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### Topic: voltage divider and current for 23k256 (Read 2139 times)previous topic - next topic

#### V_king

##### Sep 28, 2012, 11:49 pm
Hello everyone,

I am building a small circuit with an extra SRAM, for the arduino.
I am using two 23k256 and a TXB0108 logic level shifter. I am thinking of powering the chips simply by voltage divider from 5 volts, the whole circuit is using.
I wonder how much current should I leave for the chips?
I went through data sheets of both chips, and all I could find that 23k256 needs 1mA and TXB0108, barelly uses any current - 1uA. So in total I understand I need 3mA, but should I leave a bit more? as using standart 330/680 resistors I could supply 10mA, or would it be way to much and would fry the chips.

Cheers

#### AWOL

#1
##### Sep 28, 2012, 11:50 pm
Quote
I am thinking of powering the chips simply by voltage divider from 5 volts,

Stop thinking that, and use a regulator or DC-DC converter.
"Pete, it's a fool looks for logic in the chambers of the human heart." Ulysses Everett McGill.
Do not send technical questions via personal messaging - they will be ignored.

#### V_king

#2
##### Sep 29, 2012, 12:08 am
do you know any voltage regulators, which could output 5V and 3V3 from one chip? I could not find any and I am trying to save space on PCB, so though of using the voltage divider.

#### oric_dan

#3
##### Sep 29, 2012, 12:14 am
smt v.regs are pretty small, especially since you don't need a lot
of current.

#### AWOL

#4
##### Sep 29, 2012, 12:15 am
Quote
so though of using the voltage divider.

Please, you really need to stop thinking like that.
It won't work.
"Pete, it's a fool looks for logic in the chambers of the human heart." Ulysses Everett McGill.
Do not send technical questions via personal messaging - they will be ignored.

#### guix

#5
##### Sep 29, 2012, 12:20 am

Please, you really need to stop thinking like that.
It won't work.

You should explain why

#### AWOL

#6
##### Sep 29, 2012, 12:24 am
I don't need to, Mr Ohm passed a law.
"Pete, it's a fool looks for logic in the chambers of the human heart." Ulysses Everett McGill.
Do not send technical questions via personal messaging - they will be ignored.

#### guix

#7
##### Sep 29, 2012, 12:28 am
Oh right, so a voltage divider would also divide the current, that's it?

#### AWOL

#8
##### Sep 29, 2012, 12:30 am
No, just think about the impedance of the output of the divider.
"Pete, it's a fool looks for logic in the chambers of the human heart." Ulysses Everett McGill.
Do not send technical questions via personal messaging - they will be ignored.

#### V_king

#9
##### Sep 29, 2012, 01:02 am
hi,

i know, that you can't use voltage divider for low impendance circuit, because the current will just go to ground throught the circuit.
but is this the case? (just for learning purposes)

thanx anyway, will use voltage regulators.

#### dhenry

#10
##### Sep 29, 2012, 01:46 am
Quote
I am thinking of powering the chips simply by voltage divider from 5 volts,

A few possibilities:

1) See if you can power the chip at 5v. They may have maximum Vcc at 5v. Or they may be able to operate at 5v - you will need to see if you can take the changes.
2) If you have to power it at a lower voltage, see if you can drop the 5v easier. Two silicon diodes in serial, plus a resistor to ground will provide about 1.4v drop; or a resistor to an LED (white LEDs for example are around 3v). or a zener.
...

#### mauried

#11
##### Sep 29, 2012, 08:48 am
At the low currents your chips need, a simple series resistor and a 3.3 V zener diode is all you need.

#### V_king

#12
##### Sep 29, 2012, 12:32 pmLast Edit: Sep 29, 2012, 12:58 pm by V_king Reason: 1

At the low currents your chips need, a simple series resistor and a 3.3 V zener diode is all you need.

thanks for that. Zener diode sounds like a good option.

How much current should I pass throught the resistor for 3mA requirement? 5mA? 10mA to be safe?

Also, just to make sure, am I right with the resistor calculation:

R = (Vin - Vout) / Iout

Thanks

EDIT: well, small 50mA voltage regulator is half the price of the zener diode. will use the the voltage regulator.

Thanks everyone for the help. thread closed

#### BillO

#13
##### Sep 29, 2012, 06:11 pm
I've used a simple RED LED in cases like this.  It will reduce 5V to something a 3.3V  chip will be happy with.  I also use a 680 ohm resistor to ensure the LED lights up.  That way I get my pilot light (power on) too.
Facts just don't care if you ignore them.

#### Tom Carpenter

#14
##### Sep 29, 2012, 07:03 pm
For this case a zener diode would work well.
Have a look here:
http://www.reuk.co.uk/Zener-Diode-Voltage-Regulator.htm

It provides a tool to calculate the required resistor value.
~Tom~

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