# Negative volt w/o C.T.

Discussion in 'General Electronics Chat' started by Zeebit, Nov 8, 2013.

1. ### Zeebit Thread Starter Member

Nov 8, 2013
72
0
Title: Negative voltage W/O center tap

Hello everybody!

I know that this has been asked hundreds of times so I would like to apologize for posting this question again.

I am post this because I can't find the answers to my questions.

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I will be making a bench PS with this design: http://www.quan-diy.com/projects/benchps/benchps.htm

I will be using a toroid transformer with two bifilar (0-18V 3A)windings and another two bifilar (0-15V 0.4A) windings on the secondary.

The two 0-18V windings will be paralleled an used for the main circuit.

The two 0-15V windings will be used to power a fan, the volt&amp digital display and the Vcc- of the opamp.

How do I generate a negative voltage from the 0-15V windings?

I am thinking about separating the two windings. One for the fan and displays and the other will be hooked up in reverse for the opamp.

Or I could make a rail splitter but I don't know how to make one.

Say I already have generated the negative voltage for the opamp, how do I power the opamp with this since it is already getting it's Vcc+ from an isolated winding.

I would like to use only this traffo is possible to save space. Anyone willing to lend a hand?

2. ### antonv Active Member

Nov 27, 2012
148
26
How much current does the fan, display and op-amp need?

If the sum of their currents fit in 0.4 A then you can use one 15 V winding for VCC and one for the negative supply.

3. ### Zeebit Thread Starter Member

Nov 8, 2013
72
0
Can you explain a bit more? How to I use one 15v winding for Vcc+ and the other winding for Vcc-?
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How do I wire twin secondaries to get a "center tap"?

Last edited: Nov 8, 2013

Nov 8, 2013
72
0

5. ### Zeebit Thread Starter Member

Nov 8, 2013
72
0
Got another question.

Which is better for use as a ripple filter in a linear PS? One large cap or multiple small caps in parallel? Say 10000uf vs 4-5 of 2200uf.

6. ### crutschow Expert

Mar 14, 2008
23,099
6,850
There's not a lot of difference between the two options.

Depending upon the relative ESR of the capacitors the paralleled devices may have lower relative ESR which would reduce the ripple slightly.

Likely the multiple capacitors will be more expensive.

Obviously their form factor is different and one might be better from a packaging viewpoint.

But generally there's not a big reason to chose one option over the other.