Dual rail virtual ground power supply

Thread Starter

Copey84

Joined Jul 27, 2015
198
Hi there, hoping someone could help me solve a few problems I'm having with a power supply I've built.
Basically its a dual rail power supply with a virtual ground, +12 -12 outputs. Got the design from a book, starting electronics construction by Keith brindley. Having built the circuit the -12v terminal is outputting 29v, and the + terminal is outputting 12v. The transformer is rated at 10va, and is series linked giving out 40v not underload. The regulators used are 7812 and 7912, why is it that the negative voltage is so high? Looks like voltage regulator isn't doing its job but I've replaced and still no change.
There are four capacitors two rated at 2200mf 25v, they poped so replaced with 50v 2200mf which has now made the +12 volt unstable.
The other smaller caps rated at 22 mf 16v have not poped.
Have tried supply underload and the neg voltage will drop down, but still not good for op amp circuits as they have a Max voltage 15v.
Please could someone try and explain what's gone wrong, I followed the book and checked the circuit over many times. I thought maybe transformer was outputting to high when not under load, but surley the 7912 should still be able to maintain a steady neg voltage.
Appreciate any replys if you can make sense of this, thanks.
 

kubeek

Joined Sep 20, 2005
5,753
Please show your schematic, and the board design if possible. Are you sure the negative regulator is connected correctly?
 

Thread Starter

Copey84

Joined Jul 27, 2015
198
Hi thanks for reply, can't post wiring diagram. It's a basic power supply with virtual ground, checked it many times on breadboard still gives high output on neg voltage. Could someone explain how the virtual ground works.
Circuit components-
Transformer
Bridge rec
C1 2200 mf 25v
C2 2200mf 25v
7812
7912
C3 22mf 16v
C4 22mf 16v
 

bertus

Joined Apr 5, 2008
21,330
Hello,

You can post a drawing of the schematic using the "Upload a File" button.
Does the transformer have a centre tap?

Bertus
 

kubeek

Joined Sep 20, 2005
5,753
If you are using only those components then I am pretty sure this is a standard dual supply and does not have a virtual ground.
 

Thread Starter

Copey84

Joined Jul 27, 2015
198
It's a dual transformer four pins on secondary, 20v between each set. The transformer is wired in series so there's 40v output. When not under load 29 v appears on the neg terminal and 12 on positive. Thanks for info on power supply will read over it.
 

crutschow

Joined Mar 14, 2008
27,031
It's a dual transformer four pins on secondary, 20v between each set. The transformer is wired in series so there's 40v output. When not under load 29 v appears on the neg terminal and 12 on positive. Thanks for info on power supply will read over it.
Is the transformer center tap (where the two windings are connected together) connected to the output common?
It must be for the circuit to work.
 

Thread Starter

Copey84

Joined Jul 27, 2015
198
The basic configuration should be as shown in fig 2.7 in this pdf.

Read the entire paper. It should answer all your questions...
Hi mikeml, the diagram in 2.7 is my circuit except I have two smaller caps across the terminal outputs. Think the problem mite be with transformer. I have a dual rail transformer connected in series which I think doesn't allow a negative voltage to output to the 7912 regulator. Would it be possible to configure the transformer I have or will I need a centre tap.
My transformer has four pins on secondary, 20v between two and 40v across all pins
 

Thread Starter

Copey84

Joined Jul 27, 2015
198
Could anyone out there take a few minutes to look at a wiring diagram of a power supply it should be in a link.
Still not sure how it works, have built on breadboard and I'm getting -29 volts when I should be getting -12. The supply transformer has two 20v outputs that are series linked, giving 40v total.
Does the circuits virtual ground rely on the capacitors charging an discharging the return current? Also if I was to use a centre tapped transformer wouldn't that require a link between 0v and transformer.
Appreciate any replys.1438026524075-1249590656.jpg
 

bertus

Joined Apr 5, 2008
21,330
Hello,

You are missing the link between the centre tap of the transformer and the gound connections of the regulators as shown in the picture below:

copey84_dual_supply.jpg

Bertus
 

Thread Starter

Copey84

Joined Jul 27, 2015
198
Hi there kubeek, the diagram in 2.7, is there suppose to be a ground return from 0v back to centre tap of transformer, I see how it could work with return back to supply. Do you not think my circuit will work came from a electronics book.
 

bertus

Joined Apr 5, 2008
21,330
Hello,

Without the link, the centre point between the regulators is floating.
The voltage will be entirely be dependend on the load on each side.

Bertus
 

kubeek

Joined Sep 20, 2005
5,753
Hi there kubeek, the diagram in 2.7, is there suppose to be a ground return from 0v back to centre tap of transformer, I see how it could work with return back to supply. Do you not think my circuit will work came from a electronics book.
There actually is a link already in there. There is one ground symbol at the center tap, and another between the caps. All same looking symbols in a schematic are supposed to be connected together.
 

Thread Starter

Copey84

Joined Jul 27, 2015
198
Hi there thanks for replys. This book that the circuit came from has no return link, that's why I thought it had to be a virtual ground. Didn't think book would have made such a bad mistake. The transformer I have has no centre tap, will it still work if I link between the two windings?
 
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