- 1). Put a coil in the circuit that comes from the bridge. The current coming from the bridge is actually DC, but it is varying wildly from zero to maximum value 120 times a second. The coil allows current through, but it creates a magnetic field that opposes the current. The larger the current, the larger the opposition. The result of this is that a coil tends to flatten the peaks in fluctuating current. As the current decreases, the magnetic field collapses, inducing an electric current into the wire. This tends to fill in the valleys, so the current does not fall all the way to zero. If you put a wildly fluctuating current into a coil, the output has flatter peaks, meaning the maximum voltage is less, and shallower valleys, meaning the minimum voltage is greater.
- 2). Place a capacitor across the output from the bridge. Position the capacitor just before the coil. When the coil starts opposing the increase in current, some of the electrons back up on a plate of the capacitor. The stronger the opposition, the more electrons back up on the plate. Some of the maximum energy is stored in the magnetic field created by the coil, and some of the energy is stored as a charge on one plate of the capacitor. The capacitor and coil working together have a greater smoothing effect than just the coil alone.
- 3). Modify the basic coil-capacitor filter to fit the design specification of the project you are building. If money is the prime consideration, you can replace the coil with a resistor. This means you lose the magnetic-based filtering of the coil, so the output will not be as smooth as if you used a more expensive coil. The resistor also opposes the current flow, so the total output of the power supply is reduced. If expense is less important than the smoothness of the output, you can use more than one coil-capacitor stage. With each stage, the output becomes smoother.
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