Wednesday, 20 March 2019

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


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?


Enter image description here


Enter image description here



Enter image description here



Answer



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.



No comments:

Post a Comment

arduino - Can I use TI's cc2541 BLE as micro controller to perform operations/ processing instead of ATmega328P AU to save cost?

I am using arduino pro mini (which contains Atmega328p AU ) along with cc2541(HM-10) to process and transfer data over BLE to smartphone. I...