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how do i make 75 ohm resistor a 12 ohm resistor using 150,300 and 12 ohm resistorz in parallel????????plzzzz help


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So this is SUPPOSED to do what ? Your circuit is certainly interesting.
You want a maximum output voltage of ......
You want a maximum output current of......

What are you charging ?

Lets start with this, rather than assuming you need the resistance.

Steve
kartik negandhi (author)  steveastrouk5 years ago
i want current of 800mA and an output voltage of around 6.7v
Whatever you are charging will only take the current it needs, so so long as you can supply at least 800mA you will be fine.

Again, what are you charging ? Is it your own battery, or is it a device containing a battery.

Steve
kartik negandhi (author)  steveastrouk5 years ago
i am trying to charge my cellphone.
And itz a 9v battery which v get in the market
If its IN your phone, you don't have to worry, make a charger with the right output voltage and the current will look after itself.,
kartik negandhi (author)  steveastrouk5 years ago
i hav made sum changes in the circuit and now i am getting current of 1.2A
but current requirement of my cellfone is 800mA.....
Will it be safe to use it or it will harm my cellphone..........?????
Your supply can PROVIDE up to 1.2 A, it won't NEED 1.2 A to chrage your phone, if you have the right voltage.

Steve
kartik negandhi (author)  steveastrouk5 years ago
that's what i m saying!!!
My original nokia charger gives o@/p current of 800mA
so sumhow i need to reduce the current!!!
Any idea abt how to do that????
NO. You do NOT need to reduce the current, the PHONE will do that for you.
kartik negandhi (author)  steveastrouk5 years ago
r u sure it wont harm my cellphone?itz nokia c6-00
Check your charging voltage from the phone's own data.
kartik negandhi (author)  steveastrouk5 years ago
from where shall i get my phone's charging data??can u plz search and tell me about that??/
my cell is nokia c6-00
i attatched the one which i made,there came a notification of charging,but then again the notification came to 'unplug the powersupply'which meanz that the cell is not charging!!!!
wat shall i do now????
I think you need FIVE volts, not 6.7.
kartik negandhi (author)  steveastrouk5 years ago
its written as 5 volts but i measured my charger voltage with a multimeter,it was reading as 6.7v
Try 5V
kartik negandhi (author)  steveastrouk5 years ago
tried!!but it doesnt charge
the charging lights lits for a moment and then the phone displays that the charger has been unplugged!!
Check it against a real charger and see what happens.
kartik negandhi (author)  steveastrouk5 years ago
wat shall i check???
wen i connect my circuit with my phone,it shows charging for 1 second and then agen it shows that the charger has been disconnected.......
any idea about that error????
What happens when you use a real charger. Lets work out if the phone is damaged, or there is something wrong with the charger. What happens to the voltage AT the phone, when its plugged in ?

What circuit are you using now ?

kartik negandhi (author)  steveastrouk5 years ago
when i use a real charger my phone charges to full in around 45 min

the same circuit but in placce of 12ohm resistor i replaced it by sum other resistance in parallel so as to reduce the resistance bcoz i neede a current of 800ma
As I keep saying, forget the current limiting resistor, it isn't helping, because it drops the input voltage too much.
kartik negandhi (author)  steveastrouk5 years ago
i hav made sum changes in the circuit and now i am getting current of 1.2A
but current requirement of my cellfone is 800mA.....
Will it be safe to use it or it will harm my cellphone..........?????
lemonie5 years ago
You said:
i have a cicuit which has resistance of 75 ohm (approx.)
but i want it to be 12 ohm as i am using a 9v battery and i want the current to be around 800mA.


Why?
You're talking about wasting energy as heat in resistors; for what purpose?

Why have you used the tags CELLPHONE, CHARGER without saying anything else about about those?

L
kartik negandhi (author)  lemonie5 years ago
i am trying to make a portable charger!!!
so if the current will be more it wud surely damage my cell fone!!!
so for safety reasons i hav added the reisistances
i hav used ic 7805,lm317,a12kohm pot etc..
i am just a beginner !!!
so guyz plz help!!!!
If your circuit is 75 Ohm now, the max current would be 120mA.
Adding resistors isn't going to put more current in the phone.
Can you show us the circuit in some way?

L
kartik negandhi (author)  lemonie5 years ago
the star mark is the place across which i am gettin 75 ohm r which i want to reduce to 12ohm so that i could get a current of 800mA
OK, now it makes more sense to me now, thanks. But steve' is the fella to listen to about this.

L
iceng5 years ago
You can put 3 - 150 in parallel with 3 - 300 in parallel with the embedded 75 and reduce the total to 23.0 ohms.
Or just put the 12 in parallel with the embedded 75 to get  10.3 ohms.
Those are your choices. 
I use the formula  R|| = (R1 × R2)  / (R1 + R2)

A
iceng5 years ago
plzzzzzz does not clear the confusion ? ?

Take 12 ohm and put it in place of the 75 ohm, job done ! !
kartik negandhi (author)  iceng5 years ago
i75 ohm resistance of the internal circuit
cant do anything with it,just waht i can do is attatch sum resistance in parallel and reduce the resistance!!!!
Homework can be tricky can't they ?

As can the statement of the question.

How can I make a 75 Ohm resistor from a 150, 300 and 12 Ohm resistor ?

If this IS NOT the question please restate it.

Steve
kartik negandhi (author)  steveastrouk5 years ago
1 need approximate 12 ohmz
itz not any kind of hw
i need it for the mobile charger i am making!!!
will b posting it soon....
You SAID you HAD 12 Ohm resistors in the question !!!

What HAVE you got

Steve
kartik negandhi (author)  steveastrouk5 years ago
i have a cicuit which has resistance of 75 ohm (approx.)
but i want it to be 12 ohm as i am using a 9v battery and i want the current to be around 800mA.
i hav got 3-4 reisistors each of 150,300,12 ohm each and so i wanna make use of them ........
Put 12 Ohms in parallel and it will be near enough - using resistors to drop voltage is a bad plan though, if there is any kind of varying load.

Steve