There are potential lethal voltages and fire hazards connected with this project. This project is not recommended in any manner for individuals who are not appropriately qualified.
The following information is presented without warranty or guarantee of any kind. The author assumes no responsibility for the use or inability to use this information. The author assumes no responsibility for the inability or otherwise to complete the project. This project uses potentially LETHAL voltages !!!
If you are not sure of what you're doing, DO NOT continue. This information is presented as educational information only. No guarantee is made as to its fitness for any purpose. All risk is assumed by the person who choses to use this information. The authors experience has shown that this circuit is effective, however, any attempt to build and or modify this circuit is at the BUILDERS risk. Extreme care must always be excercised, this is at the builders SOLE RISK.
The desk lamp conversion from incandescent bulb to LED project, required a mains AC to DC power supply that could fit inside the existing enclosure. This negated the use of a step down transformer. As detailed in the design note by Microchip (1) there are a number of ways to convert an AC voltage at the wall outlet into the DC voltage required by the target circuit. The "usual" transformer with rectifier or switched-mode power supply generally require a relatively large number of components and take up considerable space.
A transformerless AC to DC power supply has the advantage of requiring few components and consequently is compact and low cost. However, the disadvantages include (1) can only provide low current, typically a few millamps and generally less than 100mA, (2) no isolation from the AC line voltage (ie DANGEROUS). Nevertheless, applications such as powering LED's or microcontrollers from AC voltage at the wall outlet are potentially suited to such an approach.
The remainder of this section presents further information/theory on the two basic types of transformerless power supplies, borrowing heavily from information in the design note by Microchip (1) and that from Designer Circuits (2). The following sections relate information on the capacitive transformerless circuit that was actually built and tested for powering LED's.
Basic Types of Transformerless Power Supply
There are two basic types of transformerless power supply, resistive and capacitive, referring to the method used to limit the supply current. However, perhaps more generally, a transformerless power supply typically incorporates:
- voltage division
- current inrush limiting
Basically, the AC input voltage charges up an output filter capacitor. The AC voltage is rectified to ensure that the capacitor is only charged and not discharged by the mains. Voltage division ensures that only a small fraction of the input voltage shows up across the output capacitor. Lastly, a Zener diode in parallel with the output capacitor performs basic voltage regulation.
It is recommended that the document by Designer Circuits (2) be consulted for full details (particularly since this document points out deficiencies in the equations/information presented in the Microchip application note (AN954). The following diagram summarises transformerless power supply configurations and tradeoffs.
Since the capacitive configuration is more efficient (no heat generated by the resistor dropping voltage) and with full wave rectification, provides twice the output current, this mode was selected for further use. The disadvantage of this topology being physically larger (capacitor for voltage drop rather than a resistor) and a higher component cost. However, for DIY purposes, these disadvantages are not of consequence (and in terms of power supply for high output white LEDs, the higher current possible with capacitive/full wave rectification is the crucial factor).