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ffej
Experienced Roboteer
Joined: 22 Jun 2004
Posts: 595
Location: Kurrajong, NSW
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The other side of the resistor bank needs to go to the ground rail. Essentially all your doing is creating a dummy load, as the PC PSU will not output the maximum it can on the 12V rail without some load on the 5V rail. ATX PC PSU's need an additional dummy load on the 3.3V rail for the same reason.
The more resistors you put across the 5V rail in paralell, the lower the total resistance, and the bigger the load placed on the rail. Some power supplies require more of a load than others to get the maximum output on the 12V rail, hence the "more (resistors) might be better" conclusion.
Also, the bigger the load, the more heat your going to be dumping through the resistors, so keep cooling in mind when placing them.
BTW, I dont mean to be rude, but if you dont completely understand what you are doing, should you really be attempting to mod a power supply that connects to the mains, and could turn lethal should something go wrong? _________________ Jeff Ferrara
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Mon Aug 02, 2004 10:10 pm |
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ffej
Experienced Roboteer
Joined: 22 Jun 2004
Posts: 595
Location: Kurrajong, NSW
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quote:
The resistors are like a electric magenett arent they? a wire that is coiled around that connects the - to the + they use power but dont short same sort of concept am i right?
Sort of, the resistors your using would most likely be ceramic, which is ohmic, meaning the resistance shouldnt change with diffenced in heat (it does, but in theory it shouldnt).
Resistors, funnily enough, resist current flow, but they are still conductors. The higher the resistance, the less the current can flow at a fixed voltage (ohms law). When you short a power supply, you give an "easy" path for the current to travel from positive to ground, meaning one of very little resistance. Copper is, to some extent, a resistor, and an electromagnet, theres so many copper windings, that the resistance ends up being quite high, reducing the current flow.
No idea if that answered your question . . . _________________ Jeff Ferrara
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Mon Aug 02, 2004 10:53 pm |
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Spockie-Tech
Site Admin
Joined: 31 May 2004
Posts: 3160
Location: Melbourne, Australia
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quote:
its not the volts that kill you its the amps so i still have to be carefull i know.
This is an oft-quoted saying of basic electrical theory that doesnt quite tell the whole story, and can lead people into a false sense of security..
Yes, it takes current, not voltage to stop your muscles (and your heart), but as long as humans have dry skin, our resistance is fairly high, so a low-voltage power supply is incapable of flowing many amps through you, regardless of whats is capable of supplying when hit with a low-resistance load. If it were otherwise, a 12v car-battery (capable of many hundreds of amps) would be lethal to the touch.
Likewise a Van-deGraaf (or other static) generator that can generate 10,000 volts or more is not dangerous, because despite its ability to spark-jump 10 centimeters of high-resistance air (so skin is no problem for it), it doesnt have the ability to supply any high amount of current (flow) to back up its high pressure (voltage).
But 240v is certainly enough to cause a decent current flow through skin resistance, even more so if your skin is only slightly moist, and back it up with the ability of the wall-power circuit to supply 10 amps or more (not to mention the momentary higher currents from the charged capacitors in a power supply) and you have a lethal combination of volts to cause the flow, and the amps to feed it.
In a linear (transfomer) power supply only the primary side of the main power transformer is operating at line (240v) voltage, and the filtering caps are on the low-voltage side of the transformer, so you just have to keep you fingers away from the 240v stuff.
However, in a switch-mode. a large part of the circuitry is floating at the mains voltage and has big caps charged to this voltage, so even hours after you have powered it down, it can still be storing a dangerous wallop.
An analogy for you. low voltage - high current power is like a bucket of water thrown at you. a big splash, but not with any real pressure pushing it. High Voltage - low current is like a potato/cap gun, it goes bang and moves fast, but doesnt have killing force behind it. Medium voltage & high power is like a bullet. the speed and force combined is deadly. Think water-jet cutting machines compared to your garden hose.
The electronics techies arent repeatedly warning you about them just because we're scare mongers you know. These things are far more dangerous inside than your electric toaster is, at least when you unplug the toaster, its safe. Not so with a PC power supply.
I strongly second what Jeff (nightshade) said. If you need parallel resistors explained to you more clearly, you do NOT yet have the electronics knowledge to go tinkering inside this sort of equipment, leave it to the professionals, they're a lot cheaper than a heart-defibrillation treatment in the hospital is. _________________ Great minds discuss ideas. Average minds discuss events. Small minds discuss people
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Mon Aug 02, 2004 10:53 pm |
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