Make a 24 V power supply from two 12 V supplies

Server power supplies, like the HP DPS-750RB, are available for a low price, especially if they have been used before. This makes them ideal for powering chargers that allow for or even require a DC input voltage.

The downside is, that they only produce 12 V output voltage. Most chargers claim to be able to charge at high power, but often forget to mention, that these maximum values are only reachable at higher DC voltages. So at a certain point, the voltage needs to be increased to transfer more power, since the current is already maxed out at the given voltage.

AC or mains voltage is dangerous and can be lethal! Always make sure to unplug the device when working on it!
Even when disconnected, the capacitor(s) of a DC power supply can store a high voltage/current for a significant amount of time. Always make sure to isolate or discharge first!
If you’re unsure or feel uncomfortable working on such a devices — don’t do it!

Combining two or more devices

Under certain conditions, it is possible to combine multiple DC voltage sources. If the output voltages of these power supplies are connected in series, these voltages will add up. This way a 24 V or even 36 V power supply can be build by combining multiple 12 V supplies.

These server power supplies have the DC ground pole (also called GND, RTN) and the AC PE (protective earth, also called protection or ground wire or just earth) tied together. PE is usually also connected to the metal casing of the device, since it’s an actual safety feature.

The red dashed connection is the one that needs to go for the serial connection to work.

If a second DC power supply is connected in series, the positive pole of the first supply will basically be shorted out. Why? Because the GND of the second one is also connected to the same earth wire and therefore also connected to the GND pole of the first one. Beste case scenario: The device will shut itself down. Worst case: Explosions. I’m not kidding!

For more than two power supplies in series, every additional supply needs to have an isolated or “floating” ground.

Measuring and modifying the devices

You can easily test the current configuration of your power supply, by using a continuity tester or a Ohm-meter and measure between “GND” and the metal casing of the device and/or the middle pin of the unplugged AC connector on the power supply. If nothing has been altered yet, both should be tied together. If not, something is probably wrong.

Before connecting anything in series, all added power supplies need to have a “floating ground”. This means, there is still a voltage of 12 V between GND and +12V, but the GND output isn’t connected to the case/PE anymore. Only the first supply’s GND may be tied to earth/PE.

How to isolate the ground from PE and metal case is described in detail in my other article. This modification is essential if you are planning on running multiple supplies in series!

Putting things together

The rest of the setup is quite easy! To connect the devices, the positive pole of the first power supply is connected to the negative pole to the second one. Since all the current will pass through this connection, an adequate conductor diameter should be used and soldered properly to the pads of the power supplies.

The negative pole of the combined supplies will be the negative pole of the first device in the chain, while the positive pole of the combined supplies will be the positive pole of the last device in the chain (like shown in the images above).

This is basically the same thing as a LiPo battery with multiple cells: The large connector, that carries the whole current, is connected to the first and the last cell, while the smaller balancing leads are connected in between the cells to measure their voltage. It is possible to “access” the lower voltages (like 12 V for a dual-in-series setup) in “between” the devices, but always remember that the power supplies can only supply a limited amount of current. Make sure to stay well below the values printed on labels.

Before powering on the devices, I recommend measuring one last time for continuity and resistance, like described in my other article.

The voltage doubles but the current does not

Always understand and remember what you are actually doing: Putting two battery cells of the same type in series, doubles their voltage, but the current, they can deliver, stays the same. This is where Watts as a unit for power comes into play:

P=U \cdot I
P=1.5\,V \cdot 2\,A = 3\,W

The same for a power supply:

P=12\,V \cdot 60\,A=720\,W

This is the power of one cell and one power supply. If you add up multiple cells or power supplies (same type) in series, the power value also multiplies. So, how much amps can your new combined power supply deliver?

The voltage is known (24 V), the power is known, too (1440 W):

I= \frac{P}{V}
I=\frac{1440\,W}{24\,V}=60\,A

So basically, the power doubles and the voltage doubles, while the current stays the same.

Just FIY, if you put two power supplies in parallel, the power and the current doubles, while the voltage stays the same:

I=\frac{1440\,W}{12\,V}=120\,A

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