RICHLAND COLLEGE School of Engineering Business & Technology Rev. 0 W. Slonecker Rev. 1 (8/26/2012) J. Bradbury INTC 1307 Instrumentation Test Equipment Teaching Unit 2 Direct Current Meters Unit 2 Direct Current Meters Objectives: Describe the construction of a D Arsonval meter movement. Explain the operation of a D Arsonval meter movement. Determine the effects of shunts across the meter movement. Understand the purpose of resistance in series with the meter movement. Define meter sensitivity. Analyze a circuit in terms of voltmeter loading or ammeter insertion errors. Understand the construction of an ammeter and voltmeter. Understand the construction and operation of a basic ohmmeter. Understand ammeter, voltmeter, and ohmmeter schematic diagrams. Understand procedures necessary to safeguard meters. D Arsonval Meter Movement The D Arsonval meter movement is the basic mechanism used in analog meters. This movement is also called the permanent magnetized moving coil (PMMC) type. Developed by D Arsonval in 1881, this meter has retained general usage until very recently. It is general displaced today by inexpensive digital meters. The meter operates on the principle of a dc motor. The field of an extremely strong permanent magnet acts to repel or attract a soft iron core bobbin which is magnetized by an externally applied current. Very many turns of fine wire wrap the bobbin. The bobbin is axially mounted on a jewel bearing to minimize friction. A pointer is attached to the bobbin cylinder. Rotation of the cylinder provides the pointer deflection that indicates how permanent magnet much current is flowing in the bobbin coil. Opposite magnetic poles attract while like magnetic poles repel. The amount of motoring or rotating force determines the degree of rotation of the meter movement. The strength of the bobbin field intensity is given by current multiplied by the number of bobbin turns. Increasing current increases the movement rotation. The spiral spring provides restoring torque to return the pointer to its leftmost rest position. The spring is attached at the axis of the bobbin cylinder at one end and to a lever at the other. The lever can be manually rotated to adjust the zero position of the pointer. A small slot pointer pole shoes bobbin coil spiral spring counter weight Page 1 of 9
on the underside of the spring attachment lever can be rotated against holding friction to make slight changes in the pointer zero position. This is called Zero Adjust, and must be done with no current flowing. 1. The diagram on the left shows magnetic poles on the meter movement. Since unlike poles attract and like poles repel, the pointer is being deflected toward the South pole shoe. The bobbin rotation is countered by spring tension pushing counterclockwise. Deflection is shown for about half of the total deflection range. With zero current, the pointer is at its leftmost position. The fullscale pointer deflection is shown to the right of the pointer. Fullscale does not correspond to the absolute maximum rotation possible, but it represents the maximum linear rotation of the bobbin against opposing spring torque. The zero position, marked to the left of the pointer position represents a spring neutral pointer shown deflected ½ scale from zero position Zero position. Meter test leads are polarized. If a current were applied with polarity opposite the meter polarity, the pointer would deflect to the left of the zero line. If this connection were not immediately corrected, damage could result to the meter. Meter Safety Concerns: 1. Always set the meter range to a scale higher than you would expect to read, then change to a lower scale to obtain the best reading. 2. Always be careful to observe meter polarity when making the measurement. If you don t know the polarity, start with a short tapping connection to see if the pointer deflection is in the right direction before committing to a hard connection. 3. Current measurements require that the circuit be broken and the break bridged by the ammeter. Electron flow will be into the meter and out of the meter. Power should be off before connecting the ammeter into a circuit since the voltage will be a higher value across an open circuit. 4. Ohmmeters should never be used while circuit power is ON. Even with power off, time must be allowed for capacitors to bleed off stored charge before using an ohmmeter. Keep in mind that an ohmic measurement across a circuit resistor will include the effects of all circuit resistance paralleling the resistor. Current for FullScale Deflection: Ifsd is the current required to deflect the meter pointer to the maximum safe value on the meter. The deflection at Ifsd will be to the maximum value on the meter scale set by the range switch. The Ifsd for a Simpson 260 meter movement is 50 A. You will not see a 50 A range on the meter. The lowest range is 1mA full scale. This allows some internal protection for the meter movement. A meter movement has internal resistance, Rm, due to the resistance of the fine wire wound on the movement bobbin. The Rm of the Simpson 260 is 2k. Do not attempt to measure this resistance with an ohmmeter. A meter resistance measurement will be made in a lab experiment. For an Ifsd of 50 A and Rm of 2k, calculate the maximum voltage that can be applied across the meter movement without exceeding Ifsd and damaging the movement. 1: Vfsd = Ifsd Rm 50 A 2k = 100mV or 0.1V The full scale deflection voltage for a meter is important. It is the key parameter used in designing meter shunts to change the range of an ammeter. N N S S Fullscale Page 2 of 9
The approach to designing ammeter shunt resistance is based on the Vfsd (which is based on Ifsd and Ohm s Law). In the schematic on the right, suppose we want the meter to be at full scale when the input current, I, is equal to 10mA. It should be clear that when the voltage across the meter shunt equals Vfsd, then meter current will equal to Ifsd, or full scale deflection. We reason that if I is 10mA and is 50 A, then Ish must be 10mA 50 A = 9.95mA. We also know that the Rsh voltage must equal 100mV. So we use Ohm s law, which tells us that Rsh = Vfsd/9.95mA or 10.05. Notice that to have an accurate meter, the meter shunt must have very high precision. 2: Rsh= Vfsd ax Ifsd Using the same meter movement for a 100mA full scale ammeter gives us Ish = 100mA 0.05mA =. Rsh is then Vfsd/Ish = (1.0005 ) Rm Rsh Ish I The total inline resistance of an ammeter, Rin, is the parallel combination of Rm and Rsh. For the 10mA example, this comes to 10.05 2k = 9.99975. That is a lot less than if there were no meter shunt, but it is still a lot more than a short circuit. Remember that the power to deflect the meter pointer comes from the current being measured. This means that some power is lost to the meter (I 2 Rin). Whenever a meter is used, the circuit is changed a bit. A tech has to understand how meter use affects circuits. It is easy to see that the 100mA meter has an Rin of about an ohm, so that it would have less of an effect than a 10mA (10 ) meter. Ammeter Loading: In the circuit on the right, the 10mA meter is inserted in Rmeter series with the load to measure the current. Without the meter it is evident that the current is 1v/100, or 10mA. The meter adds another 9.995 of series resistance to the 100, making the total resistance 109.995. With 1 V the meter in the circuit, the current is 1v/109.995, or 9.0913mA. Absolute error is 10.0 9.0913 = 0.90867mA for a percentage error of 9.09%. Notice that the meter reading of 9.0913mA is close to 10mA full scale. If the load were 1000 instead of 100, the meter resistance would have a much smaller effect. However, the measured current would be a little less than 1mA (1v/1009.995 ) so that reading the scale would be less accurate. MultiRange Designs: There are two designs for a switchable resistor shunt for an ammeter. One uses a makebeforebreak rotary switch with contacts like those shown on the right. Each resistor is designed for a full scale current just as we saw above. It is important that the switch not have an interval when there is no shunt connected to the meter since that would expose the movement to large currents. 100 Ohm Page 3 of 9
The schematic diagram to the right depicts a multirange ammeter with a rotary range selector switch. If the three ranges had full scale currents of 100mA, 10mA, and 1mA, we have already calculated a 1.0005 shunt for the 100mA scale and a 10.05 shunt for the 10mA scale. The 1mA scale follows a similar calculation: 1mA 0.05mA = 0.95mA shunt current for a 100mV full scale voltage across the movement. 0.1/0.00095 = 105.236 shunt resistance for the 1mA scale. With the three shunt resistors and the meter movement, the meter is designed. Rm A B C R1 R2 R3 The second type of ammeter does not require a makebeforebreak range switch. The circuit on the right shows how the shunt resistors are connected. These shunt resistors are harder to calculate. In position B, R2 R3 shunt the meter movement, which is in series with R1. In position C, R3 shunts the meter movement in series with R1 R2. Only in position A, where the shunt resistance is the sum of the three resistors is the computation comparable to the shunt calculations for the standard switching arrangement. This design is called the Ayrton Shunt. We will not examine the design equations. Rm R1 R2 R3 B A C DC Voltmeters: The dc voltmeter utilizes the meter movement and a series resistor to change a currentreading movement into a voltmeter. Ohm s law states that V/R = I. The voltmeter resistance is the series resistor, Rs, plus the movement resistance, Rm. The full scale current of 50 A (or whatever it is for a given meter movement) is given as Vin/(Rs Rm). If we wish to calculate Rs for a full scale voltage reading of 25v, then 25/(Rs 2000) = 50 A. Or (25 Vfsd)/ 50 A = Rs. So for Vfsd = 0.1v, Rs = 24.9/0.00005 = 498,000. Rs Rm = 498k 2k = 500k and 25v/500k = 50 A. 3: Rs= Vmfs Vfsd Vmfs = meter input voltage for full scale deflection Using equation 3 it is easy to see that for a 10v full scale voltmeter the series resistance is 10 0. 1 0. 00005 =198 k A multirange voltmeter is constructed with a switch that selects the series resistor for the desired range. For protection of the meter movement, the resistor series resistor for the least range is not switched and the higher range resistors simply add their resistance to the lower range. Rs Rm Page 4 of 9
For the multirange voltmeter shown on the right, range C has a full scale deflection at 2 volts dc. The meter movement has Vfsd of 100mV and Ifsd of 50 A. We first calculate R3 for the range for the 2 volt range. R3 = (2 0.1)/ 50 A = 38k. We next calculate the sum, R2 R3, for a 20 volt range. 20 0. 1 0. 00005 =398 k so that R2 = 398k 38k = 360 k. Finally we calculate R1 R2 R3 for a 100 volt range. 100 0.1 0.00005 =1998k so that R1 = 1998k 398k = 1600 k. This completes the design of a 3range voltmeter with ranges of 2 volts, 20 volts, and 100 volts. Notice that it would DAMAGE the meter movement to connect 100 volts on the 2 volt scale. The current would be 100v/38k = 2.63mA exceeding the 50 A capability of the meter movement by a factor of 52.6 times; a sure way to destroy the meter. However, if a 2.5 volt zener diode were connected from the C switch terminal to the meter common, then the meter movement would only see 2.5v/38k = 65.8 A, probably safe for the movement. The one zener would protect the meter on all scales. Sensitivity of the Meter and Circuit Loading: The sensitivity of a meter with an Ifsd = 50 A is simply 1/Ifsd = 1/50 A = 20 k /volt. Thus the total resistance for a 100 volt range would be 100 20k = 2M. That includes the Rm of the meter, which is 2k, leaving 1998k for the total Rs on that range. It is important to know the sensitivity of a meter so that you can know how much resistance the voltmeter will place in series with the circuit being measured. 4: Meter Sensitivity = 1/Ifsd If you are measuring voltage across a 2k resistor on a 20 volt range with a 20k /volt meter, the meter resistance puts 400k in parallel with the 2k, barely changing the circuit (1.99k ). However, if the resistor were a 400k resistor, then the meter would load the circuit by changing the resistance to 200k. The measurement would be practically worthless unless you knew precisely how the loading would alter the original voltage so the alteration could be accounted for. It is important for a tech to know the loading effects of instruments used to make measurements. Rule of Thumb: If Rin of a voltmeter is 9 times greater than the circuit resistance for which the voltage is measured, the error will be 10%. If Rin of a voltmeter is 20 times greater, the error will be roughly 5%. The difference in measured voltage compared to actual circuit voltage depends on the equivalent resistance of the rest of the circuit. Remember Thevenin equivalent circuits? If one resistor is isolated in a circuit, the remainder of the circuit can be considered as a Rm voltage source, Vth, and a source resistance, Rth. The voltage measured across Rx can be found using voltage division. R2 R1 Vth R3 Rth A B C Rx Page 5 of 9
Vmeter=Vth Rx Rmeter Rx Rmeter Rth You should be able to reason from the equation that if Rth is very small compared to the loaded Rx, then the meter reading is not greatly affected by meter loading. However, when Rth is about equal to the loaded Rx, error can be 50%. Voltmeter Design Example: Using a meter movement with Ifsd = 80 A, Rm = 1.5k, design voltmeter with full scale ranges of 5v, 25v, and 100v. Use a single zener diode for meter protection. R2 R3 Rm a The circuit is shown on the right. The task is easy. First calculate the sensitivity. 1/80 A = 12.5k /v R1 B A Vrange Rmeter Rs Rrange C 5v 62.5k 61k R3 = 61k 25v 312.5k 311k R2 = 250k 100v 1250k 1248.5k R1 = 937.5k Vzener = 6v Giving the protection zener a 20% margin over the small scale voltage is appropriate. The Rmeter entries are simply Vrange times sensitivity. The Rs entries are Rmeter Rm. The Rrange entries are the actual circuit resistors. R3 = Rs; R2 = Rs R3; R3 = Rs (R2 R3) Voltmeter Loading Examples: For voltmeter loading calculations, all you need to know is the sensitivity. 1. Given: Sensitivity = 10k /volt Find Rin for meter for 100v, 50v, and 10v ranges. 100v 50v 10v Vth Rth Rx Rin = 1000 k 500 k 100 k For Vth = 25v, Rth = 50 k and Rx = 50 k, the unloaded voltage across Rx is 12.5v. Solve for meter loading for each meter range. 100v Range equivalent resistance of meter paralleling Rx = 50 k 1M =47. 62k 1050 k Using voltage division, 47. 62k 25 47. 62k 50 k =12. 1 Absolute error is 12.5 12.195 = 0.305 Percent error = (0.305/12.5)x100 = 2.44% 50v Range equivalent resistance of meter paralleling Rx = 45.55k Using voltage division, 25 45.55 k 45.55 k 50 k =11.918 Absolute error is 12.5 11.918 = 0.582 Percent error = (0.582/12.5)x100 = 4.66% Page 6 of 9
10v Range equivalent resistance of meter paralleling Rx = 33.33k Using voltage division, 33.33 k Absolute error is 12.5 9.999 = 2.501 25 =9.999 v 33.33 k 50 k Percent error = (2.501/12.5)x100 = 20% Safe Voltmeter Operation Normally the meter common lead is connected first. When touching probes to component leads this means that the or common (black) lead touches first. Observe polarity. Watch needle deflection while you touch the probes to leads. If deflection is backward, swap the probes. Always start on a higher range than you may expect to read. Now you know why this is a good idea. On a higher range the meter is better protected by a higher series resistance than on a lower range. When done with measurements, always leave the meter range switch on a high voltage range for maximum protection if it does not have an off position. The Ohmmeter: A schematic diagram for a basic ohmmeter is depicted on the right. With the inclusion of a battery, it is obvious than an ohmmeter is unlike the ammeter and voltmeter circuits which take power from the circuit being measured. The internal battery is there to supply a current through the unknown resistor, Rx. An ohmmeter should never be used on a circuit with power turned on as the ohmmeter supplied current can adversely affect the circuit being measured and the measured circuit can inject power into the ohmmeter that can damage the meter. Rs2 50% V Rs1 Rm Rsh Rx The theory behind the operation is fairly simple. The total resistance in the loop is Rs1 Rs2 Rm Rx. So the total meter current is V divided by the total resistance. The potentiometer is included as part of Rs so that the resistance can be varied to compensate for drooping battery voltage, to give the battery a longer usable life. Notice that for minimum Rx, when Rx is replaced by a short circuit, the current is maximum. So zero ohms is a full scale reading. Suppose we want the meter designed so that a 1 ohm Rx will make the pointer drop to 95% of full scale. VB/Rs = 100% full scale and VB/(Rs 1 ) = 95% full scale; a 1 change produces a 5% reduction in full scale current. If we pick Rsh, the meter shunt, to set full scale current to 20mA, then a 95% full scale is 19mA. In other words, a 1 increase in Rs corresponds to a 5% increase. This makes Rs = 20. To get 20mA with a Rs resistance 20, VB must be 0.4v. Using the same meter movement that was used to design ammeters (Ifsd = 50 A, Rm = 2k ), the shunted meter resistance is 5 (Vfsd/20mA = 5 ). So of the 20 needed in the ohmmeter circuit, 5 are provided by the shunted meter. So if Rs1 is make 12, and Rs2 is a 6 pot adjusted for 3, and the battery provides 0.4Vdc, the meter is complete. Designing the pointer scale is interesting. Full scale is 20mA; 95% full scale is 19mA. 75% full scale is 15mA corresponding to an external resistance of 6.666 (0.4v/26.666 = 15mA). Half Page 7 of 9
scale is 10mA corresponding to an external resistance of 20 (0.4v/40 = 10mA). Notice that half way between 0 and 20 is not 10 but is 6.666. The scale is not linear! 25% deflection occurs for 5mA. This happens for an external resistance of 60. To mark the scale in ohms requires the 25% tick to be 60, the 50% tick to be 20, and the 75% tick to be 6.666. This is true because the meter is measuring current. Ohm s Law states that I = V/R. R is in the denominator. Designing an ohmmeter for a higher resistance scale requires correspondingly larger resistance values and no meter shunting. Suppose we want 95% deflection for 1k. This implies that 1k changes the total circuit resistance by 5%, or Rs1 Rs2 Rm = 20k. If shorting the Rx input gives 50 A, then VB = 50 A 20k = 1.0 volts. Rs1 Rs2 = 20k 2k = 18k. The actual current with Rx = 1000 would be 1.0/21k = 47.62 A or 95.24% deflection. Half scale deflection would occur for Rx = 20k. When the external resistance doubles the total circuit resistance, the meter reads half scale. Ohmmeter Summary: Ohmmeters require an internal power source, usually a battery for portable, handheld instruments. The ohmmeter is basically a series circuit with the external resistance adding its resistance to the circuit total. Ohmmeter design is mainly concerned with the range scale calibration so that resistance values within a selected range can be read from the scale. A short circuit gives maximum current and maximum meter deflection. Design guidelines: Pick a resistor value for 95% deflection; total resistance should be 20 times that value. Pick a reasonable value for internal dc voltage. That value of voltage divided by total resistance gives the full scale deflection current for the meter shunt. Safe Ohmmeter Operation: Never connect an ohmmeter to a circuit that has power applied. Always be sure that the component you wish to measure is isolated from the rest of the circuit. Always make sure that anyone who wishes to borrow your meter knows the basic safety facts. Calibration of Meters: Shop_standard 50% V The diagram above shows how 5 ammeters may be calibrated against a shop standard meter. The shop standard is a meter that is trusted, perhaps because it has been calibrated by a calibration lab and supplied with a certificate of calibration. Such certification always has a limited expiration time, but when a freshly calibrated meter comes back from the cal lab, it s a good idea to use it to Page 8 of 9
calibrate the rest of the meters in the lab. As you can see from the diagram, the potentiometer allows you to check the meters at a number of places on the meter scale and in all ranges of operation. The first point of adjustment is the meter zero, which is the mechanical adjustment of the D Arsonval movement spring. 50% Shop_standard V RL As you would expect, the calibration of voltmeters is done with the meters in parallel. A shop standard meter is used in the same way as with ammeters. Notice the load resistor, RL. This resistor is required for the potentiometer to implement a voltage divider to vary voltage. It would not be needed if a variable dc voltage source were used. Ohmmeters can also be compared to shop standards, but this must be done one meter at a time. Why? Answer: Since each meter would be putting current through the unknown Rx, the result would be that no measurement would be correct. Having more than one ohmmeter attached to a resistor would be similar to trying to measure resistance in a circuit without first turning off circuit power. Page 9 of 9