Electrical Noise Filters for ATX Power Supply +5V and +12V DC Rails - Page 1
Forum section: electronics / projects

author: onewatt
author_rank: Contributor
timestamp: January 16, 2012, 01:01:44 am
content: While checking my computer ATX power Supply +5Volt and +12Volt DC rails, I noticed a lot of high-frequency electrical noise on my oscilloscope trace. Because the +5Volt and +12Volt DC go to all computer components, the DC rails must be the chief culprit in spreading transient noise spikes throughout the computer that cause motherboard instability, video and audio artifacts, lockups, and reboots. Motherboard engineers put decoupling capacitors close to every IC chip voltage supply, and routinely place decoupling capacitors and ferrite inductive beads all over the motherboard. But computer ATX power supply engineers have ignored electrical noise spikes on the ATX power supply DC extension cables. This glaring omission is all too obvious when viewing the +5Volt and +12Volt DC rails on an oscilloscope. That bothered me, so I decided to build a noise filter to get rid of electrical noise that's coupled via the ATX extension cables.By experimenting with different filter components, I discovered that transient voltage spikes were primarily "differential mode" and responded quite readily to bypass and decoupling capacitors. At first, I experimented by soldering various capacitors directly to the DC extension cables. I then came up with the idea of mating my noise Filter to any 4-pin Molex connector, which made installing my filter easy; that way I could use several Easy Filters (ezFilter) and spread them around to all DC rail branches.Oscilloscope screen capture 1 shows that voltage transients have an amplitude that is typically between 50mVp - 300mVp and sometimes reaching above 500mVp, both positive and negative. I designed and built an easy to install DC electrical noise filter to decouple voltage transients on the computer DC voltage rails. Oscilloscope screen capture 2 clearly shows my success; my ezFilters easily slam dunks any voltage transient.I used PVC Heat Shrink Tubing to electrically insulate the ezFilter pc board from the computer chassis and any exposed wiring. I also liked the PVC's clear, transparent, and high glossy appearance vs the rather milky polyolefin heat shrink tubing.I went cheap with a 2 layer PCB from Express pcb, USA. I liked their easy to use free pc board layout software and simple price quotes. In the past I had always etched my own pc boards, but times have changed, so this was my first attempt at using pc board cad software. Because Express pcb software was the only one I could figure out how to use, my decision to go with them was simple.http://www.expresspcb.com/ExpressPCBHtm/Free_cad_software.htmPARTS LIST:C1 - Multilayer Ceramic 0.1uF, 50VC2 - Ceramic 1uF, 50VC3 - Tantalum 1uF, 50VC4 - Metallized Polyester 0.01uF, 100VC5 - Metallized Polyester 0.001uF, 100VR1 - Metal Film Resistor 300 Ohm, .25WR2 - Metal Film Resistor 1K Ohm, .25WD1 - LED RedD2 - LED YellowThe capacitor impedance to ground should be as low as possible at the noise frequencies that need to be rejected. At the capacitor's series resonate frequency, the reactance XC and parasitic inductive reactance XL are equal. By selecting various capacitor values and different types of dielectrics, the resonant frequency bandwidth is widened to catch and shunt the most noisy high-frequencies to ground.I opted for the convenience of simple Velcro backing to attach my ezFilter to any chassis location.

author: mariush
author_rank: Super Contributor
timestamp: January 16, 2012, 01:57:21 am
content: I fail to see how this would filter anything:Otherwise, PC components already have some decoupling and filtering capacitors on the pcb, just inspect the underside of hard drives and you'll find them - didn't you think maybe this wasn't needed, if there's no such thing on the market already?

author: metalphreak
author_rank: Frequent Contributor
timestamp: January 16, 2012, 03:04:01 am
content: My OCZ PSU came with capacitors connected directly before the main ATX connector and some of the PCI-E cables. Apart from making the cables impossible to bend for nice cable routing, and the opportunity to cram more marketing BS onto the box, I don't think they do muchIf the noise on your power rails is bad enough to cause issues, then the PSU isn't up to scratchI don't see how these can make anything *worse* though. Did you have a particular issue which was fixed with these?

author: McMonster
author_rank: Frequent Contributor
timestamp: January 16, 2012, 03:19:13 am
content: Some kind of test for how it really increases the reliability of ATX supply would be great for those. What really matter are large voltage spikes, so the test should include logging the number and size of large spikes under the same load and time with and without this filter.From my experience as university lab technician a cheap and poor ATX supply is a device from the depths of hell that causes multiple failures, almost all of them are dead motherboard capacitors. And since those are often rated for ridiculously small voltages, like 6.3 V cap on a 5 V rail, they are very susceptible to voltage spikes. That's why I think this project actually makes some sense, but it's nowhere near getting a good PSU.

author: amyk
author_rank: Super Contributor
timestamp: January 16, 2012, 11:35:29 am
content: It reminds me of this:http://www.newegg.com/Product/Product.aspx?Item=N82E16814999012

author: sean87
author_rank: Regular Contributor
timestamp: January 16, 2012, 11:57:02 am
content: As a noob EE Wanna Bee I also fail to see what goal you have accomplished, unless you really saw the difference after implementing your idea. Generally, As Dave sometimes says, if your PSU is one of one hung lu products your design comes to practical use, otherwise all top rated PSU makers already took care of the noise.Also I like to know you used the oscilloscope to check the noise in parallel with the load? or no load at all?Anyway good expirience

author: mariush
author_rank: Super Contributor
timestamp: January 16, 2012, 12:27:56 pm
content: What I meant (and I think people figured it out but it's not explicitly said) is that the device is connected AFTER the hard drive in that picture, so it basically does nothing.Anyway, most devices don't really use 5v or 12v except on motors, you'll almost always find in the worst case a linear voltage regulator taking everything down to 1.8v, 2.2v, 2.5v on the circuit boards - 12v is mostly used for motors and sometimes for the laser in optical drivers.So even if you connect this device before a hard drive, the 12v rail will probably have interference from the motor after the drive, so any following device may be affected.You want better quality power for your devices? Get a more quality power supply in the first place, place some ferrite coils before each connector and maybe a capacitor and you're done.

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 29, 2012, 07:00:06 pm
content: pictures link not available anymore (within 15 days only?) rendering this thread useless!this image or video has been moved or deleted - photobucket

author: vtl
author_rank: Regular Contributor
timestamp: January 30, 2012, 03:26:20 am
content: Quote from: Mechatrommer on January 29, 2012, 07:00:06 pmWell the original device was just a PCB with some filter caps on it so you're not missing much. If you're getting so much noise on your PSU you should just buy a good quality one rather than spending the effort of doing this.I fail to see how this would make any difference. Almost all the PC components run of the 12v rail via DC-DC converters with big caps and further decoupling caps placed directly across the IC pins.Probing across your 'ezfilter' to verify its success is dubious at best. What youd want to do is probe IC pins and see the noise before and after.