Alright, I will provide this information in a very simplified and general form. I cannot be specific to any single video card since I don't know which video card the person owns for which they are doing this research. My illustrations will therefore focus only on the primary items.
The primary items for this topic are the power input, the MOSFET(s), the voltage regulation chip, the filtering device, the GPU core, the variable resistor used to perform this modification, and the ground. The whole circuit which supplies the voltage to the core is controlled by pulse width modulated voltage ("PWM"). The input to the card is +12v while the core is usually designed to only function on 1.0v to 1.2v. Therefore, it is necessary to reduce the 12v input to a lower voltage.
Here is a simple video card voltage regulation circuit:
Pulse width modulation works on the theory of duty cycle. Consider that you have 12v coming to a switch and you flip the switch with your finger at an extremely rapid rate on and off. Let's say that you flip it on in off at a rhythm which makes the switch on for the same amount of time that it is off. This rate would equate to a duty cycle of 50% since the switch is on for 50% of the time and off for 50% of the time. Now, lets say that the output of the switch goes to a filtering device which smooths out the spikes of on and off into a solid source of power. How much power would come out of the filter? 6v. By adjusting how much time you leave the switch on versus how much time you leave the switch off, you can change the duty cycle of your circuit and thus change the output of the filtered voltage. If you had the switch on for only 25% of the time (25% duty cycle), the voltage output from the filter would be 3v (12 / 4).
12v power comes into the card and goes to both the MOSFETs and the Voltage regulation chip. The voltage regulation chip is the one responsible for controlling the MOSFETs. It provides a pulse width modulated input to the MOSFETs. The MOSFETs are simply heavy duty switches which can handle quite a bit of power going through them without burning up; they don't control the circuit for the most part, they only do what they are told to do by the voltage regulator chip.
The output from the MOSFETs goes to filters which smooth out the on and off spikes into a steady voltage which is provided to the GPU core. But, some of the output from the filters is fed back to the voltage regulator chip. In this way, the circuit is a closed loop circuit. The voltage regulator chip tells the MOSFETs to do something, then it monitors the output and adjusts its commands as necessary until the output is what the voltage regulator chip is looking for.
So how do we increase the core voltage? Well, we have to make the voltage regulator chip tell the MOSFETs to stay "on" longer (increase duty cycle). How do we do that? We trick the voltage regulator chip into thinking that the filtered output voltage is lower than it actually is. Almost always there is some form of resistance between the filtered output and the voltage regulator feedback input and we use this to our advantage.
Here is a simple video card volt mod:
What we do is attach a variable resistor between the voltage regulator feedback circuit and ground after the resistance between it and the filter. By doing this, we cause the voltage in that portion of the circuit to drop. The voltage regulator senses this drop in voltage and reacts by increasing the duty cycle to the MOSFETs and thus increases the final voltage to the GPU core.
So, as you see, a video card volt mod is actually very simple. All it requires is a steady hand, the right components, and research.
So, how do you go about doing your own volt mod? The first thing you should do is a Google search to see if anyone else has already performed a successful mod to the same video card. If someone else has already done the legwork, you should just copy them. If you can't find any reliable guides, you will have to do the research yourself. To start with, you will have to know which chip on your card is the voltage regulator chip which controls the core voltage. The card will have at least two (one for the core and one for the memory) and you want to do the mod to the correct one. The chip controlling the core voltage should be the one with larger inductors and capacitors around it. Once you are confident you have the correct chip, just do a Google search for the model number written on the chip. You want to find out who manufactured the chip and then find the manufacturer's website. At the manufacturer's website, they will usually have the chip listed in a catalog and next to the listing they will often time have a detailed specifications PDF document available. Open that document and find out which pin of the chip is the feedback pin.
Next, you will want to measure the resistance between that pin and the filter output. The filter output is the same as the core voltage measurement point you will use later. Usually the easiest place to measure core voltage at is the output leg of the inductor (or any one of the inductors if multiple inductors are used in parallel). So, place one terminal of a multimeter on the inductor output and the other terminal of the multimeter on the feedback pin of the voltage regulator chip. This measurement will tell you how much resistance is between those two items. The more resistance between those items, the greater value of trimming potentiometer you will need and vice versa.
For instance, lets say that the resistance between the feedback pin of the voltage regulating pin and the inductor output is 1,000 ohms. In this case, an appropriate trimming potentiometer for the job would be rated at 10,000 ohms. On the other hand, if the resistace between the feedback pin and the inductor output was only 100 ohms, you may consider getting a 1,000 to 5,000 ohm trimming potentiometer. It is generally safe to buy a potentiometer rated for 10x more resistance than the measured resistance between inductor output and feedback pin. You don't want to use a trimming potentiometer which is 20x to 50x higher in resitance since it would be way too sensitive (each turn of the potentiometer would equate to a large amount of resistance change) nor do you want to buy a trimming potentiometer which is only 2x to 5x higher in resistance since the potentiometer may not adjust high enough to keep the core voltage from exceeding a safe voltage. YOu also want to make sure that the trimming potentiometer has 10 to 20 turns on it. This means that from the lowest resistance setting to the highest resistance setting, the potentiometer has to be turned this number of turns. For instance, lets say you buy a 10,000 ohm (10k ohm) 20 turn trimming potentiometer. The potentiometer probably goes down to around 100 ohm at the lowest setting and we know it goes up to 10,000 ohms at its highest setting. 10,000 ohms minus 100 ohms is 9,900 ohms. SO, if the trimming potentiometer was a 20 turn type, this means that for each full turn, the resistance would change by 495 ohms. We don't want the resistance to change too much for each turn as this makes the adjustment too sensitive.
So, you buy a trimming potentiometer which is rated to go up to 10x higher than the measured resistance between the inductor output and feedback pin and you make sure that the trimming potentiometer is designed to have many turns between minimum and maximum resistance. You then solder the potentiometer between the feedback pin and ground. By doing this, you can dump some of the voltage going to the feedback pin to ground and thus cause an artificial lack of voltage to be sensed by the voltage regulator chip.
Now, just to confuse you a little more, a potentiometer has three legs, but we only want to use two. SO which ones do you use? I solder the wire from the feedback pin to the middle leg of the trimming potentiometer. The other wire which goes to ground needs to be soldered to one of the outer two legs. The diagram below shows my preference.
Then, you want to adjust the potentiometer so that the resistanc between the two wires is maximum. If you use the legs I used in the diagram above, you would want to turn the adjustment screw fully counter-clockwise. Then, turn on your computer, measure the core voltage, and slowly turn the potentiometer clockwise until the core voltage increases to the amount desired. As I said, you will measure the core voltage at the inductor ouput. Place one terminal of your multimeter on the inductor ouput and the other terminal on a ground (your computer case is usually a good ground) and then measure the DC voltage present.
Well, that's that. Was that too simple? Probably. You probably still have no idea what an inductor looks like for instance. Well, tell me which card you are considering modifying, and post some high quality pictures of the back and front of the card with the heatsink removed. I will then have to figure out where your voltage regulator chip is and have you read the number off of it. From there, I can do some research and see what I can draw up for you specific to your card which will point to solder points and measurement points.
Honestly, it would be way easier for me to do the mod than for me to tell you how to do the mod. So that is always an option. You could send the card to me and have me do the mod.
Here is an example:
post edited by ty_ger07 - 2010/10/09 20:47:37