Switching Transformerless Power Supply using IC 555

A simple yet smart solution is implemented here using IC 555 in its monostable mode to control in rush surge in a  transfomerless power supply via a zero crossing switching circuit concept, wherein the input power from the mains is allowed to enter the circuit only during the zero crossings of the AC signal, thereby eliminating the possibility of surge inrushes. The idea was suggested by one of the avid readers of this blog.

Technical Specifications

Would a zero cross transformerless circuit work to prevent the initial inrush current by not allowing turn on until the 0 point in the 60/50 hertz cycle?

Many solid state relays which are cheap, less then INR 10.00 and have this ability built in them.

Also I would like to drive 20watt leds with this design but am unsure how much current or how hot capacitors will get I suppose it depends on how the leds are wired series or parallel, but lets say the capacitor is sized for 5 amps or 125uf will the capacitor heat up and blow???

How does one read capacitor specs to determine how much energy they can dissipate.

The above request prompted me to look for a related design incorporating a IC 555 based zero crossing switching concept, and came across the following excellent transformerless power supply circuit which could be used for  convincingly eliminating all possible chances of surge inrush.

What's a Zero Crossing Switching:

It's important to learn this concept first before investigating the proposed surge free transformerless circuit.

We all know how a sine wave of an AC mains signal looks like. We know that this sine signal starts from a zero potential mark, and exponentially or gradually rises to the peak voltage (220 or 120) point, and from there exponentially reverts to the zero potential mark.

After this positive cycle, the waveform dips and repeats the above cycle but in the negative direction until it comes back yet again to the zero mark.

The above operation happens about 50 to 60 times per second depending upon the mains utility specs.
Since this waveform is what enters a transformerless power supply, any point in the waveform other than the zero, presents a potential danger of a switch ON surge due to the involved high current in the waveform.

However the above situation can be avoided if the load confronts the switch ON during the zero crossing, after which the rise being exponential doesn't pose any threat to the load.

This is exactly what we have tried to implement in the proposed circuit.

Simulation and Working

Referring to the circuit diagram below, the 4 1N4007 diodes form standard bridge rectifiers configuration, the cathode junction produces a 100Hz ripple across the line.
The above 100Hz frequency is dropped using a potential divider (47k/20K) and applied to the positive rail of the IC555. Across this line the potential is appropriately regulated and filtered using D1 and C1.

The above potential is also applied to the base Q1 via the 100k resistor.

The IC 555 is configured as an monostable MV which means its output will go high every time its pin#2 is grounded.

For the periods during which the AC mains is above (+)0.6V, Q1 stays switched OFF, but as soon as the AC waveform touches the zero mark, that is reaches below the (+)0.6 V, Q1 switches ON grounding pin#2 of the IC and rendering a positive output of the IC pin#3.

The output of the IC switches ON the SCR and the load and keeps it switched ON until the MMV timing elapses, to begin a new cycle.

The ON time of the monostable can be set by varying the 1M preset.

Greater ON time ensures more current to the load, making it brighter if it's an LED, and vice versa.

The switch ON conditions of this IC 555 based transformerless power supply circuit is thus restricted only when the AC is near zero, which in turn ensures no surge voltage each time the load or the circuit is switched ON.

Circuit Diagram

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Gurpreet said…
Wish you happy vijaya dashami
Swagatam said…
Thank you and Wish you the same!
Vaibhao Ingle said…
sir can i use this circuit to drive 24nos of 1watt power led? if yes then how should be the led connections...
Swagatam said…
Yes you can use them, connect them in series at the "load" points, but please note that I haven't yet tested the above concept practically.
Seok Sothea said…
sir! the load active only when the main voltage goes zero, so how can the load operate if voltage supplier is zero? or it supplies from capacitor discharging? if so, which capacitor?
Swagatam said…
Hi Seok, here the zero crossing section is responsible for switching ON the scr during the AC zero crossing points solely for evading surge problems......once the 555 is switched ON the monostable timing decides for how long the load should be kept ON, determined by the 1M pot and C2 and this in turn determines the intensity or the output level of the connected load
Swagatam said…
I'll try to design it, and let you know soon
JIJU said…
Dear Swagatam,
What about efficiency of the circuit
Swagatam said…
Dear Jiju,

there's no question of any efficiency issue in this design, the output power will depend on the pot setting of the monostable.
Carlos said…
Estimado Sr Swagatam que tipo de circuito electronico se puede instalar en un hogar para reducir el consumo electrico sobre todo de aires acondicionados? muchas gracias
Swagatam said…
thanks Carlos, could be please translate it in English?
Patricia said…
Carlos says:
what type of electronic circuit can be installed in a home to reduce electricity consumption especially air conditioners?
Swagatam said…
for reducing electric bill of ACs, you will need to install an AC inverter which will allow you control its output by adjusting its motor speed which will in turn allow you to control its electricity consumption.
Saeed Abu said…
dear bro i want to operate 4 or 6 or 8 pairs 1 watt white LED with enough brightness by transformer less AC power supply which will be safe and 100 percent surge protected.Pls help me abt it.
Swagatam said…
dear bro, you can try the following concept and see how it responds:

omary rajabu said…
may be i not familial with one point, about time for the load to be active, it means when the monostable is at OFF condition, the load also will be OFF. if yes how can work with ON/OF power supply.
omary rajabu said…
great concept. but i have one question. according to the operation of monostable,the load will be active at same time and off according configuration of monostable. so is possible to work with power supply lake that?. so pleas can you add something for me to understanding.
Swagatam said…
Thanks, the load (LED) will also fluctuate and produce the average switched output through its illumination
omary rajabu said…
can i use this circuit for smartphone by regulating the output to the 1A with 5V by zener diode. if not help me which circuit is the best for that.
Swagatam said…
No, for charging a cellphone you should always use an SMPS based circuit, like this one:

omary rajabu said…
thanks swagatam. but why this circuit or any transformerless is not used as mobile phone charge? as you say that this circuit is not compatible for mobile phone is an technical reason?
Swagatam said…
because these are not isolated from mains voltgae, and therefore could inflict a serious shock to the user while handling the phone, and moreover the surge suppression in such circuits cannot be considered 100% reliable, which could be dangerous for the phone circuitry
Swagatam said…
hello, for 12V you can remove the 20K resistor and R1, and connect the IC pin4/8 line directly with the bridge line.

and reverse the direction of the 1N4001 diode.

for 24V, replace the 20K with 47K, remove the zener diode that's all
Swagatam said…
sorry, I just missed that it's a PNP, I thought it was NPN, in that case you must keep the 20K intact and remove only R1, and also reverse 1N4001 as suggested before.
nimetj said…
I just want to ask you if C2 is 9.2nF or 92nF, because I cant find any of it?
Can it be some other value?
Thank you and keep up with good work.
Swag said…
Hello, it seems it is 9.2nF but I don't think it is so critical, you can use any value which is able to provide a reasonably stable output.

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