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.
The Circuit Request
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 Circuit Design
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.
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.