oh....so that's why people use fuses

[PurpleTheory]

first off, i'm really sorry to ask another question about this... I built the lm3886 amp from general guitar gadgets and with some help from the forum got it working. Very cool project, but the power supply wasn't sufficient to get very loud. I had an old 36v transformer kicking around so I decided to try it out. I asked a guy at a local electronic supply shop about building a bipolar ps for the amp and he said i could do it using 2 rectifier bridges and a center tapped transformer and drew it out basically like this:

So I built it, plugged it in and.......the transformer caught fire  So I guess my question is; does my ps layout look right? Also, the transformer must have been pretty old, could it just have been a bad transformer? I have another transformer, but i hesitate to ruin it if the layout is bad. Also, what are the chances that i ruined the circuit?

thanks for all the help,
matt

[niftydog]

with a centre tapped transformer, you don't need two bridge rectifiers! Questionable skill coming from that electronics store clerk... wouldn't have been radioshack would it?!

The centre tap becomes ground. It should not be connected to the rectifier at all.

The two outer taps connect (in no specific order because AC is not polarised) to the AC inputs of the rectifier.

Then, the + out goes to the positive rail (and presumably onto the regulator) and similarly for the - out to the negative rail.

[niftydog]

with a centre tapped transformer, you don't need two bridge rectifiers! Questionable skill coming from that electronics store clerk... wouldn't have been radioshack would it?!

The centre tap becomes ground. It should not be connected to the rectifier at all.

The two outer taps connect (in no specific order because AC is not polarised) to the AC inputs of the rectifier.

Then, the + out goes to the positive rail (and presumably onto the regulator) and similarly for the - out to the negative rail.

If you're really keen, I can scan an image of a good circuit, but there should be plenty of others around on the net.

[gez]

but there should be plenty of others around on the net.

But please be sure to post a link to anything you find on the net here before you plunge in and build it.  As we all know, schematics have mistakes!

[SaBer]

This is one of my favourites...

http://www.geofex.com/Article_Folders/Power-supplies/powersup.htm

[H.Manback]

Especially the last paragraph... seem familiar?

[H.Manback]

Do you need a +18 0 -18 power supply by the way? Otherwise I would say dump the center tap and go for 1 bridge rectifier and isolate the center tap.

Edit: nevermind, I should read a bit better

Edit2: The layout (although like niftydog pointed out you don't need 2 bridge rectifiers) does look like it should work, but..... Are you sure the bridge rectifier legs are the same order as your layout? And are you sure you didn't mix up the wires from the transformer? These may look like stupid questions, but you would be surprised how easy it is to make those mistakes.

Also it is ALWAYS good practice to put a fuse in there, as you experienced first hand. I had a similar situation with a transformer and my luck was that the transformer was a toroidal (don't know if that made the difference) and it made some melting noise of the plastic before actually burning .

[Hal]

don't 386s take a max of 12v or 15v or something...not bipolar.

would +/- 18 kill one of these anyway?

edit: oooohhhh 3886.  OK.

[PurpleTheory]

Especially the last paragraph... seem familiar? 

Dejavu, yeah that does seem familiar.  Guess I learned it the hard way. Thanks for the tips guys. I was wondering how important is it to use a regulated supply? My concern is that the only regulators that i could find are rated for 1A, but the transformer is 2A.

thanks,
matt

[Paul Marossy]

My concern is that the only regulators that i could find are rated for 1A, but the transformer is 2A.

Not a problem unless you plan on having more than 1A of effects being powered up. Not a likely scenario for the average pedalboard.

[H.Manback]

Actually, a added bonus is that with a regulator you usually get a current limiter (the 1A you mentioned) for free! That way you know that at the output side a short won't burn the transformer... just everything else

[Paul Perry (Frostwave)]

If you are going to rely on a regulator as a current limiter, make sure it is a National brand. But, I can't recommend it..... they are REALLY hot when they go into overload & shut off, plus they cycle on and off slowly after that.