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Topic: Best 5volt power supply for atmega328...??? (Read 4788 times) previous topic - next topic

Jack Christensen

#15
Feb 19, 2012, 02:41 pm Last Edit: Feb 19, 2012, 02:48 pm by Jack Christensen Reason: 1

I am using a 2200uf capacitor on the input of the 7805 then on it output I am using a 10uf cap..
When I solder the 7805 on the PCB there is problem, but when I am using the same 7805 outside the PCB and supplying only 5v from the 7805 to the circuit with a long wire, then things are just fine...


There is a clue in there somewhere I think. Your description of the circuit sounds OK, although I would add a 100nF ceramic capacitor in parallel with the 2200uF on the input, and another in parallel with the 10uF on the output. Attached is a schematic of a typical 7805-based supply. The electrolytic on the input can be arbitrarily large. This one shows two 100nF capacitors on the output, these are physically located near different ICs on the board. It's very important to have a 100nF capacitor from Vcc to ground located as close as possible to the ATmega328, and another from AREF to ground. In a noisy environment like this one, I'd put one 100nF near the 7805 along with the 10uF, another close to the ATmega328, and a third on the AREF pin, just for starters.

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If it's just a 328 and a couple LEDs, it can hardly be more than 50mA, yes?


You are correct it just that much of load I have. So you are sure that it will only dray 50ma...??


I was asking more than making a statement. Hard to be sure about anything without a proper schematic.
MCP79411/12 RTC ... "One Million Ohms" ATtiny kit ... available at http://www.tindie.com/stores/JChristensen/

Jack Christensen

#16
Feb 19, 2012, 02:56 pm Last Edit: Feb 19, 2012, 02:59 pm by Jack Christensen Reason: 1

So, the circuit should be something like:

Motorcycle -> 100 uF capacitor -> 7805 -> 100 uF capacitor -> BIG INDUCTOR -> BIG CAPACITOR -> output


That's a new one on me, my understanding is that the regulator shouldn't require all that hardware on the output side. If the input voltage was bad enough to need an LC pi filter, I'd put it on the input side. Give the regulator relatively decent input and it will do its thing just fine.
MCP79411/12 RTC ... "One Million Ohms" ATtiny kit ... available at http://www.tindie.com/stores/JChristensen/

Joy


Thank you for the second last post of yours...
I will follow that...



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That's a new one on me, my understanding is that the regulator shouldn't require all that hardware on the output side. If the input voltage was bad enough to need an LC pi filter, I'd put it on the input side. Give the regulator relatively decent input and it will do its thing just fine.



But I will not agree with you in this...

I tried a lipo battery on the input of the 7805 which had the cleanest input with no noise but then still the 7805 generated a noisy output...
So I believe that the 7805 itself is generating some rippls which is generating noise on the analog pins of the atmega...

Jack Christensen


Thank you for the second last post of yours...
I will follow that...

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That's a new one on me, my understanding is that the regulator shouldn't require all that hardware on the output side. If the input voltage was bad enough to need an LC pi filter, I'd put it on the input side. Give the regulator relatively decent input and it will do its thing just fine.


But I will not agree with you in this...

I tried a lipo battery on the input of the 7805 which had the cleanest input with no noise but then still the 7805 generated a noisy output...
So I believe that the 7805 itself is generating some rippls which is generating noise on the analog pins of the atmega...


Hmmm, I'd really have to be convinced of that, probably by seeing it demonstrated with scoped output, and from more than one particular 7805 (don't think you ever said whether you'd tried more than one). Lacking that, I'm perfectly happy to agree to disagree on this one.
MCP79411/12 RTC ... "One Million Ohms" ATtiny kit ... available at http://www.tindie.com/stores/JChristensen/

floresta

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So I believe that the 7805 itself is generating some rippls which is generating noise on the analog pins of the atmega...


You are really talking about oscillation and is usually dealt with by the small value capacitors that are specified across the input and/or output pins.

Don

dc42

If the 7805 has ripple on the output, there are a few possibilities:

1. You are not using the recommended capacitors, or the wires between them and the 7805 are too long, so the 7805 is oscillating.

2. You are drawing a lot of current from the 7805 and you don't have an adequate heatsink, so it is getting too hot and the thermal current limiting is kicking in. [This happened to me once with a 7912. Fitting a better heatsink fixed it.]

3. Your 7805 is faulty.

How much current are you drawing from the 5v line? Bear in mind that some things can be run from the unregulated input to the regulator, e.g. LEDs switched by transistors, LCD backlights, relays etc. So you may be able to reduce the current needed from the 5v supply to a value low enough to use a TO-92 regulator.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

Joy

The problem is solved now...
I took the 7805 out of the case and made a small power supply and supplied 5 volt to the circuit...
changed the 7805..

kf2qd

I would look at how close the wired from the power supply to the regulator are from the signal wires that are showing noise. The 15V lines could be inducing the ripple in the sensor wires. Unless you can hook up an oscilliscope to the power and ground coming from the regulator ans confirm that the 7805 on board is the source I would think that you are getting some noise on the sensor wires.

On the Motorcycle you will need to shield from the spark cables, and you should have very little ripple because the battery is analogous to a large capacitor.

Make sure you are isolating sensor wires from the power supply wiring.

tnovak

I used this dc-dc with external fuse, diodes and cap. Works great my in my circuit requiring 800ma max.

http://parts.digikey.com/1/parts/1867503-conv-dc-dc-7-5w-36vin-5vout-oki-78sr-5-1-5-w36-c.html

retrolefty

What I've been using for my last couple of 'standalone' arduino projects are those small +5.1vdc 1 amp cell phone switching regulator/charger supply wall warts. I find these at Goodwill stores for just $1-2 all the time and they work great. Means no board space taken up with a regulator chip. Just add a few power bypass caps on the board and life is good.

Lefty

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