Maker Pro
Maker Pro

Resistance calculation

selva

Apr 26, 2010
25
Joined
Apr 26, 2010
Messages
25
Hi ,

Pls don't mind for this simple question. i got confused need some help.

The output from battery is 3.3V and 4250mA, i have to reduce it to 2.5V and 11uA which is a input to an IC. I have to connect some resistor to reduce the voltage and current from the battery.
can anyone plz help in calculating the resistor value to be connected.

Thanks in advance.

Warm regards,
Selva
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
Joined
Jan 21, 2010
Messages
25,505
unless the IC has a fixed and rather accurate 11 uA current requirement, a simple resistor won't do it.

You would probably be better off with a 2.5V voltage regulator. You could use an adjustable voltage regulator similar to a LM317 (preferably in a small package) but its quiescent current will exceed the 11uA many times over.

For such a low current, you may be able to get away with a voltage reference chip such as an LM285-2.5
 

selva

Apr 26, 2010
25
Joined
Apr 26, 2010
Messages
25
Its not that Steve, i am not gonna implement in physical, i just want to know how the calculations are made to get in to an particular resistor in specific point of a circuit,i don't want to be accurate for that current lets make it around by 11uA to 20uA or 25uA.

If i have misunderstood something wrong excuse me and inform me that.

thanks
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
Joined
Jan 21, 2010
Messages
25,505
In that case, you simply need the appropriate resistor to drop 0.8V at 11uA. See here.
 

NickS

Apr 6, 2010
367
Joined
Apr 6, 2010
Messages
367
Vout = R2 / (R1+R2) * Vin
 

Attachments

  • CropperCapture[1].Png
    CropperCapture[1].Png
    2.6 KB · Views: 128

55pilot

Feb 23, 2010
434
Joined
Feb 23, 2010
Messages
434
Vout = R2 / (R1+R2) * Vin
That assumes I-Out is zero. That is not the case here.

The full equation is:

Vout = R2 / (R1+R2) * Vin - Iout*(R1*R2)/(R1+R2)

---55p
 
Last edited:

NickS

Apr 6, 2010
367
Joined
Apr 6, 2010
Messages
367
Right you are if he taps off at Vout 55p.

I was assuming that R2 was his device, like I thought Steve had directed. So I was thinking he would approximate his IC as R2 (by knowing it requires 11uA @2.5V) and then he could solve for just the in line resistor R1.

But looking at it now that was not so obvious. I should have been more specific
 

55pilot

Feb 23, 2010
434
Joined
Feb 23, 2010
Messages
434
But looking at it now that was not so obvious.
I agree with you there. It is obvious that it is not obvious what is being asked, especially to the OP.

Did I make myself obscure enough? :D

---55p
 
Top