If an isolation transformer is used, part of the concept is that a lower voltage is used too, so it is not hazardous under normal conditions. This is usually around 32V maximum. Also there are grounding constraints and a separate fuse might be required when using a transformer.
As an appliance, consider it more like a fridge or toaster. Consider all the components can be at a hazardous voltage, including the leds and their ballast resistors. It is made safer by enclosing everything in a suitable grounded metal box or by using double insulation standards. The idea is that no electrical point can be reached by little fingers or implements poked in, and any fault inside that causes the supply to touch the grounded metal enclosure should blow a fuse, disconnecting the supply, or is held at a low voltage to ground because it is connected to ground.
There are mandatory standards and regulations to comply with, to make sure the safety works as intended. This is also so you are not liable for fires, hazards to others etc. Consider children. Another point is, this is a constant voltage supply, while leds need a constant current supply, so ballast resistors are required.
The best approach, and the only one allowed in most countries, is to use an approved power supply that encloses all the mains components and produces an isolated and lower voltage (less than 32V DC typically) with a limited current. As a constant current source is required for leds, there are suitable power supplies intended especially for leds. These deliver a regulated current rather than a regulated voltage, and no ballast resistor is needed. You will need to understand what is required. This approach is also more efficient, if using a switch mode power supply. Here are some examples:
http://www.power-supplies-australia.com....
These are more efficient, no liability to you, probably cheaper, and much smaller and lighter than a mains frequency transformer too.
Interpreting the specifications:
Here is a link with a PDF data sheet you can download as an example.
http://www.power-supplies-australia.com....
The regulated current for the PCD-16-350 model is 0-350mA, so a series string of leds will have 350mA if the voltage across the string is in the specified range. This voltage is 24V to 48V. Therefore all the leds in series should add up to somewhere between 24V and 48V. If these leds have a forward voltage (Vf) of 3.6V each when operating at the desired current (i.e. 350mA) then at least 7 are needed in series and the maximum is 13 leds in series. You will need to make sure the leds are properly rated for this current and have a suitable heat sink. It is possible there is an internal adjustment for the constant current, but you need to check that with the supplier if needed.
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.
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.
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.
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.
A xfmr does provide isolation because you don't have a direct connection to mains (240VAC?) The circuit shown will mean a dc voltage of about 240VDC which awfully high to power LEDs
Because you cannot connect the 0V line to ground. (One side of the mains is grounded for safety.)
I would like to use this circuit to power arrays of high power LED's
I understand the properties required to power LED's
I 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?
And
If possible, how can it be made safe without using a transformer.
