I would like to use this circuit to power arrays of high power LED'sI understand the properties required to power LED'sI have beginners knowledge in electric circuits and have done some research on this circuit. I have come across several posts stating it is dangerous to connect this circuit straight to mains power. I don't understand is what makes it is so dangerous?AndIf possible, how can it be made safe without using a transformer.
Your circuit will be fine without an isolation transformer if you connect a fast blow fuse, with the same current rating as your diodes or a bit less in the hot line to your voltage source. Your electric shock hazard is increased slightly without the isolation transformer. Your LEDs are vulnerable to power line spikes, even if you have an isolation transformer, but perhaps the transformer helps a little. An MOV rated about 190 volts in parallel with C, may provide some protection for your LEDs if the LED operating voltage is a bit less than 190 volts. Other MOV voltages are available at least rarely. Several hundred micro farads for C also suppresses spikes not only or your LEDs but also for other devices, connected to your ac source. A large value for C however means your diode bridge should be rated 25 amps or more.
As a measure of safety, one of the lines of the mains is at earth potential. So if you stand on earth and touch the other wire, you will get a shock. the voltage between earth and the other line is 110V. Thus the source is not a floating one but a ground referenced one. If you use a 1:1 isolation transformer, then you can touch either end of the secondary as the other end is floating. SO you will have no shock hazard. You cannot take the ac mains as floating. It is not like a battery kept on a wooden table. One end of this 110V battery is already grounded, if you want to look at an analogy. And the electricity supplier grounds one end to ensure safety, and if the current to safety earth is higher than about 5mA, a protecting mechanism is invoked. as in earth leakage detectors.
The neutral leg of the AC (assume the bottom on the diagram) is grounded to earth ground at the house service entrance and also at the center tap of the power pole transformer (in the US). If your project box was metal, and you had the 0 volt output attached to the metal case and happened to touch earth ground through you. You would get shocked since during the negative half cycle, positive voltage would go through ground, then you, the metal box, through D4, back to the AC hot (on the negative half cycle). Ouch! Always use an isolation transformer or any transformer where the secondary is not in any way tied to the primary. That way you can ground either dc lead from the rectifier to ground and not have to worry about electrocution.
Assume the bottom of your AC power source is neutral, and that your voltage source is 120 volts AC when the voltage source has reached a maximum positive voltage (+170 volts), diode D1 is forward biased and connects that to the + side of the electrolytic capacitor. This is a definite safety hazard. The negative side of the capacitor is not safe either. When the AC source reaches its most negative voltage (-170 volts), diode D2 is forward biased and connects the - side of the electrolytic capacitor to that voltage. The voltage across the capacitor will be 170 volts the circuit working just as a full wave bridge rectifier should work. But the + side of the electrolytic capacitor will have a voltage with respect to neutral (and ground since the neutral is tied to ground in the circuit breaker panel) that varies from 0 to + 170 volts this is basically a 60 Vrms AC voltage superimposed on +60 DC volts. And the - side of the capacitor will have a 60 Vrms AC voltage superimposed on -60 DC volts with respect to neutral and ground.
