V,I Measuring Circuit Upgrade I

A few upgrades have been made to the measuring circuit. This is reflected in the schematic below.

There are 4 upgrades:

1. The ballast resistor which has to cope with a comfortable 6-7 A was only 15 W rated before. (MP915). Now we have swapped to the 100 W version of the same 0.1 Ohm resistor which is more appropriate since the power rating for this resistor is ~ 12 V * 7 A = 84 W. The 15 W resistor may have survived before because current only flows through the resistor when the push switch is pressed which is momentary although to ensure the resistor doesn't break, even if the push switched is pushed for a while, we now use the MP9100.
2. I (Julian) am extremely good at hooking things up with reverse polarity and in doing so I have killed the op-amps in this circuit multiple times. Luckily we have the op-amps mounted on sockets which we can switch the op-amps in and out of. A simple fix to ensure they never need replacing from this careless error is the insertion of a diode on the ground path. D2 in the schematic is there to ensure that if the positive terminal is attached to ground no current flows. This will just be some generic diode. Since it drops ~ 0.7 V in forward voltage it was very important to define the ground in the circuit (and the adjust value of the voltage regulator) as the positive side of the diode before the forward voltage drop occurs.
3. As discussed in a previous blog post the zener drop was 11 V even though it was rated for 11.5 V. From calculations the rest of the circuit was designed to use a drop of 11.5 V and so this was an inaccuracy which should be rectified. We ordered the same diode but rated for a 12 V zener voltage (NTE142A) and the drop across the diode, considering the very small current we are drawing through it,  is 11.5 V which is prefect. We also know that temperature will affect the drop of this diode but we hope that this fluctuation is insignificant.
4. One of our major goals during this project is to allow for very easy debugging. Currently if no current is flowing from the solar panel there is no way of knowing whether this issue is the solar panel or the solar charge controller. We are going to incorporate an LED into the control box which goes directly across the terminals of the solar panel and before the solar charge controller. If the solar panel is outputting anything at all the LED will be lit which can highlight if the solar charge controller is the issue. We are however extremely confident that this solar charge controller is extremely reliable. No issues have arisen so far through lots of testing. This branch will just be a resistors and LED in series. From the solar panel we usually get 19-21 V and so we can calculate the resistor needed very easily. This Wikipedia page is very good at explaining the simple resistor-LED circuit. Using the equation there, assuming a forward LED voltage of 1.7  V and current of 15 mA, we get a resistor value of ~ 1300 Ohms.