Transformerless power supply design guide in details with calculator

Published by MKDas on

From hobbyists to professionals transformerless power supply is one of the common power supply design everyone wants to do. This type of power supply has both advantages and disadvantages. In this article, I’ll discuss in detail this subject and will see several methods with calculations. Kindly keep reading.

Disclaimer: Electricity is always dangerous. Skill required to work with electricity. Do work with your own risk. Author will not be responsible for any misuse or harmful act. This website contents is also copyright protected. Anything copied and directly posted in your website and claiming it as yours is prohibited and nonsensical. Author published all the articles as opensource to help you making your project and learning purpose only. Learn and make one for yourself. If need any help feel free to ask the author. Author will be helpful to you. Thanks.


An electrocution hazard exists during experimentation with transformerless circuits that interface to wall power. There is no transformer for power-line isolation in the following circuits, so the user must be very careful and assess the risks from line-transients in the user’s application. An isolation transformer should be used when probing the following circuits.

About transformerless power supplies:

Transformerless power supplies are very common in small AC appliances where user interface is not direct such as sensor circuits, remote controlled devices, indicators etc. But it is very useful too for small AC loads because of its light weight and low price. Only two main disadvantages are there is no isolation in these circuits from AC main line and current providing capacity is low. These are ideal power supply for a small circuit based on micro-controller and some low current accessories. That is why transformerless power supplies are so popular.

Transformerless power supplies can be designed either with resistor based or capacitor based. So in this way, we can device the transformerless power supplies in two types.

  1. Resistive transformerless power supplies
  2. Capacitive transformerless power supplies.

In this article we’ll see both on the above as well as will design some others too with mixing various components.

Referance: Application Note AN954

Capacitive power supply:

A capacitive power supply is shown here:

Here resistor R1 (470 Ω/0.5W) is used as inrush current limiting resistor. Capacitor C1 (0.47µF 250V) is used to reduce the total current. D1 (5.1V ) zener diode is regulating the output and diode D2 is rectifing to get DC output. Capacitor C2 is polare capacitor is used as the bypass capacitor.

This calculation table is created by and the The formulas and information come from Microchip application note AN954, written by Reston Condit.

The capacitor C1 must be X2 type capacitor. Class X2 capacitors are in applications where the peak voltage is equal to below 2500 volts. X3 capacitors are used where the peak voltage is less than or equal to 1200 volts. X2 capacitors are the most common. Y capacitors on the other hand are connected from line to ground.

Get your values from this table.

Input variables:
Line frequency Hz
RMS voltage V
D1 (ZD) breakdown voltage V
C1 nominal capacitance uF
R1 nominal resistance Ohms
D2 forward voltage drop V
C2 capacitaance value uF
C1 tolerance %
R1 tolerance %
Design Results:
Nominal output voltage below current limit V
Output ripple at nominal current mV
Available nominal current mA
Worst case available current mA
Best case available current mA
Worst case R1 power dissipation (pick double) W
Worst case D1 power dissipation (pick double) W
Worst case D2 power dissipation (pick double) W
Minimum rated voltage for C1 (pick double) V
Total consumed power max W

Now you can select your components very easily to design your own transformer based power supply.

Advantage of this capacitor power supply:

  1. This transformerless capacitor power supply is significantly small and light weight than the transformer based power supply
  2. Sufficient for small loads such as micro-controllers and some small sensor based circuits
  3. Definately cost effective comparing with transformer based power supply
  4. Capacitor power supply is efficient than the resistor based power supply.

Disadvantages of capacitor power supply:

  1. Not isolated from AC lines.
  2. Slightly costly than the resistor based power supply
  3. Current delivering capacity is much less than the transformer based power supplies

Resistive power supply:

The resistive power supply is designed replacing the X2 capacitor C1 of the previous circuit by a resistor.

But here you need to use a high watt resistor due to the resistive loss. That is why this type of power supply is not used so frequently. The recommended circuit is with the capacitor.

Capacitor power supply with safety:

When a good transformer-less power supply is required, we should add some protections to reduce the transient faults as well as to remove the spikes.

Here, a fuse is used at the very beginning of the circuit to protect from any short circuit and transient effects. Then VR1 is a varistor to clip any spike from a rated voltage. Then R2 across the Capacitor C1 is used as the bleeder resistor to discharge the C1 when power is turned off. This way, good safety can be ensured.


Transformerless power supplies are very useful in small applications especially for micro-controller or sensor-based wall receptacles. Both the capacitive and resistive transformerless power supply offer substantial cost and space-saving but the selection of the type of the circuit is important according to the type of loads.

I hope this project was helpful to you. If you make one for yourself, it will be a great pleasure for me. Anywhere you need help, let me know. Please share this project and subscribe to my blog. Thank you.


Mithun K. Das; B.Sc. in EEE from KUET. Blog:


Leave a Reply

Your email address will not be published. Required fields are marked *