Chinese LM2596 DC to DC voltage down converter instability - Page 1
Forum section: electronics / beginners

author: Chris Wilson
author_rank: Super Contributor
timestamp: December 16, 2013, 11:46:27 am
content: Sorry, I am here again, this forum is just TOO goodWhen experimenting on a breadboard I used a commercial el cheapo Chinese DC to DC voltage step down converter to drop 24V to 13.5 V with a load of about 650 mA. The voltage out was unstable, and the multi turn voltage level pot quite un linear in places. Touching the body of the output electrolytic on the converter made quite big voltage changes, as did touching an output lead. To get an under load 13.5V out I had to have the pot set for about 15V out, off load. Duff device? Normal, I'm a cheapskate, you get what you pay for? Or normal behavior for these circuits, I should RTFM, I don't understand how these things SHOULD work? Thanks!

author: kripton2035
author_rank: Super Contributor
timestamp: December 16, 2013, 12:11:19 pm
content: to be sure of it, I would experiment with another chinese 2596, and then with a "real" 2596to see if the rest of the circuit is not the cause of the problem ?

author: sleemanj
author_rank: Super Contributor
timestamp: December 16, 2013, 01:40:26 pm
content: Generally these 2596/2577 converters out of china do what they say, and are jolly handy, I can't say I've noticed any instability in the many I have used.If it was me, I'd look to the two electrolytic SMD caps first (and the inductor second), should be easy enough to replace them with some temporary through hole ones of the same value just with the legs soldered onto the old smd pads, it wouldn't surprise me if they are the cause of your problems. May even find that one of them has a broken solder joint.

author: Chris Wilson
author_rank: Super Contributor
timestamp: December 16, 2013, 01:54:47 pm
content: I have another one, I'll try that. I have looked at a few YouTube resumes of these since posting and none seems to exhibit the issues this first one has. I'll have a close look at the soldering and maybe do some repair work if the other behaves differently. Thanks

author: Kjelt
author_rank: Super Contributor
timestamp: December 16, 2013, 03:26:01 pm
content: Also test the potentiometer, if I need one fixed output voltage with these prints I always desolder the potentiometer and replace with two fixed 0805 resistors with the correct value (or three if two won't cut it with the E12 series).

author: redwire
author_rank: Newbie
timestamp: December 16, 2013, 03:41:40 pm
content: You have to add output capacitance (>680uf) to get them stable. The input cap is small too.Also note they use a 1A buck diode only which gets pretty hot. No way these are good to 3A.

author: Kjelt
author_rank: Super Contributor
timestamp: December 16, 2013, 03:58:54 pm
content: Quote from: redwire on December 16, 2013, 03:41:40 pmGood catch, from the ONsemi datasheet table 1 with the TS parameters:Quotethe elco out parameters are :Capacity (uF) : 470Voltage (V): 35 (datasheet says 25 but that is probably a misprint)ESR (mOhm): 65Use good low ESR output elco, recommended are Nichicon PM series.

author: Chris Wilson
author_rank: Super Contributor
timestamp: December 16, 2013, 05:02:00 pm
content: OK, I have found the fault! Thanks for the great info. The output cap is missing a leg! I have temporarily replaced it with a relatively huge 1000uF 50V electrolytic on long leads, and it now works perfectly. When I looked closely the output cap pad had obviously never had anything soldered to it. As space isn't an issue should I fit another 1000uF on the input side as well? The other unit works perfectly, although if I use it for anything at all critical I might revamp the caps on that, too. I have no need to draw more than an amp maximum. Thanks for the help, had I looked properly I should have seen the issue for myself, I continue to learn