Hi, I teach a highschool chemistry class, and recently had my students construct copper oxide solar cells. These run at approximately a quarter of a volt, and produce between 0 and 700 microamps, depending on the quality of the cells, amount of sunlight, etc.
Problem is, most of the multimeters at the school read in units of mamps. I have my own personal multimeter that is sensitive enough to read microamps, but I don't have enough to go around the whole class.
Is there a simple electronic circuit which can be used to amplify this signal in a linear way, so that it can be read in milliamps? Is this enough current to switch on a transistor? Are there transistors available the approximate a linear amplification in this range?
.25 volts is not enough to switch on a transistor by it self. how ever, it is enough to switch on a JFET transistor with very Hi input resistance. using a JFET and shunting the gate to ground with aprox 330 ohms should be enough. you can use the Drain and Source of the JFet as part of a voltage divider that can be read with a voltmeter. lower current measurements can be done by increasing the by pass resistor on the gate (shunt, Clamp)... etc...
What is the full-scale sensitivity of milliAmmeter you want to use?
What load resistance should the solar cell see during your test. (I can make it so that the solar cell sees an infinite load impedance, but I'm guessing you want to simulate driving a real load, so as to compare different solar cells...)
Jfets work very well in a differential circuit to measure very low current loads. considering only 800 ua is the max current that will ever be produced! it would mostly be desirable to be able to measure the 0.250 Max volts etc.. now i realize that you can also have a pre biased bipolar circuit and using the max 0.250 voltage to offset it will also give you some results how ever, bipolar also requires a lot more current over Fets and there for you will most likely get a much smaller window effect. i don't if you ever looked at the old VTVM or meters of the like but you will find that first there was the tube technology then came the JFETS. MOsFets etc... all giving solutions to these low voltage and low current problems when measuring or metering from these sources.
here is an OpAmp based current amplifier which uses two resistors and a
9V battery. I designed something that will work with any DC millampmeter, from 1mA to 50mA full-scale.
I'm assuming that your Solar cell will put out 0.25V into 330 Ohms. The OpAmp converts the voltage at its non-inverting input to a current through the meter. The meter current is solely determined the value of R1. To match the current gain to your meter, select R1=Vsolar/ImeterFS. e.g. if your meter is 25mA F.S., then R1 = 0.25/0.025 = 10 Ohms. The LM358 will source up to 50mA, so hopefully your meters have a full scale sensitivity better than that.
The LM358 and other parts are readily available (possibly at RatShack).
I have attached an LTSpice netlist which shows the circuit. I am also attaching a jpg of a screen shot and emailing it directly to you.
MikeM
Cut this into a file called CurrentAmp.asc and open it with LTSpice ___________________________________________________________________ Version 4 SHEET 1 880 680 WIRE 336 432 128 432 WIRE 128 448 128 432 WIRE 128 176 128 432 WIRE 128 -144 336 -144 WIRE 96 160 -112 160 WIRE 0 64 0 128 WIRE 0 336 0 432 WIRE 0 432 128 432 WIRE 96 128 0 128 WIRE 0 128 0 256 WIRE 0 -16 0 -80 WIRE 0 -80 96 -80 WIRE 208 -80 208 144 WIRE 208 144 160 144 WIRE -192 160 -272 160 WIRE 96 -80 208 -80 WIRE -112 256 -112 160 WIRE -112 160 -192 160 WIRE -112 336 -112 432 WIRE -112 432 0 432 WIRE -272 256 -272 160 WIRE -272 336 -272 432 WIRE -272 432 -112 432 WIRE 336 -144 336 208 WIRE 336 288 336 432 WIRE 128 112 128 -144 FLAG 128 448 0 FLAG -192 160 solar FLAG 96 -80 Vo SYMBOL Misc\\battery 336 192 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 9v SYMBOL Opamps\\1pole 128 144 R0 SYMATTR InstName U1 SYMBOL res -16 240 R0 SYMATTR InstName R1 SYMATTR Value 25 SYMBOL res -16 -32 R0 SYMATTR InstName R2 SYMATTR Value 35 SYMBOL res -128 240 R0 SYMATTR InstName R3 SYMATTR Value 330 SYMBOL current -272 336 R180 WINDOW 0 24 88 Left 0 WINDOW 3 24 0 Left 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName I1 SYMATTR Value {Isol} TEXT -632 128 Left 0 !.step param Isol 0 800u 50u TEXT -634 266 Left 0 !.op TEXT -632 192 Left 0 !.option itl1=1000 TEXT -24 -80 Left 0 ;+ TEXT -696 -96 Left 0 ;Note: Circle is the meter\n R1 sets the full scale current (M) through the meter.\n R1=0.25/M\n R2 represents guess of meter resistance TEXT 240 -112 VRight 0 ;OpAmp is 1/2 of LM358 (commonly avail)\nsee
formatting link
TEXT -632 40 Left 0 ;R3 is the "load" for the Solar cell.\nSolar panel current is Vsolar/330 TEXT 168 168 Left 0 ;1 TEXT 64 104 Left 0 ;2 TEXT 64 184 Left 0 ;3 TEXT 136 192 Left 0 ;4 TEXT 104 80 Left 0 ;8 CIRCLE Normal 80 96 -80 -64 _________________________________________________________________________
I agree with you except in this case 800uA is a modest current when a gain of only 10 or so is required. The impedance is a little over 300 ohms so as such loading should not be a serious issue.
My feeling was that there was no simple solution, the closest I could get was a long-tailed pair, i.e. a crude differential amp probably with bipolar transistors. Probably the best solution I could think of was a instrumentation amplifier with a fixed gain of say 10 and just measure the voltage and it's change with light. That way it can be a single IC with no external components. Do they still exist? Personally if faced with this problem I would probably use an op-amp, but with feedback resistors I feel the circuit is getting to much for this gentleman.
If you put a DMM on the cell with it set to the .2V or 200 mV range, and put a 1000 ohm resistor across the test leads, the readout will be directly in microamps. 1 V across 1k is 1 mA, and .2V across the 1k is
200 microamps.
I bought seveal DMMs for $6 apiece from Futurlec, with battery, believe it or not, and even they have a .2VDC range. So you shouldn't have a problem there.
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