Questions about earth ground?

Aris · Post by **Aris** » Wed Aug 16, 2006 4:33 pm

This isnt really a "power supply" issue, but an entire computer electrical system issue. So if you feel this is in the wrong spot feel free to move it. I couldnt think of a better place.

I'm wondering if the power supply uses earth ground in its electrical grounding?

Also, is the entire chassis of a computer case grouned to earth ground through this?

Last question. Would it be bad to have positive voltages grounded to earth ground/electrical ground, as well as negative voltages? reason i ask is cause i know electricity flows from negative to positive. So with positive volteges, ground is your negative, so the elctrons "flow" from ground to your positive volteges. With negative volteges though ground becomes your positive, so the opposite would occure. Is it bad to have electrons flowing in opposite directions of each other on the same ground line?

Howard · Post by **Howard** » Wed Aug 16, 2006 7:42 pm

I don't think ground cares what voltages are being sent to it.

Oh, and electrons go from neg to pos, but current goes from pos to neg.

darthan · Post by **darthan** » Wed Aug 16, 2006 10:16 pm

Aris, electrons won't flow in both directions. They will flow in the direction of the net voltage. That is, if you hook up a +5V and a -5V to the same ground, the result is a 10V current flowing from the +5V to the -5V. (Yes, the electrons go the opposite direction, the physics/engineering convention is that current is written and described as flowing from positive to negative though).

As far as working in a computer goes, don't worry about it. Unless you start tweaking around inside your power supply (which you absolutely shouldn't do if you have to ask these questions) it will all just work. No, I don't know how it works, but it does. The power supply does use the earth wire (note that earth and ground are different things: ground is the wire back to the power plant, earth is the emergency wire to drain short circuit current), and for its intended purpose too. The earth is used to ground the case of the PSU and through it, the case of your computer. This is the purpose of earth wire: it allows a path for current to flow from short circuits to the ground that does not involve the person touching the computer. However, a properly operating computer will not have any current flowing to the earth (nor will any other properly working electronic device).

Oleg Artamonov · Post by **Oleg Artamonov** » Thu Aug 17, 2006 7:11 am

darthan wrote:However, a properly operating computer will not have any current flowing to the earth (nor will any other properly working electronic device)

Not quite right. There will be small currents from AC noise filter (~1mA), there are two Y2-class capacitors between ground and N&L AC lines. They used to filter out common-noise currents in AC line, for details see here (725k PDF).

Aris wrote:Would it be bad to have positive voltages grounded to earth ground/electrical ground

No. It would be good

Grounding is required for proper operation of PC power supply, so just don't bother.

jaganath · Post by **jaganath** » Thu Aug 17, 2006 8:43 am

Thanks for that PDF Oleg, it is very informative.

cass · Post by **cass** » Thu Aug 17, 2006 9:27 am

Dont try to electically connect anything to the computer/power supply case (or any other grounded device) in order to sink a current (i.e. dont use it to complete a circuit) unless you really know what you are doing. If any current flows through the earth connection it will most likely trip the cicuit breaker in your house. This is because the earth connection is a safety device, the idea is if there is a short cicuit and a live wire comes into contact with with the earthed case, the current will flow to earth, instead of through anything touching the case (e.g. you). If this happens, a device called an RCD (or GFCI in america) detects that the current flowing through the live connection does not match the current through the neutral (because it is now flowing from live to ground) and cuts the power to your house.
They typically trip at about a 5-6 milliamp difference in the live and neutral currents, so if you try to sink more than this it will trip.
However, this only applies if the current you are sinking is drawn from the circuit that the RCD is protecting (e.g. the ring-main circuit in your house). So if you connect a lead from your power supply to the case, the RCD will trip, because the PSU gets its power from the mains socket, which is protected by the RCD, so current will now be flowing from the mains, via the PSU, to ground.
If the current is coming from another source e.g. a battery, there shouldnt be a problem. Similarly, if you wanted to use the earth for example shielding some variety of sensitive electrical device from electromagnetically induced currents, or other such common use for an earth connection this would also be fine.

What is it that you want to connect to earth anyway?

Oleg Artamonov · Post by **Oleg Artamonov** » Thu Aug 17, 2006 10:51 am

cass wrote:They typically trip at about a 5-6 milliamp difference in the live and neutral currents

No. Minimum is 10 mA (used in dangerous environments or in specific locations at home, e.g. in the bathroom), than 30 mA (mostly used in live environments) and 100 mA (used in industrial environments only).

cass · Post by **cass** » Thu Aug 17, 2006 12:27 pm

I stand corrected...

Aris · Post by **Aris** » Thu Aug 17, 2006 8:41 pm

ok so i dont want to use an electric system with earth ground.

is a computer enclosure grounded electrically to a PSU?

if i used a negative voltage and the chassis of the case as ground, would ground become my positive because the voltage is negative?

i want to basically do the same thing they do in a car's electrical system. The negative terminal is connected to the chassis, and then only the positive leads are needed to connect to things, and then just wire the negative lead of any device to the chassis which is now negative.

the only difference is i want to use a negative voltage so that my ground becomes my positive lead.

Tibors · Post by **Tibors** » Thu Aug 17, 2006 11:51 pm

Aris wrote:is a computer enclosure grounded electrically to a PSU?

If you open up a typical ATX PSU, then you'll see that the earth wire of the powerlead is directly connected to the casing of the PSU. The casing in turn is normally screwed directly to the case itself.

Aris · Post by **Aris** » Fri Aug 18, 2006 7:46 am

so then my only question left is:

if i used a negative voltage and then the chassis of the case as ground, would ground become my positive because the voltage is negative?

Oleg Artamonov · Post by **Oleg Artamonov** » Fri Aug 18, 2006 8:33 am

Aris wrote:if i used a negative voltage and then the chassis of the case as ground, would ground become my positive because the voltage is negative?

Please explain clearly -- what you want to do?
If you get negative voltage (-12V) from PC's power supply -- yes, you can take ground as opposite (positive).

cass · Post by **cass** » Fri Aug 18, 2006 12:44 pm

im not sure i understand what you want to do...

it sounds like a needlessly complicated way of wiring things up.

what is it that you want to power in this way?

jaganath · Post by **jaganath** » Fri Aug 18, 2006 1:01 pm

what is it that you want to power in this way?

See this thread:

Zero Moving Parts 120mm Fan Project

EsaT · Post by **EsaT** » Sat Aug 19, 2006 11:49 pm

You don't need to play with negative voltages for this:
I found a DC/DC Converter that would take a standard 12v input and output 20k volts at 4watts.
Or do you want to input 24V to it?

In fact only important thing would be polarity of its output, which should be electrically insulated from input so connecting output to input's ground wouldn't cause short circuit.

Aris · Post by **Aris** » Mon Aug 21, 2006 5:54 am

i really dont see whats so complicated about it.

A negative voltage, in my case it will be -20,000 volts, and then use ground as the positive end.

Just wanted to make sure i can use ground as a positive connection, never seen it done before.

The reason i want to work with a negative voltage is because if i used a positive voltage, i would need to apply it to the chassis, which would be bad.

I'm using a DC/DC converter to change a positive 12v input from an ATX PSU into -20,000volts.

Oleg Artamonov · Post by **Oleg Artamonov** » Mon Aug 21, 2006 6:05 am

Aris wrote:The reason i want to work with a negative voltage is because if i used a positive voltage, i would need to apply it to the chassis, which would be bad

It doesn't matter whenever you connect negative or positive voltage to the ground. There's no any difference.

But for safety reasons it would be much better not to connect high voltage to the case at all. Case must be connected to the ground, but dangerous voltages must be completely isolated from the case.

Aris · Post by **Aris** » Mon Aug 21, 2006 8:21 am

Oleg Artamonov wrote:
Aris wrote:The reason i want to work with a negative voltage is because if i used a positive voltage, i would need to apply it to the chassis, which would be bad
It doesn't matter whenever you connect negative or positive voltage to the ground. There's no any difference.

But for safety reasons it would be much better not to connect high voltage to the case at all. Case must be connected to the ground, but dangerous voltages must be completely isolated from the case.

It does matter, like i said, because if its positive for the effect that i want to work, i would have to hook up the 20,000 volts to the chassis.

So i use the case as ground, (which will be my positive), and use a -20,000 volts on the other end as my negative.

qviri · Post by **qviri** » Mon Aug 21, 2006 8:25 am

Which still means you'll have a potential difference of 20 000 volts on the case.

Oleg Artamonov · Post by **Oleg Artamonov** » Mon Aug 21, 2006 8:40 am

Aris wrote: It does matter, like i said, because if its positive for the effect that i want to work, i would have to hook up the 20,000 volts to the chassis

Since all high voltage sources are isolated from the ground (for safety reasons) it doesn't matter which voltage you connect to the case, positive or negative. In both cases you'll get 20 kV between HV source output and PC case (ground). In some cases it may be... not really dangerous for you (I don't think you HV source may produce dangerous currents), but very unpleasant for you and dangerous for your computer and its peripherals. Please remember that CMOS circuits are very sensitive to high voltages.

EsaT · Post by **EsaT** » Mon Aug 21, 2006 11:09 am

Aris wrote:The reason i want to work with a negative voltage is because if i used a positive voltage, i would need to apply it to the chassis, which would be bad.

Propably not unless you would touch - wire of 20kV source with other hand at same time.

Every AC-transformer works as galvanic decoupler and also most switching power supplies (DC to DC transformer internally) have electrically insulated input and output.
You should have somekind model numbers in that transformer so I would suggest doing Google search with those for getting more information about it but I would expect it work as insulation transformer.

But danger to PC's components is very real unless you can make sure that components can't touch outputs of 20kV source.