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Old 31st March 2016, 02:31 AM   #1
jwmatx is offline jwmatx  United States
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Default LM3886 transformer question

Hey guys. I'm building my first amp, and feeling like I'm in a bit over my head. I'm hoping some of you might be willing to point me in the right direction.

I have finished assembling the Stereo LM3886 kit from Chipamp. Now I'm ready to start connecting the transformer and I'm a bit lost. Here is the transformer that I have:

AS-3225 - 300VA 25V Transformer - AnTek Products Corp

I understand that I need to connect the dual primaries in parallel. Am I correct in assuming that I combine the two red wires and connect them to the live side of the socket, and then combine the two black wires and connect them to the neutral?

And on the secondary side, when I connect the wires to the PSB, I have two blue and two green. The board has two connections labeled AC1 and two labeled AC2. Would I just connect a green and blue wire to each set?

I'm sorry in advance for what might be an elementary question. I really have tried to research this myself, but given that this is my first attempt at doing something like this, I'm a bit overwhelmed and don't want to blow anything up!

Thanks!
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Old 31st March 2016, 03:22 AM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by jwmatx View Post
I combine the two red wires and connect them to the live side of the socket, and then combine
the two black wires and connect them to the neutral? And on the secondary side, I just connect
a green and blue wire to each set?
Yes, the primary windings are connected in-phase (that's very important) and in parallel, for 120V.
That means that the red leads are connected together, and also the black leads are together.

The pcb secondary connections each have separate full wave rectifier circuits,
so one winding (green/blue) goes to the AC1 rectifier, and the other to the AC2.
Use your ohm meter to verify that the pair of green/blue leads that you select
are actually connected to the same winding. This is important to check.
You can connect the two windings to the board in the same way, but it really
doesn't matter.

Last edited by rayma; 31st March 2016 at 03:40 AM.
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Old 31st March 2016, 07:09 AM   #3
tomchr is offline tomchr  Canada
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Quote:
Originally Posted by jwmatx View Post
I understand that I need to connect the dual primaries in parallel. Am I correct in assuming that I combine the two red wires and connect them to the live side of the socket, and then combine the two black wires and connect them to the neutral?
Correct. For 120 V operation: Red to red. Black to black. One pair goes to Live the other to Neutral. It doesn't matter which.

Quote:
Originally Posted by jwmatx View Post
And on the secondary side, when I connect the wires to the PSB, I have two blue and two green. The board has two connections labeled AC1 and two labeled AC2. Would I just connect a green and blue wire to each set?
You have to figure out which green wire goes with which blue wire. Pick one of the two green wires. Connect this to the (+) lead of your ohmmeter. Measure the two blue wires. One will have low resistance. That's the blue wire that goes with the green wire the (+) lead of your meter is connected to. Take that blue-green pair and connect them to AC1. The other pair goes to AC2.

If you don't have an ohmmeter or multimeter, you can make a continuity tester from a flashlight bulb and a battery.

Tom
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Old 2nd April 2016, 07:24 AM   #4
AndrewT is online now AndrewT  Scotland
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Use a Mains Bulb Tester to power ON any new, or modified, mains powered project.

When using this test device you can mix up any of your primary wires and the mains/you will not damage the transformer.
Similarly you can mix up any of the secondary wires and there can be no damage.

BUT FIRST. !
check using a continuity tester (ohm-meter, or battery and bulb, or beeper facility on your DMM) which wire taps are connected to which winding.
A dual primary/dual secondary has 4 windings and a minimum of 8 tappings. Label them !
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