Ave Cox said:
Why did you say that the transormer is too low ? with 78xx you only need 2volts more at the input than at the output , isn't it ?
I use the transformer for some cards (5 to 10) in a eurocard bus. For the moment, I have done one of them and it needs between 4 and 5 Watts.
The voltage that you have measured is the "no load" voltage, which is about 1.4 x the transformer voltage (25 x 1.4 = 35 volts).
This voltage will be a nice smooth DC voltage,
BUT,
as you start to put some load on the supply and take current from it, you will get some "ripple".
The ripple will give voltage dips every 10 mS, (assuming a 50 hz mains supply).
For a capacitor filter power supply like yours, the magnitude of the ripple can be estimated from the equation:
Ripple Volts = (Load Current x Ripple Period)/Filter Capacitance
So in this case, if you have a 1 amp load (maximum for a 78xx regulator), the ripple will be:
ripple = (1 x 10 x 10^-3)/4700 x 10^-6 = 2.1 volts
Which means that the input to your 7812 will be varying between 35 and 32.9 volts.
If you are going to take less than 1 amp from the supply, the ripple will be less.
You could reduce the ripple by increasing the value of the smoothing capacitor, say put another 4700uF in parallel with the one you have at the moment.
The mains voltage may be low, (everybody cooking their christmas turkey!), so you will have less voltage from your transformer.
etc, etc etc.
You may just get away with it if you are lucky, but this feels like a circuit which is on the edge of not working 100%.
JimB