# Not attaining the desire current (Adjustable current source)

#### VictorMBS

Joined Jun 13, 2021
6
Hello Guys. At the schematic below, you can see a circuit that I picked from a datasheet. In this circuit, I should expect a maximum current of 4 A flowing at the fake resistor of 0.0001 ohms. Unfortunately, after I brought the circuit to the real world, I attained a maximum current of 2.40 A. What could be the cause of this problem? I used two integrated circuits, a LM338 TO-220 and a LM317T.

#### VictorMBS

Joined Jun 13, 2021
6
Note: At LM338, I used a Heatsink with 2.32 ºC/W of thermal resistance

#### Ian0

Joined Aug 7, 2020
6,980
You should get the maximum current with the preset's wiper at the top on your drawing. In which case everything other than V2, U1, R1 and R3 are irrelevant. What is the voltage on the LM338 ADJ terminal?

#### VictorMBS

Joined Jun 13, 2021
6
You should get the maximum current with the preset's wiper at the top on your drawing. In which case everything other than V2, U1, R1 and R3 are irrelevant. What is the voltage on the LM338 ADJ terminal?
Hi Ian0! Thank you for your comment! The voltage between adjust and input terminal of LM338 was 4.95 V. Besides that, the voltage between adjust pin and output pin was 0.75 V.

#### crutschow

Joined Mar 14, 2008
31,499
What is the purpose of that rather strange looking circuit?
What "datasheet" did that come from?

#### Ian0

Joined Aug 7, 2020
6,980
It would appear to put a constant current through the preset, so that a voltage could be added to the voltage drop across the current sense resistor, creating a constant current output that could be varied down to zero.
I can think of better ways . . . .
but presumably some new graduate engineer was told to design a variable constant current source circuit that could be put in the LM338 datasheet.
But if there's only 0.75V between output and adj then something is wrong with the LM338. It would appear to have enough headroom, unless the load resistor isn't a low as it is claimed.

#### Tonyr1084

Joined Sep 24, 2015
7,247
if there's only 0.75V between output and adj then something is wrong with the LM338.
My guess is R4 creating a voltage divider, which is why you're seeing so low a voltage.

#### VictorMBS

Joined Jun 13, 2021
6
What is the purpose of that rather strange looking circuit?
What "datasheet" did that come from?
Hello! The purpose of this circuit was regulate the current that flows between the path where is R3. We should expect a maximum current of 4 A and a minimum current of 0 A. We attained the desires values between 0 to 2.4 A, but, unfortunately, we didn’t get values above 2.4 A. The purpose of the potentiometer it is regulate the current that flows through the path where is R3. I picked the circuit from LM338 texas instruments datasheet. I just make a few changes because I didn’t have the resistors that appeared in the datasheet.

#### Tonyr1084

Joined Sep 24, 2015
7,247
Hello! The purpose of this circuit was regulate the current that flows between the path where is R3. We should expect a maximum current of 4 A and a minimum current of 0 A. We attained the desires values between 0 to 2.4 A, but, unfortunately, we didn’t get values above 2.4 A. The purpose of the potentiometer it is regulate the current that flows through the path where is R3. I picked the circuit from LM338 texas instruments datasheet. I just make a few changes because I didn’t have the resistors that appeared in the datasheet.
Again, R4 is acting like a voltage divider and is probably what is causing your issue. As Ian0 said before, there's a lot of unnecessary hardware in that circuit. Lose R4 and everything below that and I predict success.
everything other than V2, U1, R1 and R3 are irrelevant.
I would add (what you're calling U3) as being necessary.

#### VictorMBS

Joined Jun 13, 2021
6
My guess is R4 creating a voltage divider, which is why you're seeing so low a voltage.
View attachment 249825
Hi Tony! In some of my tests, I didn’t put the R4 resistor and I obtained the same 2.4 A maximum current. Here is the another circuit schematic. Besides that, I can not remove everything below because I need of these components to control the current that is going to R3. If I remove all of the componentes below R4 I wouldn't be able to change the current flowing through R3.

Last edited:

#### ElectricSpidey

Joined Dec 2, 2017
2,266
I just make a few changes because I didn’t have the resistors that appeared in the datasheet.
Well that could be the problem, does the sim work with the proper values?

#### VictorMBS

Joined Jun 13, 2021
6
Well that could be the problem, does the sim work with the proper values?
Yes, the sim worked pretty well. I attained the desire current in the simulation but, in real world, I didn't attained the desired current of 4 A.

#### Ian0

Joined Aug 7, 2020
6,980
What value DID you use for R3?

#### bertus

Joined Apr 5, 2008
22,133
Hello,

Reduce the value of R2 to 120 as mentioned in the TI schematic in post #8 as R3.

Bertus

#### Ian0

Joined Aug 7, 2020
6,980
Hello,

Reduce the value of R2 to 120 as mentioned in the TI schematic in post #8 as R3.

Bertus
You must also replace the preset for a 150Ω preset. Has anyone ever seen a 150Ω preset?

I suggest that for initial testing, just move the preset to its minimum setting (so it shorts ADJ to R1) because that would give the maximum output. Then, if that works, check whether the adjustment works.

#### sagor

Joined Mar 10, 2019
796
Accordin g to datasheet, you get the 5A rating only if Vin-Vout <10V. If Vin-Vout is 30V, maximum current is 1A due to current limiting. So, your 2.4A may be current limiting due to the input voltage vs the output voltage being more than 10V. I think the simulation shows 5V, which may be ok. That said, heat generated inside the LM338 may still limit the current.
The TO-3 can design is rated 50W dissipation for the datasheet specifications, the TO-220 is rated 25W dissipation for the same specifications on the datasheet (TI datasheets).

#### Tonyr1084

Joined Sep 24, 2015
7,247
Did you try this:

#### ElectricSpidey

Joined Dec 2, 2017
2,266
The only suggestion I can make is the obvious one...get as close to the data sheet values and make sure the circuit is correct.

#### crutschow

Joined Mar 14, 2008
31,499
What is the measured supply voltage when the real circuit output is 2.4A?