DC Power Supply Output connection ( + , com , - ), what is "com" for?

Question

Hi I am curious about an output of my DC power supply output.
It has three connecions: positive, common, and negative.

I use only the positive and negative terminal, but I am wondering what the common terminal is for ? Can anyone give practical example in use?

Does it technically like centre tap (ct) in transformer, but this is in DC?

Thank you!

Answer 1

OOPS- correction:

There are 2 possibilities:

1- Most likely your power supply is a dual-output. To determine this, measure the voltage by attaching youir black probe to "COM" and the red to each of the + and - jacks. If you get equal but opposite voltages of the same value (assuming you have only 0 or 1 voltage adjustments), then it is a dual-output supply. This is a common sort of supply suitable for powering circuitry that contains bipolar opamps, etc. that require both positive and negative voltages. You may continue using just the + and - terminals if you only need one output; in fact, if you need a higher voltage than the supply is able to provide just going from COM to + or COM to -, you'll have to do this.

2- Less likely it is a single-output supply and the COM terminal is actually just a chassis ground, which is connected via a third prong to AC grond at your AC outlet, which in turn is connected to earth ground outside of the building. But usually this terminal would have the earth ground symbol (short vertical line, a short horizontal line attached under it, and 3 short diagonal lines hanging from the horizontal line) rather than being labled COM. The purpose of this terminal is to provide an earth-ground reference should you need or want it, and either + or - may be jumpered to it - just don't do it until you've done the check mentioned in situation #1 above.

In either case, you should check to see if there is 0 ohms between the COM terminal and the metal of the chassis just for the sake of knowing if the supply is earth-ground referenced. You can have 1 earth-ground in a circuit, but never more than that.

All clear? Email me if not.

Answer 2

For voltage, the common terminal represents 0 volts. It is basically ground. If the power supply is rated at say, 12 volts, you obtain the 12 volts by taking a lead from the + terminal and com (12-0 = 12). If you want -12 volts, you would take a lead from the - terminal and com (0-12 = -12).

If you were to take a lead from the + and - terminal, the voltage between the two leads would be 24 (12-(-12)=24). Use com for grounding.

Answer 3

PLC's have a com strip which is used for common power to all terminals. The field wires are on one side and common on the other side which goes to the DC power supply in the PLC cabinet.

Answer 4

Com means common and is usually used for the ground. Some power supplies have a negative supply. Sometimes you can just use the + and - if the PSU lets you. Normally it is used for analogue circuitry that needs + and - rails.

Answer 5

Common is usually a ground. You have a dual polarity power supply. The voltage from positive to com will be above zero volts, and the voltage from negative to com will be below zero volts.
If you connect to the positive and negative outputs(not com) you will get a voltage which will essentially be the addition of the voltage from pos. to com. and neg. to com.
Example
If you had 10V from pos. to com. and -10V from neg. to com., you will have 20V from pos. to neg.
Yes, the com is connected to the center tap of your transformer, which is now a virtual ground.

Answer 6

power factors are not comparable to batteries. they are comments controlled gadgets that alter their output in line with what they "see" at their output as against what they are meant to be kicking out. it is not a stable concept to sequence them.

Answer 7

Common is for when you have multiple devices connected that may not be directly connected. Sometimes its also called EARTH.