Power supplies with or without 'COM' and without ground port at all

Question

This is probably something really simple, but I wasn't able to find the answer yet.

I've been using a power supply with both COM and earth ground, like the first image below.

To power an op-amp, I was using COM for grounding.

Then, I came across a power supply like the second image below, which doesn't have COM.

Also, I've seen a power supply with no ground port whatsoever like the third image below.

I heard the earth ground is noisier. Why do some power supplies have both COM and earth ground, but others don't? And if I have to use one without COM or no ground port at all, what are my alternatives?

Answer 1

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. Various options.

Photo 1

• The first photo shows a PSU with configuration of Figure 1a. There are two isolated supplies - isolated from each other and from mains earth.
• Normal use would be to connect 5V- to COM and now you would have a dual variable supply for the analog electronics - typically +/-12 V - and a 5 V supply for the digital logic.
• If the circuit requires mains earth for any reason then connect the green post to the relevant point. Typically this is the COM.

Photo 2

• This power supply has remote sense inputs. These allow the power supply to compensate for voltage drop in the wires to a remote load.
• If not required then wire as shown in Figure 1b. Note the shorting links in your photo.
• If remote sensing is required then open the links and wire as shown in Figure 1c. The voltage between the + and - terminals will vary with load but the voltage across LOAD2 should remain at the setpoint.
• Again, if an earth reference is required then this can be achieved using the green post.

^{simulate this circuit}

Photo 3

Figure 2. A dual supply can be used in multiple configurations.

This has two independent supplies but without the earth option. These can be used independently, as a symmetrical supply or, for example, as a +12 V and +5 V supply. Note the connection (or lack of) between them in each case.

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From the comments:

So if i were to use a power supply with remote sense inputs, and if i went to use it like the first power supply with COM, i would connect what would've been connected to COM to the green post.

I think you are confused. The Photo 2 PSU has only one output with + and - terminals. It is not a dual supply as shown in Photo 1. You can think of it as a variable voltage battery with an optional earth connection. You always connect the load to red and black and add an optional link from either to the earth terminal.

Have a look at my answer to Actual electric potential at terminals of battery and it may help your understanding.

Answer 2

The first supply has a bipolar output - effectively, a positive supply and a negative supply share a common pin.

The second unit is a single supply with remote sensing connections. Neither supply terminal is connected to Earth so it can be used as either a positive or negative supply.

The third unit has two separate supplies, each with neither terminal grounded, so the two supplies may be connected in series, for a bipolar supply, or used independently.

For bench supplies such as these, we often don't care about a connection to Earth ground, or make a ground connection separately from the supplies.