Go Down

Topic: Supply for handmade arduino (Read 1 time) previous topic - next topic

streetjerk

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.  :~

Krodal

#1
Jan 19, 2013, 10:08 pm Last Edit: Jan 19, 2013, 10:11 pm by Krodal Reason: 1
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.

MarkT

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
[ I won't respond to messages, use the forum please ]

silverxxx

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.

vasquo

ATMEGA(328P) chips can still work down to 4.6 volts.

Go Up