This circuit can be used to test up to three LEDs at once, connected in series. You can easily increase that number by using a higher supply voltage. This simple LED TV Backlight Tester consists of a current source with a potentiometer that can be used to adjust the current. The current source is implemented using a type TL081 opamp. The output current of the opamp flows through the diode and R2. The voltage drop across R2 is fed back to the inverting input and compared with the reference voltage, which is set with R1 and applied to the non-inverting input.
The adjustment range is approximately 0–30 mA, which is suitable for testing all normal LEDs. If you wish, you can connect a multimeter across the LED to measure the voltage on the LED TV Backlight Tester. For the power source, a good option is to use a small laboratory power supply with the output voltage set to 5 V. It is convenient to fit the potentiometer with a scale so you can see directly how much current is flowing through the LED. In order to calibrate the scale, you can temporarily connect an ammeter in place of the LED.
The TL081 is a low-cost high-speed JFET input operational amplifier with an internally trimmed input offset voltage (BI-FET IITM technology). The device requires a low supply current and yet maintains a large gain bandwidth product and a fast slew rate. In addition, well matched high voltage JFET input devices provide very low input bias and offset currents. The TL081 is pin compatible with the standard LM741 and uses the same offset voltage adjustment circuitry. This feature allows designers to immediately upgrade the overall performance of existing LM741 designs.
TL081 features and specifications:
- Supply Voltage: 7V to 36 V
- GBP – Gain Bandwidth Product: 3 MHz
- SR – Slew Rate: 13 V/us
- CMRR – Common Mode Rejection Ratio: 70 dB to 86 dB
- Operating Supply Current: 1.4 mA
- Input Bias Current: 200pA
- Power Dissipation:670mW
- Available in 8-Pin PDIP, SOIC Packages