LM2596 down converter module with DVM. Some observations....

[kjs]

Let me first say that I am a long time lurker here (since years) but never had much to contribute which is why I never posted before.

This module which is an LM2596 based step down converter with integrated DVM for input or output voltages was mentioned here before but I haven't seen a tear-down (yet). Well, now I have a micro-tear-down of it after I bought one for $2.67 off ebay. It is more an electrical observation than a real tear-down.

The good: the converter works nice and had lower ripple than specified, ~10mV under worst case and usually quite a bit less. The display is nice, big and bright and the indicator LED's are almost blinding. It has a little bit of an issue with rapid load changes from a few milli-amps to to 1.5A (my test currents) and rings a little bit. This is most likely due to the lack of an RC feedback network to compensate for it. But it isn't a serious issue for what I intended to use the module for. The build quality was really good and an inspection under the stereo microscop showed good quality solder joints and well aligned components on their pads (sorry, don't have a camera on my microscope, yet).  In short it does what it is supposed to do and on the first glance it was just another miracle part difficult to understand how China Inc. can produce such a nice module for such a low price even including free shipping.

Now to the not so good or really bad: I mentioned the super high intensity of the 3 LEDs, especially the green one for power applied and as I didn't want to correct them by changing the series resistors I simply used a black permanent marker to tone them down quite a bit. Nice thing is if you did it too much a Q-tip with some alcohol reverses it and you can start from scratch.
Now to the real issues which are a clear design flaw! The module showed the wrong output voltage and was off by 0.3V over the entire range. Instead of the actual 5V as measured on my HP bench-top DMM it showed 4.7V. Not a problem I thought as it has a calibration mode.
To dig deeper a short explanation of the two buttons on the module. The left button toggles the DVM between on and off. The right button toggles between input and output measurements and the side measures is shown with a red LED. To get into calibration mode you have to switch to the side you want to calibrate and then hold the right button for 2-3 seconds. Upon release the display blinks and with the two buttons, left = down and right = up the displayed value can be changed. It may take a few clicks to see a change as the resolution is finer than that of the display. Once you are done hold the right button down again for 2-3 seconds and the correction is stored. Well, that's in theory! On my module everything worked like a charm for the less important input side. When trying to do the calibration for the output side the display didn't move or so I thought. When looking closer I could see that there actually was a change but it was only displayed for a few milliseconds and then went back to the original. With a bit of patients I got the short flash of correctness to be correct and stored it. It didn't make a change :-( The offset was still there but then I discovered that when I toggled between input and output measurement that the newly corrected value was actuall show on transfer for a few milliseconds (just a flash) and then went back to uncalibrated.

Not a big deal I thought as Chinese ebay sellers are usually highly motivated. I complained about it and they thought that I received a bad module and shipped me a new one for free. 4 weeks later it arrived on my bench and, you may have expected it, has the exact same firmware design flaw!
Now to the even bigger surprise. I wrote the seller again with a detailed explanation and to my surprise received a response in almost perfect English (the first one was the typical Chinglish). They have forwarded the complain to their supplier and promised me yet another module! Hopefully they are able to fix it soon.

As it is hard to come by the operation instructions for the module I attach a picture with explanations. Pretty simple once  you know how but unfortunately as of yet only working for the input side measurements.....

As there are very similar but not identical looking modules for sale it may be just the one supplier which has a problem but I don't know. If you get one make sure that the calibration actually works.

Juergen