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Hardware Forum / Motherboards / ASUS / May 2007what is the purpose of -ve voltage in PC?
Hi, I want to know what is the purpose of -ve voltages in PC ? for example -5v, - 3.3v, -12v. It knows to a common man that +ve voltages flows current and i would like to know the purpose of -ve voltages. Also can you get me some links for easy understanding for power consumption of a PC and SERVERS which can be in easily understandable manner?. In general we identifying power consumption by adding all the power consumption of peripherals like keyboard, motherboard, PCI devices etc., But May i know what power consumes generally PIII, PIV PCs consumes per hour ? Thanks in advance chandra  Signaturechandra Do a search for power supply calculators, and you will find the information there. There are a few around. With CPUs and VGA cards they will usually state the maximum power consumption. Adding up all of your peripherals will result in an over estimation, as generally that would mean all of your peripherals are running at full load all of the time, which isn't the case normally. > Hi, > I want to know what is the purpose of -ve voltages in PC ? for [quoted text clipped - 13 lines] > Thanks in advance > chandra > Hi, > I want to know what is the purpose of -ve voltages in PC ? for example -5v, - [quoted text clipped - 9 lines] > Thanks in advance > chandra In my computer, the -12V is used by the RS-232 interface chip for the serial port. They stopped using -5V years ago, but there are occasionally motherboards that still use it. -5V has been removed from pin 18, on the last couple of ATX power supply specifications. There are several sites which offer to compute the power consumption of a computer, based on the components used. But I haven't seen one of those web sites yet, that does a good job. I used to like the Takaman.jp site (because of its spreadsheet output format), but the site is closed. The other sites tend to overestimate the power. The Takaman site was archived here, but I'm not sure this is working correctly right now. Anyway, this seems to work in Firefox OK. You can select the line items, and get some idea of their contribution. http://web.archive.org/web/20040411032947/http://www.takaman.jp/psu_calc.html?english Anandtech offers some total power numbers here. So the "average" 3GHz P4 computer is 122W idle and 195W busy. http://www.anandtech.com/showdoc.aspx?i=2275&p=13 Paul >> Hi, >> I want to know what is the purpose of -ve voltages in PC ? for example -5v, - [quoted text clipped - 25 lines] > > Paul Thank you all for your replies. Much easy to understand that a PIV pc consumes nearly 195W during busy load. However regarding -ve voltages, i mean to ask in the concept of electronics that what is negative voltage. Generally even a common man understands that positive voltages means, the electron flow will be there and the power comes. But what it mean the -ve voltage and what does it do ? Thanks in advance and awaiting to get back from you all. Regards Chandra Signaturechandra Negative voltages are just voltages where positive and negative are reversed so that the electricity flows in the "other" direction. Negative voltages are not used much any more, but one on a time were common (RS-232 serial ports still use them, but once they were used by other devices as well; for example, some old CPU chips and memory chips required them). >>> Hi, >>> I want to know what is the purpose of -ve voltages in PC ? for example -5v, - [quoted text clipped - 37 lines] > Regards > Chandra Dear Barry, Thanks for your reply. So, i understand the negative voltages are just the flow in opposite direction. Hope i am on right think. How can we mean to say only particular direction the electricity flow which we say +ve voltages ? If so, Serial ports are using -ve voltages, which mean the electricity flows back from serial port to main supply or motherboard ? Whats the use in that ? Please explain. THanks in advance chandra >Negative voltages are just voltages where positive and negative are >reversed so that the electricity flows in the "other" direction. [quoted text clipped - 9 lines] >> Regards >> Chandra Signaturechandra > Dear Barry, > [quoted text clipped - 14 lines] > THanks in advance > chandra The flow is always _circular_. It is pumped round by the voltage 'source'. As I already posted, it is the +ve voltages that have the flow 'backwards'!. The electrons here, flow out along the 0v (GND) wire, and back in on the +ve wire. With the -ve voltages, the electrons flow out along the -ve wire, and back in on the 0v wire. Best Wishes >>Negative voltages are just voltages where positive and negative are >>reversed so that the electricity flows in the "other" direction. [quoted text clipped - 10 lines] >>> Regards >>> Chandra >> Hi, >> I want to know what is the purpose of -ve voltages in PC ? for example -5v, - [quoted text clipped - 34 lines] > > Paul That last link is to info 2.5 years old. >>> Hi, >>> I want to know what is the purpose of -ve voltages in PC ? for example -5v, - [quoted text clipped - 35 lines] >> Paul > That last link is to info 2.5 years old. The original question was about a P4, so I tried to find a number for a P4 computer. I've just measured my own computer, but because I don't have a Kill-o-Watt type meter, I have to use other techniques to do the measurement, and cross check the results. This computer is a P4 Northwood, running at 3.2GHz. The OS is Win2K. I'm using the Halt option for the OS idle loop, which is why the processor power is so low at idle. (I.e. Running RMclock with Halt enabled, since my idle loop is not using Halt for some reason.) These are DC measurements on the main power cable, at idle. 3.3 @ 9.67A 31.9 W 5 @ 0.63A 3.15 12 @ 0.49A (fans?) 5.88 -12 @ 0.03A 0.36 +5VSB @ 0.40A 2.00 This is the processor measurement. The current jumps to about 4 amps at idle, if no Halt instruction is used. The reason I mention this, is I've been working on my idle power consumption recently, as the computer seemed to be a bit too warm at idle. 12 @ 1.17A CPU 14.04W This is for my hard drive and optical drive. 5 @ 0.42, 12 @ 0.24 HDD 4.98W 5 @ 0.08, 12 @ 0 ODD 0.40W The video card has a separate power connector (9800Pro). Part of the video card power flows through the main motherboard power connector, so this is not the total video card power, just the component drawn through the aux connector. 5 @ 3.32, 12 @ 0.17 video 18.64W The total power is 81.35W, as measured at the output side of the power supply. The power supply gets hot while it is running, and that is a measure of its "inefficiency". The label on a typical cheap supply, might say it is 70% efficient. To estimate the power at the AC wall plug, divide by the estimated efficiency. 81.35W / 0.70 = 116W. I did measure the A.C. current at the wall plug. The current and voltage at idle were 1.669A @ 120.5 VAC. The reason for the discrepancy, is the power factor. My power supply is not PFC corrected. The power factor will be about 0.7 or so. (The current waveform on the AC input, is out of phase with the voltage waveform. The real component of the power, is about 70% of the measured current.) So, if I take (120.5V * 1.669A) * 0.70 = 140.2W If I had uses an active PFC power supply, then my ammeter reading could be used directly without an unknown fudge factor. At least, if the load on the PSU was high enough. http://www.xbitlabs.com/articles/other/display/psu-methodology_8.html So I can measure the power consumed, after a fashion, but I'm not set up to do a power measurement without a fudge factor. The processor power thing is a bit strange. I could swear I've done this measurement before, did it in both Linux and Win2K, and in both OSes, the idle processor draws 1.1 amps from the 2x2 ATX12V power connector. Now, for some reason, the Win2K install is drawing 4 amps at idle, while Linux is still at 1.1 amps. By using RMclock, that program installs its own idle loop, complete with Halt instruction. Using RMclock the idle current on the processor cable is 1.1 amps again. My suspicion is that the antivirus software I was testing, may have buggered with it, but I don't know how to verify what is going on. In any case, with Halt working, room temp right now is 23C, case temp is 25C, and CPU is 28C. With Orthos running, the AC measured current jumps from 1.669A to about ~2.5A (some fluctuation). CPU temp is 41C. Paul > Hi, > I want to know what is the purpose of -ve voltages in PC ? for > example -5v, - > 3.3v, -12v. It knows to a common man that +ve voltages flows current and > i > would like to know the purpose of -ve voltages. Think of the 'flow', like a water circuit. Water can be pumped round the circuit in either direction, but there always has to be a complete 'circuit'. In fact the actual electron 'flow' in a circuit, is the opposite way to the 'sign', so it is the -ve connection, that 'sources' electrons, and the +ve ones, have the electrons flowing towards them!. With the positive connections, electrons are flowing 'out' from the 0v wire, through the circuit, and back into the +ve connection. With the negative connections, the electrons are flowing out of these connections, through the circuit, and back into the 0v pin. It is one of the oddities of history, that we have the flow 'backwards', from what would seem intuitive... Now, just as with the water analogy, there can be places where water needs pumping 'up' (cistern in the roof for example), and others where flow in the other direction is needed. Most modern stuff, doesn't really use the -ve rails now. Instead if -ve lines are needed, the circuits will contain their own 'charge pump', to provide these rails, drawing the required power fom the +ve rails, and moving this where needed (the circuit used, works just like a 'bucket brigade' of firemen). However in the past, this was much harder to do, and so -ve rails were provided. The basic uses were: The -5v rail, was required by the original memories on the PC. Modern memories generate their own internal rail. The -12v rail, was used by the RS232 signalling standard (to improve noise immunity, quite a large signalling voltage is used on this, -12v to +12v normally at the 'source'). This rail was also commonly used by audio circuits, which want their output voltages to have 'symmetrical' swings around the 0v rail. There isn't a -3.3v rail in the AT. > Also can you get me some links for easy understanding for power > consumption [quoted text clipped - 6 lines] > > Thanks in advance Varies massively. The processor itself, depending on speed, and what is actually being done, can draw anything from about 10W, to nearly 200W. Most current units are specified to dissipate about 90W max). The second biggest user, is the video card, with some of the modern high performance cards having 'peak' demands well over 100W. Systems using two such cards in SLI, or a similar manner, can have huge demands in this area. However quite competent basic displays can be done using well under 10W, and typically cards like this will be used in server applications. Disk drives, use between about 2W (some low power designs), up to nearly 30W (15000RPM server drive). CD drives generally use about 10W. Memory, about 5W/GB (depends on the speed as well). Keyboards/mice etc., will all use under 5W (remember this is the maximum a USB device can draw without external power, and hence provides a good 'guideline'). Best Wishes > In fact the actual electron 'flow' in a circuit, is the opposite way to > the 'sign', so it is the -ve connection, that 'sources' electrons, and the > +ve ones, have the electrons flowing towards them!. Good description Roger. I always showed my schematics having the flow from positive to negative. Few could call me out on this. And when they did, I would say "You can't move an electron without a hole existing first" Which would shut them all up. No one could disk me on that bs. Thanks for the memories. William >> In fact the actual electron 'flow' in a circuit, is the opposite way >> to the 'sign', so it is the -ve connection, that 'sources' electrons, [quoted text clipped - 10 lines] > > William Oh God, not holes!.... It it is obvious that all electronics is actually divine, since at heart, it is all holey... :) Best Wishes > Hi, > I want to know what is the purpose of -ve voltages in PC ? for [quoted text clipped - 13 lines] > Thanks in advance > chandra This is going to date me. Us electronic guys used to use large voltages to get things done. Lots of voltage, so things didn't get lost in the works. Tube stuff often used +- 400 volts, and transistor circuits used +-5v, and signal paths used +-12v all the time. You had to, to get the signal from one place to another. Those serial terminals might be 200 or more feet away from the computer room, and those lines might be run alongside of who knows what. Those other cables messing things up with their own noise. So the larger the signal, the better. Made it easier to detect the signal on the receiving end. Time marched on, and noise cancellation got better, signal detection got easier, and noise levels dropped. Their was a certain "cemetery" to +- voltages. That is, voltage above "ground" and voltage below "ground" Ground being the potential of the dirt you were standing on, or the pole sticking out of the ground you hooked your trusty multi-meter to to measure whatever you wanted too. You could just as easily used 0 to +24v for serial cables and call +12v ground, (without having reference to "earth" ground). As I said, time marched on, and the need for BIG signals was lost in small distance signal distribution. Electronics got smaller, used less current, (per component) and needed lower and lower voltages to operate. Electronics went from tubes to transistors to complementary metal oxide semiconductors, (CMOS). CMOS could run on anything from ~+-19 volts to ~+-3 volts when first introduced. (I made lots of stuff out of the CD4000 series, the good old days now.) Well, time marches on, and voltage is bad. It causes heat, and other bad things to happen. (By the way, thermal expansion and contraction is the number one cause of component failure in electronics!) So manufacturers go out of their way to reduce voltage requirements of their IC's, be they CPU, RAM, Controllers, Routers, or whatever. Usually you will see higher voltages used around I/0 (Input /Output) circuits, but that will change too. Lower voltages let us use smaller components for manufacturing too. So you don't see much call for negative voltages, though that is relative, and you don't see much need for any "relatively" large voltages in computers anymore either. Computer manufacturers could stop using +12 volts if they wanted to, but they would have to double the current ratings if they did, W=VxI (watts equals volts times current). So if you half the voltage, you double the current required to do the same job. I could go on. William |
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