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Calculating resistor value for voltage regulator

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aghielan

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I'm trying to do a phone charger for car using used components. I have attach a simple/basic regulator circuit using LM317. I want to set the output of the circuit at 6v & around 500mA. Where this is the output of my indoor charger. I tried to calculate the resistor value using this formula: Vo=1.25(1+R2/R1)+IadjR2. But can't get the Iadj value. Most of the typical circuit apply 120&240 ohm resistor to R1. What is the difference between this 2 value. Please assist me on this. Thanks in advance.


Regards,
Aghie
 

Re: Voltage Regulator

And how could i add a resistor to sens the current? the purpose is to view the charging level & battery full level

Regards,
Aghie
 

Re: Voltage Regulator

Hi aghielan,

Battery Chargers have become something of an art form these days. With all the new chemistry batteries it has taken on a new life. There have been a number of charging controllers that have come on the market in the last several years. I don't think it would be advisable to simply plug the battery into a regulator and walk away. Part of the charge circuitry for the new types of batteries keep tabs on the temperature of the battery. There are also fast and trickle kinds of charge modes. Jump on line with a few of the semiconductor manufacturers and take a look at a few controller chips and you can see what I mean.


Good Luck


dfullmer
 

Voltage Regulator

hi
if you see datasheet of lm317
you can find many circuits that can help you.

you can use a resistor in series with battery to sense current.

regards
 

Voltage Regulator

If it is supposed to be a phone charger, the charger is part of the phone, you just need to supply appropriate voltage and current. Make it no less no more, than the original charger, phones are sensitive to that stuff.
 

Re: Voltage Regulator

verba, so how can i set the value of the current with this 2 variables (R1&R2).
 

Voltage Regulator

aghielan, 2 variables is not enough.
If you look at the datasheet of LM317, at figure 10 you can see battery charger. You can set the current with that Rs resistor (Resistor between the output of LM317 and R1, Vout is still at R1). (must be Rs<<R1,R2)
You'll get something like Vout = (Vref-I*Rs)(1+R2/R1)
And yet, you must know what current you'll need, check the original charger.
 

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