Lab power supply earth output function?

Hi,

What's the use of the earth output in laboratory power supplies?
There is already a ground output.

If your design needs earthing , you can connect that port to your project ,
generally speaking it depends on prototype requirement.

Is the ground output really on earth level?
This depends on the power supply. Some power supplies are able to connect them in series, and thus the gnd cannot be on earth level.

As flanello points out, most lab PSU have floating outputs so they can be stacked (connected in series to give more voltage or a lifted ground potential) and in that case only one of the PSU may have an Earth connected. The Earth connection is usually brought out to it's own connecton so the PSU configuration can be changed while still leaving the most appropriate point Earthed.

Brian.

Earth is their because some people use it to stop their "0v" rail floating up to dangerous potentials...it may get "induced" up.
But generally its because its the norm in a system to just make sure every 0v in a system is , if poss, connected to earth.
This is done in such routine, that people forget why....but generally its safety.

treez said:

Earth is their because some people use it to stop their "0v" rail floating up to dangerous potentials...it may get "induced" up.
But generally its because its the norm in a system to just make sure every 0v in a system is , if poss, connected to earth.
This is done in such routine, that people forget why....but generally its safety.

Click to expand...

So generally, I should connect ground (black) to earth(green) outputs for safety?

in a lab situation I would keep the secondary floating...so dont connect them

It really depends on your application and whether the PSU is being used alone or in conjunction with another supply that may have it's own Earth connection. A compromise might be to wire a resistor of say 100K between negative (black is not ground!) and the Earth post, it will allow a small current to flow and help to tie the negative post closer to Earth potential without risking a short circuit.

Brian.

Ive lost count of the number of co's that ive been to where they have a massive earthing wiring schema for their equipment, but they forget to connect it up and it gets shipped off, and nobody notices.

Earth ground provides a low impedance common mode shunt to stray noise generated in the differential current of a Supply and return line. When circuits connected to the lab supply have high impedance inputs they become sensitive to this CM noise if they are unbalanced and a CM current creates a CM E-field which when coupled to a single ended input creates a differential noise.

Thus earth ground to the supply or return ( + or -) shunts the CM field to reduce noise in high impedance single endded inputs.

Other methods to reduce CM noise from creating Differential Mode (DM ) noise are twisted pairs, shielding, ferrite baluns and of course shunting the CM E-fields to low impedance Earth ground or a low impedance floating shielded box etc....

As the CM noise can be coupled either inductively or capacitively into sensitive high impedance circuits, the coupling factor is frequency sensitive for coupling impedance and the idea is to shunt the noise with a low impedance like an Earth Ground or absorb it with ferrite CM choke or balun or balance it with differential balanced inputs which is often limited to << 1MHz while transient noise can be >> 1MHz making it application specific, which solution is required.

Another method for Earth ground is to suppress conducted noise on supply return to earth ground using RF Caps. This prevents forcing a DC level on any supply , leaving it floating for other requirements like high voltage isolation. <3kV or as limited by PS specs. In this case a line filter is common mode and differential mode and helps isolate large motor spikes from the shared supply getting into shared sensitive analog circuits. This is also known as decoupling, but with the added CM filter to Earth ground by RF Cap or a direct connection ( Return=ground)

your choice.

True story - I was there....

A newly built factory in South Wales called me in to investigate grounding problems. The facility was defence approved and making various military equipment and secure communication products including some highly classified technology. The builing was basically a huge Faraday cage with a copper mesh floor (~20,000 square feet of it!) but they were seeing problems of electrical leakage to the incoming power lines and couldn't figure out what was wrong. When I can only see a small part of the installation at any time it was difficult to figure out what and why there appeared to be random Earth currents flowing around virtually everywhere in the building. I eventually got the (top secret) plans for the building construction and discovered there was a single connection point for the Faraday cage. It was well away from production areas where the problems were being reported so it's no wonder it was difficult to find. Lifting an inspection cover in the floor revealed a large braided cable, coiled up and secured with cable ties - but not connected to anything!
The building contractors didn't know what to do with it so they just made it look tidy and covered it over. Connecting it fixed the problem.

So it isn't only PSUs that have grounding issues, it can be whole factories!

Brian.

its shocking that they left it disconnected...and more shocking that they only had the earth connection in one place.

betwixt said:

True story - I was there....

A newly built factory in South Wales called me in to investigate grounding problems. The facility was defence approved and making various military equipment and secure communication products including some highly classified technology. The builing was basically a huge Faraday cage with a copper mesh floor (~20,000 square feet of it!) but they were seeing problems of electrical leakage to the incoming power lines and couldn't figure out what was wrong. When I can only see a small part of the installation at any time it was difficult to figure out what and why there appeared to be random Earth currents flowing around virtually everywhere in the building. I eventually got the (top secret) plans for the building construction and discovered there was a single connection point for the Faraday cage. It was well away from production areas where the problems were being reported so it's no wonder it was difficult to find. Lifting an inspection cover in the floor revealed a large braided cable, coiled up and secured with cable ties - but not connected to anything!
The building contractors didn't know what to do with it so they just made it look tidy and covered it over. Connecting it fixed the problem.

So it isn't only PSUs that have grounding issues, it can be whole factories!

Brian.

Click to expand...

How did u find that there were "random Earth currents flowing around virtually everywhere in the building"?

Inside the factory there were rows of assembly machines and 'workstation' desks where hand assembly took place, each having connections to the incoming AC through a huge EMI filter. They were regularly checked for being static free but that revealed a potential difference between desks, chairs and even personnel who were wearing conductive shoes. It may sound like a simple case of connecting everyone/everything together but this voltage would vary from day to day and sometimes not be there at all for long periods. The 'electrostatic gradients' inside a Faraday cage full of wiring and about 100 people operating equipment doesn't bear thinking about! Even such simple things as someone lifting a soldering iron could upset the voltage in a different room. Essentially, the cage was floating and the wiring inside was grounded but anything changing the charge on the cage would propagate the problem throughout the whole building.

Brian.