Power and capacitors

Other Hardware General Electronics
1

Hi all.

I have the following options, and am asking which you would consider better :

Mega 2560 R3 and a battery supply of 13.26V DC.

I also have L7808 and L7805 voltage regulators available.

Would it be better to supply power to the board with :

a. 13V DC to the power jack
b. 8V DC to the Vin pin
c. 5V DC to the 5V pin ( althouth I think I read that this will bypass the onboard regulator so not a good idea ).

Next, when googling, I see different information about the capacitors to use with the voltage regulators. What is ideal to use ?

I have seen :
Power In ..to.. Gnd : 10uF, 0.1uF,
Power Out ..to.. Gnd : 10uF, 1uF, 1nF

2

and the datasheet ( page 8 ) at :
http://docs-europe.electrocomponents.com/webdocs/0daf/0900766b80daf4ad.pdf

shows 0.33uF and 0.1uF

is there any reason why the values from the datasheet should be changed to something better relevant to the usage ( powering the Mega ) ?

3

DaveO:
Would it be better to supply power to the board with :

a. 13V DC to the power jack
b. 8V DC to the Vin pin
c. 5V DC to the 5V pin ( althouth I think I read that this will bypass the onboard regulator so not a good idea ).

d. 8V DC to the power jack.

4

Isn't the Power Jack and the Vin pin the same thing, well at least directly connected ?

5

The regulator on the board is rated to 20 V, so you can directly power on the Vin or power jack, whichever is more convenient.

6

Thanks for the responses.

My thinking is that the regulated 8V DC to the Vin pin ( or power jack ) would be better than using the 13V DC direct from the battery pack. ( based on the fact that the 8V is regulated, and also lower power to the board would be less stress on the on-board regulator ).

Any comment on the capacitors to use on the regulators ?

7

DaveO:
Isn't the Power Jack and the Vin pin the same thing, well at least directly connected ?

Yes, but the power jack has a much better connector than the feeble Arduino header.

8

DaveO:
Thanks for the responses.

My thinking is that the regulated 8V DC to the Vin pin ( or power jack ) would be better than using the 13V DC direct from the battery pack. ( based on the fact that the 8V is regulated, and also lower power to the board would be less stress on the on-board regulator ).

Lower input voltage is always preferable for regulators (so long as it's still in range, obviously).

DaveO:
Any comment on the capacitors to use on the regulators ?

Why would you need any if you're using the Arduino's on-board regulator?

9

And if you DO decide on using the 7808 or 7805 regulators, the datasheet has the answers regarding external capacitors. I have never seen a LM780x data sheet say that the capacitors were completely optional and only in the sample drawing to annoy the designer. The Fairchild datasheet says, " Input Capacitor (Usually between 10uF to33uF) is required if regulator is located an appreciable distance from power Supply filter." How far is that before I have to worry? Not specified. Why take chances? The datasheet also says "The output capacitor (usually 0.1uF) improves stability and transient response." Let's say that a 0.1uF cap is a small price to pay for a stable regulator.

10

OK. So we go with the datasheet specs of Ci = 0.33uF and Co = 0.1uF

pwillard:
And if you DO decide on using the 7808 or 7805 regulators, the datasheet has the answers regarding external capacitors.

The "DO" in your post tends to indicate that the 7808 or 7805 would not be your choice ?

11

Actually, there may be some circuitry interposed between the Power Jack and Vin, so
they may not be at exactly the same voltage. There may be a series diode, or MOSFET
pass-transistor, etc, depending on the board.

The Vin pin will be downstream of this circuitry, and will likely not be hot unless power
to the board is brought in through the Power Jack, or the Vin pin itself.

12

If you have any ambitions on running it on batteries for more than a few hours, forget the Linear voltage regs.

Get one of these, set it at 5V and connect the output to your Arduino's 5V pin and GND.

// Per.