How to implement voltage regulators?

[walter]

Can I just slap a voltage regulator in-line, or do I need to engineer a circuit design? I am building a motor drive for a mechanical speedometer. A 2000 pulse per mile signal is produced in the transmission. I am using an arduino to translate that signal to a PWM to drive a motor connected to the back of a mechanical speedometer. It works on the test bench and now I am ready to connect it to a vehicle. The Arduino runs on 5 volts and can take up to 12 volts. During development of the code, I used a bench function generator for the 2000ppm frequencies set at 5 volt. In the vehicle, the 2000ppm signal is 12 volts, so I have added a 5 volt regulator. With the function generator set to 12 volts, the 5 volt regulator does it’s job. I’m ready to connect to a vehicle for a final test. I notice my truck produces 14 volts with the engine running. I’m thinking I can add a 12 volt regulator for both the motor and the Arduino. This seems like a simple fix, maybe too simple? I usually get away with connecting electronic components to create circuits, and they work. I skip the math part of engineering, but am still interested in learning that part. I am an electronics tech of 30 years, maybe I’ll be an engineer someday. I know this is not audio related, but I’m not sure where to ask questions about this. I am a member of an automotive site, but this is more of an electronics subject. If I had a 12 volt regulator in hand, I would implement it and see the results, but I don’t so I thought I’d ask here for feedback. I have worked on power supplies that do have adjustable voltage regulators and resistors, diodes. I’m shooting for a dedicated 12 volt reg.

 

[JohnRoberts]

auto voltage is marginal for a typical 3 terminal 12V regulator but a LDO (low drop out) regulator should work.

JR 

 

[mjrippe]

To further clarify John's reply: A typical 7812 regulator needs *at least* 14.5v in order to regulate.  An LDO works with less overhead. 

But since your Arduino will work with 5v, why not use two 5v regulators?  One less part number to order and no chance someone else will replace one with the other in the future.

 

[walter]

Thanks for the replies! The code is based on the motor running on 12 volts so I thought I could use that supply for the Arduino also. A quick search yields the LM1085IT-12, a 3A 12v LDO in TO-220-3 case. The speedometer currently tracks fairly well, but I intend on learning how to code and implement PID for a future upgrade.

 

[Bo Deadly]

Maybe just insert a mosfet and resistor as a current source in series with the supply. It's 2 parts. It will drop the voltage by Vgs and help filter out the noisy currents found in a car with solenoids and motors switching on and off. You need a mosfet with a Vgs > 3V min though and you will have to fiddle with the resistor to get it into current regulation. Too low and it will just be on. Too high and the voltage will drop below 5V. Shoot for 7-9V I'd say. As long as the Arduino draws a relatively constant amount of current, it should be good.