wat resistances shall i connect with a 9v battery to obtain a current of 800mA???

wat resistances shall i connect with a 9v battery to obtain a current of 800mA???i tried with 12 ohm but the multimeter does not show any current..,anything else shud be connected with the resistances ....????

Comments

[lemonie]

Tell us why, i.e. what you want the 800mA for / to go through.

L

[kartik negandhi]

i want it to go trrough my cellphone to charge it with a 9v battery,but when i connected the entire circuit,i am not getting the required current.the multimeter showz no current in the circuit,however if we calculate using v=ir,the required resistance shud be around 12ohm

[lemonie]

You should use a voltage regulator, do you know what the phone's charger gives in volts?

L

[kartik negandhi]

my phone charger gives 5v output....
i hav used ic 7805 ie. 5v voltage regulator,but i dont get the desired output current
i require a current of 800mA,but i dont get any current at the output.wat i get is only 5v output voltage.....

[lemonie]

You won't increase it with a resistor, 800mA is quite high load for a PP9; you're probably at the limit of what it will give.

L

[steveastrouk]

Its his phone charger. He doesn't understand that the phone does the limiting.

[kartik negandhi]

i understood wat u r trying to say,but if there is no current only then how will the phone limit the current......????

[steveastrouk]

You haven't said there is no current before.

All you have said is you seem to be insisting the resistor is needed, and I am quite sure it is not.

Have you measured it ? Have you measured it with your resistor removed ?

[kartik negandhi]

without the resistor,it shows 0.4A....

[steveastrouk]

Ah, so not 1.2 A as you were predicting....

Play with the voltage a little bit, and you will establish the right vltgae for the charging circuits to say "yes, we can charge with that"

[kartik negandhi]

can i use 2 5v voltage regulator??but then too i wil hav 2 connect resistances,rite?can u tel me hw much R should i conect 2 get around 6.7v,0.8A from a 10v battery.

[steveastrouk]

Use an LM317 regulator instead.

You CANNOT use a resistor to control current and voltage for your phone charger. It is NOT POSSIBLE

Steve

[kartik negandhi]

wat to use along with lm317....????
Can u plzz give me the circuit diagram
can u plz do me that favour....???

[steveastrouk]

http://www.reuk.co.uk/LM317-Voltage-Calculator.htm

[kartik negandhi]

can u plzz tell me wat value of resistances should be used along with lm317 to obtain the desired value.....ie.6.7v,and 0.8A

[steveastrouk]

I gave you a link to a calculator ! Use it.

Steve

[kartik negandhi]

but will i get the required cureent????
the is nothing such for calculating the current

[steveastrouk]

You already said this circuit can give you up to 1A, that will be fine.

Steve

[kartik negandhi]

but that was using the earlier circuit.and i destroyd that circuit as it was using ic 7805 which could produce only 400mA max..

[steveastrouk]

THIS circuit uses an LM317. A standard 7805 will deliver 1A, if its heatsinking is done correctly. THe current limit is determined by the regulator. It will automatically close down if you try and take too much from it, if its heatsinks are OK

Steve

[lemonie]

I prefer the straightforward questions over cryptic ones...
He wants an LM-something?

L

[steveastrouk]

He's got one, only he's buggered it around with an extra resistor, which freaks the phone's charging circuits out.