What is the power consumption of a 3.18 inch 128x64 COG LCD?
If you are looking for a straight answer: the power consumption of a typical 3.18 inch 128x64 COG LCD display, like the 3.18 inch 128x64 cog lcd display, is around 1.5 to 3.5 milliwatts (mW) under normal operating conditions, with a typical current draw of 0.5 to 1.0 mA at a 3.3V supply voltage. But that number is just the tip of the iceberg. The real power draw depends on a bunch of factors: what you are displaying, the driver IC you are using, the backlight (if any), the refresh rate, and the temperature. Let me break it down with hard data and real-world scenarios so you can actually use this information in a design.
Base Power Consumption Without Backlight
Most 3.18 inch 128x64 COG LCDs are based on STN or FSTN technology, driven by a common controller like the ST7565R, SSD1305, or NT7534. These chips are designed for low power. The LCD panel itself is a passive matrix, so it only consumes power when the segments are being switched. In a static display, the power is mainly used to maintain the bias voltages. For a typical 3.18 inch 128x64 COG module, the logic supply current (VDD) is around 0.2 to 0.5 mA at 3.3V, and the LCD drive supply (VOUT) generated by an internal charge pump adds another 0.3 to 0.6 mA. So, without any backlight, the total current is roughly 0.5 to 1.1 mA, translating to 1.65 to 3.63 mW. If you are running at 5V, the current might drop slightly, but the power goes up to about 2.5 to 5.0 mW because of the higher voltage. In sleep mode, the current can drop to less than 10 µA, which is about 0.033 mW. That is a huge range, and it is critical for battery-powered devices.
Backlight Power Consumption – The Real Hog
Here is the kicker: the backlight is where most of the power goes. A 3.18 inch 128x64 COG LCD typically uses a white LED backlight, which is a strip of 3 to 6 LEDs in series or parallel. The forward voltage of each LED is about 3.0 to 3.2V, and the current per LED is usually 20 to 30 mA. For a typical 4-LED backlight, the total current is 80 to 120 mA at 3.3V, which gives you 264 to 396 mW. That is roughly 100 times more than the LCD logic power. If you are using a backlight with a higher brightness, like 300 cd/m², the current can go up to 150 mA. But if you are using a lower brightness, like 100 cd/m², you can cut that to 40 to 60 mA. Here is a table to show the difference:
Power Consumption Breakdown (Typical 3.18 inch 128x64 COG LCD)
| Component | Voltage (V) | Current (mA) | Power (mW) | Notes |
|---|---|---|---|---|
| LCD Logic (VDD) | 3.3 | 0.3 | 0.99 | Static display, no charge pump load |
| LCD Drive (VOUT) | 3.3 | 0.5 | 1.65 | Internal charge pump, 5V bias |
| Total LCD (no backlight) | 3.3 | 0.8 | 2.64 | Typical for ST7565R |
| Backlight (low brightness) | 3.3 | 40 | 132 | 100 cd/m², 4 LEDs |
| Backlight (medium brightness) | 3.3 | 80 | 264 | 200 cd/m², 4 LEDs |
| Backlight (high brightness) | 3.3 | 120 | 396 | 300 cd/m², 4 LEDs |
| Total with backlight (medium) | 3.3 | 80.8 | 266.64 | Backlight dominates |
How the Display Content Affects Power
This is a detail most people miss. In a COG LCD, the power consumption of the LCD panel itself is not constant. It depends on the number of pixels that are turned on. For an STN display, each pixel that is activated (dark) requires a small current to maintain the voltage across the liquid crystal. In a typical 128x64 matrix, there are 8,192 pixels. If you display a full black screen (all pixels on), the current draw from the LCD drive can increase by 10 to 20% compared to a blank screen (all pixels off). In practice, the difference is about 0.1 to 0.3 mA. For a FSTN display, the effect is similar but slightly lower because of the compensation film. So, if you are displaying a mostly white screen with a few dark characters, the power is closer to the low end. If you are showing a dense graphic, like a bar chart or a bitmap, the power goes up. This is why some datasheets specify a "typical" current at 50% pixel density. For the 3.18 inch 128x64 COG, the typical LCD current is 0.8 mA at 50% on pixels, but it can go to 1.2 mA at 100% on pixels.
Driver IC Variations and Power Modes
The driver IC makes a big difference. The ST7565R is a common choice, and it has a built-in charge pump that generates the negative LCD bias voltage. This charge pump is not 100% efficient. At 3.3V input, the charge pump efficiency is about 70 to 80%, so the input current is higher than the output current. Some newer ICs, like the SSD1305, have a more efficient charge pump and lower quiescent current. The SSD1305 can draw as low as 0.15 mA for the logic and 0.3 mA for the charge pump, giving a total of 0.45 mA without backlight. That is almost half of the ST7565R. Also, most ICs have a "power save" mode that reduces the frame rate or turns off the charge pump when the display is static. In this mode, the current can drop to 0.1 mA. But if you are refreshing at 60 Hz, the current is higher. For a 3.18 inch 128x64 COG, the typical frame rate is 60 to 100 Hz, and the power consumption scales linearly with the frame rate. Dropping the frame rate to 30 Hz can cut the LCD power by 30 to 40%.
Temperature Effects on Power
Temperature is a big factor, especially if you are using the display outdoors or in an industrial setting. Liquid crystal materials have a viscosity that changes with temperature. At low temperatures, like -20°C, the liquid crystal becomes thicker, and it requires a higher voltage to switch the pixels. This means the charge pump has to work harder, and the current draw can increase by 50 to 100%. At 25°C, the LCD drive current is 0.5 mA, but at -20°C, it can be 1.0 mA. At high temperatures, like 70°C, the liquid crystal becomes thinner, and the voltage requirement drops, so the current can decrease by 20 to 30%. The backlight is also affected. LEDs are more efficient at lower temperatures, so the backlight current might drop slightly at -20°C, but the overall power still goes up because of the LCD. For a 3.18 inch 128x64 COG, the datasheet usually specifies power at 25°C, but you need to account for a 1.5x to 2x increase at -20°C if you are designing for a cold environment.
Real-World Power Scenarios
Let me give you three concrete examples to make this practical. First, a battery-powered sensor node that wakes up every 10 seconds to display a reading. The display is on for 2 seconds, then goes to sleep. At 3.3V, with the backlight off, the LCD consumes 2.64 mW during the active period. In sleep mode, it consumes 0.033 mW. The average power over 10 seconds is (2.64 mW * 2s + 0.033 mW * 8s) / 10s = 0.55 mW. That is very low, and it can run for months on a coin cell battery. Second, a handheld device with a backlight on all the time. At medium brightness, the total power is 266.64 mW. If you have a 2000 mAh battery at 3.7V, that gives you 7.4 Wh of energy. The device can run for about 27.7 hours. That is a big difference. Third, an industrial panel that is always on and has a high brightness backlight. At 396 mW, the device runs for about 18.7 hours on the same battery. So, the backlight is the dominant factor, and you should always consider using a PWM dimming circuit to reduce the backlight current when full brightness is not needed.
Voltage and Current Specifications from Datasheets
Most datasheets for 3.18 inch 128x64 COG LCDs list the following typical values. For the logic supply (VDD), it is usually 2.7 to 5.5V, with a typical current of 0.3 to 0.5 mA. The LCD drive voltage (VOUT) is generated internally and is typically 5.0 to 6.5V, with a current of 0.3 to 0.6 mA. The backlight forward voltage is 3.0 to 3.2V, and the forward current is 20 to 30 mA per LED. For a 4-LED backlight, the total current is 80 to 120 mA. Some modules have a 3-LED backlight, which draws 60 to 90 mA. The total power without backlight is typically 2.0 to 4.0 mW, and with backlight, it is 200 to 400 mW. These numbers are consistent across brands like Winstar, Newhaven, and Displaytech. The exact values depend on the specific driver IC and backlight configuration.
How to Measure Power Consumption Yourself
If you want to get the exact numbers for your application, you need to measure it. Use a multimeter in series with the VDD supply to measure the logic current, and another in series with the backlight supply. The backlight is usually a separate pin. For a 3.18 inch 128x64 COG, the backlight pins are typically labeled "A" and "K" for anode and cathode. The logic pins are "VDD" and "GND". You can also use a power analyzer to get real-time data. When measuring, make sure you are using the same frame rate and pixel density as your final application. If you are using a microcontroller, the SPI communication also draws power, but that is usually less than 0.1 mA. The total system power includes the MCU, but for the display alone, the numbers above are accurate.
Comparing to Other Display Technologies
To give you context, a 3.18 inch 128x64 COG LCD is much more power-efficient than a TFT LCD of the same size. A typical 3.2-inch TFT with a resolution of 320x240 consumes 200 to 500 mW without backlight, and 500 to 1000 mW with backlight. That is 2 to 5 times more. Compared to an OLED display, the COG LCD is similar in power when the backlight is off, but OLEDs use more power when displaying bright content because each pixel emits light. For a 128x64 OLED, the power can be 20 to 50 mW for a typical display, but it goes up to 100 mW for a full white screen. The COG LCD with a backlight is higher, but if you turn the backlight off, the COG LCD is much lower. For reflective or transflective COG LCDs, the backlight is only used in low light, so the average power is very low. This makes the 3.18 inch 128x64 COG a good choice for outdoor devices that need to be readable in sunlight.
Design Considerations for Low Power
If you are designing a low-power system, here are some hard numbers to follow. Use a 3.3V supply instead of 5V, because the logic current is lower at 3.3V. The charge pump efficiency is also better at lower input voltages. Use a driver IC with a low quiescent current, like the SSD1305 or the ST7567. Set the frame rate to 30 Hz if you can, because the LCD power scales with frequency. Use a PWM pin to control the backlight brightness, and only turn it on when needed. For a 3.18 inch 128x64 COG, a 100 Hz PWM with a 50% duty cycle reduces the backlight power by half, to 132 mW. You can also use a temperature sensor to adjust the LCD bias voltage. Some driver ICs have a built-in temperature compensation circuit that adjusts the voltage automatically. This can reduce the power at low temperatures by up to 30%. Finally, use the sleep mode of the driver IC. When the display is not updating, the charge pump can be turned off, and the current drops to less than 10 µA. This is critical for battery life.
Common Misconceptions About Power
One thing I see a lot is people assuming that the power consumption is the same for all 128x64 COG LCDs. That is not true. The size of the display, the number of segments, and the driver IC all matter. A 3.18 inch display has a larger active area, so it requires a higher LCD drive voltage than a 2.0 inch display. The charge pump has to work harder, so the power is higher. Also, some modules have a built-in voltage regulator that adds a few milliwatts. Another misconception is that the backlight power is always the same. It is not. The backlight current is set by a resistor or a constant current driver. If you are using a 20 mA LED, the current is fixed, but if you are using a 30 mA LED, it is higher. Always check the datasheet for the specific module. The 3.18 inch 128x64 COG from DisplayModule, for example, has a typical backlight current of 80 mA at 3.3V, but it can vary by 10% from unit to unit.
Power in Different Operating Modes
Let me give you a detailed table for the different modes of a typical 3.18 inch 128x64 COG LCD with the ST7565R driver:
| Operating Mode | VDD (V) | IDD (mA) | Power (mW) | Condition |
|---|---|---|---|---|
| Sleep Mode | 3.3 | 0.01 | 0.033 | Charge pump off, no display |
| Standby Mode | 3.3 | 0.1 | 0.33 | Logic on, charge pump off |
| Static Display (no backlight) | 3.3 | 0.8 | 2.64 | 50% pixels on, 60 Hz |
| Static Display (with backlight) | 3.3 | 80.8 | 266.64 | Medium brightness, 200 cd/m² |
| Full Black Screen (no backlight) | 3.3 | 1.2 | 3.96 | 100% pixels on, 60 Hz |
| Full White Screen (no backlight) | 3.3 | 0.6 | 1.98 | 0% pixels on, 60 Hz |
| Low Temperature (-20°C, no backlight) | 3.3 | 1.5 | 4.95 | 50% pixels on, 60 Hz |
| High Temperature (70°C, no backlight) |