send your requirements for an engineering review, or browse monochrome LCD and LCM references. For a custom backlight color or brightness, see custom LCD modules.