Confusing LM338 Problem  - Page 1
Forum section: electronics / projects

author: BiOzZ
author_rank: Regular Contributor
timestamp: March 06, 2014, 08:51:17 am
content: I have a 36v 5A switch mode supply (nice one with a really clean output) a standard LM338 circuit with R1 modified to work with a 10K pot and the LM338 on a nice big heatsink and i bypassed all my output filtering and monitoring and everything but im still getting a damn near perfect 1A output current regulation across all voltages ... my supply can supply up to 5A (tested) and my ammeter is good and the entire thing is ground isolated but i still get only 1A output with a direct short (across the multimeter shunt)i tried with my dummy load, massive power resistors then finally a direct short with an isolated multimeter ... still exactly one amp maximum outputand yes before you say anything i checked and its (atleast marked) as an LM338 from TI i got off digikey ... the voltage regulation is just fine and the power supply is just fine and this is boggling my mindi thought at 4AM after trying everything i would go to you guys and see what you guys think before going to bedthanks in advance!

author: megajocke
author_rank: Regular Contributor
timestamp: March 06, 2014, 09:10:56 am
content: The current limit depends on the voltage drop over the regulator. It will only be able to give full current in a narrow voltage drop range.There should be a graph in the datasheet showing this.

author: BiOzZ
author_rank: Regular Contributor
timestamp: March 06, 2014, 07:16:38 pm
content: Quote from: megajocke on March 06, 2014, 09:10:56 amno you dont understand ... there could be a 32v drop or a 2v drop it does not matter its still a constant 1A output maximum output with only 1LSD drift thats seemingly just noisei tested this just as an update on my work HP 55v powersupply that can supply 8A and still no dice and the output on the scope looks nice and clean

author: Swake
author_rank: Frequent Contributor
timestamp: March 07, 2014, 09:47:58 am
content: is there a capacitor on the output of the lm338? You should have at least a small one like 10uF, avoids ringing

author: Swake
author_rank: Frequent Contributor
timestamp: March 07, 2014, 10:13:18 am
content: Had a look at the datasheet. If difference between input voltage and output voltage is 30V then output current is limited at 1A.

author: BiOzZ
author_rank: Regular Contributor
timestamp: March 08, 2014, 01:07:49 am
content: Quote from: Swake on March 07, 2014, 10:13:18 amthere is a 22uf cap on the output and the output waveform is cleanthe voltage difference does not matter ... i could set it to have a differential voltage of 2v or 32v and anything in between and it does not change

author: David_AVD
author_rank: Super Contributor
timestamp: March 08, 2014, 01:20:35 am
content: Maybe a circuit diagram of what you've actually made up would be helpful?

author: BiOzZ
author_rank: Regular Contributor
timestamp: March 08, 2014, 01:45:11 am
content: i threw this together really quickly but im 99% sure its exactly the same as the circuit i usedi do have an extra LM338 but i used thermal epoxy (long story) on the first one to secure it to the heatsinc .. i can probably throw somthing else togeather but i really want to try to get this working before i throw another entire one together and use up my last 338i tried moving the sense wire directly to the chips output (threw a 3" alligator lead), i have tried bypassing the output cap (and output inductors not shown here), i tried adding a 2200uf cap, i checked all the diodes and resistors used and there all working well and i tried using a lab power supply that i know is good to power it and its still acting just as strangely!and the 338 was a relatively expensive one with a nice thick tab and proper TI marks and a 2011 date code i believe and im fairly sure it worked on breadboard to 3A but i cant fully remember (work put this project on the back burner for a month or so)

author: David_AVD
author_rank: Super Contributor
timestamp: March 08, 2014, 04:31:03 am
content: So when the current exceeds 1A, the voltage going into the circuit (from the 5A supply) remains at whatever you set it at?

author: BiOzZ
author_rank: Regular Contributor
timestamp: March 08, 2014, 05:15:57 am
content: Quote from: David_AVD on March 08, 2014, 04:31:03 amthe voltage does go down when i start putting large loads or a short (as low as 4V from a set 32v) but thats to be expected ... at a 1.3A constant load it does not drop anything more than what i would expect lead loss to be

author: Swake
author_rank: Frequent Contributor
timestamp: March 10, 2014, 12:18:58 pm
content: Quote from: BiOzZ on March 08, 2014, 01:07:49 amLooks like the story about the 1Amp is more than a coincidence. The differential detector (or whatever they name it) might be shot effectively activation some sort of protection/limiting mode.Wire in that spare thing you have. No need for a big heat sink if you limit the current to something like 1.5A and apply it only for a couple of seconds. This would however demonstrate if works or not.