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Author Topic: Supply for handmade arduino  (Read 1128 times)
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im reciprocating the Arduino on a breadboard. i have a problem with the 5V supply. I hav visited dozens of forums and tried all the combinations of capacitors combined with 7805(for decoupling) IC that are shown in the images by various authors.

Why do we need to use for capacitors first of all ? All the combinations give an output little less than 5V. What to do ? I need 5V exact or a little above. i need the exact combination of capacitor values to obtain an exact 5V output across 7805.  smiley-confuse
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The 7805 is good for the Arduino.
If the output is for example 4.9 V, that is not a problem.
The Arduino will work with lower voltages, but if the output of the 7805 is lower than 4.9V, the 7805 is broken.

What kind of multimeter do you use ? Is it a cheap one that could be 10% off ?
Do you use a lot of current for something. If the 7805 is getting hot, the output voltage is reduced.

The 7805 needs 100nF close to the input pin and 100nF close to the output pin. At the output a capacitor of 22 ... 1000 uF should also be used. Also such a capacitor is needed at the input, but in some cases it can be smaller.

If you need an exact known voltage, use a voltage reference.
The microcontroller on the Arduino board has an internal voltage reference. But you can buy such components if you need higher accuracy.

If you want for example 5.05V and it doesn't need to be very accurate, you can use a DC/DC converter with a adjustable output.
« Last Edit: January 19, 2013, 04:11:33 pm by Krodal » Logged

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The "voltage reference" on the ATmega microcontroller is less accurate off the shelf than a 7805, but
is temperature compensated and can be calibrated.  It is spec'd at from 1.0V to 1.2V
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The capacitor does not really affect the voltage provided by the 7805. Generally those capacitors are there to smooth transient response (response to square waves), but some regulators need them to be stable. (but not the 7805) You can look up one of the many 7805 datasheets for recommended capacitors. If you need a precise voltage you can use an adjustable lm317 and set the voltage, or you can just try a few 7805s until you find the right one.
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ATMEGA(328P) chips can still work down to 4.6 volts.
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2 things quickly 5.0 VDC for a 7805 is 'nominal' it can and will vary +/- 5% so the output can vary +/- .25 V and be in spec. There are 'grades', none A, B, C, that denote various output voltage tolerances. If you need 5.0V foa an A/D measurement then correct it by multiplying the A/D result by the actual Vcc divided by 5 '5/4.9 =' .02 or the A/D measurements are 2% low when the Vcc is 4.9V
Second, @ MarkT How is the internal AVref calibrated/changed..? I would like it to be 1.024V for obvious reasons.

Bob
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I use wallwarts in many projects, no onboard regulator. Need a DC source anyway, why not start with the right voltage?

Thru hole connector
http://www.taydaelectronics.com/dc-power-jack-2-1mm-bulkhead-type-panel-mount.html
http://www.mpja.com/prodinfo.asp?number=18549+PL

http://www.dipmicro.com/store/DCA-0510  1A
http://www.dipmicro.com/store/DCA-0520  2A
http://www.mpja.com/5-Volt-DC-Plug-Power-Supply-4A-Regulated/productinfo/18520+PS/  4A

328's are spec'ed to work at 16 MHz down to ~3.8V


* safe_operation range.jpg (88.57 KB, 960x720 - viewed 16 times.)
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Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
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This tutorial may help you ....

http://www.sparkfun.com/tutorials/297

http://www.instructables.com/id/Perfboard-Hackduino-Arduino-compatible-circuit/
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What-in-the-hell kind of program made this schematic?
It's freaking ugly.
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Don't know - my guess would be fritzing.
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Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
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And there are no 0.1uF decoupling caps on Vcc and AVcc .... Grrrr!
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AREF is also connected to AVCC, instead of being independent - with its own 0.1uF cap.
No diode across the reset resistor either.
Bare minimum designs have certainly been drawn up better:


* Bare_minimum_uC2.jpg (95.34 KB, 960x720 - viewed 24 times.)
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What's the purpose of C3 on the reset pin?
I'm not familiar with that one.
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C3 lets DTR from a USB/Serial adapter perform an autoreset during serial downloads.
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Stupid question:  What's the diode across the reset pull-up do?  I know I've heard of that before, but I have no idea what problem it's meant to solve.

As for capacitors, I use Fairchild's recommendation.  Big(ish) electrolyitc caps on input and output side -- sized to provide adequate capacity per the application, but often 470uF on input, and 100uF on output.  Parallel ceramic caps on the input and output sides of the regulator -- 100nF each (though Fairchild recommends a 330nF on the input FWIW.)  Additional 100nF ceramics on Vcc pins of all ICs, as well as the AVcc of AVR chips.

None of these change the regulator's output voltage.  In fact, they're there so the voltage doesn't change with load.
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