Welcome to part 5 of my Allstar Link node build.
If you read part four of my node build, you probably arrived here expecting to read a blog post covering the grand boxing up of the project! Fear not, this is just a short interlude and the boxing up will now commence in part 6.
I need to vent a little about my concerns regarding the "Buck Converter Module" that is commonly used in node builds. The idea of using a buck converter to regulate a DC supply voltage is fine and I intend doing this myself but there is a caveat!
If you are not aware of what a buck converter is, it is a type of switch mode Power Supply; they are highly efficient when compared to traditional linear voltage regulators.
The buck converter module that I will be referrincing is shown below and can be purchased from eBay, AliExpress and many other online sources for less than £2.
When is a LM2596 not a LM2596 ?
The short answer to this is when it's a counterfeit part, such as a re-badged or rejected component.
Counterfeit parts often emerge from the Far East and typically from China. The fake parts are inferior in specification, substandard and liable to malfunctions causing damage to other electronic parts & circuits, not to mention the potential safety risk they pose.
Submission of evidence
I have built several nodes as per my blog documentation and during the first build I had an issue where the audio being transmitted from the node was rather choppy. My initial thought was that the buck converter module powering the Baofeng radio maybe faulty.
The buck converter module was not the cause of the problem; it was, however, tracked down to a 3COM switch on my home network that had developed a fault and was dropping packets of data like they were going out of fashion!
During my fault finding the problem, I looked at the output from the buck converter on my oscilloscope. The DC waveform looked good but there was some noise/ripple on the trace. On close inspection of the AC signal content, I could see evidence of the switching waveform at a frequency of 51.02kHz
I was interested in looking at the manufactures specification data to compare my own findings so I downloaded the PDF data sheet from Texas Instruments. The documentation states that sawtooth ripple at the switching frequency should be within 0.5 to 3% of the output voltage.
Here are the results from my own testing; the results do not stack up with the manufacturer data or specifications!
|
INPUT VOLTAGE |
OUTPUT VOLTAGE |
LOAD |
MEASURED RIPPLE |
|
12 Volts |
5 Volts |
NONE |
43mV @ no load |
|
12 Volts |
5 Volts |
18R 2 Watt |
300mV @ 277mA load |
Also the switching frequency of 51.02 kHz is not that of a genuine LM2596. The manufactures data sheet clearly states that the switching frequency of the LM2596 is nominally 150 kHz, no less than 110kHz & no more than 173kHz.
Conclusion
Summing up
There is no doubt whatsoever that the switching device used on the aforementioned buck converters is not a LM2596 part. These fake devices do not meet the specifications of the genuine part and should therefore be avoided!
Being a fake device means they are very much an unknown quantity so tolerances and ratings cannot be determined or trusted.
Built in protection, like thermal shutdowns and power limiting, may not be present in these fake devices so there is a potential risk that they could fail with consequences.
If the main switching transistor(s) on the silicon substrate goes short circuit you will end up with the full input voltage present at the output, which, in turn, is likely to cause failure to subsequent devices powered by the module.
Most Allstar nodes are built around a Raspberry PI mini computer board with a power supply requirement of 5 volts. If the node box is powered by say a 12-volt supply then reduced down to 5 volts with a fake buck converter, failure is likely to destroy the node and possibly create a hazardous risk.
For
my node builds, I decided to power everything from 5 volts using a
conservatively rated Raspberry PI 4 power supply. The PSUs is rated to 3
Amps and provides a level of reassurance that should the node or the
power supply itself fail, all the needed protection measures are in
place, I can keep my node powered up 24/7 with complete peace of mind.
Safety in numbers
If you have built or are considering building a node and powering it from a buck converter, you should at least consider adding some protection after the module before it feeds the Raspberry PI, node radio and any other electronics.
As a minimum I would suggest adding a fuse and possibly some over-voltage protection, such as a crowbar circuit, especially if using the aforementioned fake modules.
If you have already built a node with a fake module, consider replacing the so-called LM2596 with a genuine component but also be mindful that other components on the PCB may also be fake or of poor quality so replace those with component types specified in the manufacturer's data sheet. I suspect the excessive ripple I measured is not only down to the fake LM2596 but also other inferior components used.
If you want complete peace of mind, do not use a buck converter and instead power your node from 5 Volts using a genuine Raspberry PI plugin power supply.
Disclaimer
The information provided here is based on my own testing and research. Views expressed and opinions formed are those of my own and provided with a view to being useful and informative.
The information provided does not constitute any responsibility, liability or warranty.
I cannot confirm or imply as to whether the device referred to as "fake LM2596" is or is not fit for purpose.
EndNote
Please feel free to post comments on my article; however, I am not interested in messages telling me that your fake buck converter works fine, facts are facts and the device is fake, it does not perform or conform to that of the genuine component!