meters power supplies generators

8/6/2019 Meters Power Supplies Generators

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Meters, Power Supplies, and Generators

1. MetersGenerally analog meters respond to the average of the signal being measured. This is due to themechanical mass of the pointer and the RC response time of the electronics. Since the average ofmost AC signals is zero, AC meters require more circuitry to measure values of time varyingvoltages. Many meters can function as bo th current and voltage meters. Usually there are

separate inputs for the voltage and current func tions. Trying to use the instrument as a voltmeter,when the leads are connected to the current input will either destroy the meter or blow a fuse.

1.1 Voltage MeasurementsVoltage is measured across a component and indicates a voltage difference or voltage drop as

shown in Figure 1.

Figure 1: Voltmeter measuring a voltage drop

An ideal voltmeter has a resistance of. Real voltmeters use a large known, stable, resistor.Figure 2 illustrates the internal schematic of a voltmeter. Because of this resistor, all voltmeters

will draw some current. A typical digital voltmeter has a resistance of 10 M while some analogmeters may be as low as 3.2k.

Figure 2: Internal schematic of a voltmeter

1.2 Current MeasurementsIn order to measure a current, all of it must flow through the ammeter. Therefore, the ammeter


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must be inserted in series with the part of the circuit under investigation as shown in Figure 3.

Figure 3: Ammete r measuring current through a resistor

Ammeters work by inserting a small resistor in series with the circuit and measuring the voltage

drop across that resistor. This is normally shown as a resistor in series with an ideal current meteras shown in Figure 4.

Figure 4: Internal Schematic View of an Ammeter

The typical digital multimeter (DMM) on a 2A range has a resistance of 100 m, on the 200 A

range k, while a nanoampere meter will be more than 100 k. Because current meters haveresistance, they will cause a voltage drop and therefore may affect the operation of the circuit.

This should be kept in mind while doing any current measurements.

1.3 Calculating Meter ErrorAnalog Meters

In analog meters most of the error is due to the mechanical meter movement. This error isconstant, across a given meter range, regardless of the magnitude of the variable being measured.

An analog meter with accuracy of 2% is accurate to 2% of the full scale deflection (FSD). Forexample, suppose a reading of 35.0 V (dc) was obtained on the 50 V range and the meter

accuracy is 2%. Then, the error equals 2% * 50 VFSD = 1 V. Therefore, V = 35.0 1 V (dc) or asa percent V = 35.0 V (dc) 2.9%. Since the absolute error of 1 V is constant for the range, thesmaller the measured voltage in relation to the full scale, the larger the percent error becomes. Toaccount for the human error, a fraction of the Smallest Division (SD) is added to the analog meteraccuracy. For example, of SD on the 50V range is equivalent to an error of V.

Digital Multimeters (DMM)

The DMM is the most common instrument you will encounter. The precision of a DMM is


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expressed in digits. A 5 digit meter means that you have five full digits capable of indicatingfrom 0 to 9 and one digit, the means only a +1, blank (0), or -1 can be displayed. There are

also 3 , 4 , and 6 digit meters.The Tektronix DMM4020 used in the lab is shown in Figure 5 and its front panel controls are

described below. DMM4020 is 5 digit meter.

Figure 5: Tektronix DMM4020


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A DMM has two sources of systematic error:

One error is due to tolerances in resistors, and is expressed as a percentage of the readingand therefore varies with the magnitude of the variable being measured.

The second error is due to the digital conversion process. It is fixed for a given meterrange and is expressed as a percentage of the meter range. (See the Appendix for the accuracytables of the Tektronix DMM 4020)

For example, consider a reading of 1.54321 V on a 2 V dc range. According to Table A.1


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in the Appendix, the DMM error for the 2 V range is rated at 0.015% of the reading + 0.002% ofthe meter range for DC volt.

Then the errors are:reading e rror: 0.015% * 1.54321 V = 0.2315 mV

range error: 0.002% * 2 V = 0.04 mV.

Therefore, V = 1.54321 V 0.2715 mV or V = 1.54321 V 0.0176%.Now consider the same measurement on the 20 V range. The reading would have been 1.5432 V

reading e rror: 0.015% * 1.5432 V = 0.2315 mVrange error: 0.004% * 20 V = 0.8 mV.

Therefore, V = 1.5432 V 1.0315 mV or V = 1.54321 V 0.0668 %.

This shows that the most accurate measurements are made on the lowest meter range that will

accommodate the measured value.

1.4 Meter LoadingEvery meter affects the circuit it is connected to by drawing current (voltmeter) or adding avoltage drop (ammeter).

Analog Meters

An analog voltmeters resistance is usually specified as a sensitivity in ohms/volt. For example,the sensitivity of the Simpson analog voltmeter (Model 260) is 20 k/volt. The resistance for a

particular voltage range is determined by multiplying the sensitivity by the full scale voltage onthat range. As long as the voltmeter draws much less current than the component acrosswhich it is connected, we can say that its effect will be small.

Ammeters are more complex. The standard shunt resistance in an ammeter (precision resistor in

the meter) has a 50 mV drop at the maximum current rating. Since this voltage is usually small,an ammeter should not affect the circuit too much. Remember that every time you change the

range, the shunt resistance changes. Since the shunt resistance, in a normal meter, can vary frommilliohms to kilo-ohms, using the wrong current range can easily affect your circuit.

Digital Meters

For all AC and DC voltage ranges, the DMM has a fixed input resistance which is typically

10M. For current measurements, the meter will usually have a 200 mV drop at the maximumcurrent, but the resistance is different for each range. Digital meters are usually more accurate,since they remove parallax and other mechanical errors. However, if you do not know how to

select the range properly, you can easily get large measurement error comparable to that of ananalog meter. The Tektronix DMM4020 used in the lab is shown in Figure 5 and its front panel

controls are described below.

2. DC Power SuppliesAdjustable supplies have a control for changing the DC output voltage. DC supplies have twooutput leads where one is indicated as being positive (with respect to the other output). Many

supplies also have current limiting. This feature decreases the voltage when the drawn current isbeyond a certain limit. Because of this, the power supply will act as a voltage source until the

current limit is exceeded. Then, it acts as a current source with DC output current fixed at the


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current limit. In the case of the Anatek supply, the current limit can be set by the user using acontrol on the front panel. This control should be set before you perform any lab experiments.

Most components will not explode, or be destroyed, with 100 mA. If you short circuit the Anatekpower supply you will see the current meter indicate the present current limit. At anytime during

the lab, a glance at the power supplies current meter will tell you if you have a short circuit.

Warning: Only perform this short-circuit test on power supplies which have current

limiting. If the power supply is not limited you may start a fire or trigger an explosion.

Power Supply ConnectionsMost power supplies are floating, which means that neither the positive nor negative outputsare connected to ground or any common point. The Agilent E3620A Dual (variable) Output

DC Power Supply used in the lab is shown in Figure 6 and its front panel controls are describedbelow.

Figure 6: Power Supply Front Panel

1) POWER: Push on.

2) & 3): Voltage and C urrentMetering selectors: Push V1 or V2.

4) V1 Voltage Control (Fully clockwise for max voltage out).

5) V2 Voltage control (Fully clockwise for max voltage out).

6) V1 + terminal.


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7) V1 - terminal.

8) Ground (common) terminal.

9) V2 + terminal.

10) V2 - terminal.

NOTE: The + or the - can be connected to the common. Hence, you can have a positivecommon (or ground) or you can have a negative common (or ground). In add ition, with a dualsupply the V1 - terminal and the V2 + terminal can be connected to add both voltage

outputs.

3. Signal GeneratorsA signal generator provides a time-varying signal to test circuits. This can be anything from asimple sine, square, or triangular wave up to a TV station signal with different types of noise

added. The symbol and basic circuit model for a single-ended (notice the grounded output) signalgenerator is shown in Figure 7.

Figure 7: Signal Generator

ROUTis the output resistance and may sometimes be called RINT or internal resistance. Signal

generators typically have output resistance of 600, 75, or 50. Whenever you draw currentfrom a signal generator, the voltage at the output terminals will change due to the voltage drop

across the internal resistance. Each generator has an amplitude and frequency control. Thecontrols generally work in ranges, and you will usually see a range control that limits theminimum and maximum frequency or voltage and a fine control to adjust within the range.

Function GeneratorsA function generator is a signa l generator that is capab le of producing more than one waveformshape. The B&K Precision model 4017 function generator, shown in Figure 8, has a 50 output

impedance. Figure 8 is labeled to indicate the control panel functions.


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Figure 8: B&K Precision model 4017

1) POWER Switch. Turns power on and o ff.2) RANGE Switch. Selects output frequency range. Seven ranges from 1 Hz to 10MHz.

Switch indicate maximum frequency of range and is adjusted with COARSE

FREQUENCY control to 0.1 times the maximum. For example, if the 100 kHz range isselected, the output frequency can be adjusted from 10kHz to 100kHz.

3) FUNCTION Switch. Selects sine, square, or triangle waveform at OUTPUT jack.4) OUTPUT LEVEL Control. Controls the amplitude of the signal at the OUTPUT jack.

Output level can be decreased by approximately 20 dB with this control.

5) DC OFFSET Control. Enabled by the DC OFFSET Switch (14). Clockwise rotationfrom center changes the DC offset in a positive direction while counterclockwise rotation

from center changes the DC offset in a negative direction.6) OUTPUT Jack. Waveform selected by FUNCTION switch as well as the superimposed

DC OFFSET voltage is available at this jack.

7) TTL/CMOS Jack. TTL or CMOS square wave, depending on the position of the CMOSLEVEL switch (15) is output at this jack. This output is independent of the OUTPUT

LEVEL and DC OFFSET controls.8) CMOS LEVEL Control. Rotating this control clockwise increases the amplitude of the

CMOS square wave at the TTL/CMOS jack.

9) VCG/SWEEP Jack. Controlled by SWEEP EXT/INT Switch (12). When SWEEP EXTis selected, jack is the Voltage Controlled Generator input and permits external control of

generator output frequency by a DC voltage input at this jack. A positive voltage will

decrease frequency.10)DUTY CYCLE Control. Enabled by the DUTY CYCLE Switch (17). Rotation from

center pos ition adjusts the duty cycle of the main OUTPUT signal.11)-20 dB Switch. When engaged, the signal at the OUTPUT jack is attenuated by 20dB.

12)SWEEP INT/EXT Switch. When engaged (INT) enables the sweep mode of operation.Sweep rate is controlled by SWEEP TIME control (18) and sweep magnitude iscontrolled by the SWEEP WIDTH control (16). When released (EXT), allows external


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control of generator output frequency by a DC voltage input at the VCG/SWEEP jack(9).

13)SWEEP LIN/LOG Switch. When engaged (LOG) selects logarithmic sweepcharacteristic and in the released (LIN) position selects a linear sweep characteristic.

14)DC OFFSET Switch. When engaged, enables operation of the DC OFFSET control (5).

15)

CMOS LEVEL Switch. When engaged, changes the TTL signal to CMOS signal at theTTL/CMOS jack.

16)SWEEP WIDTH Control. Rotation determines sweep width by adjusting sweep stopfrequency.

17)DUTY CYCLE Switch. When engaged, enables operation of DUTY CYCLE control(10).

18)SWEEP TIME Control. In sweep mode, rotation determines amount of time to sweep

from the start frequency to the stop frequency.19)FINE FREQUENCY Control. Vernier adjustment of the output frequency for ease of

setting frequency.20)COARSE FREQUENCY Control. Coarse adjustment of the output frequency from 0.1

to 1 times the selected range.21)COUNTER DISPLAY. Displays frequency of internally generated waveform.22)GATE LED. Indicates when the frequency counter display is updated. When the 100K

through 10M ranges are selected, the LED will flash 10 times per second (every 0.1seconds). When the 10 through 10K ranges are selected, the LED will flash once everysecond and when the 1 range is selected, the LED will flash every 10 seconds. As the

LED turns off, the display is upda ted.23)Hz and KHz LED. Indicates whether the counter is reading in Hz or kHz.


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Appendix

Accuracy of the Tektronix Digital

Multimeter 4020 (DMM 4020)Important note: Use the values under the 1 year column as the default accuracy values.

1- DC voltageTable A.1

2- AC voltageTable A.2


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3- DC currentTable A.3

4- AC currentTable A.4

5- ResistanceTable A.5

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