skill
Voltage regulation
Holding one voltage steady while the battery sags and the load changes.
first shows up at Level 5 · Control
A battery is not a steady voltage. A fresh 9 V reads 9.4 V, a tired one 7.5 V, and every milliamp you draw knocks a little more off through its internal resistance. Most parts don't care. Some do: a chip with a 3.3 V limit, a reference you measure against, an LED whose brightness must not wander. Regulation means making one point in the circuit sit at a chosen voltage no matter what the battery and the loads are doing.
The shunt regulator
The simplest regulator is a zener diode and a series resistor. The resistor connects the battery to the regulated node; the zener sits from that node to −, backwards, so it conducts only once the node reaches its breakdown voltage, 3.3 V for a 1N4728. The resistor turns the battery into a current:
I_series = (V_battery − V_zener) ÷ R_series — 9 V, 3.3 V, 220 Ω → about 26 mA
That current has two places to go: into the loads hanging on the node, or through the zener. The zener takes whatever the loads don't:
I_zener = I_series − I_loads
Add a load and the zener gives some up; remove one and the zener absorbs it. As long as the zener keeps at least a milliamp, the node stays within a few hundredths of a volt of 3.3 V. That is shunt regulation: the regulator sits beside the load and soaks up the leftover.
The cost
The series resistor burns (V_battery − V_zener) × I_series all the time, about 0.14 W here, whether the loads use the current or not. That is fine for tens of milliamps and hopeless for an amp, which is why real power supplies use a transistor as the series element and only burn what the load asks for. A 7805 or an LM1117 regulator chip is exactly that idea with a few more parts inside. The 3.3-Volt Rule builds the shunt version and lets you watch the zener share.