This is called a voltage divider circuit. It's just two resistors. For your purposes both resistors, R1 and R2 must be identical. I don't know what your level of electrical expertise is so if something doesn't make sense, just ask.
Connect the two resistors in series, then connect one end to ground and the other end to a circuit that is always live. Common circuits that are always on include the horn, interior lights, brake lights, and in some cases the cigarette lighter.
Since the two resistors are the same value, they will each drop an equal amount of voltage, 12 volts in this case. That will give you 12 volts at point "A" which gets connected to the memory wire on your radio. It's important to note that the memory circuit draws extremely little current so that will have almost no effect on the voltage. If you should find the voltage is too low, increase the value of R2 a little or decrease the value of R1. You can find resistors at Radio Shack, but it's cheaper to swipe some out of an old vcr or tv. They have colored bands around them to denote the value. I roughly calculated that anything over 2400 ohms would keep the current flow down to well below the industry-accepted value of.035 amps, (35 milliamps) which is the maximum allowed for all the memory circuits on modern cars' many computers. A pair of 1200 ohm resistors will produce only 10 milliamps of current. That by itself will not kill a good battery for two or three months.
You can go higher in value with the resistors to cause even less current flow but then the draw of the radio will become more of a factor. All you have to do is measure the voltage at point "A" once the radio is connected. If it's at least 10 - 11 volts, it will be fine.
The common resistor values and their color bands are:
1000 ohms (1k ohms): brown, black, red
1500 ohms (1.5k ohms): brown, green, red
2200 ohms: red, red, red
2700 ohms: red, purple, red
3300 ohms: orange, orange, red
4700 ohms: yellow, purple, red
The third band denotes the number of zeros. If you find resistors with an orange band, that means three zeros. You can try those too, but as the resistors get larger, the radio's circuitry is going to have a greater influence on the voltage at point "A".
By picking the smallest value of a pair of 1200 ohm resistors, the resulting current flow is low enough that you don't have to worry about their power rating. That means physical size is not a concern. If you go to Radio Shack, they normally stock a big assortment of 1/2 watt resistors. Those are perfect. You don't need to waste your money on big ceramic power resistors. If you have your choice, I think I'd start with a pair of common 3300 or 4700 ohm resistors and go from there.
There are two disadvantages of this circuit. The first one is if you have a poor connection on the ground, the full 24 volts will appear at point "A". (You can experiment with that if you want to, but do it without the radio connected). Most radios I work on have circuits for the memory that are tolerant of voltage that is too high but that isn't always the case. 24 volts on that wire could damage something internally so be sure to have a solid ground connection before you connect the radio.
The second problem is the voltage that is dropped across R1 is directly a result of the current flow through it. The memory circuit current is very low and constant so it has little effect. The other current flow is what goes through R2 which is also constant. The problem comes in if you tried to use a circuit like this for the radio's main power. First of all you'd need up to three amps for a normal radio. To get that much current through resistor R1 it would have to be very small. Being a small resistance, if you do the math and calculate "Ohm's Law", you would need very high power resistors. That alone is not practical for the application, but much more importantly, the current on the main power circuit will be changing wildly from very low to very high according to the audio output signals. That would result in wildly changing voltage at point "A", and that can not be allowed to happen. At the very least the sound would be horribly distorted or even not recognizable, and at the worst the radio may turn on and off or the circuitry could be damaged. That's why you need a converter like those on Amazon. Com. They will deliver a smooth and steady 12 volts regardless of how much current is flowing.
There's one last thing I'll mention but it is very rare in aftermarket radios. A few models use a low-current turn-on signal, typically from the car's Body Computer, when the ignition switch is turned on, THEN the main current comes in on the memory circuit. Those are almost always original radios that are designed for the specific car. There will be one or two wires labelled "buss" or "data buss", the memory wire, and there could still be another power wire. I've never seen an aftermarket radio like that because they're intended to be used in any car.
If your radio takes the current to run the speakers from the memory circuit, my simple voltage divider won't work for the reason I explained about the wildly varying current draw and voltage. I'm reasonably sure you won't run into that problem but if you do, you'll have to connect both the main power wire and the memory wire to the converter. The radio will play fine but you will have to reset the station presets and clock, if it has one, every time you turn on the ignition switch.
Friday, June 22nd, 2012 AT 8:55 AM