Current meter
- Introduction
- Pinout and ports
- Hardware specifications
- Supported current transformers
- Known problems
- Available sketches
- Links
Current meter is a simple carrier board for panStamp capable to read from up to three current transformers. Current readings can be taken at the desired interval and then transmitted with panStamp's low power radio. This board also contains a NTC thermistor. This board was originally designed to be powered from batteries (3.6 to 6.0 VDC) but can also be powered from an external power supply.
This picture shows the main parts forming the current meter board:
Once the panStamp module assembled, port P4 can be used to program the module from a panStick, according to this picture:
- Size: 2.16 x 1.58 in (55 x 40 mm)
- Supported current transformers : 1 VAC (SCT-013-0NN type)
- Current consumption: Around 6 uA in sleep mode. 20 mA in active mode.
- Input voltage (Vcc): 3.6 to 6.0 VDC
Only SCT-013-30 and SCT-013-005 have been tested with this board but any other split-core current transformer providing 1 VAC should work as well.
The battery voltage divider (R11 = R12 = 1MOhm) was designed to provide half the voltage level of the battery (or PSU voltage) whilst sinking very low current (around 1.5 uA). However, 1MOhm x 2 has resulted to be a too high resistance for panStamp ADC's input impedance. This makes the voltage level read by the panSTamp module go 200-400 mV under the real voltage powering the board. Of course lowering the value of R11 and R12 to something around 100 KOhm fixes the problem but then this circuit's consumption raises to 15 uA.
Assembly of jack females are resulting to be too weak for continuous jack operation. Sometimes they may pop off from the PCB when inserting sensor connectors. We recommend then to hold the female against the PCB with your hand whilst plugging the male jack. This will avoid for sure this problem.
Current-meter works out-of-the-box with the current-meter sketch
This application sends RMS current values, temperature and and battery voltage periodically, based on the value of Tx Interval. If you are powering the board from a battery pack, we recommend setting Tx Interval to something between 30 and 255 seconds. Configuring values for Tx Interval below that will probably drain the battery too quickly. In any case, if you are powering the board from an external power supply then you can set Tx Interval even below 5 seconds or replace swap.goToSleep() by a simple delay().
This application only provides RMS current in mA. If you want to get active power or power consumption then you will need to do some calculations on the receiver. For active power, simply assume an AC voltage level and a power factor. Power factor is typically assumed to be 0.8 in most domestic cases so for a 240VAC system we would apply this formula:
Active_Power (W) = RMS_Current/1000 x 240 x 0.8
Power consumption (in Wh) requires the receiver or main controller to run some accumulative work. Here the controller needs to know the time (in seconds) elapsed between transmissions and use this value in the following formula:
Power_Consumption (Wh) = Power_Consumption + Active_Power x time_elapsed/3600