[SOLVED] My Bi Polar Power Supply Problem

[gerbster]

Hi,

I'm working on a small bi polar power supply capable of supplying +/- 12 VDC. I found a couple of schematics on the web, but this one comes very close:

**broken link removed**

However, I've run into some problems ;-) For some reason the 7912 IC is not giving me the requested -12 VDC. This is what i have so far:





Voltages:

Input: 19 VAC (from an old clock radio transformer)
Across Point 1 & 2: +/- 25Vdc

With the 2 Ic's omitted:
Across Point 4 & 5: -12.8 Vdc
Across Point 3 & 5: +12.8 Vdc

With the 2 Ic's installed:
Across Point 4 & 5: -5.8 Vdc
Across Point 3 & 5: +18.6 Vdc
Across Point 5 & 6: -5.5 Vdc
Across Point 5 & 7: +11.8 Vdc

So for some reason, with the 7812 & 7912 in place the voltage isn't diveded evenly between C1 & C2 anymore. Does anyone have any pointers on this?

Thanks in advance!

 

[Tahmid]

I'm guessing you didn't connect point 5 to the transformer centre tap (as you can see in the circuit diagram). That is the problem.

 

[gerbster]

You Sir, were right.. For some reason I tought that the Center Tap was the ground. For other people struggling with this..

https://en.wikipedia.org/wiki/Center_tap#Volts_center_tapped

Thanks!