Bests & Worsts Reviews from Amazon

INTRODUCTION The LM2020 is an interesting fusion of a cira-2002 all-in-one Digital Multimeter Chip and a state-of-the-art ST-Micro STM32F401 ARM SOC. On one side, you get all those basic DMM functions handled by the older chip (a GC7721AQ-LP3, which is a copy of Fortune Semiconductor's FS9721-LP3 design, and has numerous other Chinese clones like the SEMICO CS7721CN), but instead of handing the output of those basic functions directly to the display, they get passed to the ARM processor instead. The ARM processor adds the color display, pretty fonts and oscilloscope functionality, transforming that old GC7721AQ-LP3 into a "brand new" modern LM2020 Oscilloscope Multimeter! Also note that the Mustool MT8208 is identical to the Liumy LM2020 reviewed here. HOW IT WORKS The basic DMM functions are almost entirely based on the stock FS9721-LP3 design, including the reference circuits, clock design, calibration, etc... The FS9721-LP3 is a high performance, low power consumption and 3 3/4 digits (4000 Counts) Analog to Digital Converter that built-in the microprocessor (ADC+MCU). It includes an 8-bit microprocessor, low-noise and high-stability OPAMP, AC rectifier OPAMP, voltage protection and regulated voltage power, high regulated bandgap, auto measurement switch and function control circuit, buzzer driver circuit, clock oscillation circuit, backlight display control circuit, LCD display driver circuit and so on. All of these basic DMM functions are proxied through the STM32F401 MCU, including user interface and display. The new, fancy Oscilloscope and data logger functions are essentially just creatively graphing and storing data generated from the GC7721AQ ADC chip. At its core, it's a cheap yet effective hack - I mean, do we really need to re-engineer a digital multimeter? So, they recycled the old tech and married it to high-tech to give it new life. THE BAD Cheap designs come at a cost, and unfortunately, the LM2020 (and the MT8208) suffers from the same thing that ruins a lot of Chinese copy-of-copy-of-copy designs. In an effort to keep things super cheap, they cut some corners that you need to know about. Let's start with protection. The manual mentions a "protective tube", which obviously means a fuse. In your typical Fluke DMM, you'll find 2 fuses - a high current one, and a low current one - both of which are socketed and easily replaceable, because they ARE going to get damaged at some point. In the LM2020, they cut some unacceptable safety corners here, and I considered these points alone serious enough to dock them 2 stars. Instead of providing socketed, user-replaceable fuses, they use a small rectangular 10A nano fuse that's soldered directly to the PCB. Very much not user-serviceable. If that wasn't bad enough, they replaced what should have been a 500mA SLO-BLOW fuse with a self-resetting Poly Fuse (PTC) rated at only 420mA trip (instead of 500mA). The problem here is that PTC's aren't fuses, and they add a small amount of resistance to the circuit as you approach their rated trip current. So, when you're trying to measure something with the LM2020 that's around 400mA, you're only 20mA away from tripping the Poly-Fuse and it has already started to add unwanted resistance to the measurement - ultimately making it less accurate. Real fuses don’t do that, and they act like perfect shorts right up until they blow. Speaking of calibration - The GC7721AQ reference design calls for very high-precision resistances to provide core measurement reference accuracy, like 1.001KΩ, 10.010KΩ, 101.010KΩ, 1.111MΩ and 10.000MΩ. Instead of doing the work needed to provide these high-precision resistances, they used values like 1.0KΩ, 10.0KΩ, 101.0KΩ, 1.11MΩ and 10MΩ. As a result, the non-user-adjustable "calibration" on this meter is going to be quite variable in different temperature conditions and from meter-to-meter. This is just a reminder that this is a cheap meter with poor accuracy and should be treated as such. Finally, the banana lead connections to the PCB leave a lot to be desired. It's a thin metal sleeve soldered to the ET201B+ PCB that seems insufficient for measuring a 10A load without derating slightly and adding yet more unwanted resistance to your already mediocre measurement. SUMMARY I'm still glad I got it, but it's definitely a toy. This is light-years away from being accurate for electrical design, and I tend to think of this as only useful for quick-and-dirty measurements where setting up a real DMM or a scope might not be as convenient. I'm not going to do a functionality review because they already exist. Go look'em up. Remember that this meter goes by the name of Liumy LM2020 and Mustool MT8208, but they are the exact same thing. Just copies of copies of copies... You should take a moment and pop-off the cover of your DMM. Take a look around. Compare this to a basic Fluke and you’ll quickly see why this costs $80 instead of $200+. Oh, and if you DO want to have a look in the LM2020, there’s only 4 Philips screws holding the plastic cover on the back. No need to even remove the batteries or battery cover first. MISSING USER INSTRUCTIONS For those of you trying to figure out how to turn off the SUPER ANNOYING keypress beeps, I'm happy to provide the following instructions that didn't make it into my user manual: Automatic Sleep / "Auto Power" If there is no operation for a set period of time, the instrument will get into automatic sleep. To turn off the function of automatic sleep, user may press the "F1" button and then rotate the rotary knob to turn on the instrument. In this way, the instrument is set to continuous working mode. To enable the automatic sleep function, press and hold the "F2" button and then rotate the rotary knob to turn on the instrument. Keypress Beeps / "Quiet Mode" To enable Quiet Mode (disable keypress beeps), press and hold the "F4" button and then rotate the rotary knob to turn on the instrument. To disable Quiet Mode (enable keypress beeps), press and hold the "F3" button and then rotate the rotary knob to turn on the instrument.