In various water quality monitoring devices, LEDs of different wavelengths are frequently used to measure absorbance. The accuracy and repeatability of this measurement depend on many factors, the most important of which is the luminous flux of the LED, which is often related to its driving method. To ensure that the luminous flux of the LED remains constant throughout the entire lifespan of the equipment, methods such as constant current driving of the LED and luminous flux compensation are often employed. Constant current driving of the LED is widely used due to its simplicity and ease of circuit implementation.
What is a constant current driven LED? As the name suggests, constant current driving involves using a constant current to drive the LED, ensuring that the current flowing through the LED remains constant and is almost unaffected by external factors. LEDs are current-driven devices; when current flows through them, special materials inside the LED convert this current into photons through electron migration and radiate them. The amount of photons radiated is proportional to the current flowing through the LED. Therefore, to ensure a constant luminous flux, simply driving the LED with a constant current is sufficient.
How can we achieve constant current driving for LEDs? Let's first look at the simplest and most common LED driver circuit, as shown in the figure below.
R1 acts as a current limiter. Generally, the voltage drop across an LED when it's conducting is in the range of 1.2V to 2.2V, which needs to be determined according to the LED datasheet. If we want 10mA of current to flow through the LED, the approximate range of R1 can be estimated as follows: R1 = (5-2.2)/0.01 ~ (5-1.2)/0.01 = 240~380Ω. We know that the LED voltage drop has a significant temperature characteristic, specifically a negative temperature characteristic. This means that as the LED operates for a long time, the internal heat increases its temperature, and combined with the influence of the ambient temperature, the voltage drop will become smaller. Therefore, in this circuit, once the value of R1 is fixed, the current flowing through the LED will gradually increase over time, causing the luminous flux of the LED to change throughout the device's operation, thus affecting the measurement accuracy and stability of the device. This circuit can only be used to drive LEDs for indicator purposes, not for driving spectrophotometers. To address the aforementioned issues, minimize the impact of LED temperature characteristics, and ensure constant LED current and luminous flux, the light source with spectrophotometry typically employs a constant current driving method as shown in the diagram below.
To achieve constant current, this driver circuit uses an operational amplifier for negative feedback processing, sampling the current flowing through the LED in real time to participate in closed-loop regulation. R1 and R2 divide VCC to obtain a reference voltage, while Rs samples the current. The current flowing through the LED can be calculated using the following formula:
In practical applications, R2 can be an adjustable potentiometer to facilitate adjustment of the current flowing through the LED. For example, when R1 = 10KΩ, R2 = 2KΩ, Vcc = 12V, and Rs = 200Ω, the current flowing through the LED is equal to 10mA. Through the processing of the above circuit, the LED can be guaranteed to be in a constant current driving state. This is largely thanks to the participation of the operational amplifier and transistor. Because of the negative feedback effect of the operational amplifier, even minute changes in the current flowing through the LED at any given moment can be detected and the linear operating state of the transistor can be adjusted immediately, ensuring a constant and reliable 10mA current flowing through the LED for an extended period. This also eliminates the influence of the junction temperature of the LED voltage drop and the ambient temperature.
Currently, most online water quality monitoring equipment uses the above-mentioned type of drive circuit for its light source.
However, the constant current drive circuit described above still has the following drawbacks:
1. Adjusting the current is very inconvenient and cannot be done continuously in real time. Once the required current value is determined, the resistance values of R1 and R2 are also determined. If you want to change the current value next time, you need to manually adjust the potentiometer, which is very troublesome and headache-inducing.
2. The accuracy of the constant current is affected. The accuracy of the constant current depends not only on the accuracy of the sampling resistor Rs, but also on the accuracy of R1 and R2. Furthermore, R2 is usually an adjustable potentiometer; when manually adjusted, the accuracy of its resistance value is inevitably greatly affected. Even if Rs is a precision resistor with a 0.1% accuracy level, the constant current accuracy of the above circuit will not reach such a high level. A rough estimate suggests that a constant current accuracy of 2% would be very good.
3. High-side load causes interference. In the circuit above, the LED is connected in series between the power supply and the transistor; this connection is called a high-side load. Compared to a low-side load, a high-side load has a floating ground characteristic. In reality, LED leads are usually quite long, about 1 meter long. Such long leads are highly susceptible to external electromagnetic interference, introducing instability into the equipment.
To address the aforementioned issues, Shenzhen Yiwei Automation Technology Co., Ltd. improved, optimized, and tested the circuit, enabling the light source to achieve true constant current drive and maintain a constant luminous flux. Yiwei Automation embedded the improved circuit into the Ao output section of its online water quality monitoring and control unit, making the constant current drive of the LED extremely convenient and reliable. Because it's an embedded control system, the ability to precisely adjust the LED current becomes a reality. At this point, simply assigning values to the relevant registers in the Ao output section within the control program yields a current accuracy of ±0.1%.
Shenzhen Yiwei Automation Technology Co., Ltd. has focused on embedded development in the field of online monitoring for many years, providing comprehensive solutions for the online monitoring industry. From top to bottom, from interactive interface configuration to various pumps, valve mechanisms, light sources and light sensors of different wavelengths, and then to the underlying control unit, Yiwei Automation leverages its rich experience in the monitoring field and focuses on innovative technological development to bring extraordinary experiences to its customers.
The image below shows an embedded control unit developed by Yiwei Automation specifically for online water quality monitoring.
This control unit combines the advantages of a PLC with a graphical programming interface, making it very convenient and quick to learn. In addition to the stability and reliability of industrial control products like PLCs, this control unit also features the following improvements to the light source drive section:
1. The highest-level operational amplifiers in the industry are used for driving, ensuring that the impact of offset voltage, offset current and temperature drift on the constant current drive of the light source is minimized.
2. All resistors are imported high-precision resistors, especially the current sampling resistors, whose temperature drift and stability are reliably guaranteed.
3. For commonly used constant current drive circuits, the reference voltage is obtained by adjusting the potentiometer. However, for the control unit of Yiwei Automation, the reference voltage is obtained by a high-resolution DAC (digital-to-analog converter), which truly realizes online programmable constant current source drive and avoids the failure risk caused by the error of the potentiometer mechanical contact and the vibration environment.
4. Low-end load wiring method to avoid electromagnetic interference problems caused by long LED leads.
5. All Ao output ports have short-circuit protection, so even if a load short circuit occurs, the internal circuit of the control unit will never be damaged.
The following diagram shows the light source drive circuit of this control unit:
As can be seen, the reference voltage of the constant current drive circuit is directly given by the DAC, and the value of the DAC is programmed by the control program according to the absorbance requirements of the entire equipment process. Here, we continue the data format commonly used in traditional industrial control. The constant current drive output current range of this control board is 0-20mA (which can be changed to a wider output range; please contact us). The corresponding programming data range is:
0-32000. To drive an LED load with a constant current of 15mA, simply assign the value 24000 to the register AQWx, where x represents the channel number. For channel 0, x=0; for channel 1, x=1, and so on. It's very convenient and intuitive to use, and engineers no longer need to worry about potentiometer knob malfunctions.
The image below shows the actual field application of this control unit, which is used to drive 600nm orange and 660nm red LED light sources with constant current. After long-term testing and verification, its luminous flux and absorbance measured by the equipment are very stable, and the accuracy error can reach ±0.5%.
