Circuit to control an Owl energy monitor so that it reads only when the energy is flowing in one direction.
Update August 2013
The circuit described has been proved with an Owl energy monitor. There should be no reason why it should not work with other similar energy monitors.
The circuit operates by comparing the phase of the current detected by the Owl’s clamp-on current sensor with the phase of the mains voltage.
The circuit is placed in-line with the lead from the Owl’s current sensor to the Owl transmitter unit. In my design I cut the lead between the current sensor and the transmitter and connected the ends to the circuit. There are two wires in the current sensor, red and black. Depending on whether the current and voltage are in phase or not, the circuit turns on or off the signal to the transmitter.
The power source used in the circuit is a 4.5V - 6.0V AC adaptor such as the Maplin stock item N57AT (set to 4.5V). This adaptor not only serves as the power supply, but also provides the voltage phase information. Any low voltage AC adaptor with an output in the range 4.5V – 6.0V (max) can be used. Check that the actual voltage being applied to the circuit is no more than 6Vac. For instance, when connected to this low current circuit, the output voltage of the Maplin adaptor is over 7.5V on the 6V range. Do not use the Maplin adaptor set to 6.0V!
The output from the AC adaptor is rectified (diodes D1 and D2) to produce the plus and minus 6V to 9V DC supply which is used to power two integrated circuits. The output from the AC adaptor is also sampled to check the phase of the mains supply (D3, R1,R3). The sampled output is amplified (Q1, R2) to produce a square-wave signal in phase with the mains supply.
The signal from the Owl current sensor is connected to the input of a two-way analogue switch (U1 pin 14). The square-wave signal provides the switching action. By switching in phase with the mains supply, the signals produced on the two outputs of the analogue switch (pins 12 and 13) can be compared to determine whether the current is in phase or out of phase with the mains voltage.
The two outputs of the analogue switch are averaged (R4,C4 and R5,C3) and then compared using a comparator circuit (U2a). The output of the comparator changes from plus to minus as the relative phase of the current and voltage changes. In the basic circuit only one section of U2 is used.
Q2 is used to amplify and shape the output of the comparator. This amplified output lights LED1 whenever power is flowing in the correct direction.
The output from the Owl clip-on current sensor is also connected to other input sections of the 2-way analogue switch (U1 pins 4 and 15). This signal is switched from one output to the other by the amplified signal from the comparator. One of the outputs (U1 pins 2 and 5) will carry the signal when the current flows in one direction, the other output (U1 pins 1 and 3) will have the signal when the current flows in the other. By connecting one output or the other to the Owl transmitter, the transmitter will have no input signal and send a zero reading whenever the current flows in the ‘wrong’ direction.
You can choose the ‘correct’ direction by reversing the current sense clamp on the mains cable. You could connect two Owl transmitters, one to each of the outputs, and have a display of both import and export.
This section of the circuit uses two sections of U1 in parallel so as to minimise the loss though the switch. A single section of the switch would cause a loss of around 10% because the ON resistance of the switch is about 80 Ohms. Using two sections in parallel, the loss is halved to 5%. You can change the Energy reading on the Owl to accommodate the 5% loss by increasing the voltage setting to 5% more than your mains supply.
The original circuit was designed to control an Own energy monitor so that it read only import or export. This second use of the circuit does not require the purchase of the Owl monitor, only the current sensor (about £7). However it will work correctly with the Owl monitor at the same time as driving the phase angle control block. If you do not wish to use the Owl monitor, the connection from Q2 to pins 9 and 10 of U1 can be omitted along with the associated connections to PL3 and PL4.
Connect the mains current clamp on the cable so that the LED lights when exporting. This means the voltage on pin 1 of U2 will be positive when exporting.
The second section of integrated circuit U2 is used to provide the signal to the control block. The output of the first section of U2 (pin 1) is connected via the time-constant R9,C5 to the input of the voltage follower U2b. This section of U2 is connected as a unity gain buffer amplifier. The phase angle control block used to control the power to the immersion heater uses only positive voltages to control the current, so diode D5 is inserted in line with the output on U2 pin 7 to prevent negative voltages being applied. R10 provides a 2k2 Ohm connection to 0V as required by the control block.