I am trying to design my own pcb based on the atmega 328p-pu chip and am wondering if you guys could help me out with the power supply? I am looking at the lm2937 and lm2940 5 volt regulators as it says they are automotive rated and already protect against load dumps etc.
Here are links to them
Here is also what i have for a schematic. Please let me know what you think and if i should change anything
That schematic looks a bit busy. Where did you get it?
The attached is usually enough.
Hi the first circuit is as the OP says, setup for automotive application with a crowbar type overvoltage zener on the 5V output.
Tom..... 
mitchedwards:
Here is also what i have for a schematic. Please let me know what you think and if i should change anything
This schematic will most likely work, but it could probably be simplified to reduce part count without reducing the performance significantly. There are several main sections to it: the input capacitor, the regulator, the low pass filter, the over voltage protection, and the indicator LED.
The input capacitor and the regulator are the most necessary of these. The input capacitor provides an energy reservoir near the regulator to quickly supply energy when the regulator needs it. This energy more easily comes from a capacitor because there is less impedance between the capacitor and the regulator then between the regulator and the car battery. Most regulators then also use an output capacitor as shown as well. This reduces ripple and helps make the regulator stable.
The optional parts that make it more robust but are not completely necessary are the rest of the parts listed above. Namely, the low pass filter helps smooth out any transients with the series inductor and shunt capacitors. However, this is likely overkill for the situation. The over voltage protection is probably a good idea, but it will only be used if the regulator becomes unstable or is damaged. The protection diode at the input is also a good idea, but I don't think its likely to experience long duration negative transients. All these options are up to you and how many parts you are willing to use.
Hi, firstcircuit is fine, just make sure the inductor L1 is rated for the 1A load.
I don't think people are aware of how noisy and spiky an automobile/car can be. Just open a ECU or MCU or engine control box and check out the power supply area.
Tom.....
Yeah, the first circuit is fine for an automotive application. But isn't it all redundant if using a regulator designed for automotive applications with so much of it built-in?
LM2940:
Designed also for vehicular applications, the LM2940-
N/LM2940C and all regulated circuitry are protected
from reverse battery installations or 2-battery jumps.
During line transients, such as load dump when the
input voltage can momentarily exceed the specified
maximum operating voltage, the regulator will
automatically shut down to protect both the internal
circuits and the load. The LM2940/LM2940C cannot
be harmed by temporary mirror-image insertion.
Familiar regulator features such as short circuit and
thermal overload protection are also provided.
I've successfully used an LM2940 in the car with very few additional components. Just the standard complement of input/output capacitors. Never had any problems with the regulator, or anything being powered by it.
Ian.
You can never be too careful. I rather like the extra components if they add value. When you have built as many circuits as I have , less than 10 extra components (especially small cheap ones) is no big deal.
The objection to the extra parts can only be based on the feeling that building circuits is work and each
added component is more work. When you enjoy building circuits that objection doesn't exist. It's like
an artist thinking, "I think I'll leave the tree out, it's just more work..." Never going to happen.
I don't think it's worth even bringing up the added cost in view of the cheap cost of all of the added
components. If your talking about less than $25 extra I think it's silly to say that is too much to spend
for a more reliable circuit. Those components are there for a reason and the fact that the regulator has
been used without them is like saying "I've been driving for 40 years and I've never been broadsided
by someone running a red light." Congratulations. Consider yourself lucky.
Here's the choke:
5.6V zener
22V ZENER
The Polyfuse was a little harder to find
polyfuse
Thanks so much for everyones input! I understand that the lm2940 has most of this built in but i also dont want to fully rely on it as i have heard it may not be enough in some cases. If someone has a simpler but still effective design please post it as im sure im not the only one who needs a solid automotive power supply. Cost is not a huge factor i just need it to be safe and reliable.
Thanks
Ok guys so i have been thinking about this power supply and am now wondering if i should leave the arduino on all the time or if i should turn it on and off with a switched 12v signal from the truck. Any suggestions on this?
If i do decide to have it switch i can either use a transistor circuit or relay with the LM2940 or should i go with the LM2576 as it has a built in enable pin? It has has a higher input voltage rating compared to the LM2940. The LM2576 doesnt say anything about being automotive rated where as the lm2940 does.
Please keep in mind that I want to keep this circuit as simple as possible will still providing a safe supply to the atmega328
Have a look at the Micrel MIC29151 as an alternative to the LM2940. Also the ST Micro LF50 in PPAK package. Both have enable pins that allow you to turn them off.
http://www.micrel.com/_PDF/mic29150.pdf
Ian.
Replace the zener for a 16-20v rated TVS diode if you want more protection, it's switching time is way faster than a zener.
now then... the alternator is buffered by a lead acid battery, which generally stops huge voltage spikes from killing your electronics, a 1000uf capacitor on the input entering into your circuit to reduce noise.
And I challenge anyone here to destroy a regular linear regulator just by running it from your car (lead acid battery connected) I've powered loads of things nothing ever has blow while I've used it in the car, from a bar graph ic using nothing more than a 12v zener as it's regulator... countless lm317's for music players..
I really don't believe the car to be much of an issue providing you have a basic regulator and a good filtering cap...
