exact current and potential source

K

Kari Laine

Guest
Hi,

I am testing several things and now I would need a
very exact current and voltage source.
It would be nice if it were correct to 0.001mV/mA.
Even more is better but I think that pushing a little.

I can't probably construct one myself from a schematic.
So ready devices preferred. If the prices are high
then that mentioned self building might be option...

Best Regards
Kari



--
PIC - ARM - DISPLAYS - RELAYS - MODULES - CONVERTERS - I2C - SPI -
KEYPADS - ACCESSORIES
http://www.byvac.com (I am just a satisfied customer)
 
On Apr 23, 10:51 am, Kari Laine <klai...@gmail.com> wrote:

I am testing several things and now I would need a
very exact current and voltage source.
It would be nice if it were correct to 0.001mV/mA.
Click to expand...
One can purchase pre-trimmed reference voltage components,
like AD581, with various precisions ($5 to $40). Standard cells
(with a calibration certificate) are higher.

Nothing will get you to .001 mV unless your laboratory setup
includes VERY tight temperature controls; wiring thermocouple
effects make anything below .05 mV into a difficult exercise.

A calibrated voltage source, an attenuator (some of the old
potentiometers, available on the internet at bargain prices, will
hold four-digits of accuracy easily) and a few precision resistors
makes it possible to generate lots of voltages and currents
with simple operational amplifier circuits. Alas, you DO have
to know and treat all the important error sources to do this kind
of precision work.

Voltmeters in parallel have the same applied V; ammeters in serial
have the same applied I. Everything else, you gotta keep track of
errors and error propogation.
 
whit3rd wrote:
On Apr 23, 10:51 am, Kari Laine <klai...@gmail.com> wrote:

I am testing several things and now I would need a
very exact current and voltage source.
It would be nice if it were correct to 0.001mV/mA.

One can purchase pre-trimmed reference voltage components,
like AD581, with various precisions ($5 to $40). Standard cells
(with a calibration certificate) are higher.

Nothing will get you to .001 mV unless your laboratory setup
includes VERY tight temperature controls; wiring thermocouple
effects make anything below .05 mV into a difficult exercise.
Sorry I made a mistake. I meant .001A and .001V not mA/mV so....
Click to expand...

I will google for the AD581.

A calibrated voltage source, an attenuator (some of the old
potentiometers, available on the internet at bargain prices, will
hold four-digits of accuracy easily) and a few precision resistors
makes it possible to generate lots of voltages and currents
with simple operational amplifier circuits. Alas, you DO have
to know and treat all the important error sources to do this kind
of precision work.

Voltmeters in parallel have the same applied V; ammeters in serial
have the same applied I. Everything else, you gotta keep track of
errors and error propogation.
Click to expand...
I need it for calibrating the oscilloscope.
I don't have another source for calibration.

I am writing an oscilloscope program to Velleman PCSGU250. It is at it's
early stages - ALPHA!
But can be had from http://www.lnxcore.net
If you don't have the hardware connected it starts in a mode where you
can play with sin waves and select X-Y Plot from menu. Basic principle
to me is that I will make possible to simulate scope without hardware
and therefore develop the program.

Velleman is at http://velleman.be

The frequency analyzer and bode plotter has proven to be very difficult
to me.

NOTE !
http://www.syscompdesign.com/ makes and Open Source version of
oscilloscope, which works with Windows, Linux and MacIntosh.

I am now going to spend 1-2 weeks to add support for Velleman PCSGU250
to it.

Best Regards
Kari


--
PIC - ARM - DISPLAYS - RELAYS - MODULES - CONVERTERS - I2C - SPI -
KEYPADS - ACCESSORIES
http://www.byvac.com (I am just a satisfied customer)
 
On Fri, 23 Apr 2010 20:51:53 +0300, Kari Laine <klaine8@gmail.com>
wrote:

Hi,

I am testing several things and now I would need a
very exact current and voltage source.
It would be nice if it were correct to 0.001mV/mA.
Even more is better but I think that pushing a little.

I can't probably construct one myself from a schematic.
So ready devices preferred. If the prices are high
then that mentioned self building might be option...
Click to expand...
---
What you're looking for is called a "current shunt" and is essentially a
low resistance precision resistor which is wired in series with the load
and the voltage dropped across it [the shunt] measured in order to
determine the current through it.

In order to comply with your spec, what you need is a shunt with a
sensitivity of 1 millivolt per ampere, which is readily available and is
called a "100 millivolt shunt" in the trade.

That 100 millivolts refers to the voltage which will be dropped across
it with 100 amperes through it, and an example would be at:

http://www.deltecco.com/MKA-DC.html

Unfortunately, there's an error in their literature which specifies that
an ammeter is used in conjunction with the shunt in order to measure
current, when what they mean is 'voltmeter'.

JF
 
On Sat, 24 Apr 2010 13:09:24 -0500, John Fields
<jfields@austininstruments.com> wrote:

On Fri, 23 Apr 2010 20:51:53 +0300, Kari Laine <klaine8@gmail.com
wrote:

Hi,

I am testing several things and now I would need a
very exact current and voltage source.
It would be nice if it were correct to 0.001mV/mA.
Even more is better but I think that pushing a little.

I can't probably construct one myself from a schematic.
So ready devices preferred. If the prices are high
then that mentioned self building might be option...

---
What you're looking for is called a "current shunt" and is essentially a
low resistance precision resistor which is wired in series with the load
and the voltage dropped across it [the shunt] measured in order to
determine the current through it.

In order to comply with your spec, what you need is a shunt with a
sensitivity of 1 millivolt per ampere, which is readily available and is
called a "100 millivolt shunt" in the trade.

That 100 millivolts refers to the voltage which will be dropped across
it with 100 amperes through it, and an example would be at:

http://www.deltecco.com/MKA-DC.html

Unfortunately, there's an error in their literature which specifies that
an ammeter is used in conjunction with the shunt in order to measure
current, when what they mean is 'voltmeter'.
Click to expand...
---
Equally unfortunately, there's also an error in my description, since a
100 millivolt shunt is a shunt that's made to drop 100mV with a
_specified_ current through it, not just 100A.

For example, the Delteco MKA-100-100 is a 100A 100mV shunt, while the
Delteco MKA-10-100 is a 10A 100mV shunt.

JF
 
On Fri, 23 Apr 2010 20:51:53 +0300, Kari Laine <klaine8@gmail.com>
wrote:

Hi,

I am testing several things and now I would need a
very exact current and voltage source.
It would be nice if it were correct to 0.001mV/mA.
Even more is better but I think that pushing a little.

I can't probably construct one myself from a schematic.
So ready devices preferred. If the prices are high
then that mentioned self building might be option...
Click to expand...
---
I misread your requirements, please disregard my earlier posts.

JF
 
On Apr 24, 11:09 am, John Fields <jfie...@austininstruments.com>
wrote:
On Fri, 23 Apr 2010 20:51:53 +0300, Kari Laine <klai...@gmail.com
wrote:

Hi,

I am testing several things and now I would need a
very exact current and voltage source.
It would be nice if it were correct to 0.001mV/mA.

What you're looking for is called a "current shunt" and is essentially a
low resistance precision resistor which is wired in series with the load

Unfortunately, there's an error in their literature which specifies that
an ammeter is used in conjunction with the shunt in order to measure
current, when what they mean is 'voltmeter'.
Click to expand...
Not an error at all; the shunt is one half of a pair of conductors,
the
second of which is a meter with its own internal resistance (for a
typical d'Arsonval meter, a few dozen ohms). The shunt is
taking most of the current, being lower resistance, but the meter
IS a low impedance (ammeter). Current metering shunts are
intended to have the same temperature coefficient as the meter,
so are NOT simply accurate resistors. It's a current divider
function with two elements, in the classic meter scheme.

The difference between a 'voltmeter' and 'ammeter' in this
case is just the difference between 'drops-lots-of-voltage' and
'drops-negligible-voltage'.
 
On Sun, 25 Apr 2010 11:34:34 -0700 (PDT), whit3rd <whit3rd@gmail.com>
wrote:

On Apr 24, 11:09 am, John Fields <jfie...@austininstruments.com
wrote:
On Fri, 23 Apr 2010 20:51:53 +0300, Kari Laine <klai...@gmail.com
wrote:

Hi,

I am testing several things and now I would need a
very exact current and voltage source.
It would be nice if it were correct to 0.001mV/mA.

What you're looking for is called a "current shunt" and is essentially a
low resistance precision resistor which is wired in series with the load

Unfortunately, there's an error in their literature which specifies that
an ammeter is used in conjunction with the shunt in order to measure
current, when what they mean is 'voltmeter'.

Not an error at all; the shunt is one half of a pair of conductors,
the
second of which is a meter with its own internal resistance (for a
typical d'Arsonval meter, a few dozen ohms). The shunt is
taking most of the current, being lower resistance, but the meter
IS a low impedance (ammeter). Current metering shunts are
intended to have the same temperature coefficient as the meter,
so are NOT simply accurate resistors. It's a current divider
function with two elements, in the classic meter scheme.

The difference between a 'voltmeter' and 'ammeter' in this
case is just the difference between 'drops-lots-of-voltage' and
'drops-negligible-voltage'.
Click to expand...
---
In the sense that all d'Arsonval movements require the flow of charge to
work, they are ammeters.

However, when one is used to measure voltage, it's called a 'voltmeter'
and its resistance is quite high,(ideally infinite) while when used to
measure current it's called an 'ammeter' and its resistance is quite
low. (ideally zero ohms)

In the situation you describe where a (say) 1 ampere meter is used to
measure 100 amperes, a shunt is placed in parallel with the meter so
that with 100A through the assembly, 99A is passed through the shunt and
1A through the meter, with the meter dial indicating "100" at the
full-scale 1 ampere point.

The type of shunt I was referring to, shown at:

http://www.deltecco.com/MKA-DC.html

_is_ a temperature-compensated precision resistor, with Kelvin
connections, and is used to drop a voltage which is precisely related to
the charge flowing through it, that voltage being read by a high
impedance voltmeter for essentially zero loading on the shunt.

JF
 
"John Fields" <jfields@austininstruments.com> wrote in message
news:v1gbt59khdsi71i61vvdn2u5n4kid3spq7@4ax.com...
On Sun, 25 Apr 2010 11:34:34 -0700 (PDT), whit3rd <whit3rd@gmail.com
wrote:

On Apr 24, 11:09 am, John Fields <jfie...@austininstruments.com
wrote:
On Fri, 23 Apr 2010 20:51:53 +0300, Kari Laine <klai...@gmail.com
wrote:

Hi,

I am testing several things and now I would need a
very exact current and voltage source.
It would be nice if it were correct to 0.001mV/mA.

What you're looking for is called a "current shunt" and is essentially a
low resistance precision resistor which is wired in series with the load

Unfortunately, there's an error in their literature which specifies that
an ammeter is used in conjunction with the shunt in order to measure
current, when what they mean is 'voltmeter'.

Not an error at all; the shunt is one half of a pair of conductors,
the
second of which is a meter with its own internal resistance (for a
typical d'Arsonval meter, a few dozen ohms). The shunt is
taking most of the current, being lower resistance, but the meter
IS a low impedance (ammeter). Current metering shunts are
intended to have the same temperature coefficient as the meter,
so are NOT simply accurate resistors. It's a current divider
function with two elements, in the classic meter scheme.

The difference between a 'voltmeter' and 'ammeter' in this
case is just the difference between 'drops-lots-of-voltage' and
'drops-negligible-voltage'.

---
In the sense that all d'Arsonval movements require the flow of charge to
work, they are ammeters.

However, when one is used to measure voltage, it's called a 'voltmeter'
and its resistance is quite high,(ideally infinite) while when used to
measure current it's called an 'ammeter' and its resistance is quite
low. (ideally zero ohms)

In the situation you describe where a (say) 1 ampere meter is used to
measure 100 amperes, a shunt is placed in parallel with the meter so
that with 100A through the assembly, 99A is passed through the shunt and
1A through the meter, with the meter dial indicating "100" at the
full-scale 1 ampere point.

The type of shunt I was referring to, shown at:

http://www.deltecco.com/MKA-DC.html

_is_ a temperature-compensated precision resistor, with Kelvin
connections, and is used to drop a voltage which is precisely related to
the charge flowing through it, that voltage being read by a high
impedance voltmeter for essentially zero loading on the shunt.
Click to expand...
That seems to be correct. I have never heard of a shunt that matches the
tempco of the meter, except that both are constructed to be as stable as
possible with a zero tempco. However, some analog meters are supplied with
special leads which are calibrated to be used with a particular shunt. The
meter load is only significant for shunts under 500 mA where the typical 5
mA analog meter affect the reading by about 1%.

I'm not sure what the OP means by an accuracy of 0.001mV/mA. Since he asked
for a current and voltage source, it may just mean 1 uV or 1 uA depending on
what is needed. And usually absolute accuracy is not required, especially
for calibrating an oscilloscope.

Paul
 
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