diode_current_flow.h

00001 /* Compute Current Flow across Diode
00002    using voltage drop across the diode.
00003   
00004   By Joseph Ellsworth CTO of A2WH
00005   Take a look at A2WH.com Producing Water from Air using Solar Energy
00006   March-2016 License: https://developer.mbed.org/handbook/MIT-Licence 
00007   Please contact us http://a2wh.com for help with custom design projects.
00008 
00009   
00010   Based on voltage drop across a diode calculate
00011   current flowing based on known characteristics of that diode.
00012   Unlike a resistor this can not be done with straigt ohms law
00013   because it is temperature specific and is not linear.  Instead we 
00014   use a lookup table and aproximate between elements to compute
00015   a value. 
00016   
00017   A known limitation is that if voltage drop is equal or lower 
00018   than smallest value in vDropArr Then all we know is that current is some place 
00019   betweeen 0 and that minimum current number.  Since all diodes have a minimum drop 
00020   for any current flow they are not as effective when measuring very small currents. 
00021   The diodes are best used when we do not want to add the 
00022   loss of a resistor and we needed the diode in circuit anyway especially when 
00023   there may be large current flow such as in a charging circuit.
00024 
00025   If you need a temperature specific value then declare a vDrop for multiple temperature ranges 
00026   then compute the voltage drop for the temperature range above and below current temp and 
00027   use ratio level computation to compute a weighted average between the two.
00028 
00029   Note: This would be a perfect place to use a differential ADC where 
00030   the one line is on the postive side of the Diode and the other is 
00031   on the load side.    Unfortunately mbed doesn't provide standard support
00032   for the differential. 
00033   
00034   Note: Turn off the PWM for the charge circuit and turn the charge
00035   circuit all the way on before taking this reading or it mess up the ADC readings.
00036   
00037 */
00038  
00039 //#include "ohms.h"
00040 
00041 #ifndef diode_current_flow_H
00042 #define diode_current_flow_H
00043 // T.I SM74611 Smart Bypass Diode
00044 const float SM74611VDrop85C[] = {0.020, 0.024, 0.037, 0.040,  0.050,  0.057,  0.060,  0.075,  0.080,  0.090,  0.098, 0.106, -1};
00045 const float SM74611Current[]  = {2.000, 4.000, 6.00,  8.000, 10.000, 12.000, 14.000, 16.000, 18.000, 20.000, 22.000,24.000, -1};
00046 // must be oriented as lowest current flow (lowest vdrop) to highest, must be terminated with -1 and arrays
00047 // must line up by postion.  The more numbers supplied the more accurate the computation will be down to the 
00048 // limit of the ADC.
00049 
00050 // NOTE: DEFINE your own diode perfornce characteristics from the Diode datasheet.  They vary a lot and 
00051 //  the SM74611 is a very special diode with very low vdrop so these numbers will not work for other diodes.
00052 
00053 
00054 
00055 /* compute the current flowing across the diode based on the known diode 
00056 characteristics and a known voltage drop across the diode  The two arrays
00057 must both contain floats, must be the same length and must be terminated with -1.0
00058 sentinal.  
00059 
00060 */
00061 float ComputeDiodeCurrent(float *vDropArr, float *currentArr, float vDrop) {
00062   int lastNdx = 0;
00063   while (vDropArr[lastNdx] != -1.0) lastNdx++;
00064   lastNdx--;
00065 
00066   // find first vdrop in array that is less than our specified
00067   // vdrop which gives us a range where our vDrop must be so we 
00068   // than then interpolate a current across the diode. 
00069   if (vDrop >= vDropArr[lastNdx]) return currentArr[lastNdx]; // above our greatest setting
00070   
00071   int ndx = lastNdx;
00072   while ((vDrop < vDropArr[ndx]) && (ndx > 0))
00073     ndx--;
00074 
00075   if (ndx == 0) return currentArr[0]; // below our lowest setting
00076   if (vDropArr[ndx] == vDrop) return currentArr[ndx]; // exact match
00077   
00078   // Interpolate the ratio we are between the two points found 
00079   // and use that to determine our actual current flow.  This only
00080   // works assuming it is a relatively straight line between the two
00081   // points.  If that is not true then you need to add more points.
00082   float vrange = vDropArr[ndx + 1] - vDropArr[ndx];
00083   float amtOverLower = vDrop - vDropArr[ndx];
00084   float portRange = amtOverLower / vrange;
00085   float currRange = currentArr[ndx + 1] - currentArr[ndx];
00086   float lowCurr = currentArr[ndx];
00087   float portCurAdd = currRange * portRange;
00088   return lowCurr + portCurAdd;
00089 } 
00090 #endif