rdheiliger
Member
Finaly got to doing the experiment. Did not use the 1 wire solar sensor. Put the CLD240E in series with both the 390 ohm and 4.7K ohm resistors accross a 5vdc supply. Measured the voltage drop accross the sense resistor with a digital multimeter. And, recorded the multimeter volts and my SP1065 solar energy meter reading. I have attached an excel spread sheet with the data and calculations. The chart shows the two readings follow very closely. Probly some errors in trying to read both meters at the same time.
Observation, with the 390 ohm resistor one is likely to only be able to tell when the sun comes up and goes down. The readings are in millivolts. Maximum of about 150 mv. Since the DS2438 resolution is 10 mv, this does not give much range. There would only be 15 steps between daylight and night. Using the 4.7K resistor gave more range. Since the highest voltage was 1.75Vdc, there would be 175 steps daylight to dark. One could likely go to a higher resistance value, but the day I measured was a bit overcast and would not recomend going to a larger resitor than 4.7K untill I have a very clear day.
The meter I have reads in BTU/HrFt^2, so I found a convertion factor to W/M^2. Watts seems to be a defacto standard for solar flux.
I did some trial calculations on the spread sheet and came up with a multiplier to use to get W/M^2. Remember that the reading returned from the DS2438 is the voltage drop across the sense resistor, and one needs the current. So, I = E/R to get the current. One should use a multimeter to read the real value of the resistor, espesialy if it is not 1% tolerance.
The formula:
DS2438 volts / sense resistor ohms = sensor current
sensor current * 1157598 = W/M^2 (solar energy)
Note: this number is calculated with the data from the 4.7K resistor.
The chart shows that this comes very close to the solar meter.
Look at the chart and maybe things will be more clear
I still use excel 2000, hope it comes up in a newer version.
Richard
Observation, with the 390 ohm resistor one is likely to only be able to tell when the sun comes up and goes down. The readings are in millivolts. Maximum of about 150 mv. Since the DS2438 resolution is 10 mv, this does not give much range. There would only be 15 steps between daylight and night. Using the 4.7K resistor gave more range. Since the highest voltage was 1.75Vdc, there would be 175 steps daylight to dark. One could likely go to a higher resistance value, but the day I measured was a bit overcast and would not recomend going to a larger resitor than 4.7K untill I have a very clear day.
The meter I have reads in BTU/HrFt^2, so I found a convertion factor to W/M^2. Watts seems to be a defacto standard for solar flux.
I did some trial calculations on the spread sheet and came up with a multiplier to use to get W/M^2. Remember that the reading returned from the DS2438 is the voltage drop across the sense resistor, and one needs the current. So, I = E/R to get the current. One should use a multimeter to read the real value of the resistor, espesialy if it is not 1% tolerance.
The formula:
DS2438 volts / sense resistor ohms = sensor current
sensor current * 1157598 = W/M^2 (solar energy)
Note: this number is calculated with the data from the 4.7K resistor.
The chart shows that this comes very close to the solar meter.
Look at the chart and maybe things will be more clear
I still use excel 2000, hope it comes up in a newer version.
Richard
