Transformerless power supply

Discussion in 'General Electronics Chat' started by Kenny, Dec 17, 2004.

  1. Kenny

    Kenny Thread Starter Active Member

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    Oct 11, 2004
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    A few time ago I made a question about the design of transformerless power supplies, say: 3V, 6V, etc, with current limiter; however I didn't receive any good answer.
    I understand the dangers about the use of such thing, but I ask: is it not possible to reduce the dangers by using properly current limiter components i.e. fuse?
    I saw a topic about ACLed but I think that would be very useful if someone with the knowledge dissects that circuit and make a complete explanation about the theory behind of it and how to make it safe.
    OTOH, is there any forbidden subjects? Because the only answers to my question were: don´t do it, buy an ac adapter, but the people saying this think that the only factor to consider is the price, but they don´t think that the size is also important, i.e.: can you
    imagine a 1/4 inches size circuit that compounds 2 leds fed by a 2 inches size adapter?
    Also, what is the problem about learning to design such power supply?
  2. Battousai

    Battousai Senior Member

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    Nov 14, 2003
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    You need to be more specific. Transformer-less power supply driven from what type of voltage source? Are you trying to produce an AC or DC voltage?

    If you're trying to produce a DC voltage, use a Linear regulator.
  3. dragan733

    dragan733 Senior Member

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    Macedonia
    As I understood you have to supply only LED diodes. If is so, then I give you in the attachment circuits to supply the LED
  4. Xray

    Xray Well-Known Member

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    Nov 21, 2004
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    There is an error in the diagram shown on the right side. I believe that you meant for the supply voltage to be 110 Volts. That would make sense based on the resistor value.

    The major problem, of course, with transformerless power supplies is the electrical shock hazard. A transformer provides isolation in case a person should come in contact with any part of the circuit when his/her body happened to be grounded. There is a way to use capacitors in place of a transformer for low current applications, and if designed properly could also provide some safety from electric shock.
  5. dragan733

    dragan733 Senior Member

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    Yes, the right diagram is for 110V~. I copied the left diagram and I forgot to change the voltage to 110V~ on the right diagram.
    Yes, also one can use a capacitor to reduce the voltage, but for small curent that is a solution expensive. But from othe side the resistors warm, the capacitors not.
  6. Brandon

    Brandon Senior Member

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    Dec 14, 2004
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    Its very possible and very dangerous. Just because an fuse says its 500mA doesn't mean that a spike of 10 Amps won't go through it before it pops. I remember on a digital output control board I was doing a design spec on I ran the 1 Amps fuses at 1 amp for nearly a minute before they finally began to pop.

    Additionally, your wall outlet can give you as much current as the breaker/fuses in your house will permit as well. So if you have a penny in there, you could zap the crap out of your self.

    Transformers, even 1:1, are good to isolate you and your electronics from a potentially deadly situation. Lets take a simple example. Say you did your circuit will some awesome LEDs and power resistor. Both which are rated better than the wire in you walls. You LED shorts out. Since the wire in your wall is rated lower, you now have a fire inside the walls.. good by house. You put the 1:1 transformer then connect your LED. LED shorts out but now, the wall doesn't see it. The wall gives off it AC to the transformer seeing the same load. Rather, the wiring of your LED, transformer secondary winds, etc will burn out and then go open circuit leaving your house still standing.

    So in the end it comes down to how much do you trust underpaid workers 30-60 years ago to install proper wiring in your houses? I know I don't. When I rewired my kitchen I found no reds, no blacks, only yellow wires.. for everything. ground, common and 2 phases of 110AC. Needless to say i didn;t know there we 2 phases at first and ended up wiring them together as they both read 110 VAC on my meter and a fireball out of the breaker box later with some melted wires I figured it out.

    You can do it, just be very very careful.
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