The post explains the importance of a zero crossing detection in capacitive transformerless power supplies in order to make it completely safe from the mains switch ON inrush surge currents. The idea was proposed by Mr. Francis.

Technical Specifications

I have been reading about the transformer less power supply articles on your site with great interest and if I am understanding correctly the main problem is the possible in-rush current in the circuit upon switching-on, and this is caused because switching-on does not always occur when the cycle is at zero volts (zero crossing).

I am a novice in electronics and my knowledge and practical experience are very limited, but if the problem can be solved if zero crossing is implemented why not use a zero crossing component to control it such as an Optotriac with zero crossing.

The input side of the Optotriac is low power therefore a low power resistor can be used to lower the mains voltage for Optotiac operation. Therefore no capacitor is used at the Optotriac’s input. The capacitor is connected on the output side which will be switched on by the TRIAC which turns on at zero crossing.

If this is applicable it will also solve high current requirement problems, since the Optotriac in turn can operate another higher current and/or voltage TRIAC without any difficulty. The DC circuit connected to the capacitor should no longer have the in-rush current problem.

It would be nice to know your practical opinion and thank you for reading my mail.


The Design

As rightly pointed out in the above suggestion, an AC input without a zero crossing control can be a major cause of a surge current inrush in capacitive transformerless power supplies.

Today with the advent of sophisticated triac driver opto-isolators, switching an AC mains with zero crossing control is no longer a complex affair, and can be simply implemented using these units.

About MOCxxxx Opto-couplers

The MOC series triac drivers come in the form of optocouplers and are specialists in this regard and can be used with any triac for controlling AC mains through a zero crossing detection and control.

The MOC series triac drivers include MOC3041, MOC3042, MOC3043 etc all these are almost identical with their performance characteristics with only minor differences with their voltage spces, and any of these can be used for the proposed surge control application in capacitive power supplies.

The zero crossing detection and execution are all internally processed in these opto driver units and one has to only configure the power triac with it for witnessing the intended zero crossing controlled firing of the integrated triac circuit.

Before investigating the surge free triac transformerless power supply circuit using a zero crossing control concept let's first understand briefly regarding what's a zero crossing and its involved features.

What is Zero Crossing in AC Mains

We know that an AC mains potential is composed of voltage cycles which rise and fall with changing polarity from zero to maximum and vice versa across the given scale. For example in our 220V mains AC, the voltage switches from 0 to +310V peak) and back to zero, then forwarding downwards from 0 to -310V, and back to zero, this goes on continuously 50 times per second constituting a 50 Hz AC cycle.

When the mains voltage is near its instantaneous peak of the cycle, that is near 220V (for a 220V) mains input, it's in the strongest zone in terms of voltage and current, and if a capacitive power supply happens to be switched ON during this instant, the entire 220V can be expected to break through the power supply and the associated vulnerable DC load. The result could be what we normally witness in such power supply units.... that is instant burning of the connected load.

The above consequence may be commonly seen only in capacitive transformerless power supplies because, capacitors have the characteristics of behaving like a short for a fraction of a second when subjected to a supply voltage, after which it gets charged and adjusts to its correct specified output level

Coming back to the mains zero crossing issue, in a converse situation while the mains is nearing or crossing the zero line of its phase cycle, it can be considered to be in its weakest zone in terms of current and voltage, and any gadget switched ON at this instant can be expected to be entirely safe and free from a surge inrush.

Therefore if a capacitive power supply is switched ON in situations when the AC input is passing through its phase zero, we can expect the output from the power supply to be safe and void of a surge current.

How it Works

The circuit shown above utilizes a triac optoisolator driver MOC3041, and is configured in such a way that whenever power is switched ON, it fires and initiates the connected triac only during the first zero crossing of the AC phase, and then keeps the AC switched ON normally for rest of the period until power is switched OFF and switched ON again.

Referring to the figure we can see how the tiny 6-pin MOC 3041 IC is connected with a triac for executing the procedures.

The input to the triac is applied through a high voltage, current limiting capacitor 105/400V, the load can be seen attached to the other end of the supply via a bridge rectifier configuration for achieving a pure DC to the intended load which could an LED.

How Surge Current is Controlled

Whenever power is switched ON, initially the triac stays switched OFF (due to an absence of the gate drive) and so does the load connected to the bridge network.

A feed voltage derived from the output of the 105/400V capacitor reaches the internal IR LED through the pin1/2 of the opto IC. This input is monitored and processed internally with reference to the LED IR light response.... and as soon the fed AC cycle is detected reaching the zero crossing point, an internal switch instantly toggles and fires the triac and keeps the system switched ON for the rest of the period until the unit is switched OFF and ON yet again.

With the above set up, whenever power is switched ON, the MOC opto isolator triac makes sure that the triac is initiated only during that period when the AC mains is crossing the zero line of its phase, which in turn keeps the load perfectly safe and free from the dangerous surge in rush.

Improving the above Design

A comprehensive capacitive power supply circuit having a zero crossing detector, a surge suppressor and voltage regulator is discussed here, the idea was was submitted by Mr. Chamy

Designing an Improved Capacitive Power Supply Circuit with Zero Crossing Detection

Hello Swagatam.

This is my zero crossing, surge protected capacitive power supply design with voltage stabilizer,i will try to list all of my doubts.
(I know this will be expensive for the capacitors,but this is only for testing purposes)

1-I'm not sure if the BT136 haves to be changed for a BTA06 for accommodating more current.

2-The Q1 (TIP31C) can handle only 100V Max. Maybe it should be changed for a 200V 2-3A transistor?,like the 2SC4381.

3-R6 (200R 5W),I know this resistor is pretty small and its my
fault,i actually wanted to put a 1k resistor.But with an 200R 5W
resistor it would work?

4-Some resistors have been changed following your recommendations to make it 110V capable.Maybe the 10K one needs to be smaller?

If you know how to make it work correctly,i will be very happy to correct it.If it works i can make a PCB for it and you could publish it in your page (For free of course).

Thank you for taking the time and viewing my full of faults circuit.

Have a nice day.


Assessing the The Design

Hello Chamy,

your circuit looks OK to me. Here are the answers to your questions:

1) yes BT136 should be replaced with a higher rated triac.
2)TIP31 should be replaced with a Darlington transistor such as TIP142 etc otherwise it might not work properly.
3) when a Darlington is used the base resistor could be high in value, may be a 1K/2 watt resistor would be quite OK.
However the design by itself looks like an overkill, a much simpler version can be seen below



Zero Crossing Circuit

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