APRS telemetry coefficients calculation
With current firmware of Microsat APRS devices (WX3in1 Plus 2.0 v1.00, WX3in1 Plus v1.41, WX3in1 Mini v1.10, PLXDigi v1.04, PLXTracker v1.03) there is no independant "Source:" selection for WXTelemetry/Hall.
For current input 1 you should select "(M) Current1 (5A fullscale)" and for current input 2 you should select "(M) Current2 (5A fullscale)".
Units should be set to "A" (values are shown in ampers).
Coefficients should be set as follow:
Coeff A = 0.
Coeff B = (5V/0.185V) * (1/256) = 0.1156.
Coeff C = -127 * (Coeff B) = -14.6812.
Example: Analog value for APRS telemetry is calculated with an equation: analog = measured*measured*CoeffA+measured*CoeffB+CoeffC, so for no-current-flow situation we will get: analog = 127*127*0 + 127*0.1156 - 14.6812 = 0A.
Value 127 visible in Coeff C calibration is a default value returned by a device when there is no current flow on current input. But used current measurement chips have a small random offset so it may be necessary to test other values from between 125 and 129 (please also note a minus before a value in coeff equation). This offset also seems to be related to the orientation of telemetry module relative to ground. So offset should be calibrated after mounting a telemetry module in desired position (on the wall for example).
WXTelemetry module allows you to measure two voltages, two currents and one temperature and send these values to APRS with your WX3in1 Plus, WX3in1 Plus 2.0, WX3in1 Mini, PLXDigi or PLXTracker device.
- 2 voltage inputs with potentiometer,
- 2 current inputs with Hall-effect sensors (5A range each),
- DS18B20 thermometer input,
- RS485 data interface,
WXTelemetry/Hall is a modified version of our telemetry module. In this version current is measured with ACS712T Hall-effect sensors.
Using ACS712T gives you following advantages over standard measurement method:
- full isolation of measured current from module measurement circuit,
- it allows to measure much higher current because there is no measurement resistor, no voltage drops and no energy loss,
- it allows to measure current flowing in both directions.
But there are also some disadvantages:
- lower measurement resolution because of wider measurement range (around 100mA step),
- lower measurement accuracy because of sensor characteristics and measurement method (around 5% in full measurement range),
- sensor can reports small positive or negative reading even if there is no real current flow in the circuit,
- a little higher telemetry module current consumption (around 50mA)
To sum things up:
WXTelemetry/Hall module, utilizing ACS712T Hall-effect sensors is better suited for a little less precise measurement of higher currents, with accuracy of around 5% in full measurement range. It is a good solution for currents and voltages measurement in larger solar panel systems.
Device printed circuit board
Below you can see a photo of a device PCB. It is a compact design mounted on a high-quality 1-layer PCB manufactured in Poland. Every unit is mounted and tested by me.