Ideal Power 5v Supply

WanaGo · Joined: 15 Aug 2008 Posts: 111

Hi

Just another branch for the understanding.
I am trying to read up on the ideal power source for atmega's for in a workshop.

ie I have a 10VDC transformer, and I want to regulate it down to 5v and potentially 3.3v at some stage.

There are alot of 5v regulators I know, but which ones are the best? What capacitor combination is the best?
I have read there are regulators of sorts that do not do the standard 'heat dissipation' and only use the power required. ie instead of taking in 10V and outputting 5V, and turing the excess into heat...
I think an inductor comes into the picture here...

Is anyone able to provide some information on all this.

I ideally want to make an enclosure for each microcontroller setup, each having a 5v regulated supply of some kind that will filter the power in a way that the micro will have no trouble running in a workshop. Bearing in mind there is potential HF noise from Tig Welders nearby, general 3 phase noise from motors and VSD's etc.

Does anyone have a nice working solution for an environment like this?

Another thing,
I have asked many people I know the question of knowing what Caps to use, ie on the incomming line do you have 1000uF or 2200uF etc, or why they used a particular type of cap etc... however I havent had any very good reasoning as to why choices of size were made. Ceramic vs tantalum vs electrolyic etc... Alot of the choices seem to be made based on what people have available... but I know this isnt ideal.

Thanks

rileyesi · Joined: 19 Dec 2006 Posts: 398

Hi,

There is more to a power supply design than meets the eye. You have to set the parameters, such as how much current will you need to deliver?, how much voltage ripple can you have on your output?, how accurate will you need the output voltage?, etc.

A good start for you might be here:

http://my.integritynet.com.au/purdic/power1.html

Another source of information will be in the data sheets of the voltage regulators you are considering (assuming that you are using a linear supply and not a switching power supply). I have never designed a switching supply, so I don't have much knowledge in that area.

Hope this helps.

Pete

WanaGo · Joined: 15 Aug 2008 Posts: 111

Thanks Ill have a read of that.

I know the PCB plays a large roll in the overall stability of the system also, how power is supplied to the chip via the decoupling caps, rather than power going to the power pin and a cap stuck on the side etc etc..

Im not so much concerned about the inputs and outputs of the micro, and what the quality of the power is there - im more concerned about the stability of the micro and whether it will continue to run in a stable state in the environment.

I am planning to run mega644P's in a few places, running at 20Mhz.
Taking into account that I have got the PCB layout design sorted, I just need to sort out what components make up an ideal supply for use in a noisy environment.

Thanks

WanaGo · Joined: 15 Aug 2008 Posts: 111

When I was at university, by final year project was to build something. I built a semi automated rotational moulding machine, did all the mechanical and electrical side of it, aswell as building the control system.
The control system used a Rabbitcore microcontroller. While not an AVR, we designed the board with loads of caps etc, tried to get the design right as best we could - but when we engaged the High Frequency starter on the gas burners, the micro would either reset or the display would shut down.

We tried everything, and ended up putting in a PLC based system instead, which had no trouble with the environment.

Its the difference between the micro and the PLC I want to investigate and what makes the PLC more stable. In theory the micro shouldnt have any problems, but it did.

We had metal boxes, caps, ferites, suppression filters, all sorts.

rileyesi · Joined: 19 Dec 2006 Posts: 398

Hi,

I'm not sure if this is a factor in your case, but you want to be sure that you have a good ground plane. Also, if you are using both analog and digital components, it is wise to separate them on your pcb if possible and have separate digital and analog ground planes. However, you should connect the two ground planes at one point only. I usually conntect the two planes using a single trace on the pcb.

A bad or floating ground can cause troubles just like a bad voltage supply.

Just a thought.

Pete

AdrianJ · Joined: 16 Jan 2006 Posts: 2483 Location: Queensland

At the heart of every PLC is a micro controller, but its usually hidden away, well protected from the nasties in the outside world. Very often complete opto-isolation on all signal lines, and with its own heavily filtered and bypassed power supply.
This is very different from the typical processor design most of us do, with little or no filtering on switch lines, boards with power supplies coming direct from mains to 5 volt via single transformers, other signal lines direct connected to the same board, relays and their drivers etc.
And yet it is possible to make this work, provided you take reasonable care with power supplies and signal bypassing.

Power supplies are a system in themselves, and it pays to spend a lot of time getting them right. Choice of caps, blocking diodes, transient suppressors, fuses, etc are all important and should be made by considering the environment and requirements. Bypass caps in particular MUST follow the regulator makers specs, even more so with modern low dropout and switching regulators.

Switching regs are good for getting raw high voltage, eg 12-48 volt supplies down to around 5 volts for final feeding through low-drop linear regs to either 5 or 3.3 volt for your final processor supply. You can now buy single 3 terminal switching regs which do all this in one package, no fancy SMPS to design. See http://www.recom-international.com/products.html for example.