Grounded Power Supply Help.

[perfect_disturbance]

So I'm building a simple (real simple. meaning dc12v in, 1.2 amp fuse, lm7805 to power panel meter, lm317, toggle switch, output) power supply based on an lm317.  I was going to power it off a wall power adapter I had laying around that provides 12v at 1.2A.

I decided to investigate if the power adapter had a floating output because I wanted the outputs of my supply to be floating.  After checking resistances and voltages I've figured out how it's wired.  The hot and neutral wires are across the primary winding of the transformer. The the secondary winding of the transformer is rectified and then tied to the ground plug on the input.

So here's my thought If I just break off the ground post on the wall plug would that float the output of the adapter?  Something about doing that rubs me the wrong way.  What sorts of danger am I risking if I do this?  The case of the supply will be plastic and I don't plan on grounding the circuit to the case in any way.

[alm]

Get a different 12V power supply. It's not like double insulated power bricks are hard to find. The EMI filter of this power supply will most likely have Y caps from ground to the live and neutral mains wires, so if you float the ground pin, it will be pulled to half the mains voltage. The maximum current is quite low, so it's not going to kill you, but it may still be uncomfortable, and sensitive electronics may not be happy about it.

Double insulated, designed to run without the ground pin, also has to comply to more stringent rules to ensure operator safety. A grounded power supply needs less clearance and insulation because the safety ground is supposed to provide additional protection in cause of a fault. Removing this safety ground will make it less safe.

You probably guessed by now that I consider it a bad idea .

[perfect_disturbance]

yeah I didn't think about caps in an emi filter. ok so where is a good source for a 1 amp or higher rated 12v double isolated power brick.

[perfect_disturbance]

Or would I just be better off buying a transformer bridge rectifier and some caps from mouser?

[Paul Price]

Just because it has the output negative connected to the neutral pin doesn't mean that there is a always a capacitor connection to both sides of the mains AC. You can verify this by measuring the current  AC DVM ammeter to earth ground connected to the ungrounded wallwart power supply.

Let's assume it does have a wallwart with two small-value caps to the neutral(it is not really likely that this is the case).

When you connect the power supply to a "sensitive circuit" and it is, for some reason, grounded to earth, it should not bother the circuit. It is connected to ground, so no problem.  If you connect the ground of the power supply first (which is always a good practice), then there is no problem.

Assume you are forgetful and connect the positive lead of your wall-wart powered LM317 power supply first, would you not be connecting it to a large value electrolytic bypass filter cap. on your "sensitive circuit" that is earth grounded, so this bypass cap. would shunt the tiny AC current from the wallwart to ground..again no problem.

Assume your "sensitive circuit" is not grounded to earth ground, now there is no problem because no current from the AC mains will flow.

Assume your "sensitive circuit" is connected to your DIY power supply and not grounded to earth ground and you decide to connect the grounded lead of your bench oscilloscope the gate of a unprotected MOSFET on your "sensitive circuit for some strange reason)..it could be for some destructive reason..maybe a problem.

Assume your "sensitive circuit" is connected solidly to a earth ground and you walk across the room on a rug and build up upon your body a nice static electricity high voltage and then you touch the "sensitive circuit" in a sensitive place..maybe a problem, even without your custom power supply connected at all.

Assume your "sensitive circuit" is connected to an earth ground by your oscilloscope that is 3-wire grounded and you connect your "sensitive circuit" to a long wire going to your entertainment center across the room, also earth grounded, but to a different AC circuit in the building. There could be a high current, low-voltage difference between the grounds an you burn out your "sensitive circuit" because of this earth grounded AC difference.. big problem.

Assume your "sensitive circuit" and your power supply are both connect by 3-wire cords to earth ground neutral..no problem.

Finally, assume you've got y our custom power supply securely earth grounded with a 3-wire cord connected to the earth ground. Now for some misadventure reason you are holding the power supply ground clip and by neglect or accident you somehow come in contact with your other hand to  a live AC mains connected wire on something you are working with, even though it is also earth grounded. Now the "safe earth ground" of your power supply serves to complete the path of the high current AC mains voltage through your body..big problem.  It is interesting to note that if your power supply was not earth grounded, then you would not have a chance to suffer a fatal shock in this same unfortunate situation.

In Denmark, they sell and always supply laptops and printers (and many other equipment) with 3-wire and two-wire cords to connect to their wallwarts, and I reuse these wallwarts all the time (with the supplied two-prong power cords) to power breadboards on my electronics bench with no problem and I don't have anything earth grounded. Most houses here do not have 3-wire wall outlets.

There have been many other discussions before on this topic and some responders will take much time to disagree with me again on this topic, but I always welcome both sides to the story.

https://www.eevblog.com/forum/beginners/is-it-safe-to-use-a-bench-power-supply-not-grounded/msg254146/#msg254146

[Kremmen]

On this i agree with the conclusion of Paul Price.

The so called Euro plug is a 2 prong plug that fits into the grounded Schuko socket commonly used in EU. There is no ground connection at all, and indeed it is the point not to have one. All devices using the Euro plug must be double insulated class 2 gadgets that don't need the ground for anything.
My lab power supply for instance is such a device. No ground connection whatsoever (it is all linear). Likewise the safety isolated 230VAC lab supply - not ground connected. If i specifically want a grounding then i connect GND using a grounding bar.
Tools like soldering irons, scopes etc are grounded of course, as well as the work surface using the normal ESD safety grounding gear.
If you need the grounding, as you at times do, then it is there in the Schuko sockets. Just use a normal class 1 plug.

Never had a single problem related to this grounding arrangement.

[alm]

So here's my thought If I just break off the ground post on the wall plug would that float the output of the adapter? 

Does this sound like a two prong plug as would be shipped with double insulated devices?

[Paul Price]

Yes if you have a two-prong plug to begin with.

No, it would have to have 3-prongs to be left with two prongs, after the neutral third prong was broken off.

[Paul Price]

The good news for those worried about safety is that manufacturers of wallwarts use the same wallwarts for many different countries to sell to, they simply change the plugs they supply for the country of interest.

The implication of this is simple. The wallwart has been designed not to fail or present a safety hazard with or without an earth ground.