Grounding Power Supply

[Ed B.]

Thanks for all the replies to my previous posts!
Here are my new questions:

I'd like to build the power supply from general guitar gadgets (http://www.generalguitargadgets.com/pdf/ggg_reg_power_supply.pdf).

1) I was planning to use a grounded AC power cord (from small bear).  Do I wire the AC ground line to the board ground? Do I need a grounded cord? (the schematic does not use one).
2) Do I need a heat sink anywhere (they are mentioned in the description) or does the 7809 regulator already have a heat sink as in the board layout diagram.
3) If I want to add a power switch to the circuit which switch should I use and how should it be wired.

Thanks in advance for any and all input.

Ed

[Pushtone]

1) I was planning to use a grounded AC power cord (from small bear).  Do I wire the AC ground line to the board ground? Do I need a grounded cord? (the schematic does not use one).

YES.
Yes you need a safety ground on the cord. Wire the safety ground to the board ground. Pick a point on the enclosure and dill a hole for a machine bolt and serrated locking nut. Make that your safety ground point for the power cord. Connect the green safety ground from the power cord there, then jump to the board pad. Get some spade crimps to put on the ends of the ground leads. It will be easier to stack grounds on the bolt with spades.

2) Do I need a heat sink anywhere (they are mentioned in the description) or does the 7809 regulator already have a heat sink as in the board layout diagram.

Depends on how much current you are drawing. I used my GGG reg power supply to power a pedal with two 12AX7 tubes running 12V heaters and the reg chip would burn out if there were not a heat sink. Thats around 400mA of draw.
I would include a small heat sink anyway. Even if your power draw will be under 200mA.
Perhaps hack one out of a computer power supply. The layout does not include any heat sink.

3) If I want to add a power switch to the circuit which switch should I use and how should it be wired.

A SPST switch wired to open either of the leads from the power cord (neutral or hot). I don't think it matters which one.
Have a look at the power switch wiring for a tube amp layout. Also make sure the switch is rated for 240V or higher.



Have you considered using a 12VAC wall wart in place of the 120VAC line voltage?
It's safer and won't void your home insurance if it burns down your house.
That what I did from the one I built. A 12VAC wall wart costs around $10.00


Good Luck

[brett]

Hi
for a switch, I highly recommend a DPDT switch.  Leaving the active or neutral connected isn't a good option.  Use a switch with suitable V and A ratings.

+1 for the wall-wart idea.  You can pick up old ones for about $1.  For safety and less CO2 emissions, unplug them when not in use.
cheers

[MartyMart]

Hi
for a switch, I highly recommend a DPDT switch.  Leaving the active or neutral connected isn't a good option.  Use a switch with suitable V and A ratings.

+1 on switching both Live and neutral.
Then there is absolutely nothing on the other side of the switch to circuit.
In my 240v rack stuff, I also use IEC sockets with a fuse on BOTH power lines ( live and neutral )
It means a bigger "hole" in the case but is worth it IMO

MM.

[Ed B.]

Thanks for the info.

So if I want to use the wall wart, I would wire it to the inputs of the rectifier (as you would the transformer outputs) correct?  What mA rating should the wall wart be?

Thanks again,
Ed

[Pushtone]

for a switch, I highly recommend a DPDT switch.  Leaving the active or neutral connected isn't a good option.

Sorry to offer the bad advice on the power switch.

Why do so many amp layouts show a SPST switch on the neutral?

[R.G.]

It was a common mistake from the days of two-wire AC power.

First, to everyone, not just Pushtone:
IF YOU DO NOT ALREADY KNOW HOW TO WIRE AC POWER CIRCUITS SAFELY, DO NOT TRY IT WITHOUT EXPERIENCED HELP ON THE SPOT SHOWING YOU HOW.
There are simply too many ways to get  hurt, get killed, and/or kill or hurt others who use this stuff later, just like a time bomb. This is not simple stuff where a few tricks are enough. My comments on how things are wired are not enough to give you the background or skills to do this safely by themselves.

In the USA at least, AC power comes off a large transformer nominally out on a power pole somewhere. One of the secondaries of that transformer is (usually) centertapped, and the center tap is grounded to the real, no fooling dirt in the hole the pole is standing in by a copper wire which runs down the pole to the bottom of the pole. This is a good ground, unless somebody has been playing Paul Bunyan and cut the copper wire, or stolen it to resell to scrap dealers. Three wires come to your house: Ground (that centertap) and the two hot sides of the winding. Each side is 120Vac nominal in the USA, and they're out of phase.

At your power entry panel, there is another ground, this one going to a metal pole driven into the ground. This is "safety ground", and is carried into your power distribution box. In the power distribution box, incoming ground is tied to safety ground, and this junction is the source of all the "white wire" or "neutral" wires in the premises. The two (usually black, to remind us of death. No, really.)wires become the two phases of power in most houses. Each wire feeds a hopefully equal batch of breakers in series with the actual AC sockets, light switches, etc..

At the wall socket: Any color other than white, green, or bare copper must be assumed to be hot, and must also be assumed to be live unless the breaker feeding that socket is PROVEN to be set off by meter reading the wires. In the bad old days, you had only white and black, or just two blacks. With no particular way to tell between them, you might as well open either line, right? Wrong, as we'll see. It was simpler to wire amps if you had a power switch to wire to on one incoming AC wire and a fuse holder on the other side, so it became common to put the switch in one side of the AC line and the fuse in the other.

This was a bad idea. If you flip the switch to your amp, and nothing happens, what's wrong? It  could be the fuse blown, right? So you take out the fuse and in replacing it touch a grounded metal anything nearby, and your last thoughts are of a smell of burning flesh. The fuse in the neutral line left the switch on in the hot line, and when you touched the hot line and a local earth ground, you completed the circuit and died. Or you are very careful and turn the power switch off and then start replacing the fuse. In replacing it you touch a grounded metal anything nearby, and your last thoughts are of a smell of burning flesh. The power switch was in the neutral line, you touched the hot side of the fuse holder when doing the replacement.

The safe way to do this is to always put the fuse in the hot wire, not neutral. At least then your widow can sue the electrician who wired the socked backwards.

Safer is to put the fuse in the hot side and to use a DPST switch that breaks both sides of the AC line. Then your widow can't sue the electrician because it was you who didn't turn off the switch. Even better, if you ever replace the power cord on an amp, put in an IEC cord disconnect receptacle with safety fuse tray. Now you can just pull the plug out of the back of the amp.

In my 240v rack stuff, I also use IEC sockets with a fuse on BOTH power lines ( live and neutral )
It means a bigger "hole" in the case but is worth it IMO
MM.

And this guy's going to be around for a long time - if he'll just quit playing in the street...

[MartyMart]

Blimey - RG quoted ME    ..... thanks !

That's a first, I must have learned something in the last four years then 

I'm going to go have a beer to celebrate, safe in the knowledge that my rack builds are safely double fused


Marty.

EDIT - I am VERY respectful and cautious with 240v AC and recommend we all are the same on that one !!

[brett]

Hi
safety switches are cheap and for DIY with 120 or 240V are essential BACKUP to good technique.  They detect "leakage" of tiny amounts of current to ground, including through a human body.  Although you can still kill yourself by only touching active and neutral, they prevent most electrocutions.
cheers