Why is it a problem to energize a 24VDC input with a 24VDC source that has a different common than the input?

Answer 1

Reason 1 -- if there are 2 commons, then there is no
path for the return current to flow back to the 24 V source.

Reason 2 -- there is probably a common mode voltage between the different commons (that's one reason why it is called a common mode voltage).

Measure the voltage between the 2 commons. I'll bet there is at least a couple of volts, and that is probably making the difference.

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Answer 2

In an AC circuit the common is generally considered to be the grounded current carrying conductor . So for this 24VDC ckt i will assume the negative wire is the common you speak of...Lets also look at the circuit for a moment and consider this a small transformer that is tapped on the secondary to produce 24v then by rectification becomes DC. In all cases there will be an isolated circuit for this portion of the system. The small transformer secondary is used for isolation of Spike, RFI,IN-rush and all the other general noise problems of the AC system. Now you have two distinct wires with + and - charateristics as an output . Connection of these wires with correct polarity to your 24VDC should pose *****NO***** problem . The Input of the device must be connected to the same wires as the output of the X-former..Lets look at the mistakes we could make ...Observation of polarity two + 's will not work and vica vers for -'s....using the ground from another output for the ground for yours ..yes that happens there is a loop (inter system) ground and a chassis ground that can cause the circuit to chirp or relay race or not function at all...And of course there is always the fact that the output you thought was DC is AC and until you ran it through the first diode in the chassis it was messing all downstream sections up ( yes i have even done this, twice in one day) soooo. if you are having trouble with a DC ckt My best advise is to check the simple things first .Observations contrary to good reason are generally not the answer. Watch polarity and make sure your output has polarity...Well that should do it ,,,other than some sunspot caused it ,,so have a good time with it from the E...

Answer 3

I guess the problem is physicl, you will need to insulate the case of the device with the different input polarity and connect the case to the live input and the input to the main chassis , basically dont do it, unless the device is double insulated, get the right polarity.

Answer 4

Not sure what you mean by a different common, but maybe it has a negative ground instead of positive, If the input device is not to be grounded, it should be OK. Of course, if it is polarity sensitive, you should observe polarity.

Answer 5

All voltages must be referenced to a ground. If these are not a common ground, one side need to float (be insulated).