What is Transformerless Power Supply & Its Working

In general electronic products, the DC power supply converts the AC voltage to a small DC voltage by using a step-down transformer. Switch-mode power supply or step down transformer converts higher AC to lower AC voltage and then to the desired DC low voltage. This process has the main disadvantage that the cost will be high and requires more space during manufacturing and designing a product. So, in order to overcome these disadvantages, a transformerless power supply is used. It is nothing but a switch-based power supply. This article describes the transformerless power supply with 12V.

What is a Transformerless Power Supply?

Definition: A transformerless power supply converts high AC input voltage ( 120V or 230V) to the desired output DC low voltage ( 3V or 5V or 12V) with low current output in milliamps. It is used in low power electronic applications like LED bulbs, toys, and household appliances. It is cost-effective, and need less space.


Working Principle

The basic working principle of the transformerless power supply is a voltage divider circuit that converts single-phase AC high voltage to a desired low DC voltage without any use of transformer and inductor. The whole concept of this power supply involves Rectification, voltage division, regulation, and inrush limiting. The basic circuit of the transformerless power supply is shown below.

Basic Circuit Diagram of Transformerless power Supply
Basic Circuit Diagram of Transformerless power Supply

The single-phase AC high voltage (120V or 230V) is converted into low DC voltage ( 12V or 3V or 5V). Diodes are used to rectify and regulate the desired DC voltage. The capacitor connected in series with the AC restricts the flow of AC current due to its reactance. It controls the flow of current to a specific value depending on its type.

Generally, an X-rated capacitor is used in this power supply. The resistor is used to dissipate the excess energy in the form of heat and the current. Diodes are used to rectify AC high voltage to DC low voltage. The bridge rectifier circuit removes the negative voltage and stabilizes the peak voltage by the process of rectification. A Zener diode is used to remove the ripples and regulate the voltage. An LED is connected to test the circuit.

Transformerless Power Supply Construction/Design

The construction of this power supply is very simple. It uses a non-polarised capacitor 225k/400v in series with main AC supply voltage and connected with a resistor 470k/1W in parallel to discharge the current (the circuit is switched off) and prevent from shock. The capacitor maintains the flow of current to a constant value due to its reactance. Since the reactance of the capacitor is higher than the resistance of the resistor. The X-rated capacitor is used to drop the current flow and its operating voltage is 250V to 600V.

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The bridge rectifier circuit with 4 diodes for the rectification purpose. It concerts AC to DC (220VAC to 310VDC). The capacitor C2 470μF/100V is used for filtration. It removes the ripples from the obtained output voltage and maintains the peak voltage. A Zener diode is used as a regulator to convert into desired DC voltage (5V or 3V or 12V )depending on the application. The resistor R3 220Ώ/1W is for inrush limiting and acts as a current limiting resistor.

Transformerless Power Supply Circuit Diagram

The circuit diagram of this power supply is shown below.

Transformerless Power Supply Circuit Diagram
Transformerless Power Supply Circuit Diagram

This type of power supply converts high AC voltage to low DC voltage without any use of transformer and inductor. It is mainly used in low power electronic applications. The use of a transformerless power supply reduces the cost of electronic products and consumes less space during manufacturing and designing. These are available in small size and small weight compared to the transformer-based or switch-based power supplies. The main disadvantage of this type is, there is no isolation between the input main AC high voltage to output, and this results in failure and safety issues of the circuit.

Types of Transformerless Power Supply

These are available in two types which include the following.

Resistive Transformerless Power Supply

The resistor is used across a voltage dropping resistor to drop the excess energy as heat. It limits the excess current due to its resistance. The voltage dropping resistor dissipates the power. The resistor with double rated power is used because more power is dissipated across it.

Capacitive Transformerless Power Supply

it is more efficient because the heat dissipation and power loss are low. In this type, an X-rated capacitor with 230V or 600V or 400V is connected in series with the mains to drop the voltage and acts as a voltage dropping capacitor.

The major difference between the resistive and capacitive type is, the excess energy is dissipated as heat across the voltage dropping resistor and while in the capacitive type, excess voltage is dropped across the voltage dropping resistor without any heat dissipation and energy loss

Transformerless Power Supply 12v

The above diagram represents the transformerless power supply 12V. It is nothing but converting 220V main AC voltage to 12V DC voltage with the use of the capacitor, resistor, bridge rectifier, and Zener diode. From the above figure, C1 is used as an X-rated capacitor to drop the high AC voltage. The bridge rectifier (D1, D2, D3, D4) converts AC to DC by means of rectification. It converts 230V AC to high 310V DC due to the Peak RMS in the AC signal. The capacitor C2 removes the ripples from the obtained DC voltage.

The resistor R1 removes the stored current when the circuit is switched off. The resistor R2 limits the flow of excess current and used for inrush limiting. A Zener diode is used to remove the peak inverse voltage, stabilizes, and regulates the output DC voltage to 12V. An LED is connected to the circuit to check whether it is working or not. The whole circuit is enclosed with a shockproof case to avoid electric shocks and damages. For isolation purposes from the main AC supply, a small isolated transformer can be connected at the input of the supply.

Applications

The applications of Transformerless power supply 12v include low power and low-cost applications like

  • Mobile chargers
  • LED bulbs
  • Electronic toys
  • Emergency lights
  • Voltage divider and regulator circuits
  • Television receivers
  • Analog to digital converters
  • Telecommunication systems
  • Digital communication systems etc.

Thus, this is all about the transformerless power supply 12V-definition, theory, construction, types, and applications. Here is a question for you, “What are the advantages and disadvantages of transformerless power supply

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