Zener regulator

// A Zener regulator, and the load line that explains it.
//
// There is no microcontroller here. Press Run and the simulator solves the
// circuit.
//
// WHAT IT IS. A 12 V supply, 220 ohms in series, and a 1N4733A Zener across
// the output with its band to the positive side. The Zener does nothing at all
// until the voltage across it reaches about 5.1 V, and then it conducts as hard
// as it has to in order to keep it there.
//
// THE LOAD LINE, which is the whole lesson. The resistor can only deliver
// what is left of the supply:
//
//     I = (12 - Vout) / 220
//
// That is a straight line on a graph of current against output voltage: 54.5 mA
// if the output were shorted to ground, nothing at all if the output were the
// full 12 V. The Zener's own curve is the other line: nearly vertical at 5.1 V.
// The circuit sits where the two cross. Here that is 31.9 mA at 4.98 V with
// nothing on the output.
//
// NOW THE LOAD. The 1 k across the output takes about 4.9 mA for itself, so
// the Zener gets 4.9 mA less than it would with nothing on it. The resistor's
// line has not moved, so the crossing slides down the Zener's curve, and
// because that curve is not perfectly vertical, the output falls. It falls by
// exactly the datasheet's dynamic resistance times the current that moved:
//
//     7 ohms x 4.9 mA = 34 mV
//
// With the load on, 4.94 V; take it off (set the load to 1000000 in the
// properties panel) and it is 4.98. That 7 ohms is "Zzt" on a 1N4733A
// datasheet, quoted at the 49 mA test current, and it is the number that says
// how good a regulator this is. It is why nobody builds a supply this way for
// anything that draws real current, and why it is perfect for a reference a
// few milliamps wide.
//
// THINGS TO TRY. Turn the supply down to 6 V and watch the Zener give up: below
// about 5.3 V there is no current left to spare and the output is just the
// divider of 220 ohms over 1 k. Swap the Zener for a 1N4740A (10 V) in the properties panel and the
// output follows. Or drop the series resistor to 47 ohms and watch the Zener
// take 145 mA, three quarters of a watt, which on a real 1 W part is where it
// starts to smell.