LED backlighting is used to improve visibility in the dark of displays and controls. The switching circuit of the LEDs in the backlight unit is selected depending on their color and supply voltage.
Instructions
Step 1
The voltage drop across the LED at the operating current is determined experimentally. Take a power supply that has a stabilization mode not only for voltage, but also for current. Gradually increase the current from zero to working (it is usually equal to 20 mA). Then connect a voltmeter to the diode and measure the voltage drop across it. You can do without this, knowing that the voltage drop across the diode is inversely proportional to the radiation wavelength. For infrared (700 nm), it is about 1.8 V, and for violet (400 nm) it can reach 3.6 V. For a white diode, the voltage drop is the same as for a blue one, since it is based on a blue emitting crystal.
Step 2
To ensure that the resistors dissipate as little power as possible, put as many LEDs as possible for a given supply voltage in the daisy chain. For this, the sum of the voltage across the diodes should be two to three volts less than the supply voltage. Subtracting this amount from the supply voltage, you get the voltage drop across the dropping resistor:
Ures = Upit-Udiode * n, where Ures is the required voltage drop across the resistor; V, Usup - supply voltage; V, Udiode - voltage drop across one diode; B, n is the number of diodes connected in series, pcs.
Step 3
Select the resistance of the damping resistor so that at the voltage that drops across it, a current equal to the operating current flows through it. To do this, translate all the initial data into the SI system (for example, 20 mA = 0.02 A) and substitute them in the formula:
R = Ures / I, where R is the required resistance of the resistor; Ohm, Ures - voltage drop across the resistor; V, I - operating current of the LED chain, A.
Step 4
Connect in series, observing the polarity, the required number of LEDs and one damping resistor. Prepare as many of these chains as required. Connect them in parallel, also respecting the polarity. Calculate the current consumed by the backlight unit from the power source using the formula:
Itot = Icir * N, where Itot is the total current; A, Ichain - operating current of one chain; A, N - number of chains, pcs.
The current for which the power supply is designed must be at least 1.5 of the calculated one.
Step 5
Calculate the power consumed by the backlight unit:
P = Usup * Itotal, where P - power consumption; W, Usup - supply voltage; V, Itot - total current.
Having taken the efficiency of the power supply unit equal to 0.7, divide the result by this number, and you will get the approximate power consumed when the backlight unit is operating from the power supply unit from the mains (excluding other consumers connected to it).
