Hello!
I have a wierd problem with a symmetrical linear power supply I'm making.
It's a pretty basic design - very similar to this one:

I'm using L7912CV and L7812CV as regulators. This works fine, but I also need 5V and -5V, so I added a positive regulator (L7805CV) to the positive rail, and a negative one (L7905CV) to the negative rail.
The positive rail works fine, I get 12V and 5V, on the negative however, I get -5V and -14,5V (the same voltage as the input to the regulator).
I checked all the components, played with capacitor values, loads - nothing seems to work. As soon as I connect the L7905CV to the negative rail, the L7912 stops regulating.
I've spend 2 days trying to figure this out. Is there something obvious about negative regulators I'm missing?
Thanks

 

Are you sure you have the pinouts correct on the 7905? Those negative regulators have a tendency to oscillate. Be sure you have a 0.1 uF cap on both the input and output pins to ground as close as you can to the device.

 

The circuit schematic posted shows C5 and C6 as 220μF.
Reduce those to 1μF.

 

You do realize that pins 1&2 are swapped between the negative and positive regulators.
Yo may need a mall minimum load on the first regulator in the series.

 

You didn't show your real circuit yet, please take some clear photos for the circuit(breadboard or two sides of PCB) and compress it to 800x600 jpg file and upload it, thank you.

 

Hi, thanks for your replies. Here's what I got so far:
- I played around with different capacitor values, and added 100nF capacitors directly across the leads of the regulators - no real change.
- I also tried some different loads again, no real change
- Yes, I am aware of different pinouts, I checked it probably a hundred times already, making sure I'm not making a stupid mistake.
- The schematic of my power supply is pretty much identical to mine, so I didn't bother to draw a new one, especially because I'm changing things all the time.

I've attached some pictures of the board (I know, that the wiring looks like a nightmare, but I just stopped caring after resoldering it so many times - I'll make it pretty when it works).

 

Hello,

Are the regulators isolated from the heatsink?
The tabs are usualy connected to the middle pin of the TO220 housing.

Bertus

 

It was the heatsink...
I could've sworn I checked that it doesn't cause any problems, but I guess I wasn't careful enough.
Thank you so much for your help!

 

Your aware of the T0-220 mounting kits that contain a step collar, screws and an insulator?
Sil-pads don't require silicon grease, but other insulators do (kapton, Mica).

 

Yes, I am aware, thank you.
I should be fine from now on, I just needed somebody else to notice the obvious

 

Hello,

As @KeepItSimpleStupid sais, there is mounting material.
I made some pictures of the stuff I have:



Bertus

 

on the negative however, I get -5V and -14,5V (the same voltage as the input to the regulator).

When you added the heat sink to 7905 and 7912 and there is no isolation for the regulators then the input and output pins of 7912 will be connected together, the reason are the output of 7912 connected with the input of 7905, and when the heat sink added then the input of the 7905 and 7912 will be shorted through the heat sink, so the input and output of 7912 will be shorted.

Th middle pins of 7805 and 7812 are the ground pin so when they added the heat sink still ok to work.

If you need more current then you should increase the input voltage of 7912 to -15V or more, and increase the input voltage of 7812 to +15V or more.