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Topic: LM78L05 and LM79L05 supply arduino (Read 2 times) previous topic - next topic

Grumpy_Mike

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So I don't see that using the 'LS07 improves the rise time.

True, but then I never said it would. It is the fall time that controls the switch on of a ~CE signal and that is what I think causes problems.

Papa G


You don't need the -5V supply at all. The INA122 will work down to 2.2V total supply voltage and its common mode input voltage includes ground. So ditch the 79L05 and connect pin 4 of the INA122 to ground instead. See Fig 5 on the datasheet.


Consider the situation where the unbalanced state of the bridge, Vunb, is <= 0.1V/G, where G is the gain of the instrumentation amp. While the common mode input voltage includes ground, the output voltage of the instrumentation amplifier when operated from a single supply does not.

The OP's original plan takes that into consideration. If eliminating the negative supply is important, the output could be biased to a value > 0.1V by applying a reference voltage to pin 5 (Ref). A 2.5V reference might make sense in this application.

oric_dan

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Voltage dividers can often slow down the rise time of the edge given the capacitance of the input. On the Ada wave shield their version 1 had voltage dividers but they changed to level translators for version 2


Sounds surprising. Do you know what values the Rs were in the v.divider? After all, a
2.4K:4.7K looking at a high 40 pF gives a 60 nsec time-constant, which sounds pretty quick.
And the R values could be reduced.

Do the typical SPI device loads have excessively high Cin?

oric_dan

To answer my own question, the schematics for both versions of the wave shield are
found here, click on downloads.

http://www.adafruit.com/products/94

The earlier version used 4.7K:10K v.dividers. These are interfacing to an SD Card, and not
to a typical little CMOS chip. So, the SD Card apparently has more stringent requirements
for driving. I don't know the schematic for an SD card.

Yaro


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Please, can you explain why i need capacitors to input

To lower the impedance of the voltage source to prevent oscillation of the regulator.
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1.0mF will be ok?

For those parts you linked to then yes. But only if the part is actually made by the same manufacturer who wrote the data sheet.

So i have find these:
7805
http://www.datasheetcatalog.org/datasheet/stmicroelectronics/2143.pdf
7905
http://www.datasheetcatalog.org/datasheet/stmicroelectronics/2149.pdf

7805 : 0.33 mF and 0.1 mF
7905 : 2.2 mF and 1.0 mF
I found all capacitor in tantalum but not the 1.0 mF
With what would be better to replace it? I was advised that ceramic is good too. And electrolytic is too slow.
But in these datasheet:
http://www.hep.upenn.edu/SNO/daq/parts/lm7915.pdf (it's not the same producer)
Say this: "²Required for stability. For value given, capacitor must be
solid tantalum. 25 mF aluminum electrolytic may be substituted.
Values given may be increased without limit."
So it's better 1.0 mF ceramic or 25 mF electolytic?



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How i can lower voltage to 3v3?


First off, many but not all standard-size Arduino boards have a 3.3V v.reg on board.
You can add your own on other boards, but the zener idea may work.

Secondly, if you have an Arduino board that runs at 3.3V rather than 5V, you can
probably power it off a 5V regulator ok. However, you'll be running a bit close on loading
with powering from a 78L05, which can deliver only 100-mA. Would be better to use
a 7805 v.reg, ie 1 Amp device.

Thirdly, if you have mixed systems of 5V and 3.3V, you will need level-shifters to interface
the 23K256 to 5V I/O pins on an Arduino.

[Edit: I meant to say you can probably supply 5V to the Vin pin of a 3.3V board to power
the 3.3V v.reg on the board].


I've changed to 1 Amp devices, and i'll use raw pin on arduino pro mini (3v3 ver.). It'll be ok?

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