Lm338 queastion!

[watzmann]

Hi All ,

i have tested the Lm338 regulator , and as the datasheet indicates it provides constant 5A.

my question is :

how it will deliver 5A if i connected it to Ac/Dc adapter has output of 30 volts and 500mA.

secondly , i'm trying to charge my battery using this regulator but once i touch the two batter terminals the LM338 goes hot.....why that happened

thank you

 

[Speakerguy]

It CAN provide up to 5A constant current. It can be run as either a voltage regulator or as a constant current source. It will not provide more current than it can be supplied with.

If you hook it up to a 30VDC 500mA supply, and you want to charge a battery, you need to hook it up as a constant current source that charges at a current less than 500mA. You will burn up quite a lot of power in the regulator. I would try for a AC/DC supply that is much closer to the voltage of the batteries you are charging.

LM338 gets hot because you are dissipating a lot of power in it. Hook it up as a CCS, limit the current to a low value, lower the DC input voltage, put a sink on it. Any of those will help.

 

[audioguru]

i have tested the Lm338 regulator , and as the datasheet indicates it provides constant 5A.

my question is :

how it will deliver 5A if i connected it to Ac/Dc adapter has output of 30 volts and 500mA?

It will work fine but it will become very hot if the output voltage is set low.
If the load tries to draw more than 500mA then the LM338 will supply it and the adapter will be overloaded.

i'm trying to charge my battery using this regulator but once i touch the two batter terminals the LM338 goes hot.....why that happened?

Because the input voltage from the adapter is much too high.
The regulator has 30V - 12V= 18V across it.
The regulator reduces its max outoput current when it has more than 10v across it .
There isn't anything to limit the current so the regulator will limit the current to about 4A.
The amount of heat is 18V x 4A= 72W. If the regulator is in the big metal case and has a huge neatsink then it might survive or maybe would also need a fan. The smaller package will overheat and will shutdown.

If the input voltage is 16V then the heat is only 16V - 12V= 4V x 6A= 24W.

 

[watzmann]

LM338 gets hot because you are dissipating a lot of power in it. Hook it up as a CCS, limit the current to a low value, lower the DC input voltage, .

is that mean my battery is trying to draw more current than this 500mA.

if that is right so it may burn my adapter, or i'm wrong

 

[audioguru]

A battery charger usually has something to limit the current. Yours doesn't so the adapter is overloaded and it will smoke and burn.

 

[watzmann]

so i have to limit the current

 

[audioguru]

The datasheet for your battery will say what is its recommended charging current.
Is it lead-acid, Ni-Cad, Ni-MH or lithium??

You are using a regulated voltage. Most battery chargers use a regulated current.

 

[Speakerguy]

Like audioguru says, you need to regulate current. All it takes is the LM338 and a single resistor. The 338 maintains a 1.25V drop from its output to its adjust pin. You put a resistor across those two to set the current and the adjust pin node is your output. So if you wanted 100mA constant current you would put in a 12.5ohm resistor between out and adj, and then connect adj to your load.

 

[watzmann]

i have made this schematic ...... please look at it and tell me if i have something wrong .

i think it will provide 14.5 volt with constant Amp 0.4,


thank you

 

[audioguru]

An LM317, LM338 or LM350 can be a voltage regulator or a current regulator.
You tried to combine the functions so it is a very poor voltage regulator and a very poor current regulator.

You need to have a current regulator in series with a voltage regulator for them to work properly.

The 30V input is way too high. It will just make a lot of heat in the regulator.

 

[watzmann]

what about this

 

[audioguru]

The current regulator should come first, because it needs up to 3.75V across it from in to out for it to work.

The 35 ohms resistor in the current regulator is incorrect. It should be 3.125 ohms for 400mA.

The 5k pot in the voltage regulator will make an output voltage of 27V when it is turned to half. A 1.5k pot will make 16.9V max.

 

[Speakerguy]

Delete the ground connection to the adjust pin on the current regulator. That is where you connect your load.

 

[audioguru]

Delete the ground connection to the adjust pin on the current regulator. That is where you connect your load.

The output was shorted to keep the juice from leaking out!

 

[watzmann]

The 35 ohms resistor in the current regulator is incorrect. It should be 3.125 ohms for 400mA.

.

could you please tell me how you calculate this resistor :

i have output volt of 14.5

I = V/R

I= 14.5 / 35 then I = 0.4 A

what is wrong in that

 

[watzmann]

what about this ?

but where to connect the Gnd in the first Regulator.....or ir's not required

 

[audioguru]

The LM338 current regulator has up to 1.25V from its OUT pin to its ADJ pin.
If you want a current regulator to have a 400mA output then the resistor is 1.25V/400mA= 3.125 ohms.

The LM338 current regulator doesn't connect to ground.

 

[Speakerguy]

Make sure to use at least a .5w or better yet a 1w resistor for the 3.125ohms.

 

[watzmann]

what will happen if i didn't use 1 watt for the resistor , may be i cannot find this value........will it burn the regulator

 

[audioguru]

A 3.125 ohms resistor with 400mA in it dissipates 1.25V x 400mA= 0.5W. A half-watt resistor would be extremely hot and would burn you and other things that touch it. A 1W resistor is bigger and would be cooler.

Nobody makes a 3.125 ohms resistor. You simply parallel a couple or a few standard value resistors to make one.