LM2596 CC/CV module modification + forum layout rant! - Page 1
Forum section: electronics / beginners

author: poot36
author_rank: Frequent Contributor
timestamp: January 09, 2016, 10:04:22 pm
content: I have one of the Constant Current Constant Voltage adjustable LM2596 based modules and am trying to modify it for reduced minimum Constant Current. The pot for the current adjust is at its lowest setting and I am still getting 140ma of current. I am trying to reduce it to 30ma to 50ma. Would I need to adjust the resistor attached to the feedback pin to get this to work? Or would I have to adjust the CC/CV LEDs voltage drop? Pin 2 of the LM258 op-amp is already grounded by the current adjust pot. My board is blue colored and has the text LEIVIN printed on the back. I can only find a schematic for the red version which has different values for the resistors then mine. Eg R1 in this schematic (red board):http://www.mpja.com/download/31562ps.pdfis 470 ohm where as mine is 360 ohm. I am trying to use this module for a low power solar panel 1.8W 12V battery maintainer and convert it to 5V for trickle charging of USB powered devices.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 09, 2016, 10:33:31 pm
content: The minimum current setting might be limited by the offset voltage of the OP used - the LM358 is a low cost, not very accurate version. One way to lower current would be increaing the resistor used to sense the current, e.g. using a prober 100 mOhms resistor instead of just a piece of trace. However this would also lower the upper limit.Another option would be adding an offset to pin 3 of the OP, e.g. through a resistorin the line to the "shunt" and one towards the 5 V. As something like 5 mV should be enough, one might use somthing like 100 Ohms and 100 K.

author: poot36
author_rank: Frequent Contributor
timestamp: January 09, 2016, 11:03:57 pm
content: Would a current shunt resistor out of a old ATX PC power supply work? I don't really care about the 3A output current, even having 1A would be enough.

author: poot36
author_rank: Frequent Contributor
timestamp: January 11, 2016, 06:09:32 am
content: Ok, here is what I did to fix my problem. I disabled the built-in current shunt and replaced it with 2 ~2.9 ohm resistors in parallel to get a measured 1.44 ohms on my $20 LCR meter that allowed me to set the current limit to a max of ~70ma or so. By the time I had done all that work the sun was to low in the sky to test the modification on solar power. Oh well, I should be able to test it next week assuming that the sun is out.(Begin forum rant)1.I really wish this forum was like almost every other forum and had a reply button for every topic that would then let the user log in and then post in that topic instead of logging in then finding the topic and then posting in it!2.Also have the notify me of reply's button by default selected so that new users to the forum would have a higher chance of seeing that they had been responded to or at least don't have the user have to click the attachments and other options to get to the button. Maybe the Mods could add these features? It would make this forum seem more modern.(End forum rant)

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 11, 2016, 06:52:24 am
content: Quote from: poot36 on January 11, 2016, 06:09:32 amNot sure exactly what you are asking for, but I just have my EEVblog shortcut set to:https://www.eevblog.com/forum/unreadreplies/That way, I can first check for replies to topics I have posted in (or started), and then use the "Show unread posts since last visit." link to see new topics. Seems to work quite well for me.

author: poot36
author_rank: Frequent Contributor
timestamp: January 12, 2016, 01:03:26 am
content: You have a point there. I do agree for the most part that link would do what I want.