VIR or PVI

[FatAlan]

Where's Mr Burns when you need him?

 

[SparkyChick]

You have to separate power supplies and heating elements. They are very different beasts.

A heating element is typically a purely resistive load. Purely resistive loads have a fixed resistance and consequently the current flowing through them decreases as the voltage decreases, there is a direct linear correlation between the three elements (V=IR is a linear equation, i.e. it's a straight line). Consequently, the power dissipated will go down. So yes, a heating element of 19 ohms connected across 110v will only consume 5.78A meaning it will be using an instantaneous power of 645W. It will still be working, but it will only be dissipating 645W.

PC Power supplies on the other hand are complex beasties and will be a combination of capacitance, inductance and resistance, meaning there is no direct linear relationship between voltage and current (which is where the power factor comes in).

Typically a power supply will be rated based on the power it can provide to the system it is connected to. So in the case of a PC power supply, a 500W unit will be able to deliver 500W in total across it's various output voltages to the PC it is powering. Looking at the figures above you will see that the power consumed at 240v is 720W (assuming a full load of 500W), so you have a loss of 220W due to the conversion. At 100v supply, you're consuming 6A which is an instantaneous power of 600W, so the loss is only 100W. These losses are largely a result of resistance in the switching element junctions (and other inefficiencies) and correlate directly to the input voltage because for a given junction, the higher the voltage, the higher the current flowing through it and consequently the power lost = current flowing through junction x voltage drop across it... P = I x V (or at least that's my understanding of them).

In short, for a non-linear power supply (i.e. one which does not rely purely on a transformer for the conversion) the instantaneous input power will vary based on input voltage and load because the output voltages will be fixed as will the maximum available output current (i.e. the total maximum output power).

Power is an instantaneous value based on other design factors, so for your heating elements, the power they consume will be varying almost constantly based on the slight variations in supply voltage (because their resistance is fixed - as voltage goes up, so too will current and as P=IxV, so too will the power they consume), and for your PC power supply, the power they consume will be based on input voltage and the load placed on it by the PC (because they are designed to provide their output voltages at a specified maximum current - i.e. their total rated output power - as load on the output goes up, power consumed will go up, as input voltage goes down, power consumed [ignoring losses] will stay the same but the current drawn will go up because it's a constant load so I=P/V).

Hope that helps and doesn't add confusion

 

[telectrix]

it's confused me. more than 2 lines on a post is just too much for the alcohol to dissemminate through my system. need another pint to re=read.