Auto-switching voltages

Factual Questions
1

My laptop can detect the difference between 110V and the 220V that th rest of the world uses. Heck, my electric razor knows too.

How come desktops need that little switch thingy at the back to be flipped?

BTW guess who plugged a 110V PC into a 220V supply?

2

They use different types of voltage regulators. Laptop power supplies usually use switching regulators, which is a device that turns the power on and off thousands of times a second. When you pass this through a filter circuit you get a fairly stable power. The output voltage depends on what fraction of time the switch is on (“duty cycle”). This type of voltage regulator is expensive but it can vary the output voltage over a wide range. Or put it another way, it can produce a predefined output voltage from a wide range of input voltages.

Desktop computers usually use cheaper regulators (e.g. linear regulators, which acts just like a variable resistor). They don’t use switching regulators because not many people travel with desktop computers.

At least, that’s what I’d guess from my limited knowledge of electronics.

3

While I agree with your first paragraph, I respectfully disagree with the second.

The power supply in a typical desktop computer must be light, efficient, small, powerful (around 250 watts), and inexpensive. A linear supply cannot meet all of these requirements. But a switched-mode supply can.

Every computer power supply I have ever seen (in a desktop or portable computer) has been a “switched-mode” type. (I think they used to put linear supplies in some desktop computers back in the 80’s, but I believe all manufacturers have used switched-mode since then.)

As far as input voltage goes, there are basically two varieties of switched-mode supplies:

115V / 230 V configured via a selector switch
90V to 260V (approx.) “Wide Range” input

Up until recently, almost all switched-mode power supplies were the former. Many of these units are actually 230V supplies, and use an input circuit that acts as a voltage doubler when set on 115V. The newer, “Wide Range” power supplies do not incorporate a switch; instead, they are designed to use more of the pulse width modulator’s dynamic range. They also size everything in the inputs for the extremes.

So why aren’t “Wide Range” switched-mode supplies used in desktop computers?

  1. There’s not much demand for the “Wide Range” input feature on desktop computers.
  2. It is difficult (and more costly) to incorporate the “Wide Range” input feature into switched-mode power supplies above 150 watts.
    While I’m at it: Sometimes you’ll see a linear power supply with a voltage selector switch. These units usually have a transformer with two, independent primary coils. Setting the selector switch to 115V connects the coils in parallel; setting the switch to 230V connects them in series.