I know it won't work, but I want to make sure I have the right explanation why it won't work.
Given are two devices which have a build in converter (I have this digital voltmeter in mind). They need 12 – 24 V AC or DC supply. And apparently the supply is galvanically isolated from the measurement inputs. Also given is a let's say 40 VDC power supply.
One could falsely come up with the idea of just connecting the two voltmeters power inputs in series with the 40 VDC supply in order to save two (e.g.) 24V Converters.
My explanation why this won't work is as follows: Since those digital voltmeters while have internally some sort of buck converter with a feedback-loop (due to the variable input voltage), the voltage will not divide 50:50. The ratio will either tilt to one or the other side, depending on which device has a slightly higher load when turning everything on. I would say the measurement input connectors have no influence on this and could theoretically be connected since there is a galvanically isolation anyway. The only problem is that these two feedback loops behave unstable and in the end tend to apply almost all the voltage to one of the two devices. This will most likely kill one of them (or at least the fuse).
Does this explanation sound valid, or is there a more reasonable explanation that I have overlooked?
Best Answer
The problem is what happens if your two devices want to draw different amounts of current. The feedback mechanism in their internal power regulators makes things more complicated, but it very likely leads to things not working well at all. One device might end up getting the whole 40 V and the other getting nothing, the system might oscillate, etc.
A simpler solution is get just one 24 V AC-DC converter, and connect the two devices to it in parallel.