How does a benchtop power supply measure and control current?

Question

This may be a simple question, but it is something that I can't seem to wrap my head around.

I have a +12v 5A power supply at work which has two knobs, one for voltage and one for current.

There are various devices that upon plugging them in I have to increase the current otherwise they will not work, but when I do this, the voltage level stays the same.

My question is, what exactly is happening when I turn the amperage knob on a benchtop power supply? My guess would be that we are changing the resistance being supplied to the device but I am unsure and would love a real explanation!

Thanks!

Answer 1

Here is one current-limiting circuit

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

You most likely have a switching power supply that measures the current through a shunt resistor before the output voltage is measured - because the two are in series the power supply knows the output current is the same as the shunt current

The logic then follows something like if the voltage is less than the knob set, and the current is less than the knob set, then turn on the switch, and if it's higher on either, turn off the switch - then after the switch there is a smoothing capacitor or similar to reduce the ripple to where you won't notice it

Answer 3

For most uses of a bench power supply you're interested in providing a steady voltage. That's what the voltage setting is for. The current setting is a current limiter -- if your load draws more current than you've set the limiter for, it lowers the voltage until the load draws only the amount that you set the limiter for.

The idea is to keep your electronics from frying if you've made a mistake.