Some help with mains wiring to avoid death.

[william]

I've decided to stick some of the pedals I've made into a 2u rack space, and would like to have an internal transformer for them to save space and plugs.  Also, I am planning to build a tube amp in the near future and would like the experience with something smaller and not as expensive first.  I've already gotten a 12v transformer and have a foggy clue about wiring up the transformer to earth ground, and such.  But before I begin, I want to get some more information to clear some more of the fog.  Are there any good web resources that covers this subject from a diy stand point?

Thanks william

[Mike Burgundy]

get a good DIY book on the subject - check the library for DIY electronics books - and start small.
Putting a transformer in a new housing requires some insight and building skills. You need to keep the transformers contacts well away (at least half an inch, preferably more)from the chassis and anything else while still mounting it securely, isolate the contacts with heatshrink, ground the chassis properly, use the correct fuse, etc etc.
DO NOT GROUND ANYTHING ON THE TRANSFORMER PRIMARY.
The transformer primary connects to mains, which has a hot lead and a "0" lead - but you don't know which is which. The transformer just sees the difference between the two. Connecting any primary point ground your mains supply blowing up fuses and such. If you forgot to ground the chassis, the ground connection will be made through YOU. Take care.
-How are you going to mount the transformer?
-Where does that leave the contacts?
-Can you reach them after installing the transformer, or do you have to do the wiring first?
-Where does one put the transformer?
-Where does one put the powerswitch? (Remember, this should always switch BOTH powerleads!!!)
-The fuse goes on the primary side.

I'd use an adapter for the effect rack, and get a couple of effects under your belt before getting into amps.

[Peter Snowberg]

I agree with Mike that an external transformer is very preferable. Hum is a serious issue with low level signals and if you can avoid interaction by physical isolation, you will be MUCH better off. It's also a lot safer overall.

I'll echo what he said about getting a tube amp DIY book and starting with some low voltage projects before jumping to high voltage land. The worst that can happen with effects is letting the blue smoke out of the parts.... tube amps on the other hand can kill. :shock:

One thing I'll add.... When you're working with high voltage, keep one hand in your back pocket. That will stop you from electrocuting yourself fairly effectively. A Fender Super Reverb came close to killing me once. I think the only reason it didn't was that I had some tolerance from a lot of shocks before. Not an experience that I would recommend to anybody.

Take care,
-Peter

[aron]

A Fender Deluxe also packs some serious punch. Very spooky!

[william]

Yea, starting small is my plan.  Thats why I will be using a pcb 12v radio shack transformer rather than a 350vdc tube transformer to start out with.  The most common reply I've been getting is "don't do it".  I'm asking for help guys....  Anyways, I checked with my local libraries and didn't find anything that seemed to help.  I did manage to find EPFG and EPFM though and will pick them up.  I'm thinking I might get The Ultimate Tone from Lodon Power.  Does this cover the subject of wiring a transformer correctly.  

My biggest point of confusion right now is the grounding points.  I realize there needs to be an earth ground which is the green cable.  My confusion comes in with grounding the circuit.  Star grounding in an amp seems to ground at the chassie.  Wouldn't this cause a short on the ground of the primary side of the transformer (0v I believe).  

And finally if TUT doesn't cover the subject, what are some other books I could look for?  I thought about a book on wiring homes up, but decided that probley would cover lights and such.  Thank you.

William

[Peter Snowberg]

:!: You don't want the primary connected to the chassis ANYWHERE! The mains plus provides you with a "hot" and and "neutral" which should connect to nowhere other than the transformer winding, a switch for power (best to use a DPDT switch and switch both), a fuse, and a pilot light if desired.

The ground from the mains plus should connect to the chassis and ideally all the other grounds will return to a single point for the star. The center of the star and the chassis ground point can be totally different. The ground of your input jack is a good location to put the star near.

Good luck,
-Peter

[Mike Burgundy]

re-read Peter's post several times (and mine, it's in there too
Don't worry if you don't get it right away (we all started somewhere) just don't do ANYTHING unless you know what goes where and more importantly, why.
Ground connects to, well, ground. it's often hooked up to the house water system - this conducts very well and goes into the ground sufficiently to bleed off any voltage.
This is the "safe" point - this is where you want anything you can touch to sit.
When any, *any* mains AC carrying conductor touches ground something's wrong. Very wrong. Usually this will trigger a circuit breaker or pop a fuse, or it will probably cause some destruction if that doesn't happen.
so:
two AC lines (hot and cold) go to the transformer, and *only* there. (okay, insert a power switch etc)
There must NOT be any contact between the circuit and the transformer primary. None. This is one of the main points to use a transformer: galvanic separation: NO contact between the circuit an mains.
The green (green/yellow in Europe) wire is the ground connection, which is used for chassis ground.
If you use a chassis ground, you also reference signal ground to that.
A lot of HiFi equipment doesn't utilise wall-grounding and just references signal ground to itself which works fine: the chassis is not protected however, and the lot is a little more sensitive to interference.

[william]

I just re-read what I typed, and it wasn't very clear about the short thing.  I know that the two primary leads go to the primary leads of the transformer.  The third wire is for the earth ground.  The thing I'm unclear of, is if earth ground is at the chassis, and the circuits ground is also at the chasis, doesn't that short the circuit?    

Actually as I'm typing I think I understand now.  Earth ground=0v.  although the mains wire's have a gound its not really important for the transformer(well it is, but not as ground), it just sees 115 one way, then 115 the other way......

Is that correct?  Or am I just confusing the issue?  

I plan for the primary leads to go direct to the transformer then covered with heatshrink tubing.

[william]

I think I'm making it way more difficult than it should be.  the two hot leads go to the power switch, fuse,  then the transformer.  The earth ground goes to the chassis, or a common ground point.  The circuit's ground point, then goes to common ground point as well.

[Mike Burgundy]

yah, that's it.
The trick is to realise that *earth* or *mass* or *ground* is what's all around us, mainly under our feet. You don't get electrical shocks from soil.
The earth lead in a mains connection is just there for protection: if you hook it up to certain conductive parts those parts will be at 0V exactly compared to their direct surroundings, so you won't get zapped touching them.
The primaries are about the *difference* between the two primaries: if one is at "zero" level (but NOT grounded) and one does 230VAC, or both do 115VAC (as long as phase correlation is correct) the transformer doesn't know the difference. Your fingers (connected through your entire body to ground, REAL ground) will definitely know the difference, though, no matter which wire you touch if it's 2x115VAC...
Take care,
M

[niftydog]

note that there is a difference between "earth" and "ground".

Earth is the point that your mains system uses.  In australia the earth is literally a connection to a copper pipe whammied into the ground near the fuse box outside your house.

Ground is more commonly used to refer to a particular point in a circuit.  

The two need not be connected in all circumstances.

Note that a metalic case should be connected to earth.  Hence, it should never become live.

Also note that voltages become "dangerous" (as a rule of thumb) above 30VDC.

In my case, and others, the connection from ground to earth is the source of problems.  ie; ground loops = hum.  Also, my power supply is 11 isolated supplies in one; connecting all the grounds together would not only be a hassle, but it would defeat the purpose of having built it!

Ultimate safety is acheived by;

- connecting the primary and neutral to a fuse each (to ensure protection in the case of incorrect wiring in the mains system)

- then to a DPDT switch, rated for mains.

- then, to the primary coil.

- connect the earth directly to the chassis of a metal box.  Also ensure the body of the transformer and any other metal object that's NOT electrically connected to the circuit is connected to the earthed box.

and... I know you've probably heard this before, but it must be said;  In Oz, you're not even supposed to wire up extension cables or power points without a full license to do so.  Despite that, a lot of people do it, and a lot of people get killed, even professional electricians!

BE FREAKIN' CAREFUL!

DISCLAIMER;  I, nor anyone else posting to this thread should be regarded as a definitive resource of information about mains wiring.  DO NOT under any circumstances take our word for it.  Check your countries wiring regulations, double check your theory, triple check yourself and think four times about what you're doing before begining.

Never ever work on a live circuit, and be aware that some components are lethally dangerous even long after being disconnected from a source of power.