A little help with my university project please...

[morrissey]

Hello!
I was assigned a project at my university to build a power supply that gives output +6V , -6V and 5V. I have attached it to show it to you and if you could tell my what's wrong...
I would apreciate if i could get an answer by the end of the day because i have to make the corrections and prepare it for printing it on a board.

Thank you.

 

[Nigel Goodwin]

First thing, don't post diagrams af JPG's, they are poor quality and much too large - use GIF or PNG.

As for your circuit, use only ONE bridge rectifier, across the outside two of the transformer pins, the centre pin connects to 0V. Also, you have a 100nF in series with the positive output, so it can't possibly work, and you don't have a large electrolytic on the positive supply.

 

[Hero999]

You only need one bridge rectifier with two large electrolytics both conected from the positive and negitive to the centre tap.

 

[HiTech]

Go easy on him Hero--- at least he's trying and learning in the process. We all made mistakes at once (and I still do). His attempt to design and then inquire for advice on improvements is certainly alot more admirable and constructive than the many jokers who march in here asking for us to provide them with an idea, diagram, parts list, etc.

I remember when I was a kid tinkering with a drycell, a lamp, and hookup wire to make a working light. I was so proud of my (simple) accomplishment, but my father wasn't impressed and even placed it aside from me, telling me to pay attention to other things in life. That action of his only served to further fuel the flames of my interest in electronics, eventually leading to a career in it. My mother on the otherhand recognized my interest in it and did things to cultivate me.

 

[Hero999]

Go easy on him Hero--- at least he's trying and learning in the process. We all made mistakes at once (and I still do). His attempt to design and then inquire for advice on improvements is certainly alot more admirable and constructive than the many jokers who march in here asking for us to provide them with an idea, diagram, parts list, etc.

Fair enough, post edited. I suppose universities aren't very good at teaching the pratical side of electronics.

 

[JimB]

Try this.

I have redrawn the rectifier part of your circuit, in the way that others have also suggested.
You need to be aware of the pin-outs of the voltage regulators, the 78xx and 79xx are different.

JimB

 

[Hero999]

One more thing for safety's sake the fuse an switch are always connected to the phase conductor, never the neutral.

morrissey
I don't think you have enough experiance to build mains powered circuits. I recommend you should by an AC adaptor with an AC output or a torroidal halogen lighting transformer to power your project with.

 

[morrissey]

Thank you very much for your help and your quick respond!
I will try it right on and as soon as i get it fixed i'll tell you the results..
As for the universities , and especially for the greek technological university
i study, the things are very bad...It's quite difficult to "survive" in there if you
don't try to learn things by yourself and not wait for the university to learn things to you and help you...I believe in England and the U.S. the things are far more better!
Thanks again for your help!
I'll let you know how it went

 

[Nigel Goodwin]

One more thing for safety's sake the fuse an switch are always connected to the phase conductor, never the neutral.

Actually that's not entirely true!, there are many commercial units where the fuse is in the neutral lead, and many more (using figure 8 mains leads, or non-UK two pin leads) where it depends which way you insert the mains lead.

However, I agree that I MUCH prefer to see the fuse in the live lead, and not the neutral - and anything I build has a fuse in the live.

 

[morrissey]

hello again,
I printed the board and i tested it and the results are:instead of 6V i got 2,4V , instead of 5V i got -5V and instead of -6V i got 6V
The weird thing is that lm7806(down) gets really really hot.
I downloaded multisim 7.0 so i can simulate it there.But i can't find in its librady lm7805 and 7806. Is anything else similar so i can use?
And also i can't find the bridge rectifier.
thnx

 

[morrissey]

Oh and someone mentioned, as i was reading your replies again, about lm79xx.
Maybe i should use them instead of the ones i have?

 

[JimB]

instead of 6V i got 2,4V , instead of 5V i got -5V and instead of -6V i got 6V
The weird thing is that lm7806(down) gets really really hot.

Never mind simulators, you will never learn electronics that way.

OK, -5 and -6v instead of +5 and +6v, did you have the meterconnections the correct way round?

2.4v instead of 6v, and a regulator that get hot. Is this the -6v output?
You have used a 7806, you should use a 7906. In that circuit the 7806 will get hot and is now probably in silicon heaven.
I did not notice that you had 7806 for the -6v supply, which is why I made the comment to beware of the pin-out difference between 78xx and 79xx.

JimB

 

[audioguru]

Hee, hee.
The JPG schematic is so fuzzy that nobody noticed the wrong 7806 instead of a correct 7906.

Are the pins connected the correct way around as on the datasheets for the regulators?

 

[morrissey]

Yes that must be it!
I tested it again, i didn't tell you the results right!
The top LM7806 gave me 6V! correct!
The middle LM7805 gave me 5V! Correct!
The bottom LM7806 gave -2.5V. False!
So that must be it!
Tommorrow i'm going to buy an LM7906 and place it on the circuit
and tell you how it went!
thnx again! I'll write your names on the project

 

[muki55]

try this one i edit your diagram

 

[Nigel Goodwin]

try this one i edit your diagram

It's still wrong and completely useless!.

 

[Pedro Martins]

If everything is done correctily, there is no problem with mains

I hope this comes in time

 

[Nigel Goodwin]

If everything is done correctily, there is no problem with mains

I hope this comes in time

That looks good! - you even posted it in a decent format (GIF). Don't forget though, the pinouts of a 79xx if different to a 78xx.

 

[Pedro Martins]

Just to avoid some "soldering" confusions

 

[Hero999]

What voltage transformer are you using?