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An automatic compensating instrument for measuring low electric direct currents
NUCLEAR INSTRUMENTS AND METHODS
91 (1971) 633-635 ; Q NORTH-HOLLAND PUBLISHING CO.
AN AUTOMATIC COMPENSATING INSTRUMENT FOR MEASURING LOW ELECTRIC DIRECT CURRENTS O. CLOGS and G. HEIGWER
Gesellschaftt fäir Strahleqfarschung mbH, Abteilung Biophysikalische Strahlenforschung, Frankfurt a/hl, Germany Received 13 July 1970 The circuit of an automatic compensating instrument is described. The instrument consists of two identical measuring circuits, which measure low electric direct currents by Townsend's method . The potential difference between the input of the instrument and the ground does not exceed 40 mV. This instru-
ment allows the average values of direct currents down to Mr-10 A to be measured, which are fluctuating. The relative error is at most 10-3 in automatic measurements and < lOr-4 for manual measurement.
1. Intmduction Many problems in experimental physics require measurement and comparison of two electric direct currents. Particular problems of this kind arise in measuring ionizing radiations. For example, if the particle fluence ofradiations should be measured behind absorber plates of different thicknesses, one instrument "A" serves as reference instrument (monitor), and the second instrument "B" measures the desired signal behind these absorber plates. For instance the measuring instruments may be ionization chambers, photocells, luminescence detectors etc. As the fluences of many radiations show temporal variations, one is interested in the average of the fiuence per unit time. Moreover, both signals "A" and "B" may show different amplitudes which are caused by different time constants and different damping-circuits . Therefore it is necessary to register time integrals of the signals of both measuring instruments "A" and "B" for a certain period of time and to compare their values . Beyond that, many problems of nuclear measurements demand that the potential of the input of the instrument should be identical or nearly identical with the potential of the ground. For example, the effective volume ofan ionization chamber or the sensibility of a Faraday cup depend on the potential of the input of the instrument. In order to get precise measurements, .a.. Voim ü ting ins r1~i111V.i~lt ""tiQi.s7 1-dit.. CTAy, "VÜ .C YL.~iS 9VV the conditions mentioned before and permits low electric currents to be measumd with high precision.
is used to compensate the currents. The current i carries the charge Q within a certain period of time on an interchangeable condensor C, on the input of the amplifier (1) and on the stray capacity C. amplifier shows an input voltage U, . In order to reset this voltage to zero, a charge -Q has to be influenced through the condensor C to the input. For that purpose the slider of the potentiometer P, has to be moved for the voltage U2. In this case the following relations exist U,(C+ C`) = Q = U2C. If U2 is measured and the capacity C is knoxvn . one can calculate the charge Q which has h°n eollo ¬ e during this period . 3. Function and description of the circuit Fig. 2 shows the whole circuit diagram of the instrument. A description of the two most important applications will be given in the following cations 3. and 3.2. 3.1 . MEASUREMENT OF CURRENTS WHIC'.ti INTERRUPTED
A current which can be interrupted emsts for example in the case of a particle accelerator which
2. Basic principle of the instrument The instrument consists of two completely identical measuring circuits "A" and "B" . The registration of both measured values is performed by one digital voltmeter in order to reduce costs. Townsends method of compensating, which is shown schematically in fig. 1, 633
e' _;.-
Fig. l . Scheme of Tciwnscnd's method of 4N~mgvn.,xttng.
O. CLOGS AND G . HEIGWER
634
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1
MEASURING LOW ELECTRIC DIRECT CURRENTS
produces a beam of particles for a period of time. It also exists if the beam of a continually emitting source of radiation (for example a radiactive isotope) can be interrupted by any suitable mechanism . In such a case the input of the compensating instrument will be opened, and at that time the beam of radiation will be inserted . After the measurement has been completed one may calculate the average of the current i if one measures the charge Q and the time t. The switch S1 allows the polarity to be chosen not only of the compensating voltage, but also of the input voltage as a function of the polarity of the current i, which has to be measured . The current i puts charge on the condensor C and thereby the voltage U, of the input increases and by the amplifier (1) it is fed to the discriminator (2). If the input voltage exceeds 40 mV the discriminator (2) is triggered and opens the gate (3). This enables the step motor to move the slider of the potentiometer Pt for a voltage step U2 (see fig. 1). Then the discriminator (2) returns to its starting position and closes the gate (3). This procedure will be repeated until the input current i is cut oft Now the voltage U2 of the slider P, is measured by the digital voltmeter. By the switch SS the digital voltmeter is connected to the measuring circuits "A" and "B" successively . The output of the digital voltmeter is transferred to a desk calculator, which prints the values of the two slider voltages of the potentiometers P, and their quotient . The maximum error which may happen in this way of compensating is 40 mV, the maximum relative errar at a compensating voltage of 80 V does not exceed A L"i/ U, = 0 .5 x 10 -3 . If measurements should be done with a higher precision, it is necessary to put the input of the amplifier (1) on a potential of 39 mV before starting the measurement. Then the voltage difference between this potential and the trigger point of the discriminator (2) does not exceed 1 mV. After cutting off the current i, it is necessary to put the input of the amplifier (1) again on a potential of 39 mV by means of the potentiometer Pa. In doing so, it is possible to minimize the error of U2 to 1 mV only. At a compensating voltage of 100 V, the relative errnr is nt mnct
Ill-Ill- ~- 1() -4
After
having finished the measurement, the slider of the potentiometer P, must be reset. Therefore, a 100 cps square wave generator triggers through the gate (5) the step motor in the opposite direction. Tale voltage of the potentiometer slider P, is divided and connected to the negative input of the differential amplifier (4) which acts as a comparator. The positive input of this amplifier is connected to a voltage divider by the switch
635
S2. This voltage divider is so adjusted that at a vol?a,,e of less than 40 m-V at the slider of the potentiometc the gate (5) is shut by the differential amplifier through the inverter (6). Then the step motor stops. 3 .2 . MLASURLMLNt U¬- CURRtat- s wtlICH CANNOt BE INTERRUPTED
In some cases the currents cannot be interrupted or should not be interrupted in order to measure random samples. Then the measurement can be done by the following procedure. If the switch S3 is put into the position "on" the measurement is started. The difference amplifier (4) will stop the measurement in both circuits if the potential (a) of the slider of the potentiometer P2 is equal to the potential of the other input (b) of the differential amplifier. At the same time the inputs of both circuits "A" and "B" are grounded . The values of U2 for both circuits are indicated by the digital voltmeter and handled as in 3 .1 . In this case the error of the compensating voltage d t'2 is maximum 40 mV. 3 .3 . INITIAL CHECK OF FUNCTION
For testing the instrument it is necessary tci put the switch S6 in the position "on" while the input switch S,, is open . Now the compensating voltage is connected through the resistor "R" to the input of the amplifier (1). The current is handled in the same «a~ a, the enPW current i. see section 3.1 . 3 .4 . INTERNAL CALIBRATION OF CONDFNSORS
In measuring the charges 0 one neat, a cT ;}? ; ;,, densors of different capacities . These condensers be calibrated by the instrument itself with the aict t f switch S6 , see section -. .3. In this case the voltagr is measured by the digital voltmeter and the value of the resistor "R" must be known . Then the current known and it will be possible to measure the capacit if the time of measurement is knoNvn . In rncasuring one must notice that the value of the smallest con_ densor C should exceed the third part cf the stra w capacity e`, which is the sum of the input cap~tc the amplifier (1) and the capacity of the cable . A low current which can bt" nic" asured ~, i i h densor of 10 pF and an ac` uric` ;>( :v° 1 within 10 min for t , = 1(k) V is
i
'on -
\ ii?
We wish to express our thanks
Pohlit for many discussions and suggestions . This cork is dedicated to Prof. Dr. H . [)~inrer on his o ._th, birthday .
91 (1971) 633-635 ; Q NORTH-HOLLAND PUBLISHING CO.
AN AUTOMATIC COMPENSATING INSTRUMENT FOR MEASURING LOW ELECTRIC DIRECT CURRENTS O. CLOGS and G. HEIGWER
Gesellschaftt fäir Strahleqfarschung mbH, Abteilung Biophysikalische Strahlenforschung, Frankfurt a/hl, Germany Received 13 July 1970 The circuit of an automatic compensating instrument is described. The instrument consists of two identical measuring circuits, which measure low electric direct currents by Townsend's method . The potential difference between the input of the instrument and the ground does not exceed 40 mV. This instru-
ment allows the average values of direct currents down to Mr-10 A to be measured, which are fluctuating. The relative error is at most 10-3 in automatic measurements and < lOr-4 for manual measurement.
1. Intmduction Many problems in experimental physics require measurement and comparison of two electric direct currents. Particular problems of this kind arise in measuring ionizing radiations. For example, if the particle fluence ofradiations should be measured behind absorber plates of different thicknesses, one instrument "A" serves as reference instrument (monitor), and the second instrument "B" measures the desired signal behind these absorber plates. For instance the measuring instruments may be ionization chambers, photocells, luminescence detectors etc. As the fluences of many radiations show temporal variations, one is interested in the average of the fiuence per unit time. Moreover, both signals "A" and "B" may show different amplitudes which are caused by different time constants and different damping-circuits . Therefore it is necessary to register time integrals of the signals of both measuring instruments "A" and "B" for a certain period of time and to compare their values . Beyond that, many problems of nuclear measurements demand that the potential of the input of the instrument should be identical or nearly identical with the potential of the ground. For example, the effective volume ofan ionization chamber or the sensibility of a Faraday cup depend on the potential of the input of the instrument. In order to get precise measurements, .a.. Voim ü ting ins r1~i111V.i~lt ""tiQi.s7 1-dit.. CTAy, "VÜ .C YL.~iS 9VV the conditions mentioned before and permits low electric currents to be measumd with high precision.
is used to compensate the currents. The current i carries the charge Q within a certain period of time on an interchangeable condensor C, on the input of the amplifier (1) and on the stray capacity C. amplifier shows an input voltage U, . In order to reset this voltage to zero, a charge -Q has to be influenced through the condensor C to the input. For that purpose the slider of the potentiometer P, has to be moved for the voltage U2. In this case the following relations exist U,(C+ C`) = Q = U2C. If U2 is measured and the capacity C is knoxvn . one can calculate the charge Q which has h°n eollo ¬ e during this period . 3. Function and description of the circuit Fig. 2 shows the whole circuit diagram of the instrument. A description of the two most important applications will be given in the following cations 3. and 3.2. 3.1 . MEASUREMENT OF CURRENTS WHIC'.ti INTERRUPTED
A current which can be interrupted emsts for example in the case of a particle accelerator which
2. Basic principle of the instrument The instrument consists of two completely identical measuring circuits "A" and "B" . The registration of both measured values is performed by one digital voltmeter in order to reduce costs. Townsends method of compensating, which is shown schematically in fig. 1, 633
e' _;.-
Fig. l . Scheme of Tciwnscnd's method of 4N~mgvn.,xttng.
O. CLOGS AND G . HEIGWER
634
--------------------------
q e G
a
".wa r1 C r~
Q
C
bA
IV Q
0
u :.
C
Ô
~v Ô
E ~g 1
w û V
CL O
N
a
~100
Û
.~+
w
â ô
O
.
Ç ß 0
7
a
Z7
~ E
... (.
r
ô V
-1NMetvti`p
..,1
Gd
F,
ay
:3
:3
mH
Vf
Z 4 ô ZK e
w O h
L-------------i---------
1
MEASURING LOW ELECTRIC DIRECT CURRENTS
produces a beam of particles for a period of time. It also exists if the beam of a continually emitting source of radiation (for example a radiactive isotope) can be interrupted by any suitable mechanism . In such a case the input of the compensating instrument will be opened, and at that time the beam of radiation will be inserted . After the measurement has been completed one may calculate the average of the current i if one measures the charge Q and the time t. The switch S1 allows the polarity to be chosen not only of the compensating voltage, but also of the input voltage as a function of the polarity of the current i, which has to be measured . The current i puts charge on the condensor C and thereby the voltage U, of the input increases and by the amplifier (1) it is fed to the discriminator (2). If the input voltage exceeds 40 mV the discriminator (2) is triggered and opens the gate (3). This enables the step motor to move the slider of the potentiometer Pt for a voltage step U2 (see fig. 1). Then the discriminator (2) returns to its starting position and closes the gate (3). This procedure will be repeated until the input current i is cut oft Now the voltage U2 of the slider P, is measured by the digital voltmeter. By the switch SS the digital voltmeter is connected to the measuring circuits "A" and "B" successively . The output of the digital voltmeter is transferred to a desk calculator, which prints the values of the two slider voltages of the potentiometers P, and their quotient . The maximum error which may happen in this way of compensating is 40 mV, the maximum relative errar at a compensating voltage of 80 V does not exceed A L"i/ U, = 0 .5 x 10 -3 . If measurements should be done with a higher precision, it is necessary to put the input of the amplifier (1) on a potential of 39 mV before starting the measurement. Then the voltage difference between this potential and the trigger point of the discriminator (2) does not exceed 1 mV. After cutting off the current i, it is necessary to put the input of the amplifier (1) again on a potential of 39 mV by means of the potentiometer Pa. In doing so, it is possible to minimize the error of U2 to 1 mV only. At a compensating voltage of 100 V, the relative errnr is nt mnct
Ill-Ill- ~- 1() -4
After
having finished the measurement, the slider of the potentiometer P, must be reset. Therefore, a 100 cps square wave generator triggers through the gate (5) the step motor in the opposite direction. Tale voltage of the potentiometer slider P, is divided and connected to the negative input of the differential amplifier (4) which acts as a comparator. The positive input of this amplifier is connected to a voltage divider by the switch
635
S2. This voltage divider is so adjusted that at a vol?a,,e of less than 40 m-V at the slider of the potentiometc the gate (5) is shut by the differential amplifier through the inverter (6). Then the step motor stops. 3 .2 . MLASURLMLNt U¬- CURRtat- s wtlICH CANNOt BE INTERRUPTED
In some cases the currents cannot be interrupted or should not be interrupted in order to measure random samples. Then the measurement can be done by the following procedure. If the switch S3 is put into the position "on" the measurement is started. The difference amplifier (4) will stop the measurement in both circuits if the potential (a) of the slider of the potentiometer P2 is equal to the potential of the other input (b) of the differential amplifier. At the same time the inputs of both circuits "A" and "B" are grounded . The values of U2 for both circuits are indicated by the digital voltmeter and handled as in 3 .1 . In this case the error of the compensating voltage d t'2 is maximum 40 mV. 3 .3 . INITIAL CHECK OF FUNCTION
For testing the instrument it is necessary tci put the switch S6 in the position "on" while the input switch S,, is open . Now the compensating voltage is connected through the resistor "R" to the input of the amplifier (1). The current is handled in the same «a~ a, the enPW current i. see section 3.1 . 3 .4 . INTERNAL CALIBRATION OF CONDFNSORS
In measuring the charges 0 one neat, a cT ;}? ; ;,, densors of different capacities . These condensers be calibrated by the instrument itself with the aict t f switch S6 , see section -. .3. In this case the voltagr is measured by the digital voltmeter and the value of the resistor "R" must be known . Then the current known and it will be possible to measure the capacit if the time of measurement is knoNvn . In rncasuring one must notice that the value of the smallest con_ densor C should exceed the third part cf the stra w capacity e`, which is the sum of the input cap~tc the amplifier (1) and the capacity of the cable . A low current which can bt" nic" asured ~, i i h densor of 10 pF and an ac` uric` ;>( :v° 1 within 10 min for t , = 1(k) V is
i
'on -
\ ii?
We wish to express our thanks
Pohlit for many discussions and suggestions . This cork is dedicated to Prof. Dr. H . [)~inrer on his o ._th, birthday .