Power supply & wiring up a transformer

[logictom]

This might be a dumb question, I don't know
I'm looking at building a power supply as I still only have a brick that supplies 5v on my desk but I'm looking into the actual construction.
I've read that you should use a metal case for power supplies, I didn't think this was the case so I'm after some clarification there. But if I use some sort of plastic enclosure like the ones from Polycase what do I do with the earth wire from the plug? I should really know this stuff but I haven't seen it mentioned on any power supply designs, really wish there was more practical on my course then maybe I might know this stuff.

Anyway thanks for any pointers

[logictom]

Also can someone clarify ground, earth and virtual ground and what the symbols are?

[Zero999]

You can use either a plastic or metal case.

The case has to be connected to the protective earth conductor in the mains plug.

The circuit earth does not and should normally not be connected to the case or protective earth conductor.

Unfortunately the symbol used for protective earth is the same as the circuit earth/ground.

It should be clearly indicated on the schematic what connects to the metal case and what doesn't.

[logictom]

Thanks for the reply, if the case is plastic why does it still need to be connected to earth?

[Kiriakos-GR]

Thanks for the reply, if the case is plastic why does it still need to be connected to earth?

LOL  , nice question ..  

When I was very young , I had the same questions too.  

The plastic one, does not need grounding , but all the parts inside does .
For example the AC transformer at list ..

It best to use metallic one , an special aluminum , its more safe , and it does not melt !!
Or getting fire ..

The plastic boxes are good for anything that operates with a battery ,  " or "  with low  current transformers ,
like  3 -  20W or   1A max at 24V.  

[DJPhil]

Thanks for the reply, if the case is plastic why does it still need to be connected to earth?

This is one of those areas that I'd have to navigate slowly and carefully when speaking with my electrician friend. We conceptualize electricity differently, and maybe explaining his view would help. Within your wall socket there are (typically) three conductors: the 'hot', the 'neutral', and ground. The colors, voltages, and other details may vary from one country to another, and you should do your own research where necessary as this is dangerous stuff. The hot wire is connected to the breaker, and is thought of as the wire the power is coming from. The neutral is it's normal return path, and is loosely analogous to a signal ground (sort of). The actual 'ground' connection (chassis earth) is designed to be an extremely low resistance path to the same place the neutral goes, acting as a backup for the neutral wire in case something goes horribly wrong. For a slightly more detailed (and perhaps understandable) explanation, poke around wikipedia a bit. Here's the article describing the difference between ground and neutral.

So what does that mean to us, we just see three wires! In the US, the colors are (often) black, white, and green for hot, neutral, and ground. The black and white are your power conductors, and the green should be carefully and reliably bonded (screw is usually used) to the chassis. Metal cases are often used with their case attached to chassis earth so that no matter what manner of failure occurs within, the electrical potential will neutralize through the chassis and protect nearby people and equipment. Regarding plastic cases I've often seen the earth wire terminate in a screw terminal that was attached to a large transformer's mounting screws. This provides a little extra protection in case the transformer windings fail because the frame of the transformer won't be energized, and there's often no other place to put it anyhow. In schematics, this is often represented by the ground wire from the wall plug connecting to the center of a transformer, as in the attached image. Unfortunately the schematic symbols are often used interchangeably, but the one I used in the image below is almost a sure sign of a chassis ground. The most important thing is to understand the concepts, and you can usually puzzle out what the schematic means.

You won't often want to connect any part of your circuit to chassis ground. It can introduce all manner of noise, voltage fluctuation, current spikes, and other garbage into your circuit.

Hope that helps.

[logictom]

Thanks, that's exactly the sort of explanation I was after.
Cleared it up the confusion, hopefully now there won't be any nasty shocks

[MTron]

Do you know how many outputs your transformer has, or more specifically, does it have a center tap?

if the secondary coil has 3 wires coming out, one will likely be a center-tap, you can test this with a meter. Look at the voltages between all the wires, for example my transformer has 70V between 2 of the wires and 35 volts between either of those and the third wire (the center tap wire).

If the transformer is this style, and you are interested in the full voltage available, you can use this type of rectifier. These are easily found in a variety of ATX power supplies. I pulled one out of a ATX ps i has lying around, it was rated at 30A max http://www.diodes.com/datasheets/ds23018.pdf




also here is a good site for more info

http://www.eleinmec.com/article.asp?16

[DJPhil]

i talked to my younger brother sometime ago, he is a certified "high voltage" technician, i am a certified mechanical engineer. i said to him, since neutral line is at 0V (when measured using test pen or multimeter neutral to ground), so there should be no harm if both neutral and earth wire are connected. he said "NO", there will be a big explosion. i said (with my EGO as an engineer and as an elder brother), i dont trust you and he keeps insisting. so we decided to make a test. i took a small piece of wire, shorting neutral and ground, and what do you know? the wire splattered apart in a nice firework with a "big bang" sound (the same thing when you connect the HOT/life to earth wire). he is right, i was wrong. i demanded explanation. he said something about the center of delta-y in turbine generator and impedance difference which he was unable to fully explain it to my understanding. so anybody here can explain it in more detail? my curiosity still remains. if there is schematics, i think it will help further, tq.

Whoa!
I honestly have no idea, and I would have smugly put money down right alongside you (and lost it). I'd like to know why I'd have been wrong too!

Would you be willing to give more details about your test? What part of the world are you in? Was your test socket in an industrial building, or a house? What kind of circuit protection does the outlet have (fuse, breaker, GFI, etc.)?

I'm going to do some looking on this myself, and I'll post again if I find anything.

Thanks for the heads up

[Kiriakos-GR]

shafri lives probably in UK , and their lines could be had some sort of special design.

Even if in my ears, this story sounds as joke too  , still I can not make any judgment.

In Europe the neutral is grounded , and there is no voltage.

Every electrical pole , has its own ground , and every home its own , and all of them are connected as one.

In some cases , if the neutral cable are damaged we use the ground instead.

Its again all about Geography probably.   

 
 

 

[djsb]

Hi,
It's all about potential difference in the end and when it comes to electricity NOT making assumptions of any kind.

Here endeth the lesson  

David.

[DJPhil]

Hi,
It's all about potential difference in the end and when it comes to electricity NOT making assumptions of any kind.

Here endeth the lesson  

David.

Agreed. In the end you should always safely test instead of assuming. The phenomenon reported above grossly violates my understanding of safety grounding however, so I'm strongly motivated to align my expectations with reality.
Do not perform any mains testing if you don't know what you're doing and using safe equipment!

I checked into this a bit, and I think I've sorted it out. My experience is somewhat limited to the US, but the same basic concept of a safety ground/earth is used in Europe and elsewhere. Wikipedia has a heap of info on this sort of thing, so a bit of research could help the translation. I'll press on by explaining what would happen here in the US, not on the assumption that it should be the same elsewhere but because that's all I have to work with.

In the typical (properly wired) US household, all the wiring in the house comes together in the main breaker panel. Within this panel, all ground and neutral lines come together and are effectively shorted. This means that elsewhere in the house they should be at the same potential, disregarding any low level signals induced within them from RF interference or the like. The only time shorting ground and neutral will have an effect is when both have passed through a GFI (RCD) outlet or device, and should only cause the device to trip.

So what could cause a neutral to ground short to look like an explosive failure?
Perhaps it wasn't really a neutral to ground short . . .

Possibility 1 - You're dealing with a conventional 3 pin 120V household outlet in the US (may apply in Europe, don't quote me), and Hot and Neutral are swapped somewhere.
Testing - Using a safe meter and proper precaution, test hot vs. ground and neutral vs. ground. This may trip GFI outlets or other sensitive protection devices, so use caution. The larger blade connector should be the neutral, and should have very little (millivolts at most, residual RF and interference) or zero voltage to ground. The smaller blade terminal is supposed to be the hot. These are often swapped in household installations (and elsewhere) accidentally, and this is usually harmless to devices run on the circuit but poses a hazard in that it's not as expected (or compliant with electrical code).
This would cause what you reported, and such a short should have tripped the breaker or blown the fuse the circuit was attached to. If this was the case, you actually shorted hot to ground!

Possibility 2 - You're working with a type of plug that has no separate ground and neutral.
Testing - Same as above, except you should see voltage between any two pins.
Take a look at the NEMA 4 and NEMA 10 sockets. They are found here in the US on occasion and are quite common for dryers and ovens. Because of the way the electrical code is enforced in the US, old and obsolete (potentially unsafe) wiring is only updated if broken, meaning that there's a whole bunch of these still out there. Both are three pin connectors, but neither have a separate ground and neutral (hot, hot, ground, and hot, hot, neutral), so shorting what looks like the ground pin to anything else will cause fireworks. To make this worse, there's no guarantee that these plugs are carrying the voltages you expect!
This would also cause what you reported, because there's no separate neutral and ground to test.

I think that one of these (or something similar) is most likely. As far as I can tell from any source, assuming you're within an normal household environment (in the US), the only explanation for what you experienced was bad wiring or deception. You didn't bet your brother anything expensive, did you? 

[DJPhil]

hmmm. seem like a joke to most people. i just contacted my younger brother, and yea, i've some mistake, last time it was life-ground test (with explosion), neutral-ground test will result in no explosion, but will trip the circuit breaker, sorry for misinformation.

No worries at all, I was just surprised and concerned. Being a safety issue I wanted to make sure I had the right idea in my head.

i live in Malaysia, 240V 3pin home outlet socket (life (brown), neutral (blue), ground (green)). in house, we got one switchboard with many circuit breakers inside. one time, i saw in another house, all neutral lines join together in one some sort of bus (big plate of conductor), but i dont think it is connected to the ground wiring. we have 2 lines coming from electrical pole (i think its 1 phase supply).

Ah, Malaysia! Greetings from Nebraska, US.
I did some poking around on the net, and it seems a common household there has 240V, 50Hz, likely to be single phase power, with Type G sockets (that's just the US designation for them) being the most common. The wiring color convention is the same as . . well . . just about everywhere in the world except the US and Canada. I don't know why we're so stubborn, I'd rather be using the metric system. 

i just re-confirmed using volt multimeter test:
life-neutral = 240V
life-ground = 240V
neutral-ground = 0V

Excellent! We had the right idea all along.

and we got this device called "earth circuit breaker". its just i dont know whats the difference with the normal "circuit breaker", i'm not the high volt specialist, so i leave it to ppl who more involved in it.

The earth circuit breaker is probably a ELCB, or Earth Leakage Circuit Breaker. This is an older and slightly different version of a Residual Current Device (RCD or GFI/GFCI in the US). The wikipedia article on different earthing systems describes the different types and explains in part why Malaysia uses the system that it does. It's a bit hard to mire through the jargon if you're not used to it, but if you can digest that page you'll have an idea of how the neutral and ground wires connect in different installations.

yea i got mixed up with this AC issue, still not fully understand it. sorry for those who really take my previous statement seriously.

It's no trouble at all, sorting this out helped me understand this better than I did before.

[Kiriakos-GR]

I have talk in my work with another three electricians about that matter.

The most possible explanation are , that at this home at the time of the test, there was an active device.
That consumes lots of energy.
The voltage between neutral and ground , can be described as "returned " voltage .

And yes its possible to happen , at any installation.

[Polossatik]

The point is simply that they serve different purposes.

Basically in "household" the "Ground wire" is intended to provide a lower resistance circuit than the human body to the earth. That's it.

"Neutral" is in housing intended as "zero", the reference ( for systems with more then 2 wires especially).

While idealy both should be on the same potential this is often not the case. simply use a multimeter and measure the difference between your ground wire and neutral will almost always give some kind of potential differential in any building .
The last thing you want to start doing is attaching "ground" and "Neutral" everywhere

To get back to your PS Q.

An Iron cage has a few advantages, beeing an excellent shield (less noise to pick up) , better heat conductor of you can get away with using an internal heatsink (can be disadvantage also - you don't want to heat up your PCB's ).

That it's inherent "safer" is simply not true. It all depends on the mechanical construction (metal or not).
Simply put, anything metal that MAY conduct "mains" but is not "ground/zero" of the DC circuit needs to be grounded - with an iron cage this is typically then whole cage, with a plastic cage this is almost nothing.

This does not mean you need to "ground" the "zero" of your PS *after* the transformer. Some PS have a "ground" socket on the front (to use for shielding or so) but this is *not* connected to the "zero/ground" of your PS DC output.
With a metal case you need to be especially carefull to not cause any shortcircuits trough the connectors of the DC output or the heatsink.

Things you want to do are mainly common sense:
* make sure your mains is properly fused.
* make sure the mains cables to the transformer are not to long but also not under "mechanical stress" (to short), decently secured from moving using plastic wraps and any soldering/blank metal carrying mains is isolated using heat shrink tubes.
* if you have holes where you can stick things like screwdrivers in, then make sure those cannot reach any blank metal carrying mains (see pervious point).
* if you are using and old style Laminated steel transformator (use Toroidal core ones they are a bit more $$ but worth it) that has those hughe metal casing/square thing and it has a "ground tap" then connect that or ground it using the screws used to secure it in place.
* make sure your mains is properly fused .
* keep mains and low voltage way from each other - not only about safety, more about not putting 50/60hz noise in your cables/Dc output.
* use oversized copper wires (2.5 ² mm or so "electrical wires") to connect the DC outputs of the PS to the PCB never hurts. you don't want to loose voltage over a silly cable.
* make sure the heatsink (this is normally put externally ) is electrically shielded from the backend of power fet's/transistors using mica (and use heat paste). You can ground it but don't loose your sleep over it if you don't.
* make sure your mains is properly fused .
* use a decent mains on off switch properly reachable (not at the back..) - if things start smoking in your setup you want to get there fast..
* make sure the pcb's and any cables are decently secured.
* connect the amp and volt meters (you did considered to put a pair in the case did you?) as close as possible to the ouput clamps.

just all IMHO of course, additions/critic welcome

ha and did I mention you need to properly fuse your mains?