Hi I'm trying to build a (0-30Vdc) voltage source, I did the transformer and the circuit by myself, almost I have finished the project but I have a question,

1. should I connect the center tap to the ground conection of the electrical outlet's ground(earth)?.

2. I know wood is an insulator but, should I connect the ground(0V) to the box where I'm going to put all inside... just in case?.

3. Can I use the negative voltage source as a possitive if I use the ground as positive and positive as ground withouth damage the circuit ?

This is the circuit

 

1) It depends. I would say, "no" because sometimes you want a circuit that has nothing to do with planetary ground.
Some of the old designs used about 2.2 meg between secondary center tap and earth ground just in case of static charge build-up.
I never found static to be a problem.
2) Your words are scrambled.
Your circuit zero will always be more positive than your negative output. You can use the supply that way.

Oh, by the way, R2 and R5 are 240 ohms. That's for the 117 version. You have the 317 version and they need a 10 ma load.

 

1) It depends. I would say, "no" because sometimes you want a circuit that has nothing to do with planetary ground.
Some of the old designs used about 2.2 meg between secondary center tap and earth ground just in case of static charge build-up.
I never found static to be a problem.
2) Your words are scrambled.
Your circuit zero will always be more positive than your negative output. You can use the supply that way.

Oh, by the way, R2 and R5 are 240 ohms. That's for the 117 version. You have the 317 version and they need a 10 ma load.

Ok thanks for answering, let me know if now you undestand the second quenstion

 

OK. I'll try again. If you use the zero voltage point as the positive end of a circuit connected from the negative output to the center tap (zero voltage point) you can not use the positive output as ground.

 

OK. I'll try again. If you use the zero voltage point as the positive end of a circuit connected from the negative output to the center tap (zero voltage point) you can not use the positive output as ground.

I mean the question that was scrambled

 

Oh. I missed the middle question. I've been answering #3 as if it was #2.
Yes, earth ground on metal boxes is usually good. You can use insulated connectors to deliver the outputs to the outside of the metal box. That gives you a choice of when you want the zero voltage point connected to earth ground, or if you don't want it connected to earth ground.

 

Connecting earth ground to a wood box serves no useful purpose except to physical anchor floating green ( US )
pigtail.