Measure milli amperage / current
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I am trying to measure current from 4 to 100 milliamp. I have tried
2 methods.
1) Transimpedance Amplifier
I get a voltage out, but it does not seem to comply to:
Vout = Iin x Rf
I do see a gradual increase of Vout as Iin increases, and they are
both linear. But it seems wierd that it dosen't fit the afore
mentioned formula. (Probably my fault)
2) Max471
This seems to be for a totally different purpose.
Does anyone have an ideas of how to measure 4 to 100 milliamps with
voltages from 12 to 24 volts, via the Basic Stamp or via Current ->
Voltage -> ADC - > Basic Stamp?
2 methods.
1) Transimpedance Amplifier
I get a voltage out, but it does not seem to comply to:
Vout = Iin x Rf
I do see a gradual increase of Vout as Iin increases, and they are
both linear. But it seems wierd that it dosen't fit the afore
mentioned formula. (Probably my fault)
2) Max471
This seems to be for a totally different purpose.
Does anyone have an ideas of how to measure 4 to 100 milliamps with
voltages from 12 to 24 volts, via the Basic Stamp or via Current ->
Voltage -> ADC - > Basic Stamp?
Comments
voltage drop with an ADC across a well known resistance and then:
I=V/R
You would have to prescale the voltage reading before doing the
calculation or else the stamp would always return zero. For instance
suppose:
V=15V
R=200
then I=15/200=0 as far the stamp is concerned.
However using a prescale of 100 then
1500/200=7 as far as the stamp is concerned
At this point you would have to interpret the result as 70mA since
you prescaled by 100 and bear in mind the stamp always drops the
fractional portion of a quotient so the actual current is 75mA.
This may or may not be accurate enough for your application. Hope
that helps. Maybe somebody else will have a better idea? Barring that
you may have to use another micro or get a floating point
coprocessor. Best regards.
-Dave
--- In basicstamps@yahoogroups.com, "FIRSTMORNINGSTAR"
<firstmorningstar@y...> wrote:
> I am trying to measure current from 4 to 100 milliamp. I have
tried
> 2 methods.
>
> 1) Transimpedance Amplifier
> I get a voltage out, but it does not seem to comply to:
> Vout = Iin x Rf
> I do see a gradual increase of Vout as Iin increases, and they are
> both linear. But it seems wierd that it dosen't fit the afore
> mentioned formula. (Probably my fault)
>
>
> 2) Max471
> This seems to be for a totally different purpose.
>
> Does anyone have an ideas of how to measure 4 to 100 milliamps with
> voltages from 12 to 24 volts, via the Basic Stamp or via Current ->
> Voltage -> ADC - > Basic Stamp?
interface to a Boiler in a major manufacturing company.
Thanks for your input, I will give it some thought.
--- In basicstamps@yahoogroups.com, "nuclearspin2000"
<nuclearspin2000@y...> wrote:
> What's your application? As described you could just measure the
> voltage drop with an ADC across a well known resistance and then:
>
> I=V/R
>
> You would have to prescale the voltage reading before doing the
> calculation or else the stamp would always return zero. For
instance
> suppose:
>
> V=15V
> R=200
> then I=15/200=0 as far the stamp is concerned.
>
> However using a prescale of 100 then
>
> 1500/200=7 as far as the stamp is concerned
>
> At this point you would have to interpret the result as 70mA since
> you prescaled by 100 and bear in mind the stamp always drops the
> fractional portion of a quotient so the actual current is 75mA.
>
> This may or may not be accurate enough for your application. Hope
> that helps. Maybe somebody else will have a better idea? Barring
that
> you may have to use another micro or get a floating point
> coprocessor. Best regards.
>
> -Dave
>
> --- In basicstamps@yahoogroups.com, "FIRSTMORNINGSTAR"
> <firstmorningstar@y...> wrote:
> > I am trying to measure current from 4 to 100 milliamp. I have
> tried
> > 2 methods.
> >
> > 1) Transimpedance Amplifier
> > I get a voltage out, but it does not seem to comply to:
> > Vout = Iin x Rf
> > I do see a gradual increase of Vout as Iin increases, and they
are
> > both linear. But it seems wierd that it dosen't fit the afore
> > mentioned formula. (Probably my fault)
> >
> >
> > 2) Max471
> > This seems to be for a totally different purpose.
> >
> > Does anyone have an ideas of how to measure 4 to 100 milliamps
with
> > voltages from 12 to 24 volts, via the Basic Stamp or via Current -
>
> > Voltage -> ADC - > Basic Stamp?
> I am reading variable amperage output that is coming from a PLC
> interface to a Boiler in a major manufacturing company.
>
The spec for the National 831 ADC has an app note about using it to measure
current. Go to http://www.national.com/pf/AD/ADC0831.html and click on
"receive by e-mail" to get a copy of the spec. Check page 21. Their circuit
shows a current that is 2A full scale, but with a little study of the other
app notes you ought to be able to puzzle out how to reduce the span to the
range you expect.
Good luck,
Gary
If you already have a PLC then just get an analog input module and read the
voltage drop across a resistor. The Math can be done in the PLC. (Unless it is
one of the cheappie 'Brick' PLC's without an expansion slot.)
Then if you need the stamp using the prescale method works well. I use it in
some of my industrial interfaces. It works very good.
Alan Bradford
Plasma Technologies
In a message dated 11/20/03 12:32:55 AM Eastern Standard Time,
firstmorningstar@y... writes:
> I am reading variable amperage output that is coming from a PLC
> interface to a Boiler in a major manufacturing company.
>
> Thanks for your input, I will give it some thought.
>
>
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