Parallel psu after one full wave bridge

[Max999]

hi stompboxers

Would it be possible to feed two parallel branches of power off one full wave bridge rectifier?
The lm317 branch would feed relays. I thought about feeding them from the brute force part but this would probably create a ( audible) voltage drop through the voltage dropping resistor everytime I click them on or off.
I havent seen examples of this parallel thing, and maybe for a good reason?

[Rob Strand]

Looks OK, that idea has been used.   Probably want to feed the LM317 from after the first cap.  Also don't connect between the grounds of the two circuits.

[merlinb]

I havent seen examples of this parallel thing, and maybe for a good reason?

You have, you just didn't think of it that way. Nearly all electronic devices do this -several different things supplied from one main power source.

[Max999]

Thanks!

Rob the parallel circuit is already sharing a ground in the schem. You are referring to using one ground connection to prevent ground loops?
Also I don't see what would be the difference between connecting it before or after the first filter cap ..

Merlin! Teacher of wonderfull things. I am using a schematic of yours for the relays, small world!

[ElectricDruid]

Also I don't see what would be the difference between connecting it before or after the first filter cap ..

No, one side of the cap or the other wouldn't make any difference, I agree. I think they mean connecting it to the R1/C2 junction. If you've got more smoothing, why not use it to save the regulator work too, right?

[antonis]

Rob the parallel circuit is already sharing a ground in the schem. You are referring to using one ground connection to prevent ground loops?

I think Rob means totally seperate ground returns..

[Rob Strand]

Rob the parallel circuit is already sharing a ground in the schem. You are referring to using one ground connection to prevent ground loops?

The two power supplies each have a two wires (or tracks).   Those wires travel some distance over the wires and end up at each of the two powered circuits.   At the circuit end of the wires you don't want any ground wires to connect across to the other circuit that would form a loop which mixes up the quiet and noisy power supply grounds.   The result is hum, but also the possibility of clicks, whine, buzz, noise depending on the circuits.

Read up on star grounding.

Also I don't see what would be the difference between connecting it before or after the first filter cap .

When you have a rectifier feeding a capacitor very large current pulses pass between the rectifiers and the filter capacitor.   By connecting circuits along that wire you can inject noise into the circuit.   The tracks after the first cap don't contain anywhere nears the same level of noise.   If you want to avoid the risk of hum and buzz problems you should connect after the first cap.

Have a look at the PCB layouts of power amplifiers which have rectifiers and larger filter caps, they will never tap power rails off the wires/tracks between the rectifiers and the filter caps.

[ElectricDruid]

Also I don't see what would be the difference between connecting it before or after the first filter cap .

When you have a rectifier feeding a capacitor very large current pulses pass between the rectifiers and the filter capacitor.   By connecting circuits along that wire you can inject noise into the circuit.   The tracks after the first cap don't contain anywhere nears the same level of noise.   If you want to avoid the risk of hum and buzz problems you should connect after the first cap.

Aah! I see! He was talking about the PCB layout! Ok, that makes sense.

[Max999]

Thanks Rob, it's such a bad habit that I always forget about the currents. I will feed the current back to the node that feeds it.