When connected to an AC mains power supply, most LED lamps need a rectifier. Most of the time, the LED lights also need to be electrically separated from the mains. But rectifiers make noise when they switch and add to the price. Here is a simple circuit for an AC-powered LED lamp that doesn't need rectifiers. Low-power mains transformers come with one or more secondary voltages, such as 2.5 Vrms (3.5 Vpeak), 3 Vrms (4.2 Vpeak), 3.3 Vrms (4.7 Vpeak), 4.5 Vrms (6.4 Vpeak), 5 Vrms (7.1 Vpeak), 6 Vrms (8.5 Vpeak), and 6.3 Vrms (8.9 Vpeak), and power ratings from 1.3 VA to 12 VA. The primary current of these transformers is usually between 15 and 80 mA, which is low enough that resistors can be used to limit it without making a lot of heat. Most of these transformers are easy to use because they are mounted on PCBs. On the other hand, LED lamps with a power rating of 1 W to 10 W are enough to light your workstation and a small room. Depending on the technology and color, most LEDs today have a forward drop voltage of between 1.6 V and 3.7 V. Most of the time, these LEDs need between 20 mA and 100 mA of current to work, and sometimes they need

more. The LEDs' specs are very close to those of the small 2.5Vrms-6.3Vrms transformers.. Circuit and how it works With a transformer, this LED lamp doesn't need rectifiers. The current going through the LEDs is limited by the resistance of the transformer and low-cost protection resistors. This page talks about the four-power-level LED lamp. In Fig., the circuit of an LED lamp made with a transformer X1 and a secondary voltage of 4.5 V is shown. The voltage and current peaks over transformer X1 are capped by resistor R1. The noise from the mains doesn't get into the transformer because of R2 and C1. Even though R1, R2, and C1 are not strictly necessary for the lamp to work, they should not be taken out, especially in noisy urban or industrial settings. Switch S2 controls how much power goes to the LEDs. In position 4, the full amount of power is used, but in positions 3, 2, and 1, the power and light are less. The LEDs are connected in series and in pairs so that they can get the full power supply from the transformer. This is because the forward voltage drop of LEDs is about half of the secondary output voltage of the transformer. For a consistent forward voltage drop, all of the LEDs on one branch should be the same type. The type of LEDs and transformer X1 determine how many branches there are for the LEDs. The values of resistors R6 through R8 are chosen based on how much current the LEDs can handle in the forward direction. After choosing the LEDs and transformer X1, it's easy to figure out the values of the current-limiting resistors and how much power they use. The circuit doesn't need any adjustments; when switch S1 is turned on, it will work right away.

After putting the circuit together on a PCB, put it in a good case. Set up all the LEDs so that they stick out of the case they're in. Use wires on the outside of the PCB to connect the four-way rotary switch S2 to it.