DC-DC Buck Converter Step Down Module LM2596 Power Supply - Page 1
Forum section: electronics / beginners

author: MichaelW
author_rank: Contributor
timestamp: January 07, 2014, 01:15:38 pm
content: So, I bought a few of these things to play around with/use some day. The cost was cheap, $11 for 10 of them, so I don't mind blowing up a couple.The input voltage is 4V-35V and the output voltage is 1.23V-30V, so I set my supply to 12 volts and connected it to the input side and connected my multimeter across the output side. I got pretty much the same voltage as the input voltage, ever so slightly lower. I tried twiddling the adjustment pot to no effect.Should I have a load to get a different voltage across output? I don't think so, but wanted to ask just in case. Does hooking this kind of converter up with no load damage it? Could it just be bad out of the bag? The latter wouldn't surprise me, it's a cheapo straight from China. :-)I included a picture of the module and a picture of the "specs" for convenience.Is the listed 92% efficiency pretty decent? It says "highest" in quotes because it will vary, I assume the efficiency will drop the greater the difference is between the input voltage and the output voltage?I read the wikipedia article on a buck converter, but I'm still not sure what makes a buck converter a buck converter. Is it because it's a DC to DC converter? Or is it due to the topology of the circuit?Thanks for any insight anyone may provide.

author: rolycat
author_rank: Super Contributor
timestamp: January 07, 2014, 01:40:11 pm
content: A buck converter is a step-down DC-DC converter, as opposed to a step-up boost converter.Your module is probably working fine - the trimpot will be a 20-turn model and with only a 12V supply a lot of the output voltage range will be unavailable. You need to keep turning it anticlockwise until the set output voltage is well below the supply voltage. You should then be able to adjust it to provide the voltage you require.

author: Dantali0n
author_rank: Contributor
timestamp: January 07, 2014, 01:44:55 pm
content: I have 2 of these, they work fine. I first didn't get why they wouldn't work either. But it turned out you have to turn the pot A Lot!. It took me atleast 30 seconds of turning before the voltage would drop. I recommend turning them for a little longer. Mine also work without load.Hope this helps.sidenote ~ first post

author: madires
author_rank: Super Contributor
timestamp: January 07, 2014, 02:03:49 pm
content: That kind of DC DC converter doesn't need a load attached for proper function. The efficiency of 92% is the optimum. For 12V input and 3V output with a 3A load attached it's going to be around 73%. The buck function is defined by the way the choke and diode are used/switched. The PCB seems to lack a small cap parallel to R2 (top side of the pot) as suggested by the reference design.

author: JoeO
author_rank: Frequent Contributor
timestamp: January 07, 2014, 03:06:33 pm
content: Is that a 35V capacitor on the input? And the input can be as high as 35v?

author: rolycat
author_rank: Super Contributor
timestamp: January 07, 2014, 04:18:32 pm
content: Quote from: JoeO on January 07, 2014, 03:06:33 pmThese converters are just over a buck each delivered. I don't imagine safety margins were at the top of their design priority list.

author: Bored@Work
author_rank: Super Contributor
timestamp: January 07, 2014, 05:36:58 pm
content: Quote from: rolycat on January 07, 2014, 04:18:32 pmAnd it is already the revised spec. At some point in time they were advertised for up to 40 V input voltage - and had a 35 V cap on the input ...BTW, these modules can easily be used unchanged to generate a negative voltage. Just swap output and ground. And pay attention not to exceed the maximum 40 V, which in this configuration includes the output. E.g. the more negative the output the less the maximum input voltage can be.

author: Kjelt
author_rank: Super Contributor
timestamp: January 07, 2014, 06:47:07 pm
content: Place and extra ceramic capacitor (50V) parallel to the output and input elco for extra low ESR because those cheap elco's usually don't meet the spec of the LM2596 datasheet and uncooled never draw more then 1 amp.If you need a safe long time single output voltage for some project replace the trimpot with two matching (SMD) resistors, the trimpot is also crap quality and I wouldn't trust it for any long time.

author: MichaelW
author_rank: Contributor
timestamp: January 07, 2014, 11:31:12 pm
content: Thanks for the info guys, I goofed a little. When I posted that I wasn't at home so I just grabbed a picture of a module that looked like mine from Ebay and used that instead. The ones I got are a little different. Mine have 50v input caps. Im sure they are just as crappy though, as was mentioned at a hair over a buck a piece, one can't expect top quality parts. :-)Thanks for the info and advice. I hooked the one I thought might be bad back up, gave it a 20v input and put some spins on the pot and sure enough, down the output went. I was just being way to conservative with the pot.Edit: I should mention that I added the tiny heatsinks.

author: Wingspinner
author_rank: Newbie
timestamp: July 18, 2015, 11:26:31 pm
content: Quote from: Bored@Work on January 07, 2014, 05:36:58 pmThe input and output grounds are common so they really aren't very useful for generating a negative voltage. The only way they'd work for that is if the system under power ground is isolated from the input power.

author: ez24
author_rank: Super Contributor
timestamp: July 19, 2015, 01:19:59 am
content: QuoteIs there a way to get + and - 15 v for op amps using 2 of these and one 19v source? If not how about using 2 input sources (laptop power supply)tks

author: leblanc
author_rank: Regular Contributor
timestamp: July 19, 2015, 12:30:07 pm
content: http://www.ti.com/general/docs/lit/getliterature.tsp?baseLiteratureNumber=slua288&fileType=pdfThis shows you how to get a dual output out of a boost converter. I haven't had much success with this method, but I haven't put more than a couple of hours into it. I was also trying to power opamps with +/- 15V rails. If you get this working well, please share.

author: madires
author_rank: Super Contributor
timestamp: July 19, 2015, 02:14:54 pm
content: Quote from: Wingspinner on July 18, 2015, 11:26:31 pmPlease have a look at the datasheet and you'll see:- ON-Semi: Figure 22. Inverting Buck?Boost Develops ?12 V- TI: Figure 37. Inverting ?5V Regulator with Delayed Startup

author: madires
author_rank: Super Contributor
timestamp: July 19, 2015, 02:27:50 pm
content: Quote from: ez24 on July 19, 2015, 01:19:59 amThe question is, how much current do you need? The LM2596 is a 3A buck converter and the inverting circuit can deliver up to 0.7A. If you just need a few mA, there are better solutions.

author: madires
author_rank: Super Contributor
timestamp: July 19, 2015, 02:38:42 pm
content: Quote from: leblanc on July 19, 2015, 12:30:07 pmAnd there'shttp://bitsavers.trailing-edge.com/pdf/national/_appNotes/AN-1081.pdffor the LM2596That method works for most buck converters.

author: Kjelt
author_rank: Super Contributor
timestamp: July 19, 2015, 03:12:22 pm
content: Quote from: madires on July 19, 2015, 02:38:42 pmStrange AN why does C1 have to be 63VDC while it only buffers 3.3VDC?

author: ez24
author_rank: Super Contributor
timestamp: July 19, 2015, 05:26:16 pm
content: QuoteA few mA as in experimenting (and learning) with op amps. The above circuits look complicated. I have not researched this out but I will look for a better solution. I wanted to use two laptop 19v power supplies and two bucks and reverse the leads - sounds like not. tks

author: mariush
author_rank: Super Contributor
timestamp: July 19, 2015, 07:39:08 pm
content: Quote from: Kjelt on July 19, 2015, 03:12:22 pmIt's probably to make sure the esr is low enough. A 270uF 6.3v or 10v rated Nichion PL (which is already discontinued and replaced withNichicon PM, so think how old this series is) would have current ripple too low or too high esr.The application note is from 1999, in 2015 you could switch to a polymer capacitor, just make sure its technical specs match the values you calculate with the formulas in the application note.

author: Kjelt
author_rank: Super Contributor
timestamp: July 19, 2015, 07:59:31 pm
content: Quote from: mariush on July 19, 2015, 07:39:08 pmA yes ofcourse, thanks good catchPolymer capacitors are very nice for SMPS applications, though hard to find for higher voltages and can be quite expensive.A MLCC in parallel would also be a good alternative.Back on topic, has somebody already made or seen a good LP output filter for the standard chinese Ebay LM2595 pcb's?

author: cwalex
author_rank: Frequent Contributor
timestamp: July 20, 2015, 12:00:11 pm
content: Quote from: ez24 on July 19, 2015, 05:26:16 pmIf you find 2 laptop power supplies that use 2 pin mains plug with no earth then it is my understanding that you can connect them both in series and use the center connection as your circuit ground to give you +19V and -19V relative to that point.

author: madires
author_rank: Super Contributor
timestamp: July 20, 2015, 01:43:16 pm
content: Quote from: ez24 on July 19, 2015, 05:26:16 pmA simple solution for a few mA is an OPamp providing a virtual ground by halfing the supply voltage.