# design of a power supply

Discussion in 'Power Electronics' started by hamodye, Jul 3, 2011.

1. ### hamodye

18
0
Jun 28, 2011
hi my friend
I want make power supply to work load 12 v & 1500mA
power input 220 v , 60 Hz
I am using the following :
1- power supply 220 v ,60Hz
2 - transformer step-down
3- regulator 12v
4- capacitor ( but i am not sure for correct value of transformer
5- led ( to sure we have output )

you can see the picture of circuit in attachment and file of circuit in attachment
Note ( file of circuit , change extension from zip to .rar and open it )
when i am running this circuit show me error message ???!!!
what you think the error ??
and i am interesting to electrical design
what book can help in that ??
can i found book to learn design of electrical circuit ???

#### Attached Files:

• ###### Circuit1.jpg
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Last edited by a moderator: Dec 29, 2012
2. ### duke37

5,364
772
Jan 9, 2011
1. You have the rectifier connected incorrectly, this will short the transformer.
2. The led will be very bright for a millisecond. Look on this site for details of how to drive a led.
3. The rate of voltage change on a capacitor, dV/dt, is I/C.
Time is 1/120sec , current (I) = 1.5. Let the voltage change be 1V then
C = I/120 = 0.013F or 13000microF, use a standard more than this say 22000 microF

A book which is highly recommended is
The Art of Electronics by Horowitz and Hill

Why do you zip files? I find they are always a pain.

3. ### davennModerator

13,901
1,970
Sep 5, 2009
Welcome to the forums

ok here's a rebuild of your supply showing correct bridge connections, capacitor values etc

cheers
Dave

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4. ### hamodye

18
0
Jun 28, 2011
thank you very much mr.duke
but why the rectifier incorrectly ? what is right way to connect ??
which site want me looking for details ?
i am not understand what you wrote about voltage rate ?
thanks

5. ### hamodye

18
0
Jun 28, 2011
thanks mr davenn
but why yo you put capacitor before and after regulate ?
and why you take this value ??
and hoe you know the right value of resistance 27 k???

6. ### hamodye

18
0
Jun 28, 2011
how can i change the scale of time in multisim ???
if i want run circuit and i want see stable for it for long time like one year ?
how can i make the mulisim make scale and speed time of simulating ??

7. ### duke37

5,364
772
Jan 9, 2011
Davenn has shown the way to connect the bridge rectifier correctly. The arrows depicting the diodes point to the positive output. The arrows point away from the negative output. The other two connections are the AC input. Try following the current path with the current in the transformer going first one way then the other.

The correct way to drive a led comes up on this site regularly. Basically, the led needs a voltage to turn it on and if there is excess voltage, this has to be dropped with a resistor. The resistor is sized to pass 10 to 20mA to suit the led and the brightness required. Your original circuit had no series resistance so the current would be very high and the led would have a short but maybe happy life.

A voltage on a capacitor rises as current is passed into it and falls when current is taken out. The rate of change dV/dt = I/C. In other words, the bigger the current, the faster the change and the bigger the capacitance the slower the change. The reservoir capacitor is fed by the rectifier every 1/120 sec and has to provide current between these input pulses. If you chose a reasonable change of voltage, say 1 to 5V in a low voltage circuit such as this, you can calculate the size of capacitor required. For 12V out, you will need 15V into the regulator and with a capacitor ripple of say 2V means you need to charge the capacitor to 17V.There wil be up to 2V drop in the rectifier so the transformer needs to supply a peak voltage of at least 19V. Depending on the size of the transformer there will be a voltage drop in the windings so more nominal voltage will be needed to accomodate this. Choice of components should start with the output and work back to the input.

Hope this helps.

8. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,500
2,840
Jan 21, 2010
Because if you read the specs you will find that this is what you are supposed to do.

They improve the response to transients and help ensure that the regulator does not oscillate.

9. ### davennModerator

13,901
1,970
Sep 5, 2009
The value of the main smoothing capacitor (C1 in my supplied diagram) is determined by the current rating of the PSU. I dont know the specific formula, but the "rule of thumb" calculation in the electronics industry is..... 1000uF / 1 Amp of current. So for example, a 10A capable PSU you should have at least a 10,000uF smoothing capacitor.

thats the fixed current limiting resistor for the voltmeter . There is also the 10k trimpot there. Those values will depend on the meter you use. If you already have a meter that is capable of 12V or more as a full scale deflection (f.s.d. ) then you wouldnt need any of those external resistors.

cheers
Dave

Last edited: Jul 5, 2011
10. ### hamodye

18
0
Jun 28, 2011
thanks again Mr.Davenn
u hope you inform me how you thinking or know that is write design??
what i suppose to study to know and make electronic design ??

another thing if i want make same design for 12 output load in same feature ??

Last edited by a moderator: Dec 29, 2012
11. ### davennModerator

13,901
1,970
Sep 5, 2009
to repeat what Duke37 told you over a year ago.....

A book which is highly recommended is
The Art of Electronics by Horowitz and Hill

read it over and over study it till it becomes second nature for you

what do you mean by 12 output load ?
do you want to have 12 separate loads on the PSU ?

if so, on the circuit shown then the total current drawn by all the loads operating cannot exceed 1 Amp

Dave

12. ### hamodye

18
0
Jun 28, 2011
as you know Dave , the book is very huge and start from basic of electronic , and i don't wont wast my time in read from beginning, so my qeasution about which chapter toking about design ?

and about output load , i mean 12 separate output from one input ( electrical wall 220 V ) and each output 12V and 1.5 Amp

hamodyee

13. ### duke37

5,364
772
Jan 9, 2011
Would you design a house if you did not know what a brick is?

'The Art of Electronics' contains many circuits which can be modified for your own use.

To get twelve independant outputs, use twelve wall warts.
If the outputs do not need to be independant you could take twelve outputs from a 20A power supply.

14. ### hamodye

18
0
Jun 28, 2011
thanks a lot DUke37
now the idea is clear , thanks a lot .

how can i make 20 A power supply ??

15. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,500
2,840
Jan 21, 2010
After you've read The Art Of Electronics, you purchase and read a book on power electronics.

Or.

a 20A power supply is non-trivial to design and build.

16. ### Chat_Ghosty

49
0
Jun 30, 2012
PSU

I think this might work for you as a basic idea.

#### Attached Files:

• ###### High Current PSU.png
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Last edited: Jan 4, 2013