Best Practices for Connecting Commons and Grounds in Multiple DC Power Supplies

I'm no EE but I tend to create some rather sophisticated machines. Something that has always escaped me was what the best practices in regards to multiple switching DC power supplies. I typically never connect the commons of the supplies nor do I tie the common to ground, whether this is done internally in the supply or it is isolated I don't know. I tend to think that keeping them isolated seems best.


The last 2 projects I've put together involve 2 separate DC supplies one of which powers a small PC that has a DC-DC converter powering it. One for example has a 10amp 24vDC power supply for field IO and powers the PC DC-DC converter. It also has a constant current 40-70VDC LED Driver. The other has a 12vDC power supply for the PC and field I/o and a 48V 10 amp supply for stepper motor drivers. I keep all supplies separated and I don't have many issues but I am wondering if this is best.


Thanks

Hi Justin-There's not a "one size fits all" answer to your question. In my designs it's rare to have more than one power supply.  If I do have more than one supply one side (ground) is tied together between them. Even though I rarely use more than one supply I do often have one big supply with several point-of-load regulators supplying multiple output voltages.When I do have multiple supplies, it's for reasons different than yours. Examples: *High availability server with redundant power- multiple supplies tied together.   *Portable test equipment with mains power, battery power, and external 12/24 volt input - all supplies tied together.If you have multiple subsystems, and one of the subsystems creates noise and another subsystem is susceptible to noise I can see the logic in separating the supplies, but if these subsystems need to communicate with each other you have to take extra steps to isolate them.  You didn't say if the DC-DC converters in your designs are isolated or not. Also I'm wondering about the inputs to the LED driver and stepping motor driver.  I've got a suspicion that some of your designs may have the supplies inadvertently tied together through paths that you may not realize are there. 

Hi Rick,
Thanks for the response. I sometimes struggle with the full meaning of "isolated". Obviously a transformer inside the supply should isolate the side to be rectified from the input side. Other than this I tend to think of isolation between multiple supplies as "not connected". I also never refer to the negative of a supply as ground. This always confuses me as I thing of ground as path to earth and a supply that is isolated I don't believe has a path to earth, it floats. I always refer to it as "common". But then again, I ask this question because I wonder if I should tie common to earth ground or to a separate common.I don't specifically know if the "DC-DC converter" is isolated itself or not. That is actually just a DC ATX power supply for the PC, I guess it's technically a "buck" converter maybe since it has no transformer. That is powered directly from the 24v 10amp supply. The step drivers and FPGA board providing the step signals are also powered from the same 24v supply. The LED driver is powered from the AC side. It is turned on by a DC SSR. the SSR "coil" side is triggered by the FPGA on the 24v supply. In this instance the LED driver seems to be isolated from the 24v supply by means of the SSR.
In my other instance I use a 48v DC supply for only stepper driver input power. The drivers are signaled by another FPGA on a separate  12 supply. I assume quality stepper drivers isolate the signal from power input.Basically the way I think of think of these things is I will use a single supply like the 12 or 24v for mostly everything except higher current/voltage requirements. If I needed to drop 12/24v to 5v for something small I would just stick a small buck converter in there for point of use as you said. In the instance of the second system I mentioned, the 48v supply provides a higher voltage for stepper speed and I do not want the current drops to effect the stuff that "thinks".
in the first the LED driver is constant current and the voltage can swing from 40-70v. The steppers and drivers are small and draw little current so I'm not worried about current draw, as I said I assume the digital stepping drivers do a good job of isolating the signal side from the power side, though these are not electrically isolated because it's the same supply. I usually use shielded cable for the motors themselves tied to ground at one side.

Ground is an entirely misunderstood concept, and methods for creating a common point for power supplies can be quite complex.Have a look at these brief articles and the following references.What are the different "grounds"EMC, ground loops and safety hazards are the types of problems that often arise out of not sorting it out.As Rick says, if you are buying pre made equipment, you can't make assumptions about what is connected where in terms of return paths for devices demanding high currents, and data cables may end up carrying non signal current in the shields or conductors if you don't get it right. Articles and texts on EMC and low noise electronics are where you will find useful information about how to deal with power and signal commons.Cheers,Richard

Justin,If the only connection from the PC/FPGA to the LED is the SSR, then it's reasonable to assume that the LEDs are isolated from the PC/FPGA. In this case there is no need to connect the two commons other than for safety or EMI.For the stepper motors, you are connecting some signals from the FPGA to the signal side of the stepper driver. Does the stepper driver have separate common returns from the signal side and the power side? If so, you can leave the power side isolated but the signal side should be using the same common as the FPGA.If there is only one common for the stepper driver, you may be inadvertently creating a connection between the supplies using your signal paths. If this is the case (check with an Ohm meter), you can either isolate the signal paths (using optocouplers or similar) or connect the two commons together.

Just to be clear I'm not sure that I'm actually having an issue, just what besty practices are. The FPGA and the stepper drivers are (on one machine) powered by the same 24v supply. I use drivers made by Leadshine/Stepperonline. DM320T
Is the smallest example, which has "optically Isolated inputs" I guess it has to share a common ground with the FPGA but that wasn't actually my concern. Main concern was whether or not I should be tieing the common of the LED driver to the common of the 24v field Power supply which you indicated is not necessary.

From the article you linked to it sounds like my idea of "common" vs "ground" is correct.
As for "premade equipment" I assume you mean the power supplies themselves? It's not likely to find electrical drawings of the internals of a power supply prior to purchase. How would one go about doing their assessment of a power supply after the fact. Is there a procedure for this or is opening it up and tracing it out a common practice? The FPGAs I use I know are extremely well isolated. The stepper drivers supposedly (and usually) are. I tie no DC common to ground or together. Whether that is good practice is what I'm asking but I assume no connection is as isolated as it can be.
It seems to me that where the trouble you suggest may occur is when something is powered from one supply on a field power side and somehoc connected to another no the logic side. This is what I've always thought the optical isolation is for. For example the FPGA will accept a Field power input that is separate from the logic input, or their is a voltage regulator onboard that will allow 1 supply to serve both. Stepper drivers almost always have optically isolated inputs because it is very likely you will use a different power supply for the driver itself since larger motors require higher voltages than typical 5v step and direction signals. 

Actually with the stepper driver you are using, it is optically isolated and they cleverly set it up so the common is NOT used on the input side. So you could use a separate supply for the stepper motor and not have to tie the common together.The bottom line is that if your signals are isolated, the supplies can be isolated. You just have to verify that the signals are isolated or you will have problems.