Thread Starter

Đorđe Jocić

Joined Aug 15, 2019
11
Hello guys, how are you all doing?

During the past few months I've been going through the Textbooks AAC has published, experimenting with some minor circuits and even PIC microcontrollers. I've learned a lot during the process and decided to do something a bit more challenging - make a basic home security system that would be powered from the 240VAC grid.

This is my first time making something that relies on the local grid, so I decided to check with you guys before starting.

The voltage in my country varies from 220V to 240V, and has a frequency of 50Hz. I decided to use a 250V 1A fuse for overcurrent protection, and 4x 1N5408 diodes to make a rectifier circuit. The 240 DC voltage is divided using the 480 & 30 Ohm resistors respectively to bring it below the maximum input voltage required to operate a L7805 Voltage Regulator.

What do you think?

Thanks! :)
 

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ericgibbs

Joined Jan 29, 2010
9,570
hi DJ,
Note: non isolated mains supplies are not allowed on AAC.
Please use a mains step down transformer if you wish this thread to be active.
Moderation.
E

A 240Vac down to 9Vac transformer would suit your project
 

MrChips

Joined Oct 2, 2009
20,293
That is a very dangerous design. Never design any electronics with direct connection to the AC mains without isolation.
Use a step-down transformer. Period.

Any further discussion on a power supply without a transformer will cause this thread to be closed. This is a prohibited topic on AAC forums.

Edit: A simple solution is to go to a second-hand store and buy an inexpensive AC adapter, those black bricks that you plug into the wall outlet. If it is not heavy then it is a switch mode power supply. An adapter with a built-in transformer will be much heavier.
 

AlbertHall

Joined Jun 4, 2014
9,326
There are some problems with the circuit.
1. The fuse should be in series with the AC input. You have it in parallel.
2. The input to the '7805 should connect to the juction of the two resistors.
3. You will not be able to draw 1A as the increased voltage drop across the 480 ohm will reduce the input to the '7805 below the minimum required.
4. You have no smoothing capacitor across the rectified mains. The output from the '7805 will have a large amount of ripple.
5. Most seriously, if all these problems were fixed there is still a major problem as the 5V output will be at mains voltage relative to earth and thus very dangerous to touch.

PLEASE use a mains transformer to reduce the mains voltage to some much lower voltage. This transformer will also provide the isolation necessary to make the supply safe/
 

OBW0549

Joined Mar 2, 2015
3,209
There are so many things horribly wrong with your circuit that it isn't even a starting point for a real design.

Do yourself a BIG favor-- and maybe save your life-- and stop trying to design a power supply and simply buy an off-the-shelf wall-wart 5V supply. You don't know enough about electronics yet to be playing around with mains voltage.
 

ci139

Joined Jul 11, 2016
1,195
the problem with computers is that
they limit and narrow your field of view to screen "envelope" -- that limits your continuous design area
they have limited object resources and object types -- that limits your creativity and degree of freedom
they force you to fixed position looking away from real world and it's unlimited resources . . .
thus it's no wonder you were just happy getting your first draft done
? draft on paper then copy to computer -- if you use 10 sheets for adjustments it feels normal - versus - if you have to save 10 files . . . it threats to trash your hard disk space!
Untitled-1.gif
 
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Thread Starter

Đorđe Jocić

Joined Aug 15, 2019
11
It was just a thought experiment to see if I could design a viable power supply for a project whose goal was/is to expand my existing knowledge about Electronics. I honestly didn't know any better, and you helped me a lot.

Here is the revised circuit based on your suggestions - without the filtering capacitor however.

Cheers!
 

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djsfantasi

Joined Apr 11, 2010
6,188
Last question (from me at least), is what current does your 5V load take?

if it’s anything over 1A or let’s say 800mA, you’re going to have a problem, because the 7805 can only source a maximum of 1A.
 

AlbertHall

Joined Jun 4, 2014
9,326
Last question (from me at least), is what current does your 5V load take?

if it’s anything over 1A or let’s say 800mA, you’re going to have a problem, because the 7805 can only source a maximum of 1A.
It'll do 800mA but it will need a heatsink, size dependent on the transformer voltage.
 

djsfantasi

Joined Apr 11, 2010
6,188
It'll do 800mA but it will need a heatsink, size dependent on the transformer voltage.
I figured that out by reading the datasheet. My question wasn’t what the regulator circuit will do; my question was what current your load required.

If your load pulls 2A, then your circuit is going to burn up trying to supply 2A. Or simply shut down because it has an internal thermal overload circuit.

Doesn’t it?

That’s why I asked and had hoped you can answer my question.
 

Thread Starter

Đorđe Jocić

Joined Aug 15, 2019
11
It's going to be used to power PIC12F675 and an alarm consisting from two 555 timers and LM386 audio amplifier - nothing that should draw to much current. I also entertained the idea of adding two 7-segment displays, to show how many times an alarm was triggered, but it's going to take me a while to get up to that point.

What regulator do you recommend for 5V 2-3A output?
 

djsfantasi

Joined Apr 11, 2010
6,188
Not to be a jerk... or maybe yes.

“Nothing that should draw to much current” is impossible to design for. Is 1mA too much current? 10mA? 100mA? 0.1A? You need to quantify the current for all of your components.

I don’t think you know if you need 2-3A? Do you? Maybe 1A is enough. Maybe you need 1.5A? You can’t tell without answering my question.
 
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