Hello!
What are the acceptable voltage limits at VCC for the Mini Pro 328 5V?
Am I safe to wire the 6V from a (4 x AA) battery holder directly to VCC, and avoid the regulator losses - with nothing attached to “raw”? (The devices being controlled are all rated at 5V minimum, including digital clock and stepper and piezo buzzer. Stepper motor power will of course be sourced directly from this battery holder.)
My alternative is to use the 4.5V direct to VCC from a (3 x AA) battery holder.
Do I assume correctly such a battery pack would be considered “regulated” voltage?
Thanks!
Hello, and welcome to the new SaprkFun forums!
What are the acceptable voltage limits at VCC for the Mini Pro 328 5V?
VCC directly feeds the ATmega328 chip on the Pro Mini and that has an absolute maximum voltage of 5.5 volts. To be safe, you're going to want to keep voltages on that pin between 3.3 and 5.25 volts.
Am I safe to wire the 6V from a (4 x AA) battery holder directly to VCC, and avoid the regulator losses - with nothing attached to “raw”?
You're going to want to use the RAW on in this case. 6 volts directly from the battery on VCC is going to cause you trouble. It could potentially destroy the ATmega328 chip on the Pro Mini.
(The devices being controlled are all rated at 5V minimum, including digital clock and stepper and piezo buzzer. Stepper motor power will of course be sourced directly from this battery holder.)
Your clock and buzzer will probably be OK running off the Pro Mini, but a stepper motor is definitely going to have to run straight off a battery.
My alternative is to use the 4.5V direct to VCC from a (3 x AA) battery holder.
That would work just fine if you want to connect it to VCC.
Do I assume correctly such a battery pack would be considered “regulated” voltage?
A battery pack would be considered an unregulated voltage source since the voltage will get lower as the batteries discharge.
Hope this helps, and good luck with your project!
Thank you, Chris! Much appreciated!
If you had to guess, which config (4 x AA to “raw”) or (3 x AA direct to “VCC”) will give longer service on the respective AA battery pack?
I assume there’s no damage to ATmega328 if VCC drops too low…it will just shut off, right?
Thanks again!
Mark
I’d go with the 3x AA’s into VCC. That removes the losses in the voltage regulator and the ATmega will consume less current at the lower voltage. 
Low voltage won’t hurt the ATmega328, but at some point, it won’t have enough voltage to function correctly.
Thank you, Chris–I appreciate your feedback! I was thinking that may be the better way to go-so thanks for confirming.
Would you guess that the other 5V (and 5V+) devices (stepper, piezo buzzer, digital clock) will still function down to 3.3V along with the ATmega328? If so, the 3 X AA will be something of an optimum solution. (And I will try to track down precise operating voltages for each.)
But if the devices drop out at say, 4.0V then powering buzzer and digital clock and the ATmega328 through the “raw” and thus the regulator–and the stepper board directly from battery pack might be a nod for the 4 X AA configuration. How large are the regulator losses?
To explain all this concern for a seemingly minor consideration: This prototype will have a machined access door to the battery holder, and I’ll have very little time to experiment before it leaves my hands. So I’ll have to commit to 3X or 4X ahead of time.
Thanks,
Mark
Would you guess that the other 5V (and 5V+) devices (stepper, piezo buzzer, digital clock) will still function down to 3.3V along with the ATmega328? If so, the 3 X AA will be something of an optimum solution. (And I will try to track down precise operating voltages for each.)
It really depends on the device, but most should be happy with the lower voltage. A buzzer might be quieter at 4.5 volts than 5, but will probably still work just fine. Make sure your devices are not getting too much voltage though, there are 3.3 volt only parts out there and 4.5 or 5 volts can damage them.
How large are the regulator losses?
Not that bad actually. I don't have exact figures, but you're probably only loosing a few mA.
I have a variable power supply, and I’ll run some quick trials with both power configurations to check their respective current draws. While I’m at it, I’ll dial down the voltage from 6V and from 4.5V, and see how low I can go and still have viable function. That might give me a practical idea of which configuration yields better battery yield overall! Thank you, Chris–once again! You have given me lots of guidelines to work with and I really appreciate it.