understanding ground in schematics..help please

circlingcrane · Joined: Apr 16, 2011 Posts: 10 Location: IL

just getting into synth diy and have built a couple kit modules. I'm now starting to learn about converting schematics to actual working modules and have ground question. I always see the ground symbol coming off allot of designated parts in a schematic and wonder where does this ground get attached to? To the faceplate? to the neg side of the power supply? can someone help me understand ground as it relates to synth module?

much appreciated in advance for the help.

blue hell · Posted: Sat Oct 22, 2011 5:30 am Post subject:

Ground basically is a historic mistake, the word should be banned for electronics Laughing

Anyway, as it is used for electronics circuits it really is just the zero Volt supply voltage, and it is the reference to which all other voltages are usually measured.

For ease of drawing in a schematic usually the supply voltages are not drawn to be connected, but such is implied. So all the ground symbols are connected together and then go to the zero Volt out of your power supply.

There are many possible scenarios, but what you'll see mostly is either a single ended or a dual power supply.

For the single ended one, like a battery for instance, you'll see a + and a - terminal and usually the the - terminal will be used as the zero Volt line.

For a dual power supply you'll have three terminals, where usually the "middle one" is considered to be zero Volts, and then you'll usually have another terminal that's positive relative to that and the third one being negative relatively.

It's maybe good to know that there is no such thing as an absolute Voltage - Voltage is always a difference between two potentials, and so you can arbitrarily define one potential to be Zero, and that really just means that this is your reference against which all other points are measured.

circlingcrane · Joined: Apr 16, 2011 Posts: 10 Location: IL

thanks so much for the help. So in the below example, Ray is using a dual supply (+12v) (-12v) in this schematic and all the ground icons represent wires / traces that should all go back to the 0v point on the pcb? At what point do you know which grounds just get hooked to the front metal panel and don't go back to the 0v point on the pcb? is there a rule of thumb here for understanding this? again thanks so much for helping me understand this, I've been stuck trying to understand this.

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blue hell · Posted: Sat Oct 22, 2011 6:56 am Post subject:

circlingcrane · Joined: Apr 16, 2011 Posts: 10 Location: IL

sompost · Joined: Aug 17, 2010 Posts: 58 Location: Switzerland

I have a follow-up question to "understanding ground in schematics".

I am building a 19' rack unit synth with its own power supply fed from the mains power. I want to 'earth' the metal case of the unit (i.e. connect it to the earth plug from the mains connector).

My question: does the earth (from the mains power) need to be insulated from the ground (of the circuit) or are they the same? Note that the front panel will be connected to the ground (via jacks, pots and stuff) and also to the earth (via the metal case).

Thanks for your advice.

blue hell · Posted: Sat Oct 22, 2011 9:22 am Post subject:

Oh .. thats like opening a can of worms ... I'd not ground the case to safety ground, but make sure the mains wiring is well fixed and insulated. Such is allowed according to the rules in NL.

varice · Posted: Sat Oct 22, 2011 1:47 pm Post subject:

blue hell · Posted: Sat Oct 22, 2011 3:22 pm Post subject:

varice · Posted: Sat Oct 22, 2011 10:54 pm Post subject:

diablojoy · Posted: Sat Oct 22, 2011 11:55 pm Post subject:

ground loops is also a term that is misleading

what we call ground loops in reality occur because of resistance to ground
ie : a resistive ground connection can cause
a large potential difference between two items of equipement
this will mean their grounds will be different with respect to each other
this is exhibited as a voltage and can be measured it may also be heard as a hum or hiss or other
unwanted noise in an audio situation
all wire has an inherent resistance to it depending on type, guage
and length this will also cause potential differences to occur within a system
particularly larger more complex systems or over longer distances
IMHO removing the ground prong from any power cord is an insanity
more than a few people have died from the belief they are solving the problem when they are not and are actually just getting around a symptom
[sorry howard] but it's an old roadies trick that fried more than one guitarist /singer and they should have known better
yes grounding faults are a huge pain to find but better than the possible
alternative

sompost · Joined: Aug 17, 2010 Posts: 58 Location: Switzerland

I've heard about sucht things like ground loop and humming noises, too, but my concern was actually closer to the ground, so to speak.

If you've got a center tap transformer, you use the center tap as your ground level, and have your positive and negative voltages based on that. But if the center tap is not actually 0V with respect to 'earth' level (could be anything, right?) and you connect both you would have a current flowing through the transformer into earth. Would that be a problem? Am I overthinking?

I tend to ground the case, and then see if there's a problem. If there is, I'd follow the advice given above to completely insulate the power supply ("box in box"-style: mains in - raw DC out) and keep the case unearthed.

Thanks for your answers so far.

Clack · Posted: Sun Oct 23, 2011 4:48 am Post subject:

diablojoy · Posted: Sun Oct 23, 2011 1:24 pm Post subject:

ground your case to whatever ground your AC supply has
hopefully the electrician who installed the building wiring did his job properly in the first place
attach your zero volt center tap of the transformer to the exact same point
also all your board and panel grounds should all come back
via the least resistive paths possible to this same point
think of this point as the centre of the universe
in theory if you could use superconductors for all ground wiring
and every connection had zero resistance you would never run into
the problem of noise or ground hum as no potential difference could exist
between the connected points.
reality however is obviously very different
the idea is to keep any potential differences to a minimum.

Luka · Joined: Jun 29, 2007 Posts: 1003 Location: Melb.

wow - learnt a lot from this thread - thanks to all

do european connectors only have 2 pins? surely that would mean every object with an internal power supply / transformer is death trap - or is there some trick to make them safe?

diablojoy · Posted: Sun Oct 23, 2011 2:05 pm Post subject:

so called double insulation
no contactable metal parts can be live
done well it is safe unless the insulation breaks down such as due to lightning strike
done badly as in cheap knockoffs incorrectly manufactured = people die

blue hell · Posted: Mon Oct 24, 2011 12:14 am Post subject:

Clack · Posted: Mon Oct 24, 2011 12:38 am Post subject:

European connectors with earth have clips in the socket that connect with tabs on the plug.

http://www.piccolauniversitaitaliana.com/de/faqs/images/f_schuko_plug_and_socket.png

Items that power of wall warts never have saftey earth and is a way of getting around safety risks of taking mains into a unit. But they are a mess, with the chunky plugs we have in the uk you end up with a massive chunky brick of transformers!

Luka · Joined: Jun 29, 2007 Posts: 1003 Location: Melb.

ahh ok - that is the first one of those i have seen (not that ive been to europe)

weird how earth is the last connection to be made when you inset the plug - it looks like an afterthought addition

blue hell · Posted: Mon Oct 24, 2011 3:19 pm Post subject: