2 Voltages single transformer

[Objerver]

I would like to feed my B1 buffer with +18V and my source selector with +12 V.

I have at hand an 18V transformer and I'm wondering if I could get both dc voltages after the bridge using, for example, one LM317 for +18 and one 7812 for +12.
Can this be done without issues? If yes how?

Thanks in advance

 

[JMFahey]

Yes you can.
Look at the datasheet example showing how LM317 outputs stable regulated +12V while being fed a higher voltage (in this case 18V).

 

[scottjoplin]

If yes how?

Simply tee off to the two regulator circuits after the smoothing capacitor

 

[MorbidFractal]

18VAC gives you about 25VDC, ignoring ripple.

Don't forget that you can use the LM317 to get your 18VDC and then use that 18VDC to supply the 7812 which should lower and share overall dissipation in the devices. Check the voltage drop and total currents for dissipation in each device.

As hinted by JMFahey since the LM317 is adjustable you can use it in both locations. Make sure you follow what is recommended on the data sheets in terms of pre and post regulator filtering capacitance.

 

[jackinnj]

As hinted by JMFahey since the LM317 is adjustable you can use it in both locations. Make sure you follow what is recommended on the data sheets in terms of pre and post regulator filtering capacitance.

In fact, if you bypass the adjust pin of the LM317 with a 10uF electrolytic capacitor you will reduce the noise by several dB/

 

[Objerver]

thank you all for the help and the tips.

 

[nigelwright7557]

If you want to get away with fewer components you could use LM7812 and LM7818.