I'm building a project which will locate a max232 on an arduino mega prototyping shield. experience suggests that the 5 volt supply on the mega won't be enough AND I need 12 volts from some things that will be connected to the shield. There will be an lm 7805 on the shield to produce 5 volts, along with some caps, resistors and some other devices.
Has anyone come up with a slick way to get +12 from the mega, or Uno for that matter to a shield?
Obviously there could be another jack and plug, but since it's already there why not use it?
jferg:
I'm building a project which will locate a max232 on an arduino mega prototyping shield. experience suggests that the 5 volt supply on the mega won't be enough AND I need 12 volts from some things that will be connected to the shield. There will be an lm 7805 on the shield to produce 5 volts, along with some caps, resistors and some other devices.
Has anyone come up with a slick way to get +12 from the mega, or Uno for that matter to a shield?
Obviously there could be another jack and plug, but since it's already there why not use it?
Well a lot depends on how you are going to power the Arduino mega board in the first place. If you are going to use a +12vdc 1 amp wall wart type supply to the mega board to power the board, then that same +12vdc voltage source is avalible on the Vin pin on the power shield connector and thus avalibe for use on an attached shield proto-board.
One other thing: Is the available capacity of the 5 volt supply higher when connected to a 12 volt supply than with a the typical USB connection? It seems like it would be, but how to quantify? Also, if my stupid questions might be answered by reading existing text, where should I have looked?
jferg:
One other thing: Is the available capacity of the 5 volt supply higher when connected to a 12 volt supply than with a the typical USB connection? It seems like it would be, but how to quantify? Also, if my stupid questions might be answered by reading existing text, where should I have looked?
thanks for your patience.
john
It is a little hard to quantify. The USB +5vdc power is limited to 500ma by the USB standards and the 500ma thermofuse mounted on the arduino board. The on-board +5vdc regulator is limited by it's own internal current and limited value and the heat dissipation created by the voltage drop between your external power, say +12vdc in your case and +5vdc. You can get more +5vdc current from the regulator is you power your board with say 8vdc then if using 12vdc, but then your shield voltage via the Vin pin won't be +12 as you stated you required.
looks like what i was going to do originally would likely be better - namely take the 12 volts to the shield and use an lm7805 to get the local 5 volts for the max232.
I'm going to locate a gps and an LSM 303 compass in a separate enclosure that will be located about 20 feet from the arduino. I imagined that running 5 volts out to them and expecting to get usable signals back was a poor idea, so I'll run 12 volts out, reduce it to 5 locally for the compass and gps and then route their signals back to the board. I was lucky enough to find some 7 conductor 18 gauge shielded stranded cable for this so hopefully voltage drops shouldn't be a problem.
jferg:
One other thing: Is the available capacity of the 5 volt supply higher when connected to a 12 volt supply than with a the typical USB connection? It seems like it would be, but how to quantify? Also, if my stupid questions might be answered by reading existing text, where should I have looked?
There are two limiters of current: The power source (wall wart, or 12V jack, or whatever,) and 5V regulator.
If your 12V supply is a marine/car battery or similar, then the "wall wart" is not a limitation here. However, with a linear regulator, you cannot get more current out of the 5V than you draw from the jack.
Second, the regulator will overheat if you draw too much, and may have current limiting to prevent that. Typically, you'll get between 500 mA and 1500 mA out of a TO-220 device, depending on the specifics. You may also want a heatsink to actually get to the full specified current. For example, getting the specified 950 mA max current out of a 1117V50 LDO regulator when powered by more than about 7V requires a heat sink screwed onto it.
You can do all this math by reading the data sheet for the regulator you're using. The things you care about are max current limit, internal resistance/dissipation, and heat conductivity to ambient. The resistance of the regulator is simply how it regulates power. If you get 12V in, and 5V out, at 0.9 A, it has to dissipate (12-5)*0.9 == 6.3 Watts. With an un-heat-sunk thermal resistance of something like 80 C per W (making this up), you'd heat up to 500 C(!), and go up in flames. With a 15 C per W heatsink, you instead heat up by about 100C, which at 20C ambient is (just) within spec.
If you want to get better efficiency, and/or draw more current at lower voltage than you put in at high voltage, you need a switching regulator/converter.
One of the great beauties of working at this level is the cheapness of components. I built a lm7805 regulated 12 to 5 converter, hooked up the max232 and ran it for a day - it is only converting a single signal-stream. Although I had a heat sink ready, it didn't seem to be necessary so I may have lucked out here.
the incoming signal is 12 volt and the PNP inverter (if that is a correct term) requires a +12 connection. And then the 12 volt was going to be sent out to power the remote gps and compass. What I write shouldn't be worried over. I don't know much, but have mocked all this up and am now building the "final" installation. sometimes I make incorrect assumptions forgetting that if i change two things simultaneously, i won't know which change had the effect I'm now confronted by.
i re-read the max232 data sheet, and as you say, it uses little power.