EC-6.4 Diodes in Circuits: Rectifiers, Clippers, Clampers and the Zener

What this is and why it exists

This is the first circuit topic in the area where a device is not linear, and the method has to change because of it.

Everything you learned in circuit analysis assumed you could write one set of equations and solve them. You cannot do that here. A diode's behaviour depends on which way current is trying to go through it, and you do not know that until you have solved.

So the method becomes four steps. Decide which diodes conduct, replace each with a simple model, solve the linear circuit that remains, then check the assumption was consistent. That habit is worth more than any individual circuit in this topic.

The vocabulary

• Ideal diode model — a perfect switch: no drop when conducting, no current when not.
• Constant-drop model — a switch with a fixed forward voltage across it.
• Piecewise-linear model — a fixed drop with a series resistance.
• Rectifier — a circuit that turns alternating input into one-sided output.
• Bridge rectifier — four diodes arranged to use both halves of the input.
• Ripple — the residual variation left on a smoothed output.
• Clipper — a circuit that removes part of a waveform.
• Clamper — a circuit that shifts a waveform without changing its shape.
• Zener diode — one designed to be operated in reverse breakdown at a controlled voltage.

The mental model

Choose the crudest model that answers the question. The ideal switch is enough for most logic and for a first estimate. The constant-drop model, usually two thirds of a volt for silicon, is enough for almost all analysis. The piecewise-linear model with a series resistance is for when the drop's variation matters. Reaching for the exponential equation when a switch would do is the commonest way to make a short problem long.

Now the method, and it is always the same three steps.

Assume. Guess which diodes conduct and which do not.

Solve. Replace each conducting one with its model and each non-conducting one with an open circuit. What remains is a linear circuit you already know how to handle.

Check. Every diode you assumed conducting must come out with forward current through it. Every one you assumed off must come out with reverse voltage across it. If any fails, the guess was wrong — not the arithmetic — so change the guess and go again.

With that in hand, the circuits are short.

A half-wave rectifier is one diode. It passes one half of the input and blocks the other, giving an output that is one-sided but only present half the time.

A bridge rectifier is four diodes arranged so that both halves of the input reach the load with the same polarity. The output average roughly doubles. It costs two diode drops instead of one, which is negligible from a mains transformer and significant from a three-volt supply.

Smoothing is a capacitor across the output. It charges to the peak and then holds the voltage up between peaks by discharging into the load. The ripple left over follows from the load current, the capacitance and the time between peaks. That is enough to choose a value without any further theory. Bigger capacitor, less ripple, larger surge current at switch-on.

Clippers and clampers are often confused and are genuinely different. A clipper removes part of a waveform: once the input passes a threshold the diode conducts and holds the output there, so the shape changes. A clamper adds a constant offset. A capacitor charges to a fixed value and shifts the whole waveform, so the shape is unchanged and only its position moves.

Finally the Zener. Drive a junction far enough in reverse and it breaks down. But it does so at a well-controlled, repeatable voltage, and it holds that voltage while the current varies widely. That makes a simple voltage reference. Breakdown is not itself destructive; destruction comes from the power dissipated, so a Zener always needs something else limiting the current.

What you should now be able to explain or do

• Choose an appropriate diode model for the question in front of you.
• Apply assume, solve, check to a circuit with several diodes.
• Compare a half-wave and a bridge rectifier and say what the bridge costs.
• Size a smoothing capacitor from an acceptable ripple.
• Tell a clipper from a clamper, and use a Zener as a reference safely.

Check yourself