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[on:]

Hi!
I'm currently designing an arduino-compatible board based on the Atmel 1284 and I'm wondering what the pros and cons are of using an external 5VDC power supply. What I like about this approach is that the power is generated externally, i.e. the heat is outside the project enclosure, the power supply can be inexpensive yet powerful, and I save some board space. There is no need for higher-voltages, i.e. I am not trying to switch high-power relays that require 12VDC, for example.

The only cons I can think of is that unless I use a USB connector, it is possible that someone might at some point in the future install a power supply whose output exceeds the 5VDC required input yet whose 5mm/2.1mm plug fits, allowing all the magical smoke on board to be released at once. Additionally, should someone install a large but badly regulated power supply (i.e. not the well-regulated switch-mode wall-warts I am looking to use) the input voltage may once again be way too high. But that's about it and so far, it seems that a well-regulated 5VDC power supply seems like the better option.

So I respectfully ask the legions here who have far more knowledge about this subject than I ever will what their experiences with either approach were and which one they recommend using. Many thanks in advance. Constantin

[Reply #1 on:]

There is no single answer to your question. It depends on what is most important to you, robustness or cost? If it's cost, specify an external 5V supply, keep it all of board, and if someone plugs in anything different, tough.

If you want a better user experience (i.e., better robustness), it will "cost". You can add a regulator to your board and specify an external 7V supply. That will keep heating to a minimum. Or spend a few more pennies and specify an LDO regulator (e.g., LM1117) and specify an external 6V supply, for even less on-board heating.

Or specify an external 5V supply and add protection circuitry that guards against overvoltages. No heating, but costs more money.

I would start the conversation by adding a Schottky diode and LM1117 LDO to provide reverse voltage protection and input voltages between 6V-20V or so (depending on how much current you draw). Board space requirements are minimal, costs are minimal, and you've already guarded against the most common (and unfortunately likely) user errors: reverse voltage and overvoltage.

--
The Ruggeduino: compatible with Arduino UNO, 24V operation, all I/O's fused and protected

[Reply #2 on:]

I prefer external. Switching walwarts are inexpensive
http://www.dipmicro.com/store/DCA-0510
http://www.mpja.com/prodinfo.asp?number=18520+PS

with board mount power supply jack
http://www.dipmicro.com/store/KLDX-0202-A
or a panel mount jack
http://www.mpja.com/prodinfo.asp?number=18549+PL

Maybe add a 5.5V Zener across the line for overvoltage protection.

[Reply #3 on:]

@crossroads, sorry - bit of a newbie here. Is the zener diode connected to +v and -V  before the jack to prevent any voltage higher than 5.5v from being supplied to the circuitry? ie. If I supply 7.5v will the zener output 5.5v to the rest of the circuit?

[Reply #4 on:]

If I supply 7.5v will the zener output 5.5v to the rest of the circuit?

Yes, until the zener blows open and then the 7.5vdc will be back! A zener needs a current limiting resistor to act as a voltage regulator, or a fuse designed to blow open before the zener does.

Lefty

 

[Reply #5 on:]

As usual you guys are the best. I'll opt for the Diode + LM1117 approach for safety reasons as mentioned above. The additional board space for a DPAK is pretty small, ditto for a 0.5A SMD Schottky. Thanks again!