exact current and potential source
in [Basics]
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From: whit3rd on 25 Apr 2010 14:34 On Apr 24, 11:09 am, John Fields <jfie...(a)austininstruments.com> wrote: > On Fri, 23 Apr 2010 20:51:53 +0300, Kari Laine <klai...(a)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'.
From: John Fields on 26 Apr 2010 12:47 On Sun, 25 Apr 2010 11:34:34 -0700 (PDT), whit3rd <whit3rd(a)gmail.com> wrote: >On Apr 24, 11:09�am, John Fields <jfie...(a)austininstruments.com> >wrote: >> On Fri, 23 Apr 2010 20:51:53 +0300, Kari Laine <klai...(a)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. JF
From: Paul E. Schoen on 27 Apr 2010 00:30
"John Fields" <jfields(a)austininstruments.com> wrote in message news:v1gbt59khdsi71i61vvdn2u5n4kid3spq7(a)4ax.com... > On Sun, 25 Apr 2010 11:34:34 -0700 (PDT), whit3rd <whit3rd(a)gmail.com> > wrote: > >>On Apr 24, 11:09 am, John Fields <jfie...(a)austininstruments.com> >>wrote: >>> On Fri, 23 Apr 2010 20:51:53 +0300, Kari Laine <klai...(a)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. 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 |