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An electronic constant current shock generator for low current levels
Physiology and Behavior. Vol. 7, pp. 635-637. Pergamon Press, 1971. Printed in Great Britain
BRIEF COMMUNICATION An Electronic Constant Current Shock Generator for Low Current Levels J. F. REUS, V. P. H O U S E R A N D W. P. PAR]~ Pavlovian Research Laboratory, Veterans Administration Hospital, Perry Point, Maryland 21902, U.S.A. (Received 4 M a y 1971) REUS, J. F., V. P. HOUSERAND W. P. PARI~. An electronic constant current shock generatorfor low current levels. PrlVSlOL. BEHAV.7 (4) 635-637, 1971.--A simple, low cost electronic shock generator capable of producing constant current 60 Hz square wave outputs between 15-200 ~tA is described. The unit is designed using a penta-grid converter tube which regulates extremely well at low current levels. A circuit diagram is included along with a detailed description of the performance characteristics of this generator. The unit is capable of maintaining a constant current across an animal even when the animal's resistance alters radically as when shock is applied to a moving animal through a grid. The unit is extremely useful in obtaining sensory or motivational thresholds in small animals. Shock generator
Constant current
Electronic
Small animal
Threshold determinations
disadvantage to the basic pentode circuit, however, is that it regulates poorly at extremely low current levels. The shock generator to be described below is an adaptation of the pentode circuit which incorporates the advantage of low operating voltages with an ability to regulate current flow extremely well at low current levels. Figure 1 presents the circuit diagram for an electronic constant current shock generator that can regulate at low current levels. The design of this generator is based on the typical pentode constant current vacuum tube circuit, but in this case use was made of a penta grid converter tube which regulates at much lower current levels. By using pin 4, normally the oscillator plate as a screen grid, and pin 8 as a kind of super screen, while tying pin 1, normally a suppressor to the plate (pin 3), a pentode-like circuit is produced which functions very well at current levels of 15 to 200 ~A. The generator is powered by a half-wave doubler circuit which makes use of a 125 V half-wave transformer to supply a maximum of 350 V d.c. The positive output is put at ground potential, allowing low shock risk to persons handling the terminals or shock grids. The animal forms the plate load of the pentode circuit. The switching signal is applied to the control grid from one side of the transformer secondary through a 33 kf~ resistor, 1 ~ Farad capacitor and shunt rectifier to direct only the negative half cycle to the tube. Since this negative half-wave is large enough to drive the tube into cut-off, the output of the tube becomes a square wave. Current level is set by a 500 k~l potentiometer in the cathode circuit and is monitored by a microammeter. The metering circuit is placed in series between the animal and the tube
T8~ TYPICAL constant current shock generator available commercially for use with small animals accomplishes current regulation by placing a large fixed resistance in series with the animal. Thus, any changes in the animal's resistance (e.g., if the animal moves across a grid) will not substantially affect current flow in the circuit since the animal makes up such a small proportion of the total resistance. To insure maximum current regulation, however, the fixed resistance should be large, and as the circuit resistance is increased, the source voltage must also be increased to provide sufficient potential to force the current through this higher resistance. As Campbell and Masterson [1] have pointed out this technique has serious drawbacks. If the shock is being delivered through a grid upon which an animal can freely move, high source voltages lead to an increase in current density. Under high voltages the electric potential employed can jump nearly a mm causing a spark (arc) to jump from the grid to the animal's body. This increase in current density might be perceived as more painful and may explain why animals tend to freeze on shock grids. Remaining relatively immobile on a grid may insure that the available current will be distributed evenly across a wider skin area, thus reducing current density and the possibility of voltage arcs. Soon after these observations were made various types of electronic constant current shock generators which used a pentode vacuum tube to produce a constant direct current stimulus were introduced. The regulation o f this type of generator is equivalent to or better than an A C generator which is powered by a 5000 V source, yet the maximum voltage applied to the rat never exceeds 500 V [1]. Thus, current regulation is kept at a maximum without introducing high voltages or the accompanying current density problem. One 635
636
REUS, H O U S E R A N D PARI~
"•
SUBJECT
PUSHBUTTON N C PUSH TO INCREASE SENSITIVITY
FROM 200MICRO AMP FS TO SO MICRO AMP FS
METER ~0 M I C R ' ~ TRANSFO RMER S TANCOR PA
8o~,
~
MF/6OOV AMp , ,~Jl r~- - l l z" - - ~£STS"q~.._ B II~%olv I ~
33K / 2W ~
_5oo:,o* ,.~,..~ :r_L,o
,,-,, II~--__o
?
<~
I I ~.L
m:oOo /oov S
zsov
RS: I/3 RMETE R
lOOK '",
:
J
_L --
f . ,o.,
IN3194
OUTPUT - CoOHz.SQUARE WAVE lOOK
SOOK POT
FIG. 1. Circuit diagram for an electronic constant current shock generator which produces currents from 15 to 200 izA.
plate, and consists of a 50 ~A d.c. meter movement shunted to 200 t~A. The shunt is removable by actuating a pushbutton switch. This circuit measures peak to peak current and thus is an accurate reflection of what the animal is actually receiving. In the present description no attempt has been made to include in the circuit a method for controlling shock duration to the animal. This can be accomplished by simply running the generator terminal outputs through a relay to the animal. Relay closure can then be controlled by external timing devices. The advantages of this generator are economy, small bulk, ease of construction and maintenance, and finally excellent regulation within specified limits without resorting to high
200
source voltages. Figure 2 presents current regulation curves at three different reference settings. The reference settings were made by placing a dummy load (55 kf~) across the generator output terminals, to represent an animal. The 500 k t~ potentiometer was then adjusted until the current meter read either 50, 100 or 200 ~A. Once the reference settings were made various resistances were substituted across the generator terminal outputs to determine how changes in animal resistance would affect current flow. Figure 2 presents the results of these manipulations plotting changes in current as a function of changes in an animal's resistance at three various current levels. As can be seen from Fig. 2, current regulation is extremely good
"'-'----o~.~..~,,
REFERENCE SETTING IN MICROAM PERES
175
,
•
•
•
•~ .
150
50 100 200
m125 ~100 o Ix -~ 75 IE
N ....~.
50 25
1.0
2.2
4.7
ANIMAL
10
22
47
RESISTANCE IN
100
220
470
THOUSANDS
1000 2200 4700 10000 OF
OHMS
FIG. 2. Current regulation curves for three different reference settings plotting changes in current as a function of the animal's resistance.
CONSTANT CURRENT SHOCK GENERATOR
637
when an animal's resistance is between 1.0 kf~ and 1.0 raft. Regulation breaks down, especially at the higher current settings when the resistance increases above 2.2 raft. It should be noted, however, that a rat's resistance, as measured on a grid, normally varies from about 3 to 500 k f~ depending on the amount of current used as well as various local skin conditions [I]. Thus the current regulation provided by this shock generator is excellent in the normal range of resistance encountered with a freely moving rat on a shock grid. This regulation is accomplished, moreover, with extremely low source voltages. If the shock generator, for example, is
set at 200 ~zA the voltage across the animal will vary from 44 to 196 V when the resistance of the animal varies from 2.2 kf~ to 1.0 raft. At lower current levels the voltage is even less (e.g., under most conditions when the current is set below 100 ~A the voltage to the animal is under 50 v). Thus, the problems associated with changes in current density are minimal with this type of shock generator. This constant current generator is valuable whenever controlled shock of low intensity is desired. It is, therefore, useful in determining either sensory or motivation thresholds.
REFERENCE
1. Campbell, B. A. and F. A. Masterson. Psychophysics of punishment. In: Punishment and Aversive Behavior, edited by B. A. Campbell and R. M. Church. New York: Appleton-CenturyCrofts, 1969.
BRIEF COMMUNICATION An Electronic Constant Current Shock Generator for Low Current Levels J. F. REUS, V. P. H O U S E R A N D W. P. PAR]~ Pavlovian Research Laboratory, Veterans Administration Hospital, Perry Point, Maryland 21902, U.S.A. (Received 4 M a y 1971) REUS, J. F., V. P. HOUSERAND W. P. PARI~. An electronic constant current shock generatorfor low current levels. PrlVSlOL. BEHAV.7 (4) 635-637, 1971.--A simple, low cost electronic shock generator capable of producing constant current 60 Hz square wave outputs between 15-200 ~tA is described. The unit is designed using a penta-grid converter tube which regulates extremely well at low current levels. A circuit diagram is included along with a detailed description of the performance characteristics of this generator. The unit is capable of maintaining a constant current across an animal even when the animal's resistance alters radically as when shock is applied to a moving animal through a grid. The unit is extremely useful in obtaining sensory or motivational thresholds in small animals. Shock generator
Constant current
Electronic
Small animal
Threshold determinations
disadvantage to the basic pentode circuit, however, is that it regulates poorly at extremely low current levels. The shock generator to be described below is an adaptation of the pentode circuit which incorporates the advantage of low operating voltages with an ability to regulate current flow extremely well at low current levels. Figure 1 presents the circuit diagram for an electronic constant current shock generator that can regulate at low current levels. The design of this generator is based on the typical pentode constant current vacuum tube circuit, but in this case use was made of a penta grid converter tube which regulates at much lower current levels. By using pin 4, normally the oscillator plate as a screen grid, and pin 8 as a kind of super screen, while tying pin 1, normally a suppressor to the plate (pin 3), a pentode-like circuit is produced which functions very well at current levels of 15 to 200 ~A. The generator is powered by a half-wave doubler circuit which makes use of a 125 V half-wave transformer to supply a maximum of 350 V d.c. The positive output is put at ground potential, allowing low shock risk to persons handling the terminals or shock grids. The animal forms the plate load of the pentode circuit. The switching signal is applied to the control grid from one side of the transformer secondary through a 33 kf~ resistor, 1 ~ Farad capacitor and shunt rectifier to direct only the negative half cycle to the tube. Since this negative half-wave is large enough to drive the tube into cut-off, the output of the tube becomes a square wave. Current level is set by a 500 k~l potentiometer in the cathode circuit and is monitored by a microammeter. The metering circuit is placed in series between the animal and the tube
T8~ TYPICAL constant current shock generator available commercially for use with small animals accomplishes current regulation by placing a large fixed resistance in series with the animal. Thus, any changes in the animal's resistance (e.g., if the animal moves across a grid) will not substantially affect current flow in the circuit since the animal makes up such a small proportion of the total resistance. To insure maximum current regulation, however, the fixed resistance should be large, and as the circuit resistance is increased, the source voltage must also be increased to provide sufficient potential to force the current through this higher resistance. As Campbell and Masterson [1] have pointed out this technique has serious drawbacks. If the shock is being delivered through a grid upon which an animal can freely move, high source voltages lead to an increase in current density. Under high voltages the electric potential employed can jump nearly a mm causing a spark (arc) to jump from the grid to the animal's body. This increase in current density might be perceived as more painful and may explain why animals tend to freeze on shock grids. Remaining relatively immobile on a grid may insure that the available current will be distributed evenly across a wider skin area, thus reducing current density and the possibility of voltage arcs. Soon after these observations were made various types of electronic constant current shock generators which used a pentode vacuum tube to produce a constant direct current stimulus were introduced. The regulation o f this type of generator is equivalent to or better than an A C generator which is powered by a 5000 V source, yet the maximum voltage applied to the rat never exceeds 500 V [1]. Thus, current regulation is kept at a maximum without introducing high voltages or the accompanying current density problem. One 635
636
REUS, H O U S E R A N D PARI~
"•
SUBJECT
PUSHBUTTON N C PUSH TO INCREASE SENSITIVITY
FROM 200MICRO AMP FS TO SO MICRO AMP FS
METER ~0 M I C R ' ~ TRANSFO RMER S TANCOR PA
8o~,
~
MF/6OOV AMp , ,~Jl r~- - l l z" - - ~£STS"q~.._ B II~%olv I ~
33K / 2W ~
_5oo:,o* ,.~,..~ :r_L,o
,,-,, II~--__o
?
<~
I I ~.L
m:oOo /oov S
zsov
RS: I/3 RMETE R
lOOK '",
:
J
_L --
f . ,o.,
IN3194
OUTPUT - CoOHz.SQUARE WAVE lOOK
SOOK POT
FIG. 1. Circuit diagram for an electronic constant current shock generator which produces currents from 15 to 200 izA.
plate, and consists of a 50 ~A d.c. meter movement shunted to 200 t~A. The shunt is removable by actuating a pushbutton switch. This circuit measures peak to peak current and thus is an accurate reflection of what the animal is actually receiving. In the present description no attempt has been made to include in the circuit a method for controlling shock duration to the animal. This can be accomplished by simply running the generator terminal outputs through a relay to the animal. Relay closure can then be controlled by external timing devices. The advantages of this generator are economy, small bulk, ease of construction and maintenance, and finally excellent regulation within specified limits without resorting to high
200
source voltages. Figure 2 presents current regulation curves at three different reference settings. The reference settings were made by placing a dummy load (55 kf~) across the generator output terminals, to represent an animal. The 500 k t~ potentiometer was then adjusted until the current meter read either 50, 100 or 200 ~A. Once the reference settings were made various resistances were substituted across the generator terminal outputs to determine how changes in animal resistance would affect current flow. Figure 2 presents the results of these manipulations plotting changes in current as a function of changes in an animal's resistance at three various current levels. As can be seen from Fig. 2, current regulation is extremely good
"'-'----o~.~..~,,
REFERENCE SETTING IN MICROAM PERES
175
,
•
•
•
•~ .
150
50 100 200
m125 ~100 o Ix -~ 75 IE
N ....~.
50 25
1.0
2.2
4.7
ANIMAL
10
22
47
RESISTANCE IN
100
220
470
THOUSANDS
1000 2200 4700 10000 OF
OHMS
FIG. 2. Current regulation curves for three different reference settings plotting changes in current as a function of the animal's resistance.
CONSTANT CURRENT SHOCK GENERATOR
637
when an animal's resistance is between 1.0 kf~ and 1.0 raft. Regulation breaks down, especially at the higher current settings when the resistance increases above 2.2 raft. It should be noted, however, that a rat's resistance, as measured on a grid, normally varies from about 3 to 500 k f~ depending on the amount of current used as well as various local skin conditions [I]. Thus the current regulation provided by this shock generator is excellent in the normal range of resistance encountered with a freely moving rat on a shock grid. This regulation is accomplished, moreover, with extremely low source voltages. If the shock generator, for example, is
set at 200 ~zA the voltage across the animal will vary from 44 to 196 V when the resistance of the animal varies from 2.2 kf~ to 1.0 raft. At lower current levels the voltage is even less (e.g., under most conditions when the current is set below 100 ~A the voltage to the animal is under 50 v). Thus, the problems associated with changes in current density are minimal with this type of shock generator. This constant current generator is valuable whenever controlled shock of low intensity is desired. It is, therefore, useful in determining either sensory or motivation thresholds.
REFERENCE
1. Campbell, B. A. and F. A. Masterson. Psychophysics of punishment. In: Punishment and Aversive Behavior, edited by B. A. Campbell and R. M. Church. New York: Appleton-CenturyCrofts, 1969.