# Changing voltage in a car

1. Sep 8, 2011

### s.dunn

Hi everyone.

I have been poking around a bit and I couldn't find the answer to my question, likely as I had no idea where to look. This is a practical problem and I don't really even understand the theory (physics was a LONG time ago)!

Basic Problem:

I am doing a project in my North American Car. The whole electrical system is based on 12v power, reality is of course around 13v while the car is running.

I have a relay that reacts to an "input" and sends out a "12v" signal as its reaction. As long as the "input" is present it sends its 12v out. As soon as the "input" ceases the relay stops sending. I need to hook this 12v output into something that is designed to take around 9 or 10 volts maximum.

What resistor would I have to put "in line" with the output of the relay to lower the voltage to 9 or 10 v?

Full Circumstance:

I have a car that did not come with factory navigation. The radio has an input for the factory navigation comprised of a MONO input and a mute sensor, such that when a 9 or 10 v current is applied to the mute sensor it turns off the stereo in the front speakers and plays what ever is coming in on the mono input through the front speakers.

I have a TomTom navigation system hooked into a VOX relay via the headphone jack and a splitter. One goes to the VOX relay, the other to the stereo. When the VOX relay "hears" the TomTom output its sound it sends a 12v signal out. The radio in the car however was only designed to take 9 or 10 v in.

What resistor would I have to put "in line" with the output of the VOX relay to lower the voltage to 9 or 10 v by the time it hits the stereo?

2. Sep 8, 2011

### gsal

I think there are a couple of solutions.

One, like you said, might be to place a resistor in line, so that the mute sensor will only see, say, 10V...to do this, you need to put a resistor that develops 2V across itself when the current flows through it...what current? whatever current the mute sensor uses...so, if you can figure out what current the mute sensor draws, then with Ohm's Law V= IR , you have R=V/I = 2/? ....if the current is 1 milliamp, then you will need a 2kilo-ohm resistor.

The other way to go around this is to put a voltage divider, so that you can pick the fraction of 10/12 out of it...you can do this with small capacitors that will charge quickly; actually, you could even do it with resistor in parallel to the mute sensor, but you may end up drawing more current than desire out of the device providing the 12V...it may not be so bad if all this happens in a second and things are done before you know it.

Note: Oh, the voltage divider (either capacitors or resistors) goes in parallel to the mute sensor, though, not in-line...just in case it was not clear

3. Sep 9, 2011

### Delta Kilo

You can use resistor and http://en.wikipedia.org/wiki/Zener_diode" [Broken]. Standard 9.1V zener and about 1K resistor (for a few ma current) should do it. Make sure you put the zener the right way up, it operates in reverse bias mode :)

Last edited by a moderator: May 5, 2017