Simple Transformerless Power Supply Circuit

The article describes how to create a circuit for a small power supply that transforms high voltage wall socket electricity into lower voltage electronic supplies.

Without using a larger transformer the design does this in an innovative manner.

WARNING: Building circuits with high voltage can be dangerous.

Only do this while an adult is watching.

What is a Transformerless Power Supply:

A transformerless power supply circuit is a type of power supply source that operates without using a standard transformer.

A transformer divides the input and output circuits and adjusts the voltage level in a standard power supply.

On the other hand circuits for transformerless power supplies do not require a transformer which makes them smaller as well as less costly.

Transformerless Power Supply Concept:

As the name suggests a transformerless power supply circuit uses neither an inductor nor a transformer to transfer high voltage AC from the mains into low DC.

The basic idea is to lower the AC voltage to a adequate level for the connected electronic circuit by placing a high voltage capacitor, like a 105 to 400V capacitor in series with one of the primary inputs.

Component Details:

To ensure safe operation the RMS peak voltage rating of the chosen capacitors should be higher than the peak of the AC mains voltage.

For example transformerless power supply circuits use a 1uF 400V capacitor.

The voltage stays constant but the mains AC current is limited by the capacitors reactance.

Addressing Voltage Peaks:

To deal with the issue of overpowering peak voltage a Zener diode is added to the bridge rectifiers output.

The power rating of the Zener diode should be selected properly based on the approved current from the capacitor in order to maintain the voltage and to maintain safe functioning.

Note on Usage:

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

For external devices they are not designed to be used as AC to DC converters.

Advantages:

For low power applications the transformerless power supply circuit proves out to be a cost effective choice.

Particularly for situations where the required current is less than 100 mA it easily replaces standard transformers.

Disadvantages:

Despite its benefits the circuit has disadvantages too.

It is unable to generate large current outputs and designs with exposed outputs or metal cabinets which are most at risk due to the lack of split from deadly AC mains potentials.

Also, the circuit causes voltage spikes that might harm the supply circuitry as well as the powered circuit.

Circuit Working:

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.

A zener diode stage maintains the rectified output and capacitor C2 filters it to produce a clean controlled DC output.

Detailed Component Functions:

• Reducing excess from the mains to the right level, requires the use of capacitor C1.
• To avoid high voltage shocks when the circuit is disconnected resistor R1 gives C1 a discharge way.
• Bridge rectifiers which are made up of diodes D1 and D4 transform low current AC into a peak DC voltage.
• A Zener diode makes sure the output voltage stays within the desired range.
• Resistor R2 acts as a current limiting resistor that protects against damage when the power switch is turned on.
• Capacitor C2 ensures a clean DC output by acting as a filter to reduce ripples from the rectified bridge.

Enhancements to the Circuit:

Three circuit adjustments are presented in the article:

• Current limiting resistors R2 and R3
• An extra capacitor C3 to absorb switch ON surge current
• And a higher wattage zener diode for better stability.

Formulas and Calculations:

The first step to figure out a capacitors reactance is to calculate its reactance which is basically its resistance.

The formula for reactance is:

X_c = 1 /( 2πf_C)

where,

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

The output in ohms from the previously mentioned calculation is easily applied to our earlier discussion of ohms law.

Lets go through the following example to learn more about how to apply the earlier discussed ideas:

Let us see how much current a 2uF capacitor can deliver to a particular load at its maximum:

As of right now we know the following:

π = 22/7 = 3.14

f = 50 Hz (mains AC frequency)

and C= 2uF or 0.000002F

Using the above provided data the reactance formulas can be solved as follows:

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

= 1592.35  ohms approximately

We can obtain the following by adding this comparable resistance value into our ohms law calculation:

R = V/I

or I = V/R

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

We get:

I = 220/1592.35

= 0.13 amps or 130 mA approximately

The previous part provides a detailed explanation of how to calculate a capacitor current in any applicable circuit particularly transformerless capacitive power sources.

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.
• One should wear proper safety gear including insulated gloves and goggles.
• Keep the work area dry and clean.

Necessary Components:

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

Circuit Diagram:

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

Placing Components

• Place the capacitor C1 in series with one of the mains inputs most likely the phase line of the AC.
• Connect the zener diode at the output of the bridge rectifier.
• Install resistors R1, R2, R3 at their correct 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.
• Be careful to the polarity of diodes and capacitors.
• Ensure a secure and neat wiring layout.

Additional Capacitor:

• Connect the additional capacitor C3 to comply with the circuit design if you are using it for surge protection.

Insulation:

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

Testing:

• Because the circuit is not separated from the AC mains operate with extreme care during testing.
• Before testing make sure the circuit assembly is properly enclosed and insulated.
• Keep an eye on the output voltage and look for any issues.

Adjustment:

If needed adjust the resistor values or Zener diode selection to get the proper output voltage.

Safety Warning:

Extra care must be taken while testing these circuits because they are not separate from the AC mains and provide a serious risk of electric shock.

Every connection that is visible needs to be well insulated.

Conclusion:

Following safety rules and paying close attention to specifications are essential while building a transformerless power supply circuit.

It is best to get help from an electronics expert if you are not comfortable with working with mains voltage.

When working with electrical components always put safety first and stick to standard procedures.

References:

Transformerless power supply

Transformerless power supply calculations