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meter sensitivity versus internal resistance
Is there a simple relationship between a meters internal resistance and its
sensitivity (ohms per volt). Maybe this is trivial but I don't see it. Uwe |
Uwe Langmesser wrote:
Is there a simple relationship between a meters internal resistance and its sensitivity (ohms per volt). Maybe this is trivial but I don't see it. Uwe The higher the resistance, the lower the operating current, but there is no simple formula. You need a current limiting resistor in series with the meter, and a known supply voltage. Start at the maximum resistance, and slowly reduce it till you have a full scale deflection. Disconnect the voltage, then measure the total resistance of the meter and adjustable resistor. Then use Ohm's law to determine the meter's sensitivity. Its simple and only takes a couple minutes to test. Use a low voltage, and it is a good idea to have different variable resistors to put in series. Start with a higher value resistor than you think you need, so you don't damage the meter, and work your way down. I use an expensive, six decade resistor with an adjustable regulated power supply. I can adjust the resistance in steps, from zero ohms to one megohm in one ohm steps. -- We now return you to our normally scheduled programming. Michael A. Terrell Central Florida |
Uwe Langmesser wrote:
Is there a simple relationship between a meters internal resistance and its sensitivity (ohms per volt). Maybe this is trivial but I don't see it. Uwe The higher the resistance, the lower the operating current, but there is no simple formula. You need a current limiting resistor in series with the meter, and a known supply voltage. Start at the maximum resistance, and slowly reduce it till you have a full scale deflection. Disconnect the voltage, then measure the total resistance of the meter and adjustable resistor. Then use Ohm's law to determine the meter's sensitivity. Its simple and only takes a couple minutes to test. Use a low voltage, and it is a good idea to have different variable resistors to put in series. Start with a higher value resistor than you think you need, so you don't damage the meter, and work your way down. I use an expensive, six decade resistor with an adjustable regulated power supply. I can adjust the resistance in steps, from zero ohms to one megohm in one ohm steps. -- We now return you to our normally scheduled programming. Michael A. Terrell Central Florida |
Uwe Langmesser wrote: Is there a simple relationship between a meters internal resistance and its sensitivity (ohms per volt). ============================ Yes. Couldn't be more simple. It is the internal resistance of the meter divided by the voltage range the meter is set to. When the meter is at full-scale deflection the current flowing through the meter is always (Voltage Range) / (Ohms per Volt). The internal resistance of the meter, on any voltage range, is the resistance of the moving coil plus the series meter-multiplying resistance. --- Reg, G4FGQ |
Uwe Langmesser wrote: Is there a simple relationship between a meters internal resistance and its sensitivity (ohms per volt). ============================ Yes. Couldn't be more simple. It is the internal resistance of the meter divided by the voltage range the meter is set to. When the meter is at full-scale deflection the current flowing through the meter is always (Voltage Range) / (Ohms per Volt). The internal resistance of the meter, on any voltage range, is the resistance of the moving coil plus the series meter-multiplying resistance. --- Reg, G4FGQ |
See URL:
http://www.tpub.com/neets/book16/68e.htm Too lengthy to repeat here -- but gives the full explanation -- Incognito By Necessity (:-( If you can't convince them, confuse them. - - -Harry S Truman "Uwe Langmesser" wrote in message ... Is there a simple relationship between a meters internal resistance and its sensitivity (ohms per volt). Maybe this is trivial but I don't see it. Uwe |
See URL:
http://www.tpub.com/neets/book16/68e.htm Too lengthy to repeat here -- but gives the full explanation -- Incognito By Necessity (:-( If you can't convince them, confuse them. - - -Harry S Truman "Uwe Langmesser" wrote in message ... Is there a simple relationship between a meters internal resistance and its sensitivity (ohms per volt). Maybe this is trivial but I don't see it. Uwe |
Uwe Langmesser wrote in message ...
Is there a simple relationship between a meters internal resistance and its sensitivity (ohms per volt). No. Maybe this is trivial but I don't see it. Not trivial at all. The internal resistance of a meter depends on the type of meter (D'Arsonval, moving iron, electrodynamometer, etc.) and its design. As a simple example, imagine two identical 0-1 mA D'Arsonval meters. Now replace the springs in one of the meters with new ones that require more force. The meter with the "stronger" springs will need more current for fullscale deflection even though it has the same internal resistance. --- There *is* a relationship between a milliammeter's fullscale reading and the ohms-per-volt when it is used with a series resistor to make a voltmeter. Ohms-per-volt = 1/current for full scale deflection (in amps) So, when used with the appropriate series resistor(s): a 0-1 mA meter will give 1000 ohms-per-volt a 0-100 uA meter will give 10,000 ohms-per-volt a 0-50 uA meter will give 20,000 ohms-per-volt etc. 73 de Jim, N2EY |
Uwe Langmesser wrote in message ...
Is there a simple relationship between a meters internal resistance and its sensitivity (ohms per volt). No. Maybe this is trivial but I don't see it. Not trivial at all. The internal resistance of a meter depends on the type of meter (D'Arsonval, moving iron, electrodynamometer, etc.) and its design. As a simple example, imagine two identical 0-1 mA D'Arsonval meters. Now replace the springs in one of the meters with new ones that require more force. The meter with the "stronger" springs will need more current for fullscale deflection even though it has the same internal resistance. --- There *is* a relationship between a milliammeter's fullscale reading and the ohms-per-volt when it is used with a series resistor to make a voltmeter. Ohms-per-volt = 1/current for full scale deflection (in amps) So, when used with the appropriate series resistor(s): a 0-1 mA meter will give 1000 ohms-per-volt a 0-100 uA meter will give 10,000 ohms-per-volt a 0-50 uA meter will give 20,000 ohms-per-volt etc. 73 de Jim, N2EY |
Think "Ohm's Law." The meter movement responds to a current. To read
voltage, you put a resistor in series with the meter movement so that V(full scale) = R(total)*I(meter, full scale). R(total) is the sum of the meter's internal resistance and the external series resistor. So a 1mA meter movement always gives 1kohms/volt, and a 20uA meter movement gives 50kohms/volt. Cheers, Tom Uwe Langmesser wrote in message ... Is there a simple relationship between a meters internal resistance and its sensitivity (ohms per volt). Maybe this is trivial but I don't see it. Uwe |
Think "Ohm's Law." The meter movement responds to a current. To read
voltage, you put a resistor in series with the meter movement so that V(full scale) = R(total)*I(meter, full scale). R(total) is the sum of the meter's internal resistance and the external series resistor. So a 1mA meter movement always gives 1kohms/volt, and a 20uA meter movement gives 50kohms/volt. Cheers, Tom Uwe Langmesser wrote in message ... Is there a simple relationship between a meters internal resistance and its sensitivity (ohms per volt). Maybe this is trivial but I don't see it. Uwe |
You are right Tom.
But my question was that when the sensitivity, for example 1 kohms/volt, is known, can one then also know, without further measurement, the internal resistance. And from what I gather the answer to that question is NO. thank you all Uwe in article , Tom Bruhns at wrote on 2/13/04 1:26 PM: Think "Ohm's Law." The meter movement responds to a current. To read voltage, you put a resistor in series with the meter movement so that V(full scale) = R(total)*I(meter, full scale). R(total) is the sum of the meter's internal resistance and the external series resistor. So a 1mA meter movement always gives 1kohms/volt, and a 20uA meter movement gives 50kohms/volt. Cheers, Tom Uwe Langmesser wrote in message ... Is there a simple relationship between a meters internal resistance and its sensitivity (ohms per volt). Maybe this is trivial but I don't see it. Uwe |
You are right Tom.
But my question was that when the sensitivity, for example 1 kohms/volt, is known, can one then also know, without further measurement, the internal resistance. And from what I gather the answer to that question is NO. thank you all Uwe in article , Tom Bruhns at wrote on 2/13/04 1:26 PM: Think "Ohm's Law." The meter movement responds to a current. To read voltage, you put a resistor in series with the meter movement so that V(full scale) = R(total)*I(meter, full scale). R(total) is the sum of the meter's internal resistance and the external series resistor. So a 1mA meter movement always gives 1kohms/volt, and a 20uA meter movement gives 50kohms/volt. Cheers, Tom Uwe Langmesser wrote in message ... Is there a simple relationship between a meters internal resistance and its sensitivity (ohms per volt). Maybe this is trivial but I don't see it. Uwe |
In article , Uwe Langmesser
writes: But my question was that when the sensitivity, for example 1 kohms/volt, is known, can one then also know, without further measurement, the internal resistance. And from what I gather the answer to that question is NO. thank you all In actual practice, d'Arsonval meters vary in internal resistance. The "Ohms/Volt" rating is only approximate. D'Arsonval meters consume DC power dependent on the meter motor (the coil winding driving the needle) position. Miniscule power to be sure but finite power demand. If the power of movement and display is the criterion for "sensitivity," then the meter resistance must be measured or known. That is imperative if the meter is to be shunted for a higher actual current indication. A millivoltmeter can measure the meter motor voltage drop at a given current through it. Law of Resistance will then apply to find meter motor resistance from current and voltage. To shunt a meter for higher current the external shunt resistor must have a value equal to the meter motor voltage drop divided by the total of desired current minus the meter current. At high current full scales the shunt resistance becomes quite low and the meter motor resistance is negligible for accuracy of high current. At low current full scales approaching that of a meter alone, the meter motor resistance is an appreciable part of the shunt resistance. In voltmeter applications, a series resistor is approximated by the "Ohms per Volt" value on relatively high full-scale ranges. That series resistor should be (for precision) the quantity total voltage drop minus the meter motor voltage drop, all divided by the meter current. Fortunately, the meter motor resistance is small and the voltage drop is quite small compared to the total needed voltage drop. The difference in voltmeter resistance values between approximate and precision becomes less as the meter current becomes less for a given full scale. Note: For full scale voltmeter readings of 100 V and up, the approximation of "Ohms/Volt" is quite good...one needs the meter motor resistance for an accurate voltmeter indication of 5 V and below to fit modern digital logic rail voltages. One can expect most d'Arsonval meters to exhibit about 50 mV motor drop at full scale...but could be almost any value from 20 to 100 mV. Len Anderson retired (from regular hours) electronic engineer person |
In article , Uwe Langmesser
writes: But my question was that when the sensitivity, for example 1 kohms/volt, is known, can one then also know, without further measurement, the internal resistance. And from what I gather the answer to that question is NO. thank you all In actual practice, d'Arsonval meters vary in internal resistance. The "Ohms/Volt" rating is only approximate. D'Arsonval meters consume DC power dependent on the meter motor (the coil winding driving the needle) position. Miniscule power to be sure but finite power demand. If the power of movement and display is the criterion for "sensitivity," then the meter resistance must be measured or known. That is imperative if the meter is to be shunted for a higher actual current indication. A millivoltmeter can measure the meter motor voltage drop at a given current through it. Law of Resistance will then apply to find meter motor resistance from current and voltage. To shunt a meter for higher current the external shunt resistor must have a value equal to the meter motor voltage drop divided by the total of desired current minus the meter current. At high current full scales the shunt resistance becomes quite low and the meter motor resistance is negligible for accuracy of high current. At low current full scales approaching that of a meter alone, the meter motor resistance is an appreciable part of the shunt resistance. In voltmeter applications, a series resistor is approximated by the "Ohms per Volt" value on relatively high full-scale ranges. That series resistor should be (for precision) the quantity total voltage drop minus the meter motor voltage drop, all divided by the meter current. Fortunately, the meter motor resistance is small and the voltage drop is quite small compared to the total needed voltage drop. The difference in voltmeter resistance values between approximate and precision becomes less as the meter current becomes less for a given full scale. Note: For full scale voltmeter readings of 100 V and up, the approximation of "Ohms/Volt" is quite good...one needs the meter motor resistance for an accurate voltmeter indication of 5 V and below to fit modern digital logic rail voltages. One can expect most d'Arsonval meters to exhibit about 50 mV motor drop at full scale...but could be almost any value from 20 to 100 mV. Len Anderson retired (from regular hours) electronic engineer person |
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