Need help on constant Voltage and current

[plmrguest]

I am trying to control voltage range from 1.5 to 34 using LM338 voltage regulator with 1000u capacitor at the input 430u capacitor at the output with potentiometer @ the adjustment terminal.

I have 37v and around 5 Amps current at the Vout but when i try to reduce the voltage using potentiometer,resistor 120 ohms burns out. Please suggest me how to control and get constant Voltage & Current.

 

[Colin Mitchell]

Where is the 120R ?

 

[plmrguest]

in between Vout and potentiometer

 

[Colin Mitchell]

Constant voltage and constant current are two completely different circuits.
You cannot get both at the same time. The best you can do is select a voltage and have current limiting - which really means only being able to supply a maximum current and if you try to take more current, the voltage drops.

 

[plmrguest]

Yes i totally agree, for my LED's working voltage would be 28 - 34V dc. so i would like to reduce / keep voltage constant at 33V from 37V. Current should be changeable using Jumpers (apparently using resistor mechanism).

 

[Colin Mitchell]

Do you realise that once you go below 25v, the LEDs will turn off completely.
The first thing you do is put a 100R (3watt) in series with the output and measure the voltage across it.

 

[GPG]

in between Vout and potentiometer

Connected to adj pin? What mfr? Heard plenty abt knockoffs (crap)

 

[Colin Mitchell]

"Connected to adj pin? What mfr? Heard plenty abt knockoffs (crap)"

No.

What has happened is the pot has gone open circuit and the 120R has been taken to 0v.

 

[GPG]

,resistor 120 ohms burns out.

Cannot see how that could happen unless the wiring is incorrect or the regulator is faulty or the regulator is oscillating. (.1 μ ceramic on the input)

 

[davenn]

Cannot see how that could happen unless the wiring is incorrect or the regulator is faulty or the regulator is oscillating. (.1 μ ceramic on the input)

agreed .... lets see your circuit, plmrguest

 

[plmrguest]

Here is my schematic for you reference i have both voltage and current , since we can't control both voltage & current so i need to keep the voltage constant 32V and current should be adjustable.