Hello,

Here is the graphic explanation of what kubeek is trying to say:



Bertus

 

The transformer I have has no centre tap, will it still work if I link between the two windings?

Two windings with the same voltage are basically equivalent to a center tap, they just give you a bit more choice on how you will connect them - you can have two windings in series aka center tap, or two in parallel, or two completely separate.

 

Thanks for replys. This dam book has caused a lot of confusion, see how circuit works now. There is also a separate 5v circuit in same power supply with it own transformer. Book claims that voltages can be added and subtracted to give multiple voltage outputs, do you think it is possible?

 

This book that the circuit came from has no return link,

YES it does. Read post #19 again. Schematic diagrams use a standard language, with rules of usage and syntax just like any other language. All ground symbols on a schamatic are known to be connected together. If you don't understand this, no schematics will make sense to you.

Yes, the connection between two secondary winding is the same as a single centertap connection. BUT pay attention to the phase dots on the transformer datasheet. If with windings are not connected correctly, the ripple voltage on the filter capacitors will be much greater than intended.

ak

 

Hi there, still having problems with power supply. I placed link between mid point of transformer windings and 0v side of circuit, the voltage has came down to around -19v which is still not right. The + voltage remains stable at 12v, replaced 7912 with a 7915 regulator but it still couldn't stabilise the voltage.
Does anyone know why this circuit would behave like this, any help much appreciated.

 

Was it -19V with both the 7912 AND the 7915? If so I'd say there might be a connection bypassing the regulator.

 

Was it -19V with both the 7912 AND the 7915? If so I'd say there might be a connection bypassing the regulator.

Hi, I switched the 7912 for 7915 to see if there was a fault, no change, both had around -19v. Do you have any ideas why this would be?

 

Checked all connection again all ok, 7912 isn't bypassed. With the link back to centre of transformer I now have 27v input at positive and negative regulators, the regulators then output + 12 and -19.
Also voltage will drop a bit more to around -16v with a 7905 regulator in place

 

You said your transformer has two 20V windings yet your diagram showed two 15V windings. Which is it? With 15Vrms windings you should have around 20V DC at the input to each of the regulators (between input and ground) if they're unloaded.

 

Put up a pic of the wiring diagram from book I've been working from. As you can see there is no link back to transformer, how is it suppose to work? Made up 5 v circuit no probs with it, book says voltages can be combined to give out multiple values along with 5 +-12

 

Hi, the transformer I got is rated at 10va, which outputs 5va per winding that can be wired in series and parallel. Its a PCB mount type, voltage is 20 per winding 40 across both. It to the spec listed in book, thought higher voltage was because it wasn't under load. Since I put link to mid point of transformer voltage is now at 27v

 

View attachment 89367 Put up a pic of the wiring diagram from book I've been working from. As you can see there is no link back to transformer, how is it suppose to work? Made up 5 v circuit no probs with it, book says voltages can be combined to give out multiple values along with 5 +-12

That schematic is in error. The connection point between the two windings does need to be connected to the 0V (common) point of the circuit, as others have stated, otherwise it won't work.

 

Even with link in voltage is to high at 7912 output. Can you think of anything else that would cause this?

 

Even with link in voltage is to high at 7912 output. Can you think of anything else that would cause this?

Incorrect wiring.
Double-check that you have the correct pinout for the package and proper connections to the 7912.

 

Hi there, does it matter where the link from the mid point of transformer connects to on the 0v line? Does it have to go between capacitors of 2200 micro farad value? I have a link soldered between mid point and ground at 7812, its all common so thought it should work.
Checked traces on PCB with dmm all ok, voltage levels as follows
Transformer 40v across, 20v from either end to centre
Bridge rec has 20v in then outputs -27v and +27v dc
7812 vin 27 vout 11.8 dc
7912 vin -27 vout 18.7 dc
All seems OK apart from 7912 output, have tried various value 79 regulators none can output correct voltage, they couldn't all be faulty! Don't know what else to try, any help much appreciated.

 

I don't understand your comment about the connection to the 2200uF caps.
If you want further help then you need to post the exact circuit you have including the physical pins that are connected on the 7912.
There appears to still be an error in your wiring and further discourse is not likely to help.

 

Hi there I have included pics with this post showing wiring diagram and pin outs of voltage regs.
In previous post I wanted to know does it matter where the link from centre point of transformer is connected to on 0v line?
I have the link in place from transformer to the ground pin of 7812 as shown on diagram I inculded, to my knowledge this should be fine but just wanted to verify it was OK.
Have checked pins on voltage regs and are wired according to PIC included. All connections have been tested and are OK.
Appreciate any replys.

 

Have you tried swapping the 7912 for a new one?

 

Don't have any spare 7912, ordered few yesterday should have them before weekend. All 79 voltage regs I have came in same pack, be very suprised if they were all faulty.
Do you know is the link i have from transformer to 0v line OK, think it's irrelevant as to where its connected on that line as its all common, but thought I would ask just to be sure.

 

Doesn´t matter where exactly the connection is made, just make sure everything really is connected the way its drawn.