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It is common practice to connect multiple power supplies in series to obtain a higher voltage; examples: 1, 2, 3, 4. Many power supplies are designed to handle this application well. However, users are cautioned to check the manual of a power supply if it is really recommended to use it in series with another one.

One issue that might arise is called polarity reversal, i.e. a negative voltage being forced on the output of the "weakest" supply in the chain of series-connected devices. How is this possible?

Note: Same story for batteries. One should make sure that the same type of battery is used together with another, and that multiple series-connected batteries are in equally good (or mediocre) shape.

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zebonaut
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The condition to consider for this to happen is a load beyond the rating of the supplies in question. Most severely, a short circuit. A short circuit need not be permanent and can even be a part of normal use: A motor, when starting, or a capacitor, when being charged from 0 V, looks like a short circuit, too.

To explain how a negative voltage can appear on one supply's output, let's use a diagram with some arrows. Supply A's output voltage shall be called VA, supply B's output VB, and the voltage across the load shall be called VL.

enter image description here Source

With KVL:

\$V_A + V_B = V_L\$

This is true for normal operation, when we want to use, for example, two supplies with VA = VB = 12 V for a load that needs VL = 24 V:

\$12V + 12V = 24V\$

For a short circuit, VL = 0 V, and KVL still applies. Thus, \$V_A + V_B = 0\$ or

\$V_A = -V_B\$

If we want to avoid negative voltages on any of the two supplies, the only possible solution is

\$V_A = V_B = 0\$

However, because of \$V = R \cdot I\$, and because any real power supply has a non-zero internal resistance, this also means that no current must flow. Now, a power supply will try to deliver current. Since the current or power limit of the two supplies will not be exactly equal, the "stronger" supply will be able to maintain a small, positive output voltage, and the only way to solve the KVL equation is by back-feeding the other supply, with a negative voltage across its output.

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