Transformerless AC-DC

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.

Disclaimer

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.

Background

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:

• rectification
• voltage division
• regulation
• filtering
• 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.

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  Transformerless Power Supply Types

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).

A capacitive type transformerless AC to DC power supply was selected for construction, as this topology is more efficient, i.e., the capacitor impedance for dropping the AC voltage avoids the heat loss associated with the alternative resistive method. 1.5A glass passivated bridge rectifiers (eg WG06 max RMS 420V) are readily available in a small form factor package at low cost. Therefore, full-wave rectification was selected as this mode enables twice the current compared to half-wave rectification.

Circuit Operation

Refering to the schematic in the following section, the capacitive reactance of C1 drops the 240 volt AC mains to a lower voltage AC. It is important that a X2 rated metallised film capacitor is used for C1, as this capacitor will be directly connected to the mains (choose voltage appropriate to the mains voltage in question). Capacitors with X2 rating are specifically designed for AC voltage. The value of C1 is calculated using the formula Xc = 1/(2 * π * f * C) where Xc is capacitive reactance in ohms, f is mains frequency in Hertz and C is capacitance in farads. A mains frequency of 50Hz with a 1uF capacitor gives Xc approximately 3183 ohms. This with a mains Vrms of 230 volts would then provide a current of approximately 230/3183 = 0.071A. To be slightly more accurate, the value of R3 can be added to the value of Xc.

The resistor R2 is a safety precaution to bleed capacitor C1 when the circuit is turned off. Otherwise, there is the potential for capacitor C1 to store a lethal shock. Resistor R3 is to reduce inrush current. The value of R3 is chosen so that it does not dissipate to much power, yet is large enough to limit inrush current. From (2) if a capacitor is connected directly to the mains at an instant when the AC voltage is at a peak value, the large voltage will rapidly charge up the capacitor, which appears (albeit briefly) to be a short circuit. The high current that charges up the capacitance can potentially exceed upstream circuit breaker current limits and produce undesirable arcing at the moment the device is plugged in. A Vp for 240 Vrms is approximately 325V, so with a value of 100 ohms for R3, inrush current should be limited to ~3.25A, well below the typical 10-15A circuit breaker. Based upon a value of 1uF for capacitor C1 (therefore 71mA) and 100 ohms for R3, the power loss due to this extra resistance will be approximately 0.5W as heat. So R3 should be a 1W rating.

The bridge rectifer BR1 converts the AC voltage to a DC voltage (full wave rectification). Since mains AC is involved, a W06G rectifier is used. This provides 1.5A with a surge overload of 50A peak and a maximum RMS input voltage of 420V. While four individual diodes (eg 1N4005) could be used, a single W06G or similar provides a smaller component count, easier layout/connection, for roughly equivalent cost.

Capacitor C2 provides output ripple reduction. With a 50Hz AC input voltage and full wave rectification, the output ripple will be 100Hz with a peak voltage equal to the zener voltage. The magnitude of voltage ripple will vary directly with the amount of load current; more load current will result in a higher magnitude of voltage ripple.

The zener diode and R1 provides the required voltage regulation. The Zener diode introduces a non-obvious drawback common to all transformerless power supplies: constant power consumption regardless of load. Constant average input power input is incurred for a transformerless power supply regardless of whether or not the load draws current. In the worst case-scenario (when the load current is zero) the maximum current for the transformerless supply is being passed through the Zener diode. The power dissipation in the Zener will be the Zener voltage multiplied by the rated output current of the transformerless supply. Thus, the higher the Zener voltage and the higher the power rating of the supply, the more heat the Zener will generate (and therefore the thermal properties of the zener need to be selected appropriately for the circuit/components in question).

The schematic also includes two "optional" components that provide some safety precautions. The inclusion of the fuse is obvious. The MOV (metal oxide varistor) in normal conditions has a very high resistance. When the connected voltage gets higher than the specification of the varistor the resistance immediately goes extremely low. Therefore, the MOV is used to protect the circuitry from over-voltage. The MOV is simply added to the power supply input. When high voltage surges and spikes appear the MOV will short them and protect the following components. The value of the MOV is the clamping voltage and energy dissipation (rather than the resistance). In the case of the transformerless supply, the Suntan TSV20D431K (Jaycar part number RN3404) provides operating voltage of 275VAC, clamping voltage of 710V, peak current 6500A, max energy 190J.

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  Transformerless Supply Schematic

The circuit was constructed on prototyping bread board (see the Photographs Section) and used to power 1W LED's. A zener diode was not used as the 4 x LED's provided the "diode drop" necessary for the voltage regulation component of the circuit.

A number of X2 rated capacitors were available from the "junk box" and were tried with the circuit (see results below). I had a 2.2uF X2 capacitor but this was not used as the ~1uF capacitor was already causing sufficient current (~70mA)to cause heating of the LED's. The LED's were only soldered to a piece of veroboard and did not have heat dissipation fins/capability. At ~70mA the temperature of the LED's increased from ~28^oC to ~38^oC after about 5 minutes (after which testing was stopped).

meas=measured with multimeter

calc=calculated (using measured values as appropriate)

5 x 1W LED's in circuit (forward voltage 3.2-3.6V)

71.5mA = LED very bright (blinding)

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ref001: http://ww1.microchip.com/downloads/en/AppNotes/00954A.pdf

ref002: http://www.designercircuits.com/DesignNote1a.pdf