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Posted by daestrom on August 26, 2006, 6:12 pm
 


The choice of how/where to use a resistor depends on the meter you're using.
If you have two meters, you can do a little experiment.  Set one meter up
for reading mA and set the other up for reading resistance.  Connect the two
meters together.  The 'ohmmeter' will pass a small current through the
'milliammeter'.  You should see this on the 'milliammeter'.  At the same
time, the 'ohmmeter' will show you the resistance of the 'milliammeter'.  It
is that resistance that you should be concerned with.

The voltage drop across your milliammeter as it measures different currents
is simply E=IR.  So with that resistance, you can figure out what the
voltage drop across the milliammeter will be for various currents.  For
example, say the resistance of your milliammeter is 50 ohms when on the 0 -
10 mA scale.  So when reading 10 mA, the meter will have a voltage drop of
10 mA * 50 ohms = 500 mV (0.5 Volt) across it.  Calculate the voltage for
several different currents to get an idea what the I-V line for your
particular meter would look like.

Take those results and plot them on the I-V curve for your solar cell.  The
various calculated meter values should form a straight line from the 0 mA, 0
V point upward and through the cell performance lines.  If they intersect
the cell performance lines in the area where the cell I-V lines are
straight, then your milliammeter will give you nice, linear results.

But if the 'meter line' intersects the cell performance lines in the middle
of the curve, or way over where all the cell performance lines tend to
converge, then your meter has too much internal resistance to give you
linear data.

One way around this is what 'chuck-wright' is saying.  And that is to
'shunt' the current with a very low resistor across the cell.  Then simply
measure the voltage developed across the resistor.  The resistor used should
be a small enough resistance that it's line of current versus voltage drop
forms a line on the cell performance curve that is steeper and thus
intersects the cell curve in the area left of the 'knee' where current is
constant regardless of connected load.

More sophisticated circuits can be devised that would allow for long
distances.  (the resistance of long leads between the cell and the shunt
resistor add to the total circuit resistance and move things back towards
the 'knee' of the cell I-V performance curve).  Simply placing the shunt
resistor right at the cell, and setting the remote meter to read *voltage*
will work pretty well.

It is important that if you're using a resistor in parallel with your meter,
set your meter on a *voltage* scale.  This makes the meter a very high
resistance so negligible solar cell current will flow through the meter.

If you're using a resistor in *series* with your meter, then you want the
meter set on *current* scale.  But chances are, the cell output current fed
directly into the meter will not over-range the meter.  For example, a small
Radio-Shack cell might have an output current of 20 mA, and your meter has a
0-200 mA scale, you could just connect the meter directly to the cell with
*no* resistor.

daestrom


Posted by schooner on August 27, 2006, 10:47 am
 
Ended up that just reading the amps from the small panel directly gave the
best reading, again not so concerned with calibration as just a way to
baseline each day for comparison.  Going to give this a try and see how it
relates to panel performance over each day.



Posted by SJC on August 27, 2006, 3:01 pm
 

best reading, again not so concerned with calibration

and see how it relates to panel performance over each

  You should be able to correlate rather nicely and compare different panel
designs well. I have not heard much about mildly overcast days and IR on
here. I am starting to think that you can get some heat even on those days,
when PV is not going to produce much.


be on its own, for use to compare one day to the

temp diff and air output it is hard to compare one

given day.

shack, a 10 ohm resister, and a millimeter to read to

varies from minute to minute even when the sky looks

some basic level of calibration, knowing when you

to use one such as this one:

If you have two meters, you can do a little

resistance. Connect the two meters together.  The

see this on the 'milliammeter'. At the same time,

that resistance that you should be concerned with.

is simply E=IR.  So with that resistance, you can

currents. For example, say the resistance of your

the meter will have a voltage drop of 10 mA * 50

different currents to get an idea what the I-V line for

various calculated meter values should form a straight

If they intersect the cell performance lines in the

you nice, linear results.

of the curve, or way over where all the cell

resistance to give you linear data.

the current with a very low resistor across the

resistor used should be a small enough resistance that

curve that is steeper and thus intersects the cell

connected load.

distances.  (the resistance of long leads between the cell

back towards the 'knee' of the cell I-V performance

remote meter to read *voltage* will work pretty

set your meter on a *voltage* scale.  This makes the

through the meter.

meter set on *current* scale.  But chances are, the

meter.  For example, a small Radio-Shack cell might have

just connect the meter directly to the cell with *no*


Posted by Morris Dovey on August 27, 2006, 3:21 pm
 SJC (in ktiIg.3658$p23.2903@trnddc04) said:

| I have not heard much about mildly overcast days and IR on
| here. I am starting to think that you can get some heat even on
| those days, when PV is not going to produce much.

Datum:

In the neighborhood of 41N91W, a 2'x6' panel sitting in the shade 12'
inside my shop and aimed north through the open door produces a
measured 7-9F gain. The variance in gain is the variance between
overcast and clear skies.

--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA
http://www.iedu.com/DeSoto



Posted by schooner on August 27, 2006, 5:54 pm
 Anyone ever use this meter?
http://amep.com/standarddetail.asp?cid 28



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