Zarcon Dee Grissom's Idea Page
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Updated 23MAR2008
Home > Ideas > RFI Killer HDD Power plugs.
RFI Killer Hard drive Power plugs.
The longer the wire, the greater the Inductance induced Voltage fluctuations are.
RFI plug v0.1
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Idea#013
The location for this was a toss up, OSWATT Power BUSS, or Idea013.

Foot notes, before we break something expensive.
Some mods, should only be attempted when all other options have been exhausted.

0) I'm still brain storming the SATA plug things.

1) the Molex(tm) power plugs are only rated for 50, thats fifty and no more, plug/unplug cycles. So the solution needs to be cheep enough to throw away when worn out. "O" and there only rated for like 5Amps, so the buss plugs the power supply plugs into, needs to be ganged up. like one plug for each 5Amp draw. The expensive plugs at Mouser Electronics can handle 8amps, so two of them will allow me 16amps on the buss. Thats 16amps on the 5V (80 Watts), plus 16amps 12V (192 Watts).

2) The ATX standard calls for 16AWG wire for the "Auxiliary" (HDD, CDROM, etc) Power connectors, 18AWG for the floppy things. I was going to use excessive 12AWG (5V&12V), and 10AWG (ground) for the long runs up and down the tower. With a breakout box like things to the individual plugs, to reduce power loss. Voltage drop, and RFI was already on my mind before OCZ came out with there excellent product. The thought crossed my mind to order a slew of them for "breakout-cables" to the devices. 18 plugs * unit-price (about $25) = approximately  $450... for a throw away item, Ouch! I'll settle for just one for my graphics card.

3) There are calculators on-line to calculate the inductance of straight runs of round wire. Coiling up the excess cable in the computer only increases the inductance. Making the internal noise transmitting of the wire worse. It is NOT clad-shielded (Coaxial) stuff. Plane and simple, all you need to know is the thickness of the conductor (excluding the insulation) in mm or inches (decimal not fractions), and the length of the wire stretched out straight from the PSU to the last plug (in inches or mm), for each run of cable. This txt file is what I came up with for my computer.

4) With DC power... When the device demands an increase of power draw threw an inductor, the voltage drops. When the device decreases pulling power, the inductor only cares about Amps flow staying the same, thus increasing the voltage at the device to keep the Amps flow constant. Allot of Buck/Current-Boost/Flyback switching regulators take advantage of the inductors properties in a good way. In DC power distribution to devices, inductance without addiquit protection is BAD. Voltage will drop during the increase of power draw, stabilize at constant power draw, then spike during decrease of power draw. The quicker the power draw is changed with a given inductance, will increase the severity of these voltage fluctuations.

5) before we go over kill with "snubbing caps" as some call them. The ATX standard also states the minimum amount of capacitance the Power supply needs to be able to charge during the start-up cycle, to have the voltages stable in "X" amount of time. This implies there is a maximum amount of capacitance the given power supply can charge up as well. An excessive amount of capacitance will probably trick the power supply into thinking there is a short, and thus not turn on in the best case. The lovely little shot-key diodes in the power supply, or something else in the computer might just release it's carbon in the worst case (POOF! plume of smoke rises from expensive computer).

6) And last of the footnotes. The closer we can get the caps to the RFI source (the Hard drives), the less wire between them is transmitting that nasty RFI. Additional chokes are tricky items. OCZ did it right with a toroid. Toroids reduce the surrounding magnetic fields far better then simple bar inductors. The last thing we want to do is degauss the hard drive platters.

How long are your wires, and what are the effects?
clipboard notes
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Finding Cable lengths that work in tall computer cases often leads to multiple feet of power cable runs. Anything less, and the plugs wont reach. All that cable length comes at a price. Wire Inductance induced, Voltage fluctuations. Like Any DC power system, combating Voltage fluctuation is done with capacitance. Now I know there are products out there that have capacitors in them to reduce noise from the Hard drive or VGA card. They Also contain additional Inductors. Adding Inductance in the power cabling without a capacitor before the inductor, would be counter-productive with such long wire runs.

HDD test lead
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Plane and simple, three feet of 16AWG wire, has an inductance of over one micro henry (1uH). In electronics and RF terms, Thats a significant amount of inductance. More then enough inductance for the changing power draw of a single hard drive, to throw the voltage at the hard drive well out of spec. No simple multimeter will see these high frequency inductance induced spikes. However even with the most primitive Oscilloscope, they become blatantly obvious. As I sit here and watch the bar graph on my Fluke periodically peg to the beet of the hard drive chatter, and the numeric display stays solid at 12.073VDC.

Why don't the capacitors in the hard drive dampen these spikes? The capacitors in most consumer computer gear is there to reduce EMI/RFI interference, in accordance to various regional standards. There not there to compensate for a lacking power system. In engineering terms, adding more capacitance to the product to correct the voltage in a hand full of rare situations would not be cost affective. In management terms, Most people don't have more then three feet of power cable between there PSU and the hard drive. Most people don't need a better, more expensive to produce, less profitable product.

This leaves me with a power supply and hard drives, that was not designed for that extra length of cable. The solution sounded simple at first, till I took some mechanical measurements, and fired up ELSIE to run some numbers. In my case, I needed to add a measly 0.057uf (0.01uf + 0.047uf) to each plug on the Power extension cable. The larger 1uf caps can be pushed back further on the cable for a convenient mounting location. The problem is, there is not enough room around the plug to attach the small capacitors in the same way on every plug. Either there is no room under a plug, less then enough room behind the plug, or there precariously stacked on top of the plug blocking the hard drive above it.

There is the other issue of the cable radiating the hard drive noise All over the inside of the computer case. Making nasty lines on my screen, and digital noise in my sound card, when ever I'm browsing my hard drives. At first I thought the Power at the motherboard was noisy, till I tested and found the power well with in spec, everywhere except at the lower hard drive rack. So the caps need to be as close to the hard drives as possible, without voiding the hard drives warranties.

So now I'm thinking about modifying the plugs on the power extension to make the capacitors fit in the plugs. Why not, 0.01uf and 0.047uf Polly-film caps are rather small, and inexpensive. Before I ruined my expensive computer, I grabbed a dead PSU, and a hacksaw. I cut up the dead PSU's plugs, to see how close to the wires I could get the caps. Then I stole the Polly-film caps out of that dead PSU, to try to fit them in various ways in the plugs. I also drew them up in ExpressPCB as a crude mechanical drawing while brain storming the solution.

dead power supplies
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cut up plugs
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ExpressPCB draft
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ExpressPCB_File
last test
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last test
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last test
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So for the moment, All I have is this ugly hacksaw-job mechanical test dummy thing, and a pending order for allot of brand new Polly-film caps from an electronics supplier.

soldering on caps
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test example
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UPDATE:
OSWATT Wire harness...
Well the plugs are looking cool, hopefully by next week, I'll have that harness in OSWATT, and more in the works for the Proxies, and LASI&SPICE-drones errr SETI-Cluster.


BTW, Protecting the conductors?
I think plugging these things into a dead drive while I'm hot-gluing the exposed conductors to protect them, might ensure that the plug pins somewhat lineup when the plug is used. Hot glue has some give to it, not much.

Conclusion.
T.B.D. The O-scope I had access to, gave up the ghost before I had a chance to use it... Ahgggggg!
This mod also might help the RFI emissions of acrylic PC cases as well, just a thought.


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