How Much Current Do I Need?

By: Haut^Karl

Sunday, October 13, 2002


You have an old BX motherboard and power supply that could do with a Tualatin upgrade but can they handle the power requirements of such a new processor? Well, we have to take a look at the two components seperately: the power supply(PS) and the motherboard(mobo). While you may have a beefy 400 watt PS, the step down power regulators on your mobo may not be able to handle the continuous AMPs the Tualatin processors require. "What you say?"

Okay. Your power supply basically provides 12, 5, and 3.3 volts yet your nifty Tualatin processor needs between 1.45 and 1.5 volts. Thats where step down voltage regulators come in. These babies reduce the 5 or 3.3v down to the necessary voltage set by your VID pins or by you in the BIOS. In the CeleronI and PentiumII days, you were using 2.00v and low AMPs to run your processor. Today, the Tualatin processors with the low voltages require higher AMPs. This can cause you serious troubles if your power regulators are not rated for the higher AMPs. The power regulators will simply melt off your motherboard!

In the above picture we have a motherboard that was mounted vertically as in most computer cases. The step down voltage regulators in the 'blue' box simply overheated and gravity did the rest. At the same time the capacitors in 'yellow' began to leak and/or become domed. I tried to repair another mobo with the same problem but too many components were destroyed. This one is now used for parts!

Also, your power supply may not be rated for the AMPs consumed by your new processor and peripherals. A good rule of thumb is: constant working load should not exceed 70% of the power supply's rated AMPs for that voltage. Of course your particular PS manufacturer will have the specific details for your PS. Take for instance my P.O.S. power supply below.

Depending on the design of your motherboard, 5 or 3.3v may feed your power regulators. I like to convert to watts(5v*16A=80watts) and multiply by 70% to get my constant working load(56 watts). These same 5 and 3v sources also power other parts of your mobo and peripherals so don't think all 56 watts are available for the processor. I believe the 3.3v mostly powers the AGP, PCI slots and RAM but I'm not 100% sure. So now what?

How much current will your Tualatin processor require? A little scouring of Intel pdfs revealed enough information to get a good idea of what you will need.



Max Speed

Max Total AMPs

Max Total Watts

PentiumII w/512k external cache


450 MHz



CeleronI w/128k on-die cache


533 MHz



Pentium3 EB w/256k on-die cache


1.1 GHz



CeleronII w/128k on-die cache


1.1 GHz



Pentium3 Tualatin w/256k on-die cache


1.33 GHz



Pentium3 Tualatin w/512k on-die cache


1.4 GHz



Celeron3 Tualatin w/256k on-die cache


1.4 GHz



So what processor was your old mobo designed to take? My old ABIT ZM-6 was designed for the CeleronI processor so it will probably do okay with a Celeron3 1.4 GHz but I wouldn't want to overclock it since that would mean greater voltages and AMPs. Since my P.O.S. 230w power supply worked for a CeleronI 500 MHz and a Pentium3 933 MHz, it will work fine for the 1.2 GHz Tualatin Celeron3 we bought.

For those of you who overclock, you may want a power supply >250w and a motherboard that was designed to take Pentium3 processors. If you choose to ignore this, you may suffer stability problems. Also, keep an eye on those power regulators. They are going to get very, very hot!

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