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Old 6th May 2007, 11:32 PM   #1
ebenai is offline ebenai  United States
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Default Transformer wiring problem

I have tried (without success) to wire a Hammond transformer for a LM4780 power supply (Audiosector kit). I have followed the manufacturer's wiring instructions but every time I try the transformer overheats or the fuse blows up. If I understood well, both the primary and secondaries should be wired in paralel. I have attached a diagram.

HAMMOND 182P22
PRIMARY 2 x 117
VA 225
SECONDARY
2 x 22V @ 5.1 A
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Old 7th May 2007, 01:49 AM   #2
BWRX is offline BWRX  United States
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ebenai,

The transformer you have should have the primary windings connected in parallel for 120VAC operation. In other words, connect the orange and white wires to AC neutral. Then connect the black and brown wires to AC line. Make 100% sure this is NOT plugged into the wall before wiring it up.

You want a split rail supply for your LM4780 amp. That means you need a positive and a negative voltage rail. The LM4780 board has 4 AC connections. The transformer secondary windings connect to the spots on the supply board. Following the PCB on the left half of the image on Peter's site: http://www.audiosector.com/images/4780.gif

Connect the red wire to the upper AC1 through hole.
Connect the blue wire to the lower AC1 through hole.
Connect the green wire to the upper AC2 through hole.
Connect the yellow wire to the lower AC2 through hole.

The DC voltage between V+ and PG+ should be around 32V.
The DC voltage between V- and PG- should be around -32V.

Test the DC voltages BEFORE connecting the power supply board to the amp board. If the voltages are not correct you could risk damaging the amp. You may need to connect a resistor between V+ and PG+ to load it down slightly in order to get a correct reading. Likewise, you may need to connect a resistor between V- and PG- to load it down slightly in order to get a correct reading. I would use a 10kohm 1/4W since that's a pretty standard value to have around. You may use a smaller value resistor but don't use one much lower than 5kohm unless it rated higher than 1/4W.

It's good you are using a fuse! You mentioned the transformer is getting hot before the fuse blows up though. About how hot is it getting? What is the rating of the fuse you are using?
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Old 7th May 2007, 04:36 PM   #3
ebenai is offline ebenai  United States
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Default Transformer

Thanks Bryan:

I have tried the configuration you suggested several times. I just cannot make the connection without blowing up the fuse (3 A, 250V). The first time I tried it was w/o fuse and it got hot (you cannot hold it for more than 2 secs). The first time I connected it was w/o fuse after installing it to the chassis. To the best of my knowledge it was properly grounded. After that I took it out and tested it out of the enclosure. Every time I've tested it has been w/o any load to measure the output voltage directly out of the secondaries before connecting it to the rectifier board. I'm afraid I did something wrong and perhasp damage it. I think there is a short inside the transformer. Is there a good way to test it w/o conecting it to the mains? I tried measuring the resistance and got 1.5 R from one primary, about 2.1 from the other primary and around 0.2 R from the secondaries.
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Old 7th May 2007, 06:56 PM   #4
BWRX is offline BWRX  United States
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Default Re: Transformer

Quote:
Originally posted by ebenai
I have tried the configuration you suggested several times. I just cannot make the connection without blowing up the fuse (3 A, 250V). The first time I tried it was w/o fuse and it got hot (you cannot hold it for more than 2 secs).
So the transformer gets too hot to hold? This was just with the transformer connected to the mains with nothing connected to the secondaries, correct? If so, that's definitely not good.

Quote:
Originally posted by ebenai
I'm afraid I did something wrong and perhasp damage it. I think there is a short inside the transformer. Is there a good way to test it w/o conecting it to the mains? I tried measuring the resistance and got 1.5 R from one primary, about 2.1 from the other primary and around 0.2 R from the secondaries.
The windings of most toroids usually measure the same. You measured 0.2ohms across each secondary which is a good - they are the same low resistance value. The primary windings use more turns of smaller wire than the secondary windings, so their measured resistances should be slightly higher and very similar. Your readings of 1.5ohms and 2.1ohms show a pretty big difference in resistance for two windings that should be very close in value.

To determine which primary winding is bad simply connect one primary winding at a time and see if the secondary voltages are correct. Load down each secondary with a 10k or so resistor and measure the AC voltage across the resistors. If the secondaries measure about 22VAC then that primary is good. If they don't or the transformer gets real hot you know that primary winding is bad.

If you have one good primary winding you can still use the transformer with just that primary winding connected, but the VA rating of the transformer will be halved.
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