Lecture 19: Building a Voltage Multiplier

What this lecture covers

The lecture explains how a charge pump can produce a higher voltage than its power supply without a transformer, using only capacitors, diodes, and a switching signal. It sits alongside the channel's 6502 breadboard computer series as background for later videos that need higher voltages (such as for RS-232 signaling) from a 5-volt supply.

By the end, you can explain why alternating a capacitor between charging and series positions raises voltage, why diodes are needed to stop the charge from flowing backward, and how stacking stages multiplies the effect further.

Key ideas

• Manual voltage stacking: physically charging a capacitor to the battery voltage, then reconnecting it in series with the battery, doubles the output voltage for as long as the capacitor holds its charge.
• A second holding capacitor: adding a capacitor across the output stores charge so the output voltage does not drop to zero while the first capacitor is being recharged.
• 555 timer as the switch: replacing a manual switch with a 555 timer's fast square wave lets the charge-and-transfer cycle happen many times per second, smoothing the output.
• Diodes prevent backflow: without a diode, a charged capacitor would immediately discharge back into the lower-voltage source; a diode only lets current flow the direction that charges the next capacitor.
• Dickson charge pump topology: alternating a clock signal and its inverted copy across a chain of diode-capacitor stages lets each stage add roughly one supply voltage to the output, multiplying voltage with each additional stage.
• High voltage, low energy: a charge pump built from small capacitors can reach tens of volts but stores very little energy, so touching it is not dangerous even though the voltage looks alarming.
• Practical use case: a charge pump can generate the higher voltages (for example, plus/minus 10-15V for RS-232) needed by a circuit that otherwise only has a 5V supply.

Walkthrough

Manually stacking a charged capacitor (0:00)

The lecture starts with a 9V battery and shows that physically moving a charged capacitor into series with the battery produces almost 18V, though the voltage drops as the capacitor discharges and drops to zero while it is disconnected to recharge.

Adding a holding capacitor and a mechanical switch (1:00)

A second capacitor across the output stores charge so the voltage no longer drops to zero during recharging. A hand-built switch lets one capacitor alternate between charging and series positions, and flipping it faster produces a smoother, roughly 18V output — this arrangement is named a charge pump.

Automating the switch with a 555 timer and diodes (3:01)

A 555 timer's square wave output replaces the mechanical switch, and the lecture works through the theory of using a diode instead of a mechanical connection: when the clock is high, a charged capacitor in series with the supply produces double voltage; when low, the capacitor recharges through the diode from the supply. Building this on the breadboard and observing it on the oscilloscope confirms the node alternates between roughly 9V and 18V.

Stabilizing the output with a second diode-capacitor stage (8:05)

A further diode and capacitor to ground smooth the fluctuating 9-18V signal into a relatively steady higher voltage, confirmed on the scope as roughly 15V (below the theoretical 18V due to real diode drops).

Adding stages with an inverted clock (10:06)

To push the voltage even higher, the lecture builds a MOSFET-based inverter to generate a second clock signal opposite the first, then adds another diode-capacitor stage driven by this inverted clock, reaching roughly 27V in theory. It shows that further stages simply alternate between the two clock phases.

A many-stage charge pump and a safety demonstration (13:07)

A larger breadboard circuit with many stacked stages produces around 110V from a 9V battery. The lecture touches the terminals to demonstrate that despite the high voltage, the tiny capacitors store very little energy, so no shock is felt, and closes by noting charge pumps are useful for signaling voltages (such as RS-232) rather than delivering power.

Before you watch

• Basic capacitor and diode behavior (charging, discharging, one-way current flow).
• Familiarity with the 555 timer as a square-wave source, covered in an earlier video on this channel.
• Comfort reading an oscilloscope trace with multiple channels.

Check your understanding

1. Why does a charge pump need a diode instead of just a switch to move charge between capacitors?
2. Why does the output voltage stay relatively steady with a holding capacitor even though the input clock is a square wave?
3. Why is an inverted clock signal needed to add additional stages to the charge pump?
4. Why can a charge pump reach a high voltage yet deliver a harmless shock when touched?