Increasing Available Current

A Possible Solution

With the increased usage of high capacity lithium polymer or A123 batteries in more and more electric powered aircraft, the current demands of the newer chargers used to maintain these batteries have likewise increased. Individuals that have converted PC power supplies to use as DC power sources are finding that the available current is not sufficient to allow the chargers to function properly or, in some instances, charge at all. The question then becomes, "Are there any steps that can be taken to modify my present converted PSU or adjustments made to a future conversion to provide more current?". The short answer is, "Possibly."

For the interested reader, a little history of power supply design follows:

Around 1995, the existing power supply designs were simply running out of power as newer motherboards were beginning to consume more and more current. INTEL's approach was to modify the internal circuitry and move from dual 6 pin connectors (AT) to the more familiar 2x10 (20 pin) power connectors (ATX) commonly found on many motherboards today. At the same time, a 3.3v rail was added to provide support for a lower processor core voltage in an attempt to reduce the operating temperature of the relatively new Pentium class processors. However, processor development continued at a feverish pace and by the late 1990's, power demands were again beginning to surpass the capabilities of the existing ATX designs. Recalling that power (watts) equals volts x amps, the power hungry motherboards required more amperage on the 12v and 3.3v rails than the connectors and wiring could safely support. Because each of the terminals on the standard Molex 20 pin connector were rated at 6 amps, the rated capacity was around 250 watts. Motherboards and associated adapter cards were drawing more than the rated amount and manufacturers were now building supplies in the 300 watt plus range. Without a change in connector design, overheating and terminal melting could and did occur. INTEL again revised their Design Guide to include a six pin auxiliary power connector. Although INTEL added the auxiliary power connector to the Design Guide, the Guide is a recommendation and not a mandate, consequently, very few board designers implemented it and few power supplies included it. (The connector ratings were for 3.3v @ 33 watts and 5v @ 25 watts. It is important to note that this 6 pin connector lacked a 12v rail.)

The Pentium 4 was introduced in 2000 and brought with it higher power consumption than any of the prior processor designs. As the power demands rose, board designers were faced with the problem of meeting the desired power levels without overextending the capacity of the standard 20 pin connector. Recalling the power formula, I can move the same measured wattage across my connectors using low voltage, high amperage or with a higher voltage and lower amperage. One workable solution was to utilize the 12 volt rail to drive dropdown regulators mounted on the motherboard. These regulators, sourced at a higher voltage, required fewer amps to supply the same amount of power to the processors, which were now running with 1.6 to 1.7 volt cores. The downside was that the standard 20 pin motherboard connector carried a single 12v terminal.

INTEL stepped in with another Design Guide revision and added an additional 4 pin connector designed to deliver up to 192 watts to the on-board regulators. This revision is known as the ATX12V version and may be identified by the existence of a 4 pin Molex connector carrying two 12v rails (yellow w/ black stripe) and two ground wires (black). Designed with pins rated at 8 amps, each 12v rail can deliver up to 96 watts. With the new 12v power connector now supplying the onboard voltage regulators with sufficient current, the power needs on the 3.3v and 5v rails associated with the motherboard connector dropped dramatically eliminating the need for the 6 pin auxiliary connector. Consequently, INTEL dropped the 6 pin plug from the Design Guide in 2000.

2004 saw a new development, the PCI-Express bus. PCI-Express is a dedicated serial bus aimed at high speed data movement for specific network and audio adapter cards. However, the primary application is in PCIe multichannel high bandwidth video cards targeted as a replacement for the PCI and AGP video protocols. Graphics rendering today has become exceedingly complex and much of the computational load has been transferred from the computer's primary processor to a video processor housed on the video adaptor itself. The upside is an exceptional improvement in the complex graphics rendering engines commonly associated with high end gaming. Unfortunately, the old 20 pin motherboard connector lacked sufficient capacity to supply enough power to drive the PCIe components.

What about the ATX12V 2x2 auxiliary power connector -- why can't it supply the needed power for the PCIe cards? It's not available due to dedicated usage by the primary processor and associated voltage regulators. Consequently, INTEL made yet another revision, moving from a 2x10 (20 pin) connector to a 2x12 (24 pin) primary connector for the motherboard. However, to maintain backward compatibility, this connector is many times packaged as a 20+4 pin connector by the power supply manufacturers. The new 4 pin plug is populated with a 3.3v (orange), 5v (red), 12v (yellow) and ground (black) allowing a substantial amount of additional power to be delivered to the motherboard. Generally, it is separate from the older 20 pin connector, although usually bundled with zip ties to the connector wire bundle. The 20+4 design allows its use with an older 20 pin motherboard or with the newer boards that support the PCIe protocol. The designation for this type of supply is ATX 12V2.0, sometimes abbreviated to ATX12V2. The physical dimensions of the power supply were retained to avoid a change in case dimensions.

Increasing Available Current

If you elected to skip the historical development of the ATX design and arrived directly here, "Shame on you!" The ATX12V2 guidelines do not offer much help, but the earlier ATX12V design changes open another gateway to the power supply and provide a rail from which we may tap additional power.

The 12v lines associated with the motherboard connector and other peripheral power connectors (hard drives and optical drives) are tied to the same common point, i.e., all yellow 12V1 wires have the same current rating and same current source. The four pin ATX12V2 connector (12 volts, yellow w/ black stripe) will have a separate connect point and current source. Power distribution in these supplies is a study in itself and vendors may deviate from the recommendations when marketing their designs. Where does the additional current originate and how much can I expect to draw from my converted power supply? The simplest answer is while some rails may provide their full current rating alone, when combined with other rails, there is a maximum obtainable power ceiling which will usually fall below the sum of individual power ratings.


For example, the specification placard on a 350W PSU made by a well known manufacturer lists the following rail ratings: 3.3v @ 22.0A; 5v @ 21.0 A; 12V1 (yellow) @ 18.0A and 12V2 (yellow w/ black stripe) @ 16.0A. Individual power calculations yield 72.6 watts at 3.3v, 105 watts at 5v, 216 watts on 12V1 and 192 watts on 12V2 for a whopping 586 watts!! However, the placard also lists the maximum combined power, i.e., how many watts are available from two rails at the same time. The 3.3v and 5v rails together will supply 130 watts max. The 12V1 and 12V2 rails have a maximum power rating of 300 watts when both are loaded. Even so, the sum of these combinations is still significantly above the overall rating of 350 watts for the PSU.

On this particular supply, combined power is about 73% of what one might expect from simply accumulating the output levels. The obtainable power is a result of the internal distribution circuitry and will vary from manufacturer to manufacturer, so do not expect to realize the listed current from any of the supplies when all rails are in use. INTEL does specify how this algorithm should behave, but the guide, as noted previously, is not a mandate.

A possible solution for low current:

If the 12V1 or the 12V2 rail cannot supply sufficient current to your charger individually, you may combine the two by wiring the rails in parallel. Before attempting this, measured voltage on each rail should be the same or very close. Because we are preparing to draw relatively heavy current loads from the power supply, doubling the wiring would be advisable. The proposed solution is to connect both the 12V1 (yellow) and 12V2 (yellow w/ black stripe) rails to the positive binding post -- any two solid yellow and both of the 12V2 wires for a total of four wires. For the DC ground wire (black), bundle at least two and possibly three wires to attach to your negative post -- all ground wires are common, so the selection is immaterial.

Using the example PSU from above, placard ratings stated a combined maximum 12V1 and 12V2 power output of 300 watts, which translates to a current of 25 amps. This is below the 34 amps one might have expected, but has given a substantial increase over the 18A and 16A ratings of the individual rails. If you do elect to combine rails for higher current, it would be prudent to keep a close eye on your PSU early on to ensure it can carry high charging loads without overheating or dropping voltage.

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