Hi there,

I am wanting to make a +/- 5V power supply, but need a centre tap transformer (I think!) -- similar to the following design:
http://www.reconnsworld.com/power/multipowersupply.gif

The only problem is that I don't have a centre tap transformer, but just a normal two-winding step down transformer.

• Is it possible to use a normal transformer as a centre tap transformer? Or how else can I get -5V out?
• Also, is the 100uF cap on the output neccessary?

Thanks

Edit: another question on the side:
If I have a raw signal of about 0.1-1uV coming in to a band pass filter, is it better to amplify it (by about 1000) before or after the filter?

 

No need for ct, just use half wave rectification with one end of the transformer ground. Each diode ( 2 ) points in the opposite direction ( conventional current flow ).

 

Sorry ... I'm lost :/ How do you get +/- 5V from that?

 

See the attached schematic.

If you wish, you can add a 10uF capacitor to the outputs of each regulator. This will help to improve transient response.

C1 and C2 are very minimal. I suggest doubling their size.

If your load requirements are small, you could substitute 1N400x diodes for the 1N540x diodes.

As far as your 2nd question, please start a new thread with a suitable subject line; you will receive much better help that way.

 

Take warning that the pinouts for 7805 and 7905 regulators are different so their respective wiring will be different.

 

Thanks so much for your help. Problem solved!

 

Your C2 in the dwg, seems to be upside down

 

Well spotted that Wildebeast.

 

Actually an Eland , about the weight of an American buffalo.(Big grin)

 

I stand corrected.
I will take my demerits along with Wookie.

 

Your C2 in the dwg, seems to be upside down

You are correct on that point.

Thank you for pointing out the error.