# Need help on constant Voltage and current

#### plmrguest

Nov 20, 2015
4
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

Aug 31, 2014
1,416
Where is the 120R ?

#### plmrguest

Nov 20, 2015
4
in between Vout and potentiometer

#### Colin Mitchell

Aug 31, 2014
1,416
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

Nov 20, 2015
4
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

Aug 31, 2014
1,416
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

Sep 18, 2015
452
in between Vout and potentiometer
Connected to adj pin? What mfr? Heard plenty abt knockoffs (crap)

#### Colin Mitchell

Aug 31, 2014
1,416
"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

Sep 18, 2015
452
,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

Moderator
Sep 5, 2009
14,260
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

Nov 20, 2015
4
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.

#### Attachments

• power-supply.png
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