When the solar node is receiving energy from sun light there is a variable amount of current that might be produced. The solar panel is relatively consistent in its 21 V output but the current can range from 0-6+ A depending on the intensity of the sunlight. We have not tested midday sunlight yet but we hope to get a rough 7 A out of the panel.
When the node is deployed in an un-electrified village it is likely that the energy produced by the node would be in high demand from not only the community clients but also the house hold owners of the node. Obviously, during the day is the only time available to charge the battery and given enough sun light there should be enough current produced to charge a few phones and charge the batter at a reasonable rate. If there are 4 phones charging each drawing 0.5 A and the solar panel is outputting 7 A we will have 5 A left over to put into the battery. The battery we are using is 40 Ah and so given 8 hours of good sunlight we could charge the battery almost entirely.
But there will be times when there is not a steady 7 A coming from the solar panel. If the day is particularly cloudy or it is winter time there will inevitably be a lower current output. This will lead to times when a decision needs to be made based on the owner's priorities for either satisfying the load of charging phones or charging the battery so that the house can have night time lighting. To make this decision an indication of the current being outputted from the solar panel will need to be available which motivated the design of a circuit to indicate current flow with just 3 red LED's.
A circuit to measure current turned out to be quite hard to make. You need to place the circuit in series and attempt to make the circuit have as little resistance as possible so as to not change the current flow when the circuit is inserted. The diagram below is the first prototype which will be tested.
The circuit is deigned to indicate 2, 4 and 6 A of current flow. There are three comparators (OP1, OP2, OP3) which all supply their V+ to output when the voltage across R1 is surpassed. OP1 has a reference voltage (into IN-) of 0.2 V which means it will turn on LED 1 when there is at least 2 A passing through R1. OP2 and OP3 have reference voltages of 0.4 and 0.6 respectively to turn on for 4 and 6 A. The reference voltages are taken from a voltage regulator which drops the 12 V battery supply to 3 V (although the diagram says 2.85 V). It is very important to use a voltage regulator here because the battery voltage will vary with charge level and the reference voltages need to be very secure for good measurement.
R1, the MP915, has been ordered from Caddock through Element14 for 3.20 $ a piece. This resistor is also rated at 15 W which is a good safety buffer considering how much power this will need to dissipate. If 7 A is going through the circuit the power will be 4.9 W as calculated by the P = I2R law. The vision for this circuit is that it will live in the command module of the node with the LEDs visible to the user. There will be a push-to-make switch that will engage the ammeter when pressed. This will ensure that the current is only momentarily measured and not constantly measured. This should prevent too much heat building up on R1 and the voltage regulator as well as reducing unnecessary energy loss.
When the parts arrive this circuit will be tested. If this circuit works well it should be very easy to expand the circuit to accommodate more current increments. Additionally a circuit will be built to display the voltage of the battery via a series of LEDs. This will be shown in a later post.
16th April: Just did a preliminary test with one opAmp to test the theory of the circuit. Worked perfectly but used the MCP6002 opAmp configured as a comparator. Also used a 5V regulator, the LM7805C which was used for the phone charging circuit, which can regulate a maximum input voltage of 35 V. This means that the battery will not need to be used as an external power source as shown in the diagram but the power to drive the ammeter can come directly from the source it is measuring if there is also a link to ground. This is the case for many of the components and so will significantly reduce wiring hassle.
When the node is deployed in an un-electrified village it is likely that the energy produced by the node would be in high demand from not only the community clients but also the house hold owners of the node. Obviously, during the day is the only time available to charge the battery and given enough sun light there should be enough current produced to charge a few phones and charge the batter at a reasonable rate. If there are 4 phones charging each drawing 0.5 A and the solar panel is outputting 7 A we will have 5 A left over to put into the battery. The battery we are using is 40 Ah and so given 8 hours of good sunlight we could charge the battery almost entirely.
But there will be times when there is not a steady 7 A coming from the solar panel. If the day is particularly cloudy or it is winter time there will inevitably be a lower current output. This will lead to times when a decision needs to be made based on the owner's priorities for either satisfying the load of charging phones or charging the battery so that the house can have night time lighting. To make this decision an indication of the current being outputted from the solar panel will need to be available which motivated the design of a circuit to indicate current flow with just 3 red LED's.
A circuit to measure current turned out to be quite hard to make. You need to place the circuit in series and attempt to make the circuit have as little resistance as possible so as to not change the current flow when the circuit is inserted. The diagram below is the first prototype which will be tested.
The circuit is deigned to indicate 2, 4 and 6 A of current flow. There are three comparators (OP1, OP2, OP3) which all supply their V+ to output when the voltage across R1 is surpassed. OP1 has a reference voltage (into IN-) of 0.2 V which means it will turn on LED 1 when there is at least 2 A passing through R1. OP2 and OP3 have reference voltages of 0.4 and 0.6 respectively to turn on for 4 and 6 A. The reference voltages are taken from a voltage regulator which drops the 12 V battery supply to 3 V (although the diagram says 2.85 V). It is very important to use a voltage regulator here because the battery voltage will vary with charge level and the reference voltages need to be very secure for good measurement.
R1, the MP915, has been ordered from Caddock through Element14 for 3.20 $ a piece. This resistor is also rated at 15 W which is a good safety buffer considering how much power this will need to dissipate. If 7 A is going through the circuit the power will be 4.9 W as calculated by the P = I2R law. The vision for this circuit is that it will live in the command module of the node with the LEDs visible to the user. There will be a push-to-make switch that will engage the ammeter when pressed. This will ensure that the current is only momentarily measured and not constantly measured. This should prevent too much heat building up on R1 and the voltage regulator as well as reducing unnecessary energy loss.
When the parts arrive this circuit will be tested. If this circuit works well it should be very easy to expand the circuit to accommodate more current increments. Additionally a circuit will be built to display the voltage of the battery via a series of LEDs. This will be shown in a later post.
16th April: Just did a preliminary test with one opAmp to test the theory of the circuit. Worked perfectly but used the MCP6002 opAmp configured as a comparator. Also used a 5V regulator, the LM7805C which was used for the phone charging circuit, which can regulate a maximum input voltage of 35 V. This means that the battery will not need to be used as an external power source as shown in the diagram but the power to drive the ammeter can come directly from the source it is measuring if there is also a link to ground. This is the case for many of the components and so will significantly reduce wiring hassle.
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