This is a follow up post to the previous post on how to convert a computer ATX power supply to a shop DC supply. This article describes how to get more voltage combinations and goes into more detail on how to work with the unit.
When you convert a computer ATX power supply into a DC bench supply it can include up to 5 voltage outputs plus a ground. An ATX PSU (power supply unit) will always have four voltage rails — one each for +3.3 volts, +5 volts, +12 volts and -12 volts and sometimes there will be a fifth rail for -5 volts.
These five voltages are made by connecting the negative DC terminal to the GND output and the positive terminal either to the +3.3v, +5v, -5v,+12v or -12v output.
The unit is not limited to just these voltages. It is possible to have as many as 22 different voltages by connecting the positive and negative DC terminals in different combinations. Like any other shop tool, it helps to know how to get the most out of the unit. Understanding a little more about these combinations could make it one of your most often used manufacturing tools.
There are 30 possible combinations for 22 different voltages (some of the 22 combinations can be made two different ways for a total of 30). Below is a chart that shows how to make different voltage combinations:
The columns from left to right list each output for connecting the positive DC terminal and the rows from top to bottom list each output for connecting the negative DC terminal. The voltage is displayed at the intersection of a row and column. For example, by using a multimeter with a red positive lead and a black negative lead and by connecting the black lead to the GND terminal and the red lead to the +12v terminal, you would get +12 volts. But, if you were to reverse the leads and connect the red lead to the GND terminal and the black lead to the +12v terminal, you would get -12 volts.
Below is a list of the number of possible voltage combinations:
|+1.7v||1 combination||-1.7v||1 combination|
|+3.3v||1 combination||-3.3v||1 combination|
|+5v||2 combinations||-5v||2 combinations|
|+7v||2 combinations||-7v||2 combinations|
|+8.3v||1 combination||-8.3v||1 combination|
|+8.7v||1 combination||-8.7v||1 combination|
|+10v||1 combination||-10v||1 combination|
|+12v||2 combinations||-12v||2 combinations|
|+15.3v||1 combination||-15.3v||1 combination|
|+17v||2 combinations||-17v||2 combinations|
|+24v||1 combination||-24v||1 combination|
Chart Color Codes
You may wonder about the funny color codes used in the chart. The colors have to do with the available amps for the different voltage rails when combined in different ways. If you examine the specification placard on the side of the PSU it will tell you the available amps for each rail:
Notice that in the above example, the -5v rail and the -12v rail have the least amount of power — less than one amp each. The other rails have many times more available amps. A view of the inside of the power supply reveals that there is a relationship between the number of wires and the available amperage for each rail:
There are many wires for the +3.3v, +5v, and +12v rails and the ground but there is only one wire each for the -5v and -12v rails. This brings us back to the color codes in the previous chart:
- Red is for the high amp combinations (both positive and negative DC terminals have multiple wires)
- Yellow is for the medium amp combinations (one or the other of the positive or negative terminals have multiple wires and the other terminal has only one wire)
- Green is for low amp combinations (both positive and negative DC terminals have a single wire)
The terms used are just generally “High” “Medium” or “Low” because the exact number of amps will vary from one PSU to the next, but the rails with the lowest amperage will be the -5v rail and the -12v rail and will usually only have one wire each. The remaining voltage rails plus the ground will have multiple wires.
Choose the Combination With Appropriate Amperage
You may have noticed that when a certain voltage can be made with more than one combination (e.g. +12v and -12v), then it may be better to choose the combination that supports a higher amperage. When connecting devices and placing the unit under load it is usually better for the unit to be able to handle more current than less current. Still, some devices may draw as much current as you feed it and they may overload and fail as a result.
Be careful about using the low amp combinations. You shouldn’t have to worry too much, though, because most ATX PSUs have over current protection and the unit will simply shut down if the current load exceeds the limits set by the manufacturer.
There are only two voltage combinations that fall into the “Low amp” category: -7v and +7v (the ones color coded green in the chart above). There are high amp combinations available for -7v and +7v (the ones color coded red) and might be a better choice in most situations. Perhaps the low amp combination should be avoided for this reason.
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