Well, I have +12V, but I only have 1.8 V on the negative side. The transformer and the LM320 get pretty hot. I'll prolly just go get a LM7912.

According to the datasheet, you need extra capacitors.

My transformer isn't cooling down with the power cord disconnected and switch off...it stays hot to the touch....what's wrong?

 

The transformer may take a while to cool down when it's been overheated. After all, it's a chunk of metal, and the wires are metal (copper) with insulation of various types on them. You don't want it to get too hot, or the insulation will melt/burn.

Did you un-solder the input connections to the regulator ICs? (both of them)

If you did, and got those readings, then something is wrong with your rectifier bridge connections. As I've mentioned before, I've purchased surplus rectifier bridges that were incorrectly marked; it takes a number of measurements to determine what the actual configuration of the internal diodes are.

If you did not, you need to start by disconnecting your regulator ICs' inputs, and leave the capacitors on the rectifier bridge output connected for the moment.

With your meter on the 20VDC scale, you should be reading about +17V between the transformer center tap and the + output of the bridge rectifier, and about -17V between the transformer center tap and the - output of the bridge rectifier.

 

Okay, you say I have to connect the input lead with the tab since they are both input? Is the tab basically the heatsink part? So I solder the input lead (middle) to the anywhere on the metal heatsink?

The problem right now is I'm not really sure of the GND, IN, AND OUT connections on the LM320MP-12.

 

http://www.rose-hulman.edu/%7Eherniter/Data_Sheets/LM120.pdf

http://users.otenet.gr/~athsam/database.htm

Opps, posted without looking. Sorry.

Is there something unusual about this package style?

 

The transformer may take a while to cool down when it's been overheated. After all, it's a chunk of metal, and the wires are metal (copper) with insulation of various types on them. You don't want it to get too hot, or the insulation will melt/burn.

Did you un-solder the input connections to the regulator ICs? (both of them)

If you did, and got those readings, then something is wrong with your rectifier bridge connections. As I've mentioned before, I've purchased surplus rectifier bridges that were incorrectly marked; it takes a number of measurements to determine what the actual configuration of the internal diodes are.

If you did not, you need to start by disconnecting your regulator ICs' inputs, and leave the capacitors on the rectifier bridge output connected for the moment.

With your meter on the 20VDC scale, you should be reading about +17V between the transformer center tap and the + output of the bridge rectifier, and about -17V between the transformer center tap and the - output of the bridge rectifier.

I disconnected the regulator inputs and tried getting the readings you told me, then the rectifier started smoking up. Time for a new rectifier huh? =\

 

Yes, it sounds like the rectifier bridge is likely fried.

Do you have a digital camera? Can you take a photo of what you've built and post it?

 

The IN and GND on a negative regulator are usually opposite that of a positive regulator. You have to pay careful attention to the datasheet for pin numbers, their order/placement on the package, and their definition -- and, be careful whether the drawing representation is looking from the top or bottom side of the part. Also, especially when heatsinking, be careful whether the case or tab is at gnd or some voltage.

If you connected the negative regulator incorrectly, then it's probably bad now, so replace it with a new one using the proper connections. Refer to page 8 of the attached LM320 datasheet that you pointed to earlier.

Look at page 2 of the LM7812 datasheet I've attached for the positive regulator pinout, and also page 25 for a sample circuit (although it is +/-15V and not +/-12V). The reverse diodes are used with split supplies in case one supply goes bad, the other supply cannot backfeed the bad supply more than a diode drop.

 

Well, the way I connected and soldered the components and wires is very confusing and so I doubt a picture of my circuit will help. (This is my second time soldering and first time making something that I plug into the wall outlet) But I replaced the bridge rectifier and I get the +12V correctly and the rectifier doesn't heat up anymore.

The negative voltage regulator still heats up and I get 15V on the negative output side, so I will prolly go with murphy's suggestion and replace it with a new one. I double checked all the other connections and everything else seems fine. It must just be a bad negative voltage regulator.

Thanks murphy for the datasheets. I'll see if a new neg. regulator will help and keep you guys posted..

 

Another gotcha for negative regulators is the polarity of the capacitors. Be sure to put the positive side to ground as shown on the datasheet. John

 

Well, my +-12 V power supply worked! I think I could improve the ripple better..but i have a solid plus 12 volts and minus 12 volts.

The problem all along was a bad negative voltage regulator since I connected it incorrectly the first time I tried it. I replaced it. I also replaced the fried rectifier.

Thanks everyone for all the help! I do appreciate it. =]

 

Congratulations

You could help reduce the ripple considerably by increasing the size of the caps at the output of the rectifier bridge. As things are with the 1000uF caps, even a small load on the output will result in output ripple.

You could also replace your 25v center tapped transformer with a 36v center tapped transformer.