Regulators don't regulate correctly

Hey all,

I'm pretty new to practical electronics, so please bear with my novice problems.

I'm constructing a symmetrical power supply, which is powered by an unregulated transfomer (nominal 12 V/800 mA) and is supposed to provide my device with +5 V, GND and -5 V.

There is no external ground connection and no bridge, and I use 7805 and 7905 regulators. I also have additional 470 µH electrolytic condensators between the ground and regulator outputs.

The problem is that my supply only manages to provide a total 8 V difference between the positive and negative outputs, and this is not symmetrical. The 7905 seems to function as intended, but 7805 only gets to 3 volts. When I measure the input voltages, the 7905 seems to hog almost all voltage, over 12 V while 7805 gets less than 4 V.

I suspected this might be due to incorrect voltage division between the 1000 µH caps, but this is not the case. The voltage is divided correctly with only the caps and with both the caps and resistors, when the regulators are disconnected. It also crossed my mind that the dc supply is too weak, since it nominally provides 12 V, but the supply stays at over 16 V at all times. I blamed defective regulators, defective caps and switched all parts. I rebuilt it entirely on a breadboard, but nothing seems to work.

Any ideas what to try next?

---------- Post added at 21:05 ---------- Previous post was at 21:04 ----------

The design is roughly as depicted here, with the exceptions mentioned in the previous post (for some reason I need to have one previous post before being able to post links...):

**broken link removed**

Capacitors are measured in Farads (F) not Henries (H)!

What do you mean by "no ground connection and no bridge", the circuit is fed from a center tapped transformer and the bridge is absolutely essential. Without it the capacitors would explode and it certainly wouldn't work. Can you draw the schematic you are actually using, including the transformer connections.

Brian.

This sounds like a similar circuit that has been discussed a few times here recently where capacitors are expected to divide DC! As Brian says, you need to show you EXACT circuit.

You've been quiet recently Brian (or maybe it's me!)

Keith

The attached schematic shows the transformer in the input too (it uses 12v regulators but operation is the same), you should use a transformer with a center tap like the pictured one with a voltage output that suits your needs,
the center tap becomes the reference point and you use a full wave rectifier to create a positive voltage and a negative one with reference to the gnd (center tap).
Then you can use the regulators to regulate this voltages.

**broken link removed**

Alex

Your output capacitors have no ground connection and are far too small.

I've got a major problem here with an oil company that dumped 1,000 litres of diesel fuel on my land and caused a very serious pollution problem. I've been fighting them in court for over three years and finally forced them to clean up at their expense (about £200K). Thankfully, I have a holiday cottage nearby, usually let out to visitors but I'm having to prepare it to be a temporary home so I can move out for a while until it's cleared up. I'm playing at being a builder as well as electronics engineer, it's taking a lot of my time! I still visit several times a day though.

Brian.