Power supply and GND

[aballen]

I have a great transformer on order from Weber right here http://taweber.powweb.com/store/pdlxfmr.gif

Since I have most of the parts already, I designed a nice circuit around it.  Note I am planning on having a separate circuit for each winding on the secondary.  I expect this should produce some super clean power.  8 of them at 9V DC, and the last one will be 9V AC for my line 6 looper.


My question about GND is the following.  Based on the layout I made, I will have 8 isolated circuits, each pedal should be completely separate.  The yellow trace has not been routed so I should be able to print this out as eight separate circuits. However I have a three prong outlet.  Should I connect the GND to each circuit to the GND wire on the outlet?  It seems like if I do this I'm creating a ground loop.  Though without it, I don't really have a GND.  What is the correct option?


Oh yeah, the secondaries go to the top holes(11v in) and the jacks connect to the bottom holes(9v out)  assume 7809s where you see 7805s

Thanks in advance.  If I get it all working I'll post finalized layouts for everyone.

[defaced]

No, they are not connected to Earth and they are not connected together; they float. The only reason that Eagle wants you to connect them is that you are using the "GND" symbol which has special meaning (just like all the other supply parts).  The GND symbol ties all points connected to it by using the same Net name.  If you went through and renamed each of the GND symbols GND1, GND2, etc, the airwire connecting them would go away.  This is because you have renamed the nets to GND1, GND2, etc, just like the symbols. 

[aballen]

Yes, I understand why eagle connected them, and I won't.  I still don't know what to do with the GND wire from the wall outlet.  What do I attach it to? Or do I leave that float as well?

[defaced]

Read up a little more and mains wiring and reply back with where you think it should attach.  I am not comfortable advising on mains wiring via a forum, and would prefer to confirm an answer you researched as opposed to tell you and somehow miss something that causes a fire or even worse, you to get hurt.  I vaguely remember Steve at Smallbear having a mains powered project on his site where he details some mains wiring considerations, I believe RG also has some on his site (sorry, I haven't verified this, you'll have to look, check his Spyder project) and there are many other pages on the net on the topic.  As a hint, where to locate the mains ground is directly related to what material the enclosure is and certain features that are on some power transformers. 

[R.G.]

As a couple of hints:
There is no true ground but ground - that is, an electrical wire to a metal rod driven into the planet itself. The word "ground" as it's used today in electronics just means "the point I will decide is the source of 0.000000V and a reference against which all other voltages are measured."

In this setup, you do not have multiple grounds. You have - and want, as a matter of fact - only a number of isolated, not-connected voltage sources. You want this to act like individual batteries, right down to not having a ground. Batteries don't have a ground terminal, they only have plus and minus terminals.

I would recommend going back into Eagle and re-naming those things Vminus1, Vminus2, etc.

As to the mains wiring: first, this is NOT a complete nor necessarily accurate description of how to do this safely. You must take responsibility for your own wiring safety. If you do not already know how to wire AC power line wiring safely, I advise you NOT to proceed until you get training or assistance to do it safely.

The ground terminal coming in from the AC power line is a real ground. It's tied through a series of metallic conductors to a rod driven into the dirt of the planet, or is if the wiring you plug into is wired correctly, but that's another story. It's called "safety ground" because it can protect you against electrocution in certain specific conditions. Those conditions are (1) you use a completely enclosing metal box around your circuits (2) that safety ground wire is solidly wired to the metallic enclosure so that any fault currents from the two other AC power wires are shorted to the enclosure. This will blow the AC line fuse. Oh, yeah: (3) there must be an AC line fuse. And (4) the AC power wiring must be wired to meet safety codes.

As an aside, you're missing a diode with anode to output and cathode to input across the voltage regulators. This protects the regulator chip if the input DC to it is shorted. Otherwise, it can die if the filter cap (for instance) shorts.

See the "Spyder" articles and "Power Supplies Basics" at http://www.geofex.com

[Govmnt_Lacky]

And (4) the AC power wiring must be wired to meet safety codes.

Funny you say this RG. As a matter of fact, it is common practice in household wiring to connect the Earth grounds and the Neutrals TOGETHER at the main breaker panel.

[aballen]

Great stuff.  I appreciate the feedback guys. I'm asking because I know how dangerous mains power is, and I will read up on the sites you recommended. 

I know ground is really just a reference.  I also knew the mains GND went to the metal enclosure.... but that connects to nothing.  I did not understand why, but short protection makes complete sense.

I'll add diodes and a fuse.  In fact the voltage drop from the diode will take some of the load off of the regulators, so that is great. 

I'm assuming I should size the fuse according to the output(300ma x 8 + 2A = 4.4A) so I'm thinking a 5A fuse should be more than sufficient.

I'll read up, the Spyder looks exactly like what I want to do.

[Govmnt_Lacky]

I also knew the mains GND went to the metal enclosure.... but that connects to nothing. 

This may be true for your power supply but, what about EFFECTS that are powered by mains? Like the older Flanger, Chorus, and ODs that had the mains run into transformers while the Earth ground was tied to the metal enclosure. The In/Out jacks were tied to the circuit/audio ground AND the earth ground.

[defaced]

The outputs of the power supply end up being referenced to Earth via through the effects pedal, through the connection cables, and into the the amp's input/FX loop jack(s) which are tied to the chassis earth connection either via it's sleeve connection to the chassis or by the amp's internal ground scheme.  Going in the other direction, the string ground inside the guitar's body (and you too by way of your fingers) are connected to Earth via the cables and amp internal grounding.  This is why people get shocked when the PA and guitar amp are on different outlets with one of them being miswired.  

A smaller fuse is better than a larger fuse.  By my math, you need something around the 3 to 4 amp range (edit: don't listen to me, listen to RG, apparently I can't do simple algebra today).  Being on the primary side, I'd use a slo blo fuse.  

[R.G.]

Funny you say this RG. As a matter of fact, it is common practice in household wiring to connect the Earth grounds and the Neutrals TOGETHER at the main breaker panel.

That's true. The low voltage side of the AC power line is grounded at the power pole with a copper wire down the side of the pole to the bottom of the pole itself, then carried into the house on the un-insulated ground wire. The house is supposed to have this re-attached to earth ground by carrying both the pole-ground and a separate earth ground wire to the house's own safety ground wire at the house itself. Both pole-ground and house-ground are attached to neutral at the main breaker box. The idea is that you want the neutral stitched to real, no-fooling earth ground at many places so it doesn't float if one comes loose, and does not float by the voltage caused by any current flowing in the neutral line.

The old two-wire system simply assumed that "neutral" was ground-y enough. We learned better.

I was trying not to get into that with someone who doesn't understand the concept of "ground" in electronics all that well (not to belittle the OP - we've all been there).

The outputs of the power supply end up being referenced to Earth via through the effects pedal, through the connection cables, and into the the amp's input/FX loop jack(s) which are tied to the chassis earth connection either via it's sleeve connection to the chassis or by the amp's internal ground scheme.  Going in the other direction, the string ground inside the guitar's body (and you too by way of your fingers) are connected to Earth via the cables and amp internal grounding.  This is why people get shocked when the PA and guitar amp are on different outlets with one of them being miswired. 

Right!

A smaller fuse is better than a larger fuse.  By my math, you need something around the 3 to 4 amp range.  Being on the primary side, I'd use a slo blo fuse. 

Correct - smaller is better for fuses as long as it will carry the current and support the start-up inrush surge. Also correct on the slow-blow for inrush.

I'd say something more in the 1/2A range. The current out is eight windings times 11Vac times 300ma, plus 9V times 2.0A. That current is smaller on the primary side by the turns ratio, which gives (11/120)*(8*0.3) + (9/120)*(2) = 0.22A + 0.067A =  0.287A at the primary. Allowing for a 2:1 inrush current and a slow-blow fuse, a 1/2 A woul be where I started.

[defaced]

I'd say something more in the 1/2A range. The current out is eight windings times 11Vac times 300ma, plus 9V times 2.0A. That current is smaller on the primary side by the turns ratio, which gives (11/120)*(8*0.3) + (9/120)*(2) = 0.22A + 0.067A =  0.287A at the primary. Allowing for a 2:1 inrush current and a slow-blow fuse, a 1/2 A woul be where I started.

Not sure where I screwed up my math, but I re-checked it and agree with RG.  Previous post updated. 

[aballen]

Ok so I've read up, and here is what I came up with.

• Attach main GND to the metal case
• Added in diodes
• Added a place to mount the fuse, to keep it isolated(and I'll go with 1/2 amp)

What do you think?

[defaced]

I'm not familiar with the diode trick, so I'll let RG comment on that.  The traces to/from the fuse will need to handle the full load of 0.5 amps, not sure if yours meets that or not.  I often like to use IEC sockets with the fuse installed in them.  This keeps me from messing up the fuse wiring and generally from having to engineer a bunch of stuff (like trace width).  This is the Mouser part number to the last IEC socket like this I bought: 693-6200.2300  There are other mounting and termination styles, this one happens to be a screw in kind with 0.25 inch blade quick disconnects.  The datasheet contains the part numbers for the other styles. 

[R.G.]

Both sides of the fuse must be separated from secondary circuits by at least 8mm in creepage (shortest distance across any surface) and clearance (any 3-dimensional straight line distance). By far the easiest way to handle incoming AC power line fusing and connection is to use an already type-approved and UL/CSA/CE/YADA... marked IEC power inlet with a fuse in a tray.

AC power line wiring gets COMPLICATED.

By now you'll be getting frustrated with me for not just telling you how to do it right. I can't. Even if I tried. A certification lab won't tell you that what you did is safe. They will say instead (if you pass) that they could not find any way they think you didn't meet the safety specifications, which is quite a different thing to say.

[aballen]

I really like that connector.  I'll be picking it up and removing the fuse.

The diode seems like overkill to protect a $2 part, but there is space so why not.  Also the way  i put it in should not create any voltage drop.  It appears to be exactly what RG suggested.

RG, not frustrating at all.  I appreciate the suggestions, and I get the whole don't wanna be responsible if this guy electrocutes himself thing.  I have by the way and I really want to avoid doing it again.  Looks like the transformer I'm using is very close to what is listed in the spider tutorial, if not the same one.

So one last question.  The 9v ac secondary.  Do I just hook those wires directly to a jack to power my line 6 looper?

[R.G.]

So one last question.  The 9v ac secondary.  Do I just hook those wires directly to a jack to power my line 6 looper?

Yes.

However, a 9Vac output will kill 9Vdc pedals dead if you happen to accidentally connect it to them and they use the standard shunt-diode polarity protection. Using the same kind of jack for the 9Vac as the 9Vdc is asking for sudden pedal death if you ever accidentally do this. Cliff tells me that all 9Vac connectors have 2.5mm center prongs, instead of the 2.1mm center prongs on 9Vdc pedals. However, sometimes you can push one in there. Sudden smoke results. Some people have accidentally plugged 9Vac into a daisychain. 

[Jdansti]

Going in the other direction, the string ground inside the guitar's body (and you too by way of your fingers) are connected to Earth via the cables and amp internal grounding.  This is why people get shocked when the PA and guitar amp are on different outlets with one of them being miswired.  

Yep.  I remember many times as a kid getting shocked on the lips from a mic while I was playing guitar. It's kinda hard to sing while that's happening!  One time we were playing a little concert under a gazebo that had a concrete slab.  I decided to kick my shoes off (I was a kid) and found that there was a grounding problem.  Current was passing through the concrete, through my feet and through my guitar.  It was just barely tingly so it took a while to figure out what was happening.

[aballen]

ok I've updated it again, made the layout tighter, added diodes, removed fuse(I'll get one built in to the outlet)



and the traces mirrored

[R.G.]

One last question - can you actually get 1000uF caps that fit those spaces on the PCB?

Sorry - it just now struck me that the physical size of the 1000uFs may be too tight.

[aballen]

Oh no way, I was thinking 100 uf and 10 uf.  I think I could go to 220uf/100uf, but that about as big as I can fit with this layout.

The datasheet for 780* suggests 1uf and .33 uf 

We are just talking 300ma(max) per secondary, so I'm thinking this is enough for ripple protection, am I wrong?