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[Reply #30 on:]

Perfect.  I really appreciate it.  had you not clarified this, I would have just used the 2.1 mm round connection.  Thanks again!!

[Reply #31 on:]

what happens if we connect one external power source to the 2.1 round connection and another external power to the VIN connection?  Will the Arduino get too much power and break or will it just choose one?  Just wanted to know. 

[Reply #32 on:]

good point. the voltage output from an adjustable switching regulator will sag with relation to the input voltage.

actually these switching supplies won't vary like that within a wide range of voltages.  Sorry about the link earlier, the data sheet is actually here.  The section on the characteristics of the variable type happens on p15 where it talks about calculating the resistors for the voltage feedback loop.  In the ebay board this is replaced with a trimmer pot so it can be dialed into the voltage you desire. 

My primary reason for suggesting a fixed one was so the OP didn't need to do the voltage setting step granted 5V is the only level needed, and that this is one of the available fixed-output variants.

Geoff

[Reply #33 on:]

what happens if we connect one external power source to the 2.1 round connection and another external power to the VIN connection?  Will the Arduino get too much power and break or will it just choose one?  Just wanted to know. 

the linear regulator most likely wont enjoy that.  If you want to do dual inputs, make your 5V input arrive via the USB socket as the UNO has circuitry to deal with choosing between those two power sources.

Geoff

[Reply #34 on:]

Nice.  The reason I said to take the 5 output volts and put it on the VIN pin is- that the Arduino's voltage regulator will not do anything as it's already 5 volts. 

In other words, if you take a car battery and connect it to one of the voltage regulators we discussed and connect that to the VIN pin (instead of the 5v pin). Than, even if it goes through the Arduino's voltage regulator, you shouldn't lose any efficiency as it's already proper voltage that can be used by the Arduino.  Am I mistaken?

[Reply #35 on:]

Nice.  The reason I said to take the 5 output volts and put it on the VIN pin is- that the Arduino's voltage regulator will not do anything as it's already 5 volts. 

If you're putting power through it, why would you presume it would do nothing?

Am I mistaken?

Quite.  There are a couple of issues with this.  Firstly, in this scenario where you're chasing every ounce of efficiency there's a loss to the operation of the regulator, even if it doesn't need to divert excess power to heat.  The cost of running (or quiescent current) for a common L7805 5V regulator is 6mA, according to table 4 of the datasheet (link to datasheet) or, more than 1/10th your project's total power consumption as calculated by CrossRoads earlier in this thread.

Also a linear regulator requires a particular minimum voltage in order to guarantee the desired regulated output.  In the case of that same example linked above, this minimum is 2V above the regulated output voltage, or 7V.  If you google 'dropout voltage' you'll find examples where 5V input will deliver 4.9V approx output (for the 7805 regulator linked above) and get a better explanation of what this means.

Bottom line is there's no need to regulate your input voltage twice, and if you're wanting maximum life out of this battery set-up it's best to power only the components you actually need.

Geoff

[Reply #36 on:]

Thanks Geoff and everyone that replied to this thread!!  I wish the books I read would have covered this; experience matters. I appreciate your time.