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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?

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schematic

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.

schematic

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.


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.

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  • \$\begingroup\$ Thank you so much for your in-depth explanation. So if i were to use a power supply with remote sense inputs, and if i want to use it like the first power supply with COM, is there a way to use the 2nd type like the 1st type?. \$\endgroup\$ – Blackwidow Aug 25 '18 at 17:16
  • \$\begingroup\$ See the update. \$\endgroup\$ – Transistor Aug 25 '18 at 17:23
  • \$\begingroup\$ My apologies, I did not phrase my question correctly. I am trying to run a transimpedance amplifier I made on a protoboard. I need +15, -15V to power the op-amp and also ground the positive pin. If I had the 1st type of power supply, I would just connect COM to the positive pin. What if I only have the 2nd type of power supply though? (without COM). Would I still be able to run the circuit? (It seems not) \$\endgroup\$ – Blackwidow Aug 25 '18 at 17:26
  • \$\begingroup\$ @Blackwidow: You can use the third unit as a +/- supply - connect the + terminal of one supply to the - terminal of the other, and use that point as "common". \$\endgroup\$ – Peter Bennett Aug 25 '18 at 17:28
  • \$\begingroup\$ @PeterBennett Thank you. I think it makes perfect sense now. What happens if you just connect mains to the + pin of the op-amp and connect +,-15V to power the op amp I think I just destroyed the op amp. Since mains isnt connected to anything else, I am not sure what actually happened to the op-amp when I pretended mains (the green port) was the ground that the + pin was supposed to connect to \$\endgroup\$ – Blackwidow Aug 25 '18 at 17:32
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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.

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  • \$\begingroup\$ Thank you. I am trying to drive an op-amp with the positive pin grounded (transimpedance amplifier). What if I only have the second and third units? Since I don't have COM, is it impossible to drive an op-amp unless I have the first unit? \$\endgroup\$ – Blackwidow Aug 25 '18 at 17:23

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