Transformerless power supply design guide in detail with calculator

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. So let’s start our “Transformerless power supply design with calculator”.

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***Warning:

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 the 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 lightweight and low price. The only two main disadvantages are there is no isolation in these circuits from the AC mainline and the current providing capacity is low. These are ideal power supplies for a small circuit based on a micro-controller and some low-current accessories. That is why transformerless power supplies are so popular.

Transformerless power supplies can be designed either resistor-based or capacitor-based. So in this way, we can devise 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.

Reference: Application Note AN954

Capacitive power supply:

A capacitive power supply is shown here:

Here resistor R1 (470 Ω/0.5W) is used as an 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 rectifying to get DC output. Capacitor C2 is a polar capacitor is used as the bypass capacitor.

This calculation table is created by http://www.nomad.ee and 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.

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Get your values from this table.

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 to replace the X2 capacitor C1 of the previous circuit with 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.

Conclusion:

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.

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