Hey all,
New member here, looking for some help with a very simple circuit. See below. The same circuit applies to the LM338T regulator, except it can handle higher currents (up to 5A).
I have used this circuit before quite successfully many times, using a .50 Ohm resistor, three 10W LEDs (Vf 3.0-3.3V each), and a 12V bench power supply (adjustable from 11.0 to 15.0V). Typical behavior has the current entering regulation at 12.5V. After this point, the output current remains rock-solid at 2.5A as I increase supply voltage up to the maximum 15.0V. The excess voltage turns into excess heat. Thus, the circuit requires about 2.5V over the combined LED voltage to remain in regulation. These chips were labeled Fairchild Semiconductor LM338T JM14RG P+.
Here's the issue. I recently purchased a new batch of regulators from a new supplier. I connected one of them exactly the same way, and at 15.0V I am only seeing 1.8A. I am not shorting any of the leads, and the setup is identical to what I used before. The new chips are ON Semiconductor labeled LM338T RBQ209G.
So, I bought yet another batch, thinking they were bad, from the supplier I originally purchased from. These are Fairchild Semiconductor labeled LM338T JM32RG P+. I connected it in the same way as before, and they still only reach 1.8A.
So I am using the same chips, from two different suppliers, in the same configuration, and getting the same results, both of which are different than they should be.
I do not have an electrical engineering degree and struggle with the theory of how this circuit works. I know there is a 1.25 reference voltage created between the output and adj terminals, which is how you calculate the resistor required.
Can anyone shed some light on why this isn't working, and what I can possibly test for to diagnose why?
edit: Here is the data sheet for this type of linear regulator. http://www.ti.com/lit/ds/symlink/lm138.pdf
New member here, looking for some help with a very simple circuit. See below. The same circuit applies to the LM338T regulator, except it can handle higher currents (up to 5A).
I have used this circuit before quite successfully many times, using a .50 Ohm resistor, three 10W LEDs (Vf 3.0-3.3V each), and a 12V bench power supply (adjustable from 11.0 to 15.0V). Typical behavior has the current entering regulation at 12.5V. After this point, the output current remains rock-solid at 2.5A as I increase supply voltage up to the maximum 15.0V. The excess voltage turns into excess heat. Thus, the circuit requires about 2.5V over the combined LED voltage to remain in regulation. These chips were labeled Fairchild Semiconductor LM338T JM14RG P+.
Here's the issue. I recently purchased a new batch of regulators from a new supplier. I connected one of them exactly the same way, and at 15.0V I am only seeing 1.8A. I am not shorting any of the leads, and the setup is identical to what I used before. The new chips are ON Semiconductor labeled LM338T RBQ209G.
So, I bought yet another batch, thinking they were bad, from the supplier I originally purchased from. These are Fairchild Semiconductor labeled LM338T JM32RG P+. I connected it in the same way as before, and they still only reach 1.8A.
So I am using the same chips, from two different suppliers, in the same configuration, and getting the same results, both of which are different than they should be.
I do not have an electrical engineering degree and struggle with the theory of how this circuit works. I know there is a 1.25 reference voltage created between the output and adj terminals, which is how you calculate the resistor required.
Can anyone shed some light on why this isn't working, and what I can possibly test for to diagnose why?
edit: Here is the data sheet for this type of linear regulator. http://www.ti.com/lit/ds/symlink/lm138.pdf
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