Simple DC-DC Converter
Have you ever wanted to build a circuit to run off a single 9 Volt battery only to find you needed levels like +12 and -12 Volts? The thought of multiple batteries might have put you off. Why not use a simple DC to DC converter?Just to prove a point, here’s a DC to DC circuit put together just from parts found on the test-bench. All it took was two transistors, two resistors, an audio transformer, a bridge and two caps. It may not be the ultimate in performance, but it does work.
The key to how it works is the transformer. As you can see, two transistors drive the transformer primary with the base drive for each coming from the collector of the other. When power is applied, suppose Q1 turns on a few nanoseconds faster than Q2.
As Q1 turns on two things happen: The Q1 collector voltage drops shutting off Q2, and Q2 collector voltage rises turning Q1 on more. Q1 collector voltage drops due to the inductive reactance of the primary coil.
As current flows through the transformer primary, a voltage is induced in the secondary by the expanding magnetic field in the transformer core. But at some point the magnetic field stops expanding, because either the transistor reached the maximum collector current it could pass or because the transformer core reached the maximum magnetic field it could hold. Either way, the inductive reactance of the primary drops causing the voltage on the collector of Q1 to rise.
Since the collector of Q1 drives the base of Q2, Q2 turns on which in turn shuts off Q1. Now current is flowing the opposite way through the primary causing the magnetic field in the core to reverse itself, which induces an opposite voltage in the secondary which continues until the field stops expanding and the process switches again. Basically, the circuit is a square-wave oscillator.
This circuit is perfect for enabling students to become familiar with the basics of DC to DC conversion. Use the parts listed, or use what you have. Use a scope to look at the voltage wave-shapes on the primary and secondary. Measure DC output with and without load.
See if you can improve it. Have some fun with it! And check our web site at www.elexp.com for more information on such circuits. NOTE: Use an Ohm-meter to measure both sides of the transformer. The side with the lower resistance, should be used as the primary. Both sides need a center tap.
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