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Topic: voltage divider and current for 23k256 (Read 1 time) previous topic - next topic

V_king

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.
Any comments welcome.

Cheers

AWOL

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

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

smt v.regs are pretty small, especially since you don't need a lot
of current.

AWOL

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


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


You should explain why :)

AWOL

I don't need to, Mr Ohm passed a law.   8)
"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

Oh right, so a voltage divider would also divide the current, that's it?

AWOL

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

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

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

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 pm Last 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

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

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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