How to calculate capacitor values for a 12V DC to 5V DC power supply?

[Duff_Man]

Hello everyone,

I'm trying to design a system that will be used in cars, so I need a power supply unit that could convert the 12V used in cars to 5V to power the MCU. I'm using LM7805 regulator for this But how do you calculate the correct capacitor values? How many capacitors do I need? I've been searching for days, but I just can't seem to find the right answer. Any help would be greatly appreciated.

Cheers

Duff_Man

 

[Nigel Goodwin]

No calculation required - and not at all critical. I generally just use 1uF multilayer ceramic on input and output these days, in the past I generally used 10uF electrolytics..

 

[ronsimpson]

What is the load on the 5V? What are you powering?
Have you thought about getting a automotive phone charger? Automotive 12V to 5V 2A.
By the way automotive 12V is not really 12 volts but as high as 18V. (14.5 is typical when the generator is on)

 

[throbscottle]

The more normal approach is to use a buck converter, often followed by a LDO regulator. You're asking the 7805 to drop a minimum of 7v with the engine off, The car's 12v can go up to 14v or more with the engine running, so consider the power dissipation at that voltage, and it can spike much higher than that because you can get a "load dump" when systems are switched off, which momentarily pushes the voltage right up. So you're better to drop some of that voltage in an efficient way. Buck regulators are easy to build..Depending on the system you're designing you might want to just build a 5v buck, or you might want go a bit higher and stick a 5v linear regulator on its output, which gives a cleaner supply.

In a car you've got alternator noise and a load of other noise on top of that. It's typically all fairly high frequency. Depending on how noise-sensitive your application is, I'd go with the manufacturers recommendation for the regulator I was using (typically a ceramic cap close to the regulator, and an electrolytic one. You can add extra filtering if needed.