A car radio designed to operate with a 6.3V battery uses 4.5A of current,
a) If we wanted to use a 12.6V battery, what resistance should be placed in series with the radio to keep the same operating conditions?
2006-10-12 07:32:34 · 7 answers · asked by Anonymous in Science & Mathematics ➔ Engineering
Ohm's Law, R=E/I
Voltage = 12.6V
Amperage = 4.5A
Resistance would need to be 2.8 ohms
2006-10-12 07:49:45 · answer #1 · answered by londonhawk 4 · 0⤊ 0⤋
resistance of the radio is 6.3/4.5 = 1.4 ohms
place 1.4 ohms in series and do a voltage divider and you will find that the voltage on the radio is now 6.3V
This is a BAD idea though because the resistance of the radio is not constant. It changes as factors inside the radio change(ie volume). The best way to go would be to get a LM7806 voltage regulator from radioshack(~$1) and hook it up. It will regulate the voltage to the radio no matter how much draw. 4.5A is alot of current for a radio though.
2006-10-12 14:44:58 · answer #2 · answered by Joel D 2 · 0⤊ 0⤋
Sorry to be the wet blanket. In fact it is possible that the ideas of the others may work. However most of the circuitry in the radio is non-ohmic (semiconductor) and may not voltage divide as a resistor would. You can get voltage regulators speciffically designed for the 6.3v from 12-12.6V.
The fun way would be to get a string of diodes in forward bias. Each would drop approx 0.6V irrespective of current. You would need 10 in series. You will need to size them to suit the current (still 4.5A).
You may need to provide a heat sink as well.
2006-10-12 20:46:59 · answer #3 · answered by slatibartfast 3 · 0⤊ 0⤋
You cannot solve this problem by placing a resistance in series with the radio. The radio power consumption would depend on radio output, so the current will vary too. So if you base your resistance as per full radio output your current would be 4.5 amps. But a lower volume output would cause lower power supply current as well, so the voltage divider rule would not work. You have to use a DC/DC power supply - input 12.6, output 6.3 Volts.
2006-10-12 22:18:09 · answer #4 · answered by Borti 1 · 0⤊ 0⤋
6.3/4.5 = 7/5 = 1.2 Ω
So, use a 1.2Ω, 25W resistor with a good heat sink, and good luck!
Even then, every time you turn the radio on, you shock it with 12.6V. That's likely to damage components eventually.
2006-10-12 14:43:39 · answer #5 · answered by Helmut 7 · 0⤊ 0⤋
double the resistance for twice the voltage assuming the radio resistance is constant. go with 2.8 ohms.
2006-10-12 17:55:02 · answer #6 · answered by BRUCE W 1 · 0⤊ 0⤋
1.4 ohm
the original resistance is 6.3/4.5=1.4
you are doubling the voltage, so to keep the current the same you need to double resistance. 2x1.4=2.8
1.4 + 1.4=2.8
2006-10-12 14:48:28 · answer #7 · answered by Anonymous · 0⤊ 0⤋