Transformer wiring problem

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sumacSK

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Joined 2004
#1
Hi all,

I'm in process of building LM3875 stereo amp based on brian's rev.3 board. I've ordered a transformer (200VA, 2*19V). It has arrived today, but it is not transformer with dual secondaried (as I thought it will be) but it looks exactly like this: http://ezk.cz/images/tor16.jpg . If I understand it correctly, it is center-tapped transformer. So now, I'm little unsure how to wire it to rectifier board. Can anybody explain it to me?

Thanks Martin
 
#2
Put the Two black wires on mains ( put fuse in-line)
Then measure the ac voltage on the other side.

Then (as I can see) the red wires are your secondaries with the center one being ground. The center one is just the windings that are already connected.

The center tap is the wire that always gives a lower voltage.

Just wait for someone to confirm this... or be VERY VERY carefull.

I dont want to kill you with bad advice.
 
#3
Apparently the red wire with the 4 leads is the center tap, the red ones with the 2 wires are the top and bottom leads (which are interchangeable of course). The black ones are the mains leads.
I am not familiar with your particular board, but the center tap should go to ground (no surprise here!) while the other ones should go to each end of the double-phase rectifier. If you post a schematic of your rectifier setup we can exactly tell you where they should go.

Jan Didden
 
#4
Hi,

here is schematic of rectifier board, but it is using transformer with dual secondaries: http://www.audiosector.com/lm4780 psu.pdf Here is users guide this gainclone: http://chipamp.com/nigc_kit-users_guide.pdf . There is also shown how to wite center tapped transformer, so if I uderstand it correctly, those 2 red wires (top and bottom leads) goes to AC1N and AC1H, AC2 stays unused, and center tap goes to PG+ and PG- . Also only 4 diodes should be used.

Is that right?

Thanks Martin
 
#7
sumacSK said:
here is schematic of rectifier board, but it is using transformer with dual secondaries: http://www.audiosector.com/lm4780 psu.pdf Here is users guide this gainclone: http://chipamp.com/nigc_kit-users_guide.pdf .
Click to expand...
Ok.... I'm not trolling, just some observations from viewing the above schematics. And Yes I heard wiring ct trans up as I'm about to suggest will let the smoke out.

Here goes:
Wiring sec 1 to AC2H, center tap to AC2N and AC1H, sec 2 to AC1N. Why can't the center tap of trans be wired to AC2N and AC1H? I see V+ to PG+ as plus rail and PG- to V- as neg rail.

To go a step farther, couldn't PG+ and PG- be spliced together. I say this because I enlarged the pdf file looking at the pcb's and I don't see a short. Can someone please tell me were the short occurs?

To go a step farther, page 15 of the manual shows PG+ and PG- are common to the amp board. This narrows the possible short to the rectifier layout.
 
#13
janneman said:
Apparently the red wire with the 4 leads is the center tap
Click to expand...

Hi sumacSK,

As Jan has said, I suspect the red with 4 leads will be the CT and I guess it would be simple with a multimeter to work out which 2 belong to each secondary winding, then you will have 2 separate seondaries.

Usually the leads of transformers are insulated with enamel and only the last 15 or 20 mm is stripped for soldering. Check the insulation of each of the CT leads and use a multimeter to check that there is no connection between all the 4 leads.

regards
 
#14
Tried it...

MrLots2do:

I did the very thing you said thinking "it's gotta work". I wired it up with the cetertap to both connectors. I measured the correct +/- voltage prior to connecting the amp. But once I connected the amp, a fuse would blow (I have fuses on the Hot and Neutral - kept blowing the Neutral). I went through 12 fuses in two days before I just removed the extra four diodes and jumpered the missing diodes. Works fine now. I would hate to think what would have happened had I not had the fuse.

Why don't you try it, you realize it won't work correctly.

My 2 cents
 
#15
if you try to use a CT transformer with the center tap connected to both sides of a dual rectifier board, you will cause a short on half the cycle. the short actually is through some of the diodes. and the common ground. so it might appear to work untill you hook up the amplifier.

either use it with 4 diodes as a single bridge circuit, or, you can modify your transformer to dual secondary. i did the latter, and it worked fine but BE CAREFUL! I would not want anyone to ruin a perfectly good transformer or zap themselves.
 
#16
Re: Tried it...

john65b said:
I did the very thing you said thinking "it's gotta work". I wired it up with the cetertap to both connectors. I measured the correct +/- voltage prior to connecting the amp. But once I connected the amp, a fuse would blow (I have fuses on the Hot and Neutral - kept blowing the Neutral). I went through 12 fuses in two days before I just removed the extra four diodes and jumpered the missing diodes. Works fine now. I would hate to think what would have happened had I not had the fuse.
Click to expand...
Didn't see the micro-short (as I'm calling it) until you described it in detail, thanks John65b. Built a 4 diode rectifier board after reading your writeup. What I did come to realize was only a single rectifier is needed per power supply secondary. Quess I could build more amps now that I have extra rectifier pcb parts.

Thanks again
 
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