Monday, June 11, 2012

Configuring a Split Rail Power Supply

In this post we will look at creating a split rail power supply that provides both a positive and negative voltage output. This is a trivial task if you have a multiple output power supply that has a negative output lead. This task becomes more challenging when all you have at your bench is single quadrant power supplies that can only output positive voltage and positive current.

This challenge of creating a split rail supply can be overcome by combining two single quadrant supplies in series. If we call the supply that creates the negative supply voltage V1 and the supply that creates the positive voltage V2, the high lead of V1 will be connected to the low lead of V2. This forms the common or "rail" node of the supplies, which can be tied to ground or to the common of the DUT. The simple split rail power supply setup can be seen in the figure below.

Note that the high lead of V2 is then connected to the positive supply input of the DUT and the low lead of V1 is connected to the negative supply input of the DUT. Lets look at an example where we want to create a split rail supply with a 15 V output and a -15 V output. both outputs were connected to a DMM and the resulting measurements can be seen in the figure below.


Note that the common or low leads on both DMMs are at the same potential. When implementing this setup be sure that the low lead of the negative voltage supply (V1 in the first figure) is not tied to ground. Additional supply outputs, either positive or negative, can be easily added. This is done by either tying the additional power supply low lead, in the case of a positive output, or high lead, in the case of a negative output to the rail node.

In this post we looked at how to configure two single quadrant power supplies to power a DUT that requires a split rail power supply. If you have questions please shot me an email and if you have any personal insights to add to this post please use the comments section below.

12 comments:

  1. This comment has been removed by a blog administrator.

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  2. My PSU is Coler Master Real Power 550W and I plan to buy new graphic card Gainward GTX 460 GS GLH. Minimum PSU for GTX 460 is 450W with two 6-pin PCI-E power connectors and CM PSU have them. So, I worry about amperage needed to power this card... My PSU has tree v12+ rails at 360W power. 1st and 2nd rails are at 12A and 3rd is at 6A. In total 30A. DOES CARD USES AMPS ONLY FROM ONE RAIL, OR FROM THREE RAILS COMBINED?
    Thanks, @Rimi from Ac Dc Adapter UK

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  3. If each rail's current is spec'd individually, then they only supply that spec'd current. You could connect the rails in parallel for higher current, but you would first want to check the power supply's data sheet to ensure it supports paralleling.

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  4. Hello, I appreciate your writing. I was looking for a 24v power supply for my headphone amp when I came across a 24v 6.5a unit locally, for a good price. I looked into it and as the picture shows, it has two outputs for V+ and two for V-, along with a ground. I started thinking about using it not only for the headphone amp, but for my phono preamp which require +15 and -15 dc. Could I take the +V and -V lines and regulate each of them down with a lm317 circuit to +15v and -15V?
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