A Simple Boost Circuit Guide

Introduction

Boost converters aren’t just theory—they’re practical, powerful, and fascinating. In this article, we’ll explore a real-world boost circuit originally designed to power a toy solenoid, and show how it can convert just 3V from a battery into an impressive 85V output using flyback voltage and a toroidal transformer.

We’ll rebuild the circuit on a breadboard, measure key signals with an oscilloscope, and explain how energy transfer, flyback effect, and transformer behavior lead to such a dramatic voltage increase.

Overview of the Boost Circuit

This circuit is based on an oscillating boost converter configuration (source: Easybom). It utilizes a triode (transistor) and a toroidal transformer with three sets of coils. The transformer’s feedback and flyback characteristics enable voltage multiplication far beyond the input.

Input Voltage: 3V DC (from batteries)
Output Voltage: Up to 85V after a short delay
Main Components:

• Triode/transistor
• Toroidal transformer (with three windings)
• Electrolytic capacitor (1000 µF)
• Diode for rectification
• Feedback coil

Measurement Results and Oscilloscope Observations

The toroidal transformer has three sets of coils, per the oscillating circuit schematic diagram. The output coil’s number of turns divided by the collecting coil’s number of turns is 18:5. About 10V should be the output voltage if this ratio is adhered to. The output coil’s voltage waveform as seen through an oscilloscope, is shown below.

In the circuit, measure the voltage waveform at the output coil port. The positive voltage is roughly 10V, the negative pulse is over 100V, and the oscillation frequency is 138 kHz. This voltage is about appropriate for the oscillator circuit’s transformer ratio. The electrolytic capacitor has a charge voltage of approximately 11V.

Based on the voltage waveform that was observed. On the same end of the output coil, replace the nameplate. Utilize the flyback voltage produced when the triode is turned off to rectify. This results in a comparatively elevated voltage. This is the output coil’s voltage waveform following tuning. It’s evident that the output voltage is rising continuously. As a result, this circuit may produce large voltages independent of the transformer’s ratio. Rather, it makes use of the energy stored in the toroidal transformer as well as the flyback voltage produced when the triode fails. These days, the magnetic toroidal transformer functions as a tool for storing electrical energy.

When the triode is working, energy is stored in the power supply for the toroid. When the triode cuts off, the stored energy in the toroid charges the electrolytic capacitor. Eventually, a high voltage develops.

The voltage across the capacitor reaches 85 V after around two minutes. The regulator diode, which is connected in parallel to the capacitor, finally clamps this voltage. As can be observed, the boost circuit in question is not dependent on the coil passing 3V through the variable ratio in order to accomplish the boost. Instead, when the triode cuts off, it uses the energy that has been stored in the coil to produce a flyback high voltage.

What therefore ought to be this circuit’s maximum boost voltage? The oscilloscope was upgraded to include the triode base voltage waveform. There is a reverse breakdown at the base, which is roughly -12 V, during the triode cutoff phase. Through the base feedback coil, the inductor’s energy can also be released following the breakdown. The output’s maximum voltage is established by this technique.

Below are the voltage waveforms at the oscillating triode’s base and collector. The polarity should be reversed because they are connected to different co-sign terminals on the transformer. The base voltage is represented by the cyan signal waveform, and the collector voltage is shown by the yellow signal waveform. The reason why the collector voltage is more complex than the base voltage hasn’t been fully understood for a while.

Conclusion

This experiment clearly shows how a basic oscillating boost converter using a triode and a toroidal transformer can amplify a small 3V input into a powerful 85V output. The secret lies not just in coil windings but in the flyback voltage created when the switch (triode) cuts off, combined with feedback-driven oscillation.

Such circuits are commonly used in:

• Ignition coils
• Piezo drivers
• Electric lighters
• High-voltage experimental devices

FAQ