SMD 0805 LED power supply current, current limiting resistance and brightness
The relationship table of current, voltage and brightness of an SMD 0805 LED:
Vf If（算） 亮度
1.74v 0.46mA 做指示灯不刺眼刚刚好（推荐）
1.81v 1.7mA 同上
1.86v 4.14mA 还可以，估计晚上看刺眼
1.89v 8mA 好刺眼（可以放在半透明的东西里当指示灯）
1.91v 14mA 同上（电阻好烫）
1.90v 18.1mA 同上，发热大
1.90v 20mA 同上，发热大，光衰加快，寿命较短</span>
in conclusion:
If: It can work normally within 20mA, but the greater the current, the greater the heat generation and the shorter the life span. The light can be seen at 0.1mA, and it is recommended to use about 1mA when the indicator light is used;
Vf: The voltage variation range is less than 0.2v.
If used as an indicator, generally 2mA-5mA is sufficient. The current-limiting resistor needs to be calculated, your working voltage-LED voltage drop (usually 2V for red, 3V for green and blue, depending on the specific situation) / the current you need, the resistance value of the current-limiting resistor is obtained.
For example: your working voltage is 5V, the red voltage drop of the diode is 2V, and the design current is 2mA, then the resistance value = (5-2)/0.002=1500, then take a 1.5K resistance.
After testing, the results are as follows:
The basic turn-on voltage drop of the three LEDs, BLUE, GREEN, and WHITE, is above 3V, which may seem to be 3V;
The two conduction voltage drops of RED and YELLOW are slightly less than 2V.
It is convenient for everyone to calculate the current-limiting resistance.
The parameters of the highlighted led provided:
Red voltage drop: 1.82-1.88V Current: 5-8mA
Green voltage drop: 1.75-1.82V Current: 3-5mA
Orange voltage drop: 1.7-1.8V current: 3-5mA
Blue voltage drop: 3.1-3.3V Current: 8-10mA
White voltage drop: 3.0-3.2V current: 10-15mA
The above are all 5mm parameters, measured by ourselves, power supply voltage: 5V, for reference only.
Q: It is stated in the specification that the normal current is allowed to be 25mA for a long time and the instantaneous current is 0.1ms/100mA.
I now design a 40mA current with a 2ms pulse width, and I don’t know if there is any problem.
In actual use, there are about 1W grains, and there has been no problem after aging for 2 hours, but it does not mean that it will not be damaged in the future.
I asked the manufacturer’s technicians and said that there were no such parameters, and they had not done any tests.
Is the life of the LED tube only related to the heat dissipation power?
A: 2ms pulse width 40mA current, whether there is any problem depends on the duty cycle. If the duty cycle is 100%, the long-term continuous working current cannot exceed 20mA at most. If the duty cycle is 50%, a pulse current of 40 mA should be no big problem. You can equivalently convert it into the power that needs to be consumed during long-term continuous work, because the life of the LED is directly affected by the useless work (heating).
Q: The duty cycle is 1/7, the entire scanning cycle is 12.8ms, and the lighting time of each line is 2ms less than 1.8ms.
A: 1/7, equivalent to 12.5%. 40mA is converted to 5mA in time.
Is the brightness enough?
Q: The brightness is enough. If the instantaneous current is reduced, the brightness is not enough. But the lighting time of each line in the program can only be so long, and it will jitter if it is longer.