# Weak Power-Supply

The current computer ‘Phoenix‘ (I own several computers) has suffered from a string of malfunctions in the past, which I had trouble diagnosing the cause of.

I think I’ve found the problem: Its power-supply is weak in some way, which can also lead to low-voltage conditions that it subjects the high-speed electronics to. When high-speed logic circuits are fed low supply-voltages, the computer can spontaneously crash.

The reader may wonder how I know this.

When I’ve left the computer idling and come back to it, thus entering my password to dismiss the screen-saver, the case-fan speed seems to be stable around 3,500 RPM. But as soon as I fire up my Web-browser, the CPU usage goes from low-usage to nearly-100% usage for less than a minute, and as soon as that happens, the case-fan speed becomes unstable, sometimes resulting in a reading of ~40 RPM, which means that ‘The fan has stopped spinning.’ Then, as soon as I allow CPU usage to go below 5% again, the case-fan speed sometimes stabilizes again, within the same sitting.

Well there is no valid logic by which the motherboard would signal for the fan to stop spinning, or to slow down, at the moment the CPU usage is high. And so the only other explanation I can think of, is that the CPU – and possibly other circuits in the box – are drawing more current, and that this is causing a temporary dip in supply-voltage, just enough for the recently-installed fan to stop spinning.

But then, such a weakness also makes this computer more susceptible to such phenomena as brown-outs. Even though my eyes can see power-fluctuations that take place within a fraction of a second, I cannot see a low-voltage condition in the A/C power we are fed, if that low-voltage condition has set in over a period of minutes.

I might start looking for a new power-supply for this old box, rather than a new case-fan.

I might also add, that the maximum number of milliseconds for which the computer can run entirely off its supply-capacitor, that is, with the A/C input at zero, is adversely affected by how nearly-overdrawn the power-supply is. A smaller power-supply will also tend to have a smaller supply-capacitor, such that if the current being drawn from it by the computer corresponds to 90% of capacity, then this can only continue for a disappointingly-short number of milliseconds, such as maybe 100ms .

If the power-supply has a higher capacity, such that the same computer is only drawing 50% of what the supply can deliver, that supply will also have a larger capacitor, so that if there is a dropout in the A/C voltage, the same computer could keep running entirely from the supply-capacitor for a longer number of milliseconds, maybe approaching about 500ms .

Dirk

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