Linear Regulators
A transistor, throttled by feedback, holds the output at exactly 5 V — and pays for that precision in heat.
Builds on: 6.2 Negative Feedback Amplifiers5.2 Rectifiers: AC → DC1.4 Power & Energy
Why raw supplies aren't enough
Your rectifier lesson (5.2) ended with DC that still ripples, and batteries sag from 9.5 V to 7 V across their life. Chips don’t tolerate that: logic wants 3.3 V or 5 V, exactly, whatever the input does. The missing block from the adapter recipe — transform, rectify, smooth, regulate — is this lesson.
The linear regulator: feedback all the way down
Inside a three-pin regulator like the classic 7805 sits everything from Unit 6: a voltage reference, an error amplifier, and a pass transistor. The amplifier compares a divided-down sample of the output with the reference and drives the transistor like a self-adjusting resistor — sagging output? open the tap. Overshooting? throttle it. It is your negative-feedback amplifier (6.2) holding a DC level instead of amplifying music.
And there is the catch. Feed a 7805 with 12 V at 1 A of load and it delivers 5 W while burning 7 W — a 42%-efficient space heater that happens to regulate. That is why linear regulators wear metal tabs and heatsinks, and why they shine only when the voltage drop is small or the current is modest. Two more habits: they need the input a couple of volts above the output (the dropout — below it, regulation quietly fails), and they answer noise with silence: no switching, no interference, just clean DC. Audio and radio circuits love them for exactly that.
Before using a linear regulator, do the ten-second check from Lesson 1.4: (V_in − V_out) × I. Under half a watt, fine bare; a few watts, heatsink; more — read the next lesson.
⚡ Lab — Precision, Paid in Heat
A 5 V linear regulator with a live power-flow bar.
- 12 V in, 1 A out: watch more power become heat than reaches the load.
- Lower Vin toward 7 V — efficiency climbs. Cross the dropout and regulation collapses.
- Find a combination where the regulator runs cool with no heatsink.