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Dennis Hui 19th April 2008 11:23 AM

Quote:

Originally posted by johnm

My transformer arrived this morning. Just to make sure I have this right... it's got two secondaries, both 0v - 12v. Do I connect the two 'inner' windings together, and this is my CT which goes to the negative rail. And the two 'outer' tags (0 and 12v) both goto the AC in section?

Cheers,

- John

Hi John,

I think you're right. Just to be clear, in the diagram here :

http://www.airlinktransformers.com/t...ing.asp?data=4

the black and yellow wires are tied together to form the CT.

Cheers,
Dennis

johnm 19th April 2008 11:25 AM

I don't have a capacitance meter unfortunately - think I'll get one from Ebay. Sounds like a useful device to have. All the caps have been used before (once) in other projects and they were fine then. I was very careful removing them so I assume they are still OK now.

I've had this with Black Gates before though and then just as I'm about to remove them they quite often come on song!

I'll give it a week and then change the crystal back. If there's no improvement it can only be one of the replaced capacitors.

Thanks for that link Dennis :)

I think I need to take a crash course in how transformers work. Still confused. When I tried out my 16v transformer Peter said it might be too much for the LM7808 regulator, unless I used a LARGE heatsink.

However, using this transformer 2 x 12v (a recommended configuration for this project), surely the regulator will have to then deal with 24 volts then? Or have I got this all wrong?

Puffin 19th April 2008 12:17 PM

Johnm. If you have 2 pairs of secondary's,you can join the two inner wires to make a CT,but I beleive that you will get 24v on the two outer wires. Check it with a meter.

If all you need is 12v ac just use one pair of the secondary's

luvdunhill 19th April 2008 12:41 PM

Quote:

However, using this transformer 2 x 12v (a recommended configuration for this project), surely the regulator will have to then deal with 24 volts then? Or have I got this all wrong? [/B]

from Wikipedia:

"Full-wave rectification converts both polarities of the input waveform to DC(direct current), and is more efficient. However, in a circuit with a non-center tapped transformer, four diodes are required instead of the one needed for half-wave rectification. This is due to each output polarity requiring two rectifiers each, for example, one for when AC terminal 'X' is positive and one for when AC terminal 'Y' is positive. The other DC output requires exactly the same, resulting in four individual junctions (See semiconductors, diode). Four rectifiers arranged this way are called a diode bridge or bridge rectifier:"

http://upload.wikimedia.org/wikipedi...ctifier.en.png

A full-wave rectifier converts the whole of the input waveform to one of constant polarity (positive or negative) at its output by reversing the negative (or positive) portions of the alternating current waveform. The positive (or negative) portions thus combine with the reversed negative (or positive) portions to produce an entirely positive (or negative) voltage/current waveform.

For single-phase AC, if the transformer is center-tapped, then two diodes back-to-back (i.e. anodes-to-anode or cathode-to-cathode) form a full-wave rectifier (in this case, the voltage is half of that for the non-tapped bridge circuit above, and the diagram voltages are not to scale).

http://upload.wikimedia.org/wikipedi...ctifier.en.png

Note: The last parathetical sentence applies to you :)

johnm 19th April 2008 01:35 PM

Thanks for that :) I'm going to print that up!

So that means I am using 12v then - cool. It's slowly seeping into the old gray matter ;)

Much appreciated!

Puffin 19th April 2008 01:45 PM

Johnm. The photo below shows a dual secondary tranny. One pair of the ac sec's is connected to one bridge and the other set of ac secondary's to the other bridge. The Bridge Rects are marked + & -, these are the dc outs. the terminals in the opppsite corners are connected together to provide the 0v voltage ref


http://i73.photobucket.com/albums/i2.../DualBRect.jpg

In the second picture I use a CT tapped tranny and only one Br Rect. This is simpler in that the two dc outs are marked +& - and the ac goes to the other two. The CT is connected to direct to the power ground.


http://i73.photobucket.com/albums/i2...BridgeRect.jpg

quan 19th April 2008 01:51 PM

shigaclone
 
Hi got mine up and running today -will allow burning in-unable to get GB from OZ and M Percy is not responding
So far upgrade power supply with full bridge rectifier-Mundorf power cap
all caps are replaced with Rubycon ZA/ZL/ZLG series
dig out-75R/75R
So far so good.

kevinkr 19th April 2008 02:09 PM

Sorry had to delete last post with pix and schematic as I noted a couple of schematic errors, and also want to post the pictures so that they are comprehensible. Will try next week. Sorry.. :rolleyes:

Anyone who download schematic please delete.

johnm 19th April 2008 02:09 PM

Thanks Puffin / Kevin :)

I'm clear now - I'll twist together the two wires in the 'center' for the CT, and - as Peter said - this will go directly to the negative of the first PSU cap.

The 'outer' 0v and 12v on the transformer will goto both AC points on Peter's PSU board.

I will be using the PSU board with only two diodes as shown in earlier pics.

Thanks,


- John


P.S. Just to avid confusion, both the JVC and the 16v torroidal I used were both single secondaries only, and for those I used four diodes. For this new dual secondary (2 x 12V) transformer I just use two - correct?

Dennis Hui 19th April 2008 03:23 PM

Quote:

Originally posted by johnm
Thanks Puffin / Kevin :)

P.S. Just to avid confusion, both the JVC and the 16v torroidal I used were both single secondaries only, and for those I used four diodes. For this new dual secondary (2 x 12V) transformer I just use two - correct?

That's right. With the centre tap configuration, only two
diodes are require.

Dennis

P.S. You guys are really quick on this project; I've had the
boombox for a week and haven't even taken it apart yet. :)


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