amplifying microamps

D

dave e

Guest
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?

Dave
 
.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...



dave e wrote:

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?

Dave
Click to expand...
 
dave e <dgenglish@hotmail.com> wrote:
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.
Click to expand...
Put it on volts, add a resistor.
 
Hi Dave,

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...)

MikeM



dave e wrote:
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?

Dave
Click to expand...
 
Won't a JFET with a threshold voltage that well specified be like rocking
'orse manure?


"Jamie" <jamie_5_not_valid_after_5_Please@charter.net> wrote in message
news:103m2frtbs15586@corp.supernews.com...
.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...



dave e wrote:

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?

Dave
Click to expand...
 
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.


Fred wrote:

Won't a JFET with a threshold voltage that well specified be like rocking
'orse manure?


"Jamie" <jamie_5_not_valid_after_5_Please@charter.net> wrote in message
news:103m2frtbs15586@corp.supernews.com...

.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...



dave e wrote:


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?

Dave
Click to expand...
 
Dave,

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
http://www.national.com/ds/LM/LM158.pdf
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
_________________________________________________________________________



mikem wrote:

Hi Dave,

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...)

MikeM



dave e wrote:

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?

Dave
Click to expand...
 
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.

"Jamie" <jamie_5_not_valid_after_5_Please@charter.net> wrote in message
news:103om8grgf68gd1@corp.supernews.com...
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.


Fred wrote:

Won't a JFET with a threshold voltage that well specified be like
rocking
'orse manure?


"Jamie" <jamie_5_not_valid_after_5_Please@charter.net> wrote in message
news:103m2frtbs15586@corp.supernews.com...

.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...



dave e wrote:


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?

Dave
Click to expand...
 
mikem <mikem@bogus.adr> wrote in message news:<c1il8g$sra$1@coward.ks.cc.utah.edu>...
Dave,

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.

snip
Dave
Click to expand...
You can get instrumentation opamps with Jfet inputs from more than one
manufacturer for free.

I bet that if you put your request on school letterhead they might
give you enough to go around.

http://www.analog.com/IST/SelectionTable/?selection_table_id=103

http://www.linear.com/pdf/instrumentation.pdf

http://para.maxim-ic.com/compare.asp?Fam=Op_Amp&Tree=Amps&HP=AmpComp.cfm&ln=&SORD=210&FT_210=1533&ITEMLIST=121127,121128,121129,121190,121191,121192,121193,121194,121195

http://www.national.com/parametric/0,1850,703,00.html

http://www.aldinc.com/online_opamps.html#opamps

HTH,
Ren
 
"dave e" <dgenglish@hotmail.com> wrote in message
news:10ffa4e4.0402231140.1bb29b1b@posting.google.com...
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?

Dave
Click to expand...
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.
 
"mikem" <mikem@bogus.adr> wrote in message
[snip]

The LM358 and other parts are readily available (possibly at
RatShack).
Click to expand...

The LM324 is four opamps which are trhe same as the 2 opamps in the
LM358. It is available at Rat Shack.

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
Click to expand...
Why don't you do us a big favor and post the graphic of the schematic to
alt.binaries.schematics.electronic.
 
"Fred" <fred@abuse.com> wrote in message
news:403cf935$0$17835$fa0fcedb@lovejoy.zen.co.uk...
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.
Click to expand...

Oh, really? How could you much simpler than two resistors and an opamp?

[snip]
 
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