Replies

Most if not all “brick” power supplies and any power supply inside a standard desktop computer is a “switching” power supplies. They develop their output by rectifying and filtering a high-frequency (maybe 100 KHz) oscillator inside. One of the several advantages of these types of power supplies is that they do not need a heavy-with-iron power transformer. If you have some device that uses a “wall wart” type of external supply, maybe 2” x 2” x 2”, you may note that such a supply is fairly heavy. That’s because it has an iron transformer inside. Those are called “linear” supplies. Should you wish to develop 6.7 amps out of a linear supply, you’d probably be looking at a fairly heavy item. Switchers are more efficient; but they tend to fail a bit more and they almost inevitably place residual noise (from the internal oscillator) on the DC power they are designed to create. In most cases, that noise does not matter, or, stated differently, the thing the power supply is designed to power up is immune from such noise by design.

Without going into further minutiae, you have the right idea. Switchers almost universally require a load to work. Linears do not. They do their voodoo without any load placed across the output. If you go to youtube and search for “PC desktop bench supply” you find multiple videos where folks converted a PC power supply into an experimenter benchtop supply. Which is pretty cool, because presumably you can get an old PC power supply for free from a discarded computer....unless the supply itself is the cause of death. You might have to pay $100 for a benchtop experimenter power supply....but of course, you don’t have the meters, the pretty case, the carry handle, the convenient binding posts. But that’s “experimenter” tradition. My point is, that EVERY one of those projects requires placing a load resistor across the output wires of the adapted PC power supply. The load resistor fools the switching power supply into thinking that the device it is supposed to power is actually there when it isn’t.