Do appliance power supplies always draw the same amount of current?

Question

I've done a little searching and I guess there are two basic types of power supplies:

1. Linear Power Supplies - I pretty much know the basics of how these work, and it seems to me that they would always draw the same amount of current, since, the power goes through a coil so it's always a closed circuit with the same resistance. Right?

2. Switched-Mode Power Supplies - I've got a fuzzier understanding of these. The power goes through other solid-state components before hitting the transformers, so I guess it's possible to adjust the current, but, do they?

For instance, a desktop PC power supply: When the processor steps to a higher or lower frequency and voltage, or when a disk drive turns off or on, does the power supply adjust the current on the AC side? What about when it goes into sleep mode?

And do those box power supplies, that printers and game systems use, still draw the same amount of current when turned off?

Wow, this turned into quite a few questions, lol.

Answer 1

There is a fundamental difference between a linear and a switch-mode power supply.  The switch-mode power supply is more efficient.  Here's why:

Let's say you want to step 48 Volts down to 12 Volts at 1 Amp.

The linear supply will draw that full 1 Amp at 48 Volts, and dissipate the wasted power (1 Amp at 36 Volts) as heat within the linear power supply.

The switch-mode power supply will chop up that 48 Volts into little bits and then transform it down with almost no losses.  That means that to give a 12V output at 1 Amp, it only needs to draw 1/4 Amp from the 48 V source (plus some minor losses in the supply itself).  It's kinda like a transformwr for DC voltages.

The power supply in your computer will draw ONLY the current it needs to power the system it needs to drive (plus some small internal requirements).  When your printer is printing, the power supply will draw what it needs to power the systems.  When it stops printing, the power supply draws almost no current.

In sleep mode, the power supply draws just a few milliamps - just enough to keep the essential monitoring circuits working.

Answer 2

For all types of power supplies, the input power is equal to the output power plus losses that are converted to heat in the power supply. The losses consist of some fixed losses and some losses that are proportional to the output power. The end result is that the input power goes up and down as the output power goes up and down, but the input power goes down to some fixed level when the output power goes down to zero.

That is a general description of the end results. If you want to know how the internal workings of the power supplies produce these end results, perhaps someone else will provide some of those details.