Dear All, I'm hoping that someone with more power supply design experience can help me out, I'm OK with digital stuff but this one is foxing me a bit. I'd like to apologise in advance for the scruffy hand drawn schematic attached but this was a quick and easy way of doing it...
This is a power supply design which I believe to be quite common, and it's not my own - I found it on the net, it's quite commonly replicated. The PSU supplies a microcontroller plus associated peripheral components, which draws about 3-400mA maximum. It's intended for use on a vehicle which I know is an electrically harsh environment, however the design as shown works fine on most vehicles - it survives all relay switching, wiper motor actuation, fuel pumps etc. without problems. I added the TVS as shown which works well and cured some initial problems.
However I have fitted this to a vehicle with a dynamo based charging system, and it can't tolerate that at all. I've looked at the vehicle's electrical system with a scope, and there are some pretty nasty spikes coming out of that generator; I should say that I believe this to be quite normal for the vehicle in question (a Harley Davidson Ironhead Sportster, actually). These are making their way through to the 5V side quite easily and are causing my MCU to continuously reset.
I've tried a few things so far that have worked for me in the past: I have fitted ferrite beads to the power lines, and I have wound the power lines around a larger ferrite. This does reduce my spiking to some extent but does not alleviate the problem sufficiently to allow the MCU to function reliably.
So what I'm looking for is a method for modifying this circuit, if possible, to provide some kind of protection against this kind of noise. I've found references to pi (LC) filters on this site, and others - but the information provided is so variable, I'm having a hard time working through it. I figured now was the time to ask for some aid!
-CF