Using information from the
UNI-T UT210E thread, I made some modifications to my Mastercraft:
- Increased full-scale counts to 8000 count instead of 6000 (auto-range up at 8001, down at 750)
- Added 80.00 mV and 800.0 mV ranges for AC and DC volt (auto-ranging) (press Select to toggle AC/DC)
- Added ability to switch between AC and DC voltage measurement without changing the rotary switch (press Select to toggle AC/DC)
- Added ability to switch between degree C and F without changing the rotary switch (press Select to toggle C/F)
- Ability to change the auto-off and backlight timeout to suit the user.
Just the mV ranges themselves are a fantastic upgrade. The extra counts are a bonus. The AC/DC and C/F switching will come in handy I'm sure, since it's easier to press the Select button than to rotate the switch, which is generally a two-handed operation.
In order to add the mV range, I needed to "steal" the Ohms switch position. I moved the Ohms (and diode and continuity) measurements to the CAP switch position; you press Select to change to the desired function. I have mine set to the following sequence: Resistance -> Capacitance -> Diode -> Continuity -> (repeat)...
The mV range has a resolution of 10 microvolts, and and input impedance of >500M on DC (higher than the 11M or 10M input impedance for other ranges) and 10M on AC. It is surprisingly accurate and noise-free. The "normal" AC and DC functions also have a 800.0 mV range which is manually selectable with the range button (it won't auto-range down below 8.000 V range). That is different than the mV-specific ranges: it has a 10M input impedance and a few digits of noise (AC) or offset (DC) with a shorted input. The operator needs to use care with using the mV ranges, the input protection is not as good as in the other voltage ranges. In the higher voltage ranges, the input goes through a 10M resistor before seeing anything else. In the mV ranges, the input is set up the same as in the Ohm or Cap measurement functions. The overload protection is via a PTC and a pair of transistors designed to conduct beyond a certain voltage, causing the PTC to rise in resistance to help protect to meter. If you connect a high voltage, high energy input like AC mains to the input in mV (or Ohm or Cap) mode, be prepared to suffer the consequences of at best a damaged meter, and at worst... explosion and injury.
The counts are limited to 8000 due to the need for AC measurements to have extra headroom. It seems the chip's ADC works up to around 12000 counts or so. For DC, we could use 9999 counts, but if you tried to use that for AC, the waveform would clip (a 9.999 V
RMS sinusoid has a peak value around 14.141 V, beyond ADC full-scale), resulting in a displayed value less than actual. Setting the limit of 8000 counts gives enough headroom for accurate measurement of a sinusoidal signal. If the AC input signal has a higher crest factor, like a triangle wave, or noise, then one should consider manually ranging down, in order to ensure ADC headroom to capture the higher peaks. This is a limitation common to all sampling-type RMS AC meters. The original 6000 count full-scale resolution accounts for a crest factor of >2.0 at full-scale, which is quite good enough. The 8000 counts reduces the margin but is a compromise to gain extra resolution.
I was not able to add RS-232 output, as the required I/O pin is being used for something else.
I will post the technical details later, including how to customize your own meter if you don't like my way. All you need is a programmer that can read/write a 24C02 I2C EEPROM, and a suitable soldering equipment and skills to either remove/replace the SOIC package or solder fly-wires.
edit: corrected input impedance for DCmV