Radio Current

Project Number: 28

Description: Install the battery eliminator connector into the battery compartment in the FM radio, as shown. Build the circuit as shown, and connect the red & black wires to it. Set the meter (M6) to the 50mA setting and the slide switch (S5) to position B. Plug the earphones into the radio and place them in your ears. Turn the volume knob clockwise to turn on the radio, press the reset button, then press the scan button several times to find a radio station (radio features may vary). Listen to the radio and notice how much current it needs. If the sound is distorted, then recharge the battery or other power sources in this kit. Use the volume knob to turn off the radio and look at the current measured on the meter. Even though the radio is off, the current may not be zero. The battery eliminator is using all the remaining current. The radio needs only 3V and can be damaged by higher voltages, so the battery eliminator has circuitry to reduce the higher voltages in this kit to 3V (for example, the B4 battery is 3.6V, the solar cell can produce 7V, and the hand crank can produce more than 10V). Your radio may have a light bulb or LED in it; if so, press the button to activate it. It may only light dimly, but notice what happens to the current and sound. Incandescent light bulbs need high current to get bright, much higher than LEDs. The radio may not work when the light is activated, due to the circuitry in the battery eliminator, which protects the radio from overvoltage. The radio light will work if you use the radio with normal 1.5V "AAA" batteries (not included).

Snappy: The 470uF capacitor (C5) is used here to improve sound quality by helping to stabilize the battery voltage. The radio needs a steady voltage to produce good sound quality, but the current it needs changes slightly as the sound changes. The capacitor can only supply a small amount of current but can react to changes faster than the battery can. Remove the capacitor from the circuit and see if you notice a difference in sound quality.