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Power Supply Idea

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vne147

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Hey everybody. Is there anything wrong with this idea?

Transformerless PS.PNG

I would like to use it to supply voltage to a logic circuit consisting of few comparators, a 555, a 4541 and a NOR gate. The ouput from the NOR gate will turn on AC outlets through an opto-isolator and triac arrangement. A portion of the circuit I'd like to power with this idea is talked about in this thread:

Thermistor and Comparator Circuit

Thanks.
 
Generally not a good idea to have direct coupling to AC mains. One of the AC leads, the neutral line (wider of two prongs) is ground bonded at fuse box where it enters house. The bridge will flip this connection at 60 Hz to your output ground. If you touch the output ground to anything grounded in environment you will have high voltage across 7805 blowing it out. If you are the path for environment ground you will carry the current.

You also are going to dissipate 1.3 watts in the 20K resistor.

Switch mode power supplies use direct coupling to AC main but have a high frequency transformer and opto coupler for control feedback to isolate D.C. output from AC mains.
 
transformer

Why not use a simple 110 / 12 Volts transformer to isolate your circuit from the mains.
 
Why not use a simple 110 / 12 Volts transformer to isolate your circuit from the mains.

I know that I can use a transformer to isolate my circuit from mains and step down the voltage at the same time but I'm trying to make it so it can be confined in a small space. Hence my ultra-simple but unfortunately impractical idea. I'll probably end up using a SMPS.
 
I know that I can use a transformer to isolate my circuit from mains and step down the voltage at the same time but I'm trying to make it so it can be confined in a small space. Hence my ultra-simple but unfortunately impractical idea. I'll probably end up using a SMPS.

hi,
This type is commonly used, beware not islolated .
 

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vne147,

For a cheap 120VAC/5VDC SMPS you might look at used cellphone chargers in thrift (i.e. Goodwill) stores. If you're careful, you can cut open the case and extract the PCB. I saw along the seam, but not quite through, except at the corners. At the corners, use a large flat-blade screwdriver with a twisting motion to pop the case halves apart. Be "gently" aggressive. ;)

Ken
 
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