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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?

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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.

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  • \$\begingroup\$ Thank you for the quick response. So if I see this correctly my explanation approach agrees with yours? \$\endgroup\$ – Mr.Sh4nnon Apr 26 '20 at 18:52
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It’s possible. Use a 21V shunt Zener regulator across the input of each device to ensure the input voltage doesn’t go too high if the loads are unequal.

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  • \$\begingroup\$ Thank you for the tip. \$\endgroup\$ – Mr.Sh4nnon Apr 26 '20 at 18:53
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The Photon's answer agrees with my explanation attempt. However, it does not state clearly a reason for this. Doing some research I found a paper which mentions the following:

It is important to note that stable operation does not result when two standard, independent dc-dc converters, without any special control for sharing, are connected in the input- series and output-series mode. In the absence of input voltage sharing control, any small disturbance causes a runaway of the converter input voltages due to the negative input resistance property of dc-dc converters. For example, if a small disturbance causes a converter input voltage to increase, its control loop reduces its duty ratio, thereby reducing the average input current. This results in further increase in the input voltage of the corresponding converter, ultimately leading to the entire input voltage appearing across this converter...

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