How to Connect a Portable Radio to a Car Battery
- 1). Check to see what the direct current (DC) operating voltage is on the manufacturer's label on the back of the radio. If it does not indicate the DC operating voltage, count how many 1.5V batteries are needed to run the radio. If the radio uses six or eight 1.5V batteries, then the DC operating voltage is 9V or 12V, respectively.
- 2). Attach the black and red wires to the radio battery compartment. Connect a black electrical wire to the spring shaped negative contact in the battery compartment by wrapping the wire around the spring. Connect a red electrical wire to the flat positive shaped contact in the battery compartment. This can be done by using a screwdriver to push the wire into the small space behind the negative contact. Another option is to remove the negative contact and connect the red wire directly to the wire attached to the negative contact.
- 3). Allow for 3 feet of red and black electrical wire, then cut both wires. Strip 1/2 inch of insulation off of the other end of the red and black wires. If the DC operating voltage of the radio is 12V, then connect the red positive wire to the red positive terminal of the battery. Connect the black negative wire to the black negative terminal of the battery, and you are done. If the DC operating voltage of the radio is less than 12V, then go to step 4.
- 4). Connect the red wire to the center terminal on the potentiometer. A potentiometer is a variable resistor, which may be purchased at an electronics store. By turning the knob on a potentiometer, you can change the resistance of the potentiometer terminals. Connect the black wire to one of the two outside terminals of the potentiometer. Make sure the radio is turned off. Connect a jumper wire between the center terminal of the potentiometer and the positive terminal of the battery. Connect a jumper wire between the negative terminal of the battery and the same potentiometer terminal that the black negative wire is connected to.
- 5). Set the digital multimeter to measure voltage. Place the meter's black negative probe on the negative spring terminal on the radio. Place the red positive probe on the positive terminal of the radio and measure the voltage across the radio terminals. Adjust the potentiometer until the voltage across the radio terminal matched the DC operating voltage for the radio.
- 6). Turn the radio on to test it. Once the correct operating voltage is found, potentiometer can be replaced by one or a combination of fixed resistors. Fixed resistors have a constant value that cannot be changed. Use an ohmmeter to measure the resistance of the potentiometer with the power off. Ohmmeters are used to measure the resistance of resistors and other electronic components.
- 7). Disconnect the negative wire at the battery terminal and turn the radio off. Place the black negative ohmmeter probe onto one of the potentiometer probes. Place the red positive ohmmeter probe onto the other potentiometer terminal. The probes should be connected to the two potentiometer terminals that have wires attached to them. Make note of the ohmmeter reading. For example, if the ohmmeter reading is 10 ohms, then the potentiometer can be replaced by one 10 ohm resistor or a group of resistors whose resistance values add up to 10 ohms.
- 8). Take the fixed resistor and set it beside the potentiometer. Work on one terminal at a time. Disconnect the wires going to the first potentiometer terminal. Reconnect those same wires to one terminal of the fixed resistor. Disconnect the wires connected to the second terminal of the potentiometer and reconnect those wires to the second terminal of the fixed resistor. Reconnect the black negative wire at the battery and turn the radio back on to test it.
Source...