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Author Topic: voltage divider and current for 23k256  (Read 1524 times)
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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
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I don't think you connected the grounds, Dave.
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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.
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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.
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smt v.regs are pretty small, especially since you don't need a lot
of current.
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I don't think you connected the grounds, Dave.
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so though of using the voltage divider.
Please, you really need to stop thinking like that.
It won't work.
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Please, you really need to stop thinking like that.
It won't work.

You should explain why smiley
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I don't think you connected the grounds, Dave.
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I don't need to, Mr Ohm passed a law.   smiley-cool
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Oh right, so a voltage divider would also divide the current, that's it?
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No, just think about the impedance of the output of the divider.
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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) smiley

thanx anyway, will use voltage regulators.
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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.
...
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At the low currents your chips need, a simple series resistor and a 3.3 V zener diode is all you need.
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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 smiley

« Last Edit: September 29, 2012, 05:58:57 am by V_king » Logged

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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.


* LEDREG.jpg (9.99 KB, 553x226 - viewed 40 times.)
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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.
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~Tom~

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