Go Down

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

oric_dan

#5
Dec 04, 2012, 03:39 am Last Edit: Dec 04, 2012, 07:43 pm by oric_dan(333) Reason: 1
Quote
Please, can you explain why i need capacitors to input?


Yes, read the datasheets for the parts. They tell you what is proper.

Quote
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].

Grumpy_Mike

Quote
Please, can you explain why i need capacitors to input

To lower the impedance of the voltage source to prevent oscillation of the regulator.
Quote
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.

Quote
but i need to power an 23k256 too, that work with 3v3. How i can lower voltage to 3v3?

You use a voltage regulator designed to give you a 3V3 output.
That part is an SPI device so you will have to lower the drive voltage from the arduino. What I have used for this is a 74LS07 open collector buffer. You get 6 in on IC package, the inputs come from the arduino and the outputs to the SRAM with a 1K8 or so pull up resistor to the SRAM's 3V3 supply. The output of this chip can be fed straight into the arduino.

dc42


That part is an SPI device so you will have to lower the drive voltage from the arduino. What I have used for this is a 74LS07 open collector buffer. You get 6 in on IC package, the inputs come from the arduino and the outputs to the SRAM with a 1K8 or so pull up resistor to the SRAM's 3V3 supply. The output of this chip can be fed straight into the arduino.


Mike, I'd be interested to know why you are suggesting using a 74LS07 instead of the simpler option of a voltage divider?
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

Grumpy_Mike

Quote
why you are suggesting using a 74LS07 instead of the simpler option of a voltage divider?

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 because of the trouble the users were getting with dividers. With a buffer ( which I already had so that was a plus factor for that part ) the falling edge could be very fast and the rising edge controllable by the size of the pull up resistor.
I used this arrangement on my SpoonDuino, worked first time.  :)

dc42


Quote
why you are suggesting using a 74LS07 instead of the simpler option of a voltage divider?

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 because of the trouble the users were getting with dividers. With a buffer ( which I already had so that was a plus factor for that part ) the falling edge could be very fast and the rising edge controllable by the size of the pull up resistor.


Thanks, but I'm still puzzled. Both the Arduino and the 74LS07 have a maximum output current of 40mA (it's under "recommended operating conditions" for the 74LS07, and is the highest current for which Vol is specified). So I don't see that using the 'LS07 improves the rise time.

Example:

1. 74LS07 with 82 ohm pullup resistor to +3.3v. 40mA sink current, right at the top of the recommended operating conditions.

2. Voltage divider with 100 ohms to ground and 51 ohms to the Ardino pin. Current sourced by the Arduino is a little under 33mA. The equivalent circuit is 3.3V fed through (51 || 100) ohms = 33 ohms.

So it appears to me that the voltage divider should give a rise time less than half that of the 'LS07, if pin input capacitance is the dominant factor. OTOH the 'LS07 will probably give a faster fall time.

I guess that if you wanted to level shift lots of signals, using t he voltage divider approach you would have to design for a lower current draw to stay within the 200mA chip limit, then the 'LS07 might be better.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

Go Up