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Electronics Projects: Block DC while Passing AC

By Doug Lowe

One of the important features of capacitors in electronics circuits is their ability to block direct current while allowing alternating current to pass.

This works because of the way a capacitor allows current to flow while the plates are charging or discharging, but halts current in its tracks once the capacitor is fully charged. Thus, as long as the source voltage is constantly changing, the capacitor allows current to flow. When the source voltage is steady, the capacitor blocks current from flowing.

In Project 3-2, you build a simple circuit that demonstrates this principle in action. The circuit places a resistor and a light-emitting diode in series with a capacitor. Then, it uses a knife switch so that you can switch the voltage source for the current between a 9 V battery (direct current) and a 9 VAC power adapter (alternating current).


The trick to successfully building this circuit is finding a 9 VAC power adapter. If you go looking for one at your local RadioShack or at a department store, you might find one, but it will probably cost $20 or more. If you search the Internet, you can probably find one for more like $10 or $15.

You can purchase at a thrift store for even less. Just pay attention to the markings on the back of the power adapter:

    INPUT: 120VAC 60Hz 15W

    OUTPUT: 9VAC 1000mA

These are the specifications you want to look for when you search for an AC adapter to use for this project. In particular, the output voltage must be listed at 9 VAC.