lm2596 module - negative voltage - Page 1
Forum section: electronics / projects

author: t0mt0m
author_rank: Contributor
timestamp: August 25, 2022, 09:06:38 am
content: Hi,I have a meanwell rt-65c power supply. Voltages are +5V, +15V and -15V.Now I also need -5V and +12V for my application.I have some lm2596 modules I falsley bought years ago lying around and was wondering if I could use them.Converting +15V into +12V is straight forward but I am not sure if converting -15V into -5V is possible and if so, how to hook it up.GND to + and -15V to - ? And then the output + to GND and - = -5V?(looking at the reference design this will not work of course)I have no clue how these lm2596 modules work internally, so I am hoping for some recommendations before I fry something.cheers

author: wraper
author_rank: Unknown
timestamp: August 25, 2022, 09:21:24 am
content: Those modules are based on fake lm2596 which are not even close to original thing. The best thing you can do is scrap them if you do care about not burning the load.

author: t0mt0m
author_rank: Contributor
timestamp: August 25, 2022, 11:00:30 am
content: Yes, they are most likely fake. But I have the same style modul in step up version for boosting 5V -> 20V for backlights and they work flawlessly for over 4 years now.But yes, I absolutely do not want to fry the load. I guess I'm better off with the old 7905 then. At least that's the alternative I have lying around atm...

author: Siwastaja
author_rank: Super Contributor
timestamp: August 25, 2022, 01:04:15 pm
content: How much current do you need? Good old 7905 is fine, but coming from -15V, any linear regulator is just max 33% efficient. Not bad if it's a low power rail, though.

author: BradC
author_rank: Super Contributor
timestamp: August 25, 2022, 02:21:24 pm
content: Quote from: wraper on August 25, 2022, 09:21:24 amOr do what I did. Buy 10 modules, 10 genuine lm2596 and upgrade them. Still worked out cheaper than me trying to source something branded and reliable, or knocking up boards myself.

author: t0mt0m
author_rank: Contributor
timestamp: August 25, 2022, 08:20:20 pm
content: Which brings us back to the question if it would even be possible to use these modules - fake or not - to convert -15V into -5V.My design isn't yet finished but the -5V rail should stay at around 300mA.Thank you all so far

author: magic
author_rank: Super Contributor
timestamp: August 25, 2022, 08:47:04 pm
content: LM2596 (or clones) is a positive buck regulator, it can only source current to its output. It can sink from the input pin, and you could perhaps put your inductor there, but then feedback would have wrong polarity.You would need a negative buck regulator which can sink current and regulate voltage with respect to its positive rail (GND), or maybe an inverting converter to go from 5V to -5V.At 300mA, a linear regulator obviously dissipates 3W under such voltages. This could be doable by LM337 or LM7905 in TO220 with a small heatsink.

author: niconiconi
author_rank: Frequent Contributor
timestamp: August 26, 2022, 01:40:48 pm
content: You can't use a positive buck converter to convert -15 V to -5 V. BUT it should be possible to convert +15 V to -5 V. Most ordinary buck converters can be "abused" as an inverting buck-boost converter by moving the ground reference.See: Working With Inverting Buck-Boost Convertershttps://www.ti.com/lit/an/snva856a/snva856a.pdfBut you'll have to cut some traces. The "GND" pin of the regulator and the surrounding LC circuit must be isolated from the input ground.