I'm trying to convert 12v from an ignition live down to around 3.5v which will be an input into my ecu. I've been looking up how to do it and have come across this circuit diagram that uses an LM317 and a potentiometer. While I can make this circuit easily enough can I not just use a potentiometer in series to reduce the voltage?
(http://i156.photobucket.com/albums/t23/dave317/index_zpsw8gxv7do.png)
You'd be better off buying a fixed 3.5v PSU (they typically come in a transistor package) that way it will output 3.5v regardless of the input voltage (ignition is anything between 11.5 off to 14.8v running)
You can't just use a variable resistor as they are only mean to handle small amounts of current. It would burn out with any sort of load on it. Also you would not get a fixed 3.5v output, it would vary with the input.
I've got it sorted now. I made the circuit but used different resistors. It works perfectly and I can set the voltage very easily.
I use this kind of voltage reducer to emulate a TPS voltage, it works very well.
m http://www.ebay.com/itm/DC-DC-LM2596-St ... 43d68c28a3 (http://www.ebay.com/itm/DC-DC-LM2596-Step-Down-Adjustable-Converter-Power-Supply-Module-1-3V-35V-F5-/291362318499?pt=LH_DefaultDomain_2&hash=item43d68c28a3) m
(http://1.1.1.1/bmi/www.photo138.com/0/FG5/MC00100-ALL-1.JPG)