# Designing power supply with Vo = 5V and then 12V and -12V

Discussion in 'Homework Help' started by holz1100, May 10, 2016.

1. ### holz1100 Thread Starter New Member

Apr 27, 2016
14
0
I need to design a power supply using a full bridge rectifier and some voltage IC's. I am using a multi output transformer connected to mains and the outputs available are (in rms) : 6v, 9v, 12v, 15v, 18v. I need a 5v rail (using lm7805ct) and then a 12v rail and -12v rail (lm7812ct and lm7912ct). The data sheet species that the lm7812ct needs an input voltage of 19v, is this absolutely necessary? If so how do i use my transformer in order to get the right input for 12v and -12v? The 5v rail needs to supply a current of about 310mA and the 12v and -12v's need to supply about 80mA. Any help in the way of circuit diagrams and explanations? Help is greatly appreciated

Mod edit: moved to Homework Help

Last edited by a moderator: May 10, 2016
2. ### rthorna New Member

Jan 31, 2016
1
1
If you take a look at page 8 of the LM7812 datasheet, you can see what conditions the output voltage parameter was tested under. In this case, it shows a Vin ranging from 14.5V to 27V. You'll be okay anywhere in this range.

Just remember, the larger the difference between the input and output voltages, the more power will be dissipated as heat. Don't forget to take thermals into account when using linear regulators.

holz1100 likes this.
3. ### holz1100 Thread Starter New Member

Apr 27, 2016
14
0
This is what I've built so far. Any advice for the capacitor values used? Especially with reference to the loads mentioned above. Also I've only got metal with surface area of about 22.5 cm^2, will this be enough for the heatsinks of the IC's?

File size:
32.4 KB
Views:
4
4. ### holz1100 Thread Starter New Member

Apr 27, 2016
14
0
This is the circuit I've built so far for anyone else who reads this thread. Any advice about the capacitor values with regards to the loads mentioned above? (310mA for the 5V output and 80mA for the -12v and +12v outputs). Also any idea how i can achieve that transformer configuration if the transformer has outputs of 0v, 6v, 9v, 12v, 15v and 18v?

5. ### dl324 Distinguished Member

Mar 30, 2015
3,250
626
If I understand your transformer description, all of the voltages are taps on the same secondary. If this is the case, you can't get what you want. You'll need an independent secondary for the negative supply.

You can use multiple taps for your positive supplies. Can't comment on current requirement without knowing current rating of the secondary.

Apr 27, 2016
14
0

7. ### holz1100 Thread Starter New Member

Apr 27, 2016
14
0

I've uploaded a photo, may not be any help but may confirm your views. Will using 2 transformers work if this is the case?

8. ### dl324 Distinguished Member

Mar 30, 2015
3,250
626
Looks like a single tapped secondary to me. You can use the 9VAC winding for your 5V supply and the 15VAC winding for 12V. You need another transformer for -12.

What is the secondary current rating?

9. ### holz1100 Thread Starter New Member

Apr 27, 2016
14
0
U

Unfortunately the secondary current was not rated on the device and I have no clue of the part number to look up the data sheet

10. ### dl324 Distinguished Member

Mar 30, 2015
3,250
626
Measure one of the secondary voltages with no load. Add increasing load until voltage drops 10%. That will give you an approximate full load current. With the rectification circuit you posted, available DC current will be about 0.6x of that value.

11. ### holz1100 Thread Starter New Member

Apr 27, 2016
14
0
I've just contacted my lecturer and she says that I must do the dual power supply for 9v rather

12. ### dl324 Distinguished Member

Mar 30, 2015
3,250
626
You need another or different transformer.

If this is a school assignment, you should post in the Homework Help forum so members don't give you answers; that could be considered cheating. Teachers usually care about what you know/can do, not what others can do.

13. ### WBahn Moderator

Mar 31, 2012
17,777
4,805
Measure the resistances between the various secondary taps. That will give a good idea of how they are wound and, in particular, if they are a single tapped secondary (which is my guess, too).

If so, it might be possible to use the 9 V tap as a center tap. That would give you positive and negative 9 V taps using the 18 V tap and the 0 V tap (be sure the 0 V tap isn't electrically connect to either of the primary taps or the case -- chances are it is not). With 9 Vrms you have 12.7 V peak. That won't give you enough headroom to use a 7x12 regulator. So it looks like you need two transformers (if this particular transformer model is all you have to draw from).