Using ultra-bright white light-emitting diodes for lighting is an excellent way to save energy. Because it takes enough LEDs to be bright enough, it can be awkward to install, and it's not very practical (about 100) to fit in the living room. If installed in the toilet, it works well. Because the toilet area is small, the brightness requirement is not high. The toilet of the author has 12 LEDs that are bright enough, but it must be scattered and re-attached to the ceiling. The distance should not be less than 10 cm. Otherwise, the effect is poor.
The 12 LEDs are connected in parallel, and I originally intended to step down with a transformer. However, the 220V/3V transformer is not easy to buy. It was only necessary to use a capacitor to step down the power after rectification and filtering, as shown in Figure 1. In this case, the circuit is very simple, but with 220V AC mains. Not safe. In addition, due to the large capacitance C1, sparks are generated when the switch K1 is operated. After using it for a while, I switched it to a switching power supply. This is once and for all.
It consists of two parts. The first half is drawn in a dashed box, which is a switching power supply that turns 220V AC into 6V DC. The power supply can be replaced by a mobile phone charger, so its working principle need not be described here. The second half is a switching power supply that uses 6V DC down to 3.2V.
In the step-down switching power supply, the integrated circuit 555 forms a rectangular wave generator and adjusts its duty cycle so that the output voltage can be adjusted. Because the square wave output from 555 charges C4 through Q1, the voltage across C4 is the output voltage. The size of the rectangular wave duty cycle determines the level of the C4 charging voltage. When the duty cycle is 50 percent, the output voltage is one-half that of 3V. Change the size of R1 in the circuit, can change the rectangular wave duty ratio, thus realize the purpose of regulating the output voltage. Since the LED is directly connected to the output, if the output voltage is too high, the LED will be burned out. Therefore, when changing the output voltage of R1, the LED must be replaced with an 18 Ω dummy load (ie 3.2V180mA) and monitored simultaneously. The voltage at both ends must not exceed 3.2V! That is to say, R1 is adjusted well and it is finalized before the LED can be connected.
The commercially available LED parameters are uneven and should be selected beforehand. The current of 15mA is required to flow at 3.2V and the brightness must be the same. In addition. Because both transistors operate in the on-off state, the losses are small and do not generate heat. So do not have to add a radiator.