Simple Transformerless Power Supply Circuit

This post shows you how to build a small power supply circuit that takes high voltage wall outlet power and turns it into lower voltage for your electronics.

It does this in a special way, without needing a bulky transformer.

WARNING: Building circuits with high voltage can be dangerous.

Only do this with adult supervision.

What is a Transformerless Power Supply:

A Transformerless Power Supply is a type of power supply design that operates without the use of a traditional transformer.

In typical power supplies, a transformer is employed to step up or step down the voltage level and isolate the input and output circuits.

However, transformerless power supply circuits eliminate the need for a transformer making the circuit more compact and often more cost effective.

Transformerless Power Supply Concept:

A transformerless power supply circuit as the name suggests, converts high voltage AC from the mains into low DC without utilizing a transformer or inductor.

The key principle involves using a high voltage capacitor, such as the 105 to 400V capacitor, in series with one of the mains inputs to drop the AC voltage to a suitable level for the connected electronic circuit.

Component Details:

The chosen capacitors RMS peak voltage rating should exceed the peak of the AC mains voltage to ensure safe operation.

For example, a 1uF 400V capacitor is commonly used in transformerless power supply circuits.

The capacitors reactance restricts the mains AC current but the voltage remains unaltered.

Addressing Voltage Peaks:

To address the high peak voltage issue, a zener diode is introduced at the output of the bridge rectifier.

Proper selection of the zener diode wattage in accordance with the permissible current from the capacitor helps stabilize the voltage and ensures safe operation.

Note on Usage:

These transformerless power supply circuits are specifically designed for powering small circuits and should be enclosed in well insulated boxes to maximize safety.

They are not intended for use as AC to DC adapters for external gadgets.

Advantages:

The transformerless power supply circuit proves to be a cost effective and compact solution for applications requiring low power.

It efficiently replaces traditional transformers especially for applications with a current requirement below 100 mA.

Disadvantages:

While the circuit has its advantages it does come with limitations.

It cannot produce high current outputs and the lack of isolation from dangerous AC mains potentials poses a risk especially for designs with exposed outputs or metal cabinets.

Additionally, the circuit allows voltage surges which may damage both the powered circuit and the supply circuitry.

Circuit Operation:

Parts List:

The working of the transformerless power supply involves the capacitor C1 limiting the mains current while allowing the full mains voltage to reach the bridge rectifier.

The rectified output is then stabilized through a zener diode stage and filtered using capacitor C2 resulting in a clean and regulated DC output.

Detailed Component Functions:

• Capacitor C1 serves as the crucial component for reducing high current from the mains to the desired level.
• Resistor R1 provides a discharge path for C1 to prevent high voltage shocks when the circuit is unplugged.
• Diodes D1 D4 function as a bridge rectifier converting low current AC to a peak DC voltage.
• Zener diode ensures the output voltage is within the desired range.
• Resistor R2 acts as a current limiting resistor to prevent damage during power switch ON periods.
• Capacitor C2 functions as a filter to smooth out ripples from the rectified bridge ensuring a clean DC output.

Enhancements to the Circuit:

The article introduces three improvements to the circuit including current limiting resistors R2 and R3 an additional capacitor C3 to absorb switch ON surge current and an increased wattage zener diode for enhanced stability.

Formulas and Calculations:

Finding the reactance of the capacitor: which is essentially its resistance is the first step in calculating the reactance of a capacitor.

The formula for reactance is:

X_c = 1 /( 2πf_C)

where,

• X_c = reactance
• π = 22/7
• f = frequency
• C = capacitor value in farads

The above calculation yields an output in ohms, which may be directly entered into our earlier discussion of ohms law.

To better grasp how to apply the aforementioned concepts, lets solve the following example:

Lets investigate the maximum current that a 2uF capacitor may provide to a specific load:

We currently possess the following information:

π = 22/7 = 3.14

f = 50 Hz (mains AC frequency)

and C= 2uF or 0.000002F

Using the information above to solve the reactance equation yields:

XC = 1 / (2 x 3.14 x 50 x 0.000002)

= 1592.35  ohms approximately

Using our ohms law calculation and substituting this equivalent resistance value, we obtain:

R = V/I

or I = V/R

Assuming V = 220V (since the capacitor is intended to work with the mains voltage.)

We get:

I = 220/1592.35

= 0.13 amps or 130 mA approximately

The calculation of a capacitor current in any relevant circuit especially transformerless capacitive power supplies is explained in detail in the section above.

How to Build the Circuit:

Building a transformerless power supply circuit involves the below mentioned steps:

Safety Precautions

• Ensure you are working in a well ventilated area.
• Wear appropriate safety gear including insulated gloves and goggles.
• Keep the work area dry and uncluttered.

Necessary Components

• Verify that you have all the necessary components and tools.

Circuit Diagram

• Refer to the above circuit diagram and double check component values.

Placing Components

• Place the capacitor C1 in series with one of the mains inputs preferably the phase line of the AC.
• Connect the zener diode at the output of the bridge rectifier.
• Mount resistors R1, R2, R3 at their designated locations.
• Arrange diodes D1-D4 in a bridge rectifier configuration.
• Place the filter capacitor C2 at the output.

Wiring

• Connect components using insulated wires following the circuit diagram.
• Pay careful attention to the polarity of diodes and capacitors.
• Ensure a secure and neat wiring layout.

Additional Capacitor

• If using an additional capacitor for surge protection C3, connect it according to the circuit design.

Insulation

• Insulate all exposed connections and points to prevent electric shock.
• Consider using an insulated box to enclose the entire assembly.

Testing

• CAUTION: Exercise extreme caution during testing, as the circuit is not isolated from AC mains.
• Ensure the assembly is properly insulated and enclosed before testing.
• Monitor the output voltage and check for any anomalies.

Adjustment

• If necessary adjust resistor values or zener diode selection to achieve the desired output voltage.

Final Enclosure

• Once testing is successful enclose the entire circuit in a well insulated box to enhance safety.

Safety Warning:

It is crucial to exercise extreme caution while testing these circuits as they are not isolated from AC mains and can pose a risk of lethal electric shock.

Adequate insulation of all exposed connections is mandatory.

Conclusion:

Building a transformerless power supply circuit requires careful attention to detail and adherence to safety guidelines.

If you are unfamiliar with working with mains voltage it is advisable to seek assistance from someone experienced in electronics.

Always prioritize safety and follow best practices when handling electrical components.

References

Transformerless power supply

Transformerless power supply calculations