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A linearized DC lamp controller
Journal o] Neuroscience Methods, 39 ( 1991 ) 203-206
203
~? 1991 Elsevier Science Publishers B.V. All rights reserved 0165-0270/91/$03.50 NSM 01264
A linearized DC lamp controller J a m e s A. G a l b r a i t h Peripheral Nert'e Research Group, Unicersity of California at San Diego, La Jolla, CA 92093-0151, (U.S.A.) (Received 7 May 1991) (Accepted 14 June 1991)
Key words: Electrophysiology; Lamp controller; Microscope; Patch-clamp recording; Tissue culture; Voltage clamp
The design of a simple and inexpensive battery powered lamp controller for microscope lamps commonly used in patch clamp and other biophysical studies is described. The circuit linearizes the normally exponential voltage-intensity curve of standard 12-V lamps by using an operational amplifier to provide feedback to the drain of a power MOSFET. When the unit is turned off, a charging circuit recharges the battery at a constant potential until the battery voltage reaches a preset level. The charging voltage is then reduced to balance the self-discharge rate of the battery.
In many biophysical studies which use isolated cells or cell membranes it is necessary to record low-level electrical signals while simultaneously observing the preparation through a microscope. Single channel recordings from patch-clamp experiments are a particular example of such requirements (Hamill et al., 1981). Under these circumstances, a Faraday cage is often employed to reduce electrical interference, and the electronic equipment is battery powered. However, the power supply which is used for either the general illumination or the microscope lamp is one source of noise which is sometimes overlooked. Since most of these lamps are rated at 12 V, this source of noise can be eliminated by simply attaching the lamp directly to a gell-cell or lead-acid battery. As this provides only full
Correspondence: James A. Galbraith, Ph.D., Peripheral Nerve Research Group, University of California at San Diego, La Jolla, CA 92[)93-9151, U.S.A.
brightness, the following simple circuit has been designed to control lamp intensity accurately. The circuit shown in Fig. 1, consists of an N-channel enhancement mode power MOSFET, a feedback and linearizing stage to drive the gate of the FET, and a charging circuit for the battery. The lamp is connected between the battery and the drain of the MOSFET, and it lights as the F E T is brought into conduction. A FET is used in this application rather than a bipolar transistor because of its high input impedance and low drain-source resistance. Since a slight voltage drop develops across the source-drain junction due to its resistance, a switch is included to bypass the F E T for full lamp intensity. Without feedback, a linear voltage applied to the gate of the F E T causes the drain voltage to follow a sigmoidal shape. After the gate-source voltage reaches threshold, the FET is rapidly brought into full conduction as the input is increased another few volts. Both the threshold voltage and the steepness of turn-on are device-
204 a 10 k12 p u l l - u p r e s i s t o r a n d a p a i r o t d i o d e s at lhe output of the op-amp.
12 V
?
~
IOK
~
Fig. 2 s h o w s a p l o t o f l a m p v o l t a g e v e r s u s l a m p intensity obtained for a typical microscope bulb
MTHSONO5E
(Osram 64425:12 V, 20 W) and a simple linear voltage divider. It reveals that the intensity is it non-linear function of the applied voltage. A common empirical expression for this intensity function, regardless of the lamp, is (Kaufman, 1984):
m
0.1
13.3 220
,~
RED
LED
12V BATi-ERY
IK
LM301A
2K
.~
GREEN LED
Fig, 1. Schematic of lamp controller and battery charger. For low-noise applications a 12-V storage battery is used to power microscope lamps. Linear intensity control is achieved by feedback from the operational amplifier to the N-channel enhancement mode power MOSFET. A modified constant potential charging circuit recharges the battery when the controller is not in operation. The charger initially charges the battery at 13.9 V until the charging current drops below 180 mA; the charging voltage is then reduced to 13 V to 'float' the battery and offset losses due to self discharge. The LEDs indicate the state of charge: red, charging; green, floating. All resistances are in g/s and have a 1/4 W rating, and all capacitances are in F.
intensity
applied vohs
, maximum intensity
. nlaxinl-t]mrat~ttvo/ts j
At 50% of full voltage, or at the midpoint of the divider's position, the lamp intensity is only 10% of its full output. For finer control, lamp intensity should be linearly proportional to the position of the trimpot wiper. This requires that the lamp voltage be an exponential function of wiper position. To achieve this, the trimpot can be given a non-linear taper by adding a shunt resistor (R~) between the wiper and either end of the trimpot, depending on the type of taper de-
10
o
Inlensily
•
Vollage
•
t~ ,r-
0.8 ¸
0
•
,t"-
t" 0.6 ¸ 0
•
0
•
O 0.4 ¸
dependent quantities which vary between different models as well as between individual FETs of the same model. To linearize the drain voltage with respect to the input and to remove device dependencies, an operational amplifier (op-amp) provides feedback by connecting the non-inverting terminal to the drain of the FET. Since the voltage at the inverting and non-inverting terminals of the amplifier are the same, the drain follows the control signal applied to the inverting terminal. To ensure that the gate reaches 12 V and full conduction of the FET occurs, there are
0 0
N
E h.
0 0.2
0 0
O Z 0
~.,-o-.o..~o..o ~, 9 °
0.0
0.0
0.2
0
0
0
i
0.4
Normalized
06
0,8
10
lamp voltage
Fig. 2. Lamp intensity as a function of applied voltage. A linear lamp voltage :causes the intensity to vary exponentially. A common empirical expression for this function is given by: (intensity/maximum intensity) = (applied volts/maximum rated volts) 34.
205 1.0
,,,w .D
t-
0.8
t-•
"O I=
o o
0.6 0
G) 0 ww..,
0 >
0.4
m .N m
E
0 0
0.2-
Z
o
Intensity
•
Voltage
o
0.0" 0,0
o i
0.2
i
0.4
Dial
i
0.6
i
0.8
i
1.0
Position
Fig. 3. Linearized lamp intensity and corresponding bulb voltage. The addition of a shunt resistor (R~) to a simple voltage divider between the wiper and ground linearizes the intensity curve. Depending on the wattage of the lamp, a ratio of between 5 and 8 for the trimpot resistance to the shunt resistance will straighten the curve. The simplest way to determine the appropriate R~ is to measure the voltage across the bulb when the trimpot is set to 50c~ and adjust R~ until the lamp voltage is 9.814 V.
sired (Rummreich, 1988). Placing R, between the wiper and ground and using a ratio of 7.5 between the trimpot resistance and R~ provided the voltage and intensity curves shown in Fig. 3. The intensity approximates a linear function over the range of the trimpot, with only slight deviations near full intensity. Due to variations in resistance between different wattage bulbs, it is sometimes necessary to adjust the value of R~ when the circuit is used with different lamps. This controller has been used with 20-W and 50-W bulbs, and the ratio of trimpot to shunt resistors has been between five and eight. The simplest way to determine R~ is to measure the voltage across the bulb when the trimpot is set to 50% and adjust R~ until the lamp voltage is 9.814 V. The choice of the M O S F E T is dependent on the maximum lamp wattage and the drain current. The MTH50N05E (Motorola Inc.) is an
inexpensive M O S F E T which has sufficient drain current (50 A) and power dissipation (125 W) ratings for lamps with wattages up to 100 W. in addition, the rj,, of 28 mg~ minimizes the voltage drop across the F E T during conduction. The F E T must have an adequate heat sink since the feedback amplifier will attempt to maintain control even during thermal runaway. The OP-90 (Precision Monolithics Inc.) operational amplifier was chosen because it operates from a single supply, draws only 12-p,A quiescent current, and accepts inputs down to negative rail. The op-amp must be able to handle this zero-volt input condition because maximum brightness is obtained when the input is at zero, and normal op-amps tend to lose control as they approach the rail. To indicate the lamp's intensity a voltmeter may be placed across its terminals; however, this will follow the same non-linear relationship shown in Fig. 2. A better approximation of intensity can be obtained by using a ganged trimpot on the input. While the first trimpot provides the control signal for the lamp, the second trimpot is a simple linear voltage divider and approximates the lamp intensity. If finer control is desired, a l()-turn potentiometer with a turns counting dial may be used to show the percentage of available intensity. Unlike the previous two methods, it will not indicate how much the battery voltage has decreased with use. Standard lead-acid storage cells have a discharge rate which follows a relatively flat timecourse over the period of their A - h rating (Salkind et al., 1984). Even though this discharge is minimal during most experiments, it is desirable to recharge the battery after use to maintain its charge and service life. A charging circuit based on a design from National Instruments is also shown in Fig. 1 (Hoffart, 1988). After the lamp controller is switched off the battery is recharged by supplying a constant voltage to the cells until the current to the battery through the 0.14/ resistor drops below 180 mA. When this occurs, the op-amp output goes high, reducing the charging voltage and 'floating' the battery at a lower voltage which is adequate to offset for its self-discharge. This modified constant potential technique helps to prolong battery life by reduc-
206 ing grid corrosion and water loss which arise when the battery is overcharged. The circuit uses an LM317 adjustable voltage regulator (maximum current 2 A) to set the charging voltage at 13.9 V, and the float level at 13 V. This ratio of charge to float voltage is set by the 100-S2 resistor in the adjust arm of the regulator and the 1-k J2 resistor in the collector of the transistor. When the battery is disconnected from the charger, the 2-k J2 trimpot should be adjusted so that the output is 13 V. The 2 LEDs indicate the status of the battery and charger. When the red L E D is lit, the battery is charging, and when the green L E D is lit, the battery is fully charged. Since the input to the charger is a rectified line voltage, it is best to either keep the circuit outside the Faraday cage during an experiment or unplug the A C connection and then reconnect it at the end of an experiment. The circuit described provides linear control of lamp intensity and is useful in optical experiments where continuous control of reduced light intensity is required. By using a DC supply the circuit removes a sometimes troublesome source of noise from the environment. The power dissipation of the control circuit is negligible, and its construction requires only a few inexpensive components. Although the circuit was designed with
microscope lamps in mind, it can also be useful for other high-current loads where a non-linear control signal is required.
Acknowledgments This work was supported by a fellowship from the Anesthesia Foundation of the University of California at San Diego. Special thanks to J.R. Galbraith for his technical advice, and R.R. Myers for laboratory facilities.
References Hamill, O.P., Marty, A., Neher, E., Sakmann, B. and Sigworth~ F.J. (1981) Improved patch-clamp techniques for high resolution current recording from cells and cell-free membrane patches. Pfliigers Arch., 391: 85-100. Hoffart, F. (1988) Charger extends lead-acid-battery life. EDN. 33(15A): 22. Kaufman, J. (Ed.). (1984) IES Lighting Handbook. Illuminating Engineering Society of North America, New York. Rummreich, M. (1988) Resistors provide non-linear pot tapers. EDN 33(15A): 69-70. Salkind, A., Mayer, G. and" Linden, D. (1984) Lead-acid batteries. In: D. Linden (Ed.), Handbook of Batteries and Fuel Cells. McGraw-Hill, New York.
203
~? 1991 Elsevier Science Publishers B.V. All rights reserved 0165-0270/91/$03.50 NSM 01264
A linearized DC lamp controller J a m e s A. G a l b r a i t h Peripheral Nert'e Research Group, Unicersity of California at San Diego, La Jolla, CA 92093-0151, (U.S.A.) (Received 7 May 1991) (Accepted 14 June 1991)
Key words: Electrophysiology; Lamp controller; Microscope; Patch-clamp recording; Tissue culture; Voltage clamp
The design of a simple and inexpensive battery powered lamp controller for microscope lamps commonly used in patch clamp and other biophysical studies is described. The circuit linearizes the normally exponential voltage-intensity curve of standard 12-V lamps by using an operational amplifier to provide feedback to the drain of a power MOSFET. When the unit is turned off, a charging circuit recharges the battery at a constant potential until the battery voltage reaches a preset level. The charging voltage is then reduced to balance the self-discharge rate of the battery.
In many biophysical studies which use isolated cells or cell membranes it is necessary to record low-level electrical signals while simultaneously observing the preparation through a microscope. Single channel recordings from patch-clamp experiments are a particular example of such requirements (Hamill et al., 1981). Under these circumstances, a Faraday cage is often employed to reduce electrical interference, and the electronic equipment is battery powered. However, the power supply which is used for either the general illumination or the microscope lamp is one source of noise which is sometimes overlooked. Since most of these lamps are rated at 12 V, this source of noise can be eliminated by simply attaching the lamp directly to a gell-cell or lead-acid battery. As this provides only full
Correspondence: James A. Galbraith, Ph.D., Peripheral Nerve Research Group, University of California at San Diego, La Jolla, CA 92[)93-9151, U.S.A.
brightness, the following simple circuit has been designed to control lamp intensity accurately. The circuit shown in Fig. 1, consists of an N-channel enhancement mode power MOSFET, a feedback and linearizing stage to drive the gate of the FET, and a charging circuit for the battery. The lamp is connected between the battery and the drain of the MOSFET, and it lights as the F E T is brought into conduction. A FET is used in this application rather than a bipolar transistor because of its high input impedance and low drain-source resistance. Since a slight voltage drop develops across the source-drain junction due to its resistance, a switch is included to bypass the F E T for full lamp intensity. Without feedback, a linear voltage applied to the gate of the F E T causes the drain voltage to follow a sigmoidal shape. After the gate-source voltage reaches threshold, the FET is rapidly brought into full conduction as the input is increased another few volts. Both the threshold voltage and the steepness of turn-on are device-
204 a 10 k12 p u l l - u p r e s i s t o r a n d a p a i r o t d i o d e s at lhe output of the op-amp.
12 V
?
~
IOK
~
Fig. 2 s h o w s a p l o t o f l a m p v o l t a g e v e r s u s l a m p intensity obtained for a typical microscope bulb
MTHSONO5E
(Osram 64425:12 V, 20 W) and a simple linear voltage divider. It reveals that the intensity is it non-linear function of the applied voltage. A common empirical expression for this intensity function, regardless of the lamp, is (Kaufman, 1984):
m
0.1
13.3 220
,~
RED
LED
12V BATi-ERY
IK
LM301A
2K
.~
GREEN LED
Fig, 1. Schematic of lamp controller and battery charger. For low-noise applications a 12-V storage battery is used to power microscope lamps. Linear intensity control is achieved by feedback from the operational amplifier to the N-channel enhancement mode power MOSFET. A modified constant potential charging circuit recharges the battery when the controller is not in operation. The charger initially charges the battery at 13.9 V until the charging current drops below 180 mA; the charging voltage is then reduced to 13 V to 'float' the battery and offset losses due to self discharge. The LEDs indicate the state of charge: red, charging; green, floating. All resistances are in g/s and have a 1/4 W rating, and all capacitances are in F.
intensity
applied vohs
, maximum intensity
. nlaxinl-t]mrat~ttvo/ts j
At 50% of full voltage, or at the midpoint of the divider's position, the lamp intensity is only 10% of its full output. For finer control, lamp intensity should be linearly proportional to the position of the trimpot wiper. This requires that the lamp voltage be an exponential function of wiper position. To achieve this, the trimpot can be given a non-linear taper by adding a shunt resistor (R~) between the wiper and either end of the trimpot, depending on the type of taper de-
10
o
Inlensily
•
Vollage
•
t~ ,r-
0.8 ¸
0
•
,t"-
t" 0.6 ¸ 0
•
0
•
O 0.4 ¸
dependent quantities which vary between different models as well as between individual FETs of the same model. To linearize the drain voltage with respect to the input and to remove device dependencies, an operational amplifier (op-amp) provides feedback by connecting the non-inverting terminal to the drain of the FET. Since the voltage at the inverting and non-inverting terminals of the amplifier are the same, the drain follows the control signal applied to the inverting terminal. To ensure that the gate reaches 12 V and full conduction of the FET occurs, there are
0 0
N
E h.
0 0.2
0 0
O Z 0
~.,-o-.o..~o..o ~, 9 °
0.0
0.0
0.2
0
0
0
i
0.4
Normalized
06
0,8
10
lamp voltage
Fig. 2. Lamp intensity as a function of applied voltage. A linear lamp voltage :causes the intensity to vary exponentially. A common empirical expression for this function is given by: (intensity/maximum intensity) = (applied volts/maximum rated volts) 34.
205 1.0
,,,w .D
t-
0.8
t-•
"O I=
o o
0.6 0
G) 0 ww..,
0 >
0.4
m .N m
E
0 0
0.2-
Z
o
Intensity
•
Voltage
o
0.0" 0,0
o i
0.2
i
0.4
Dial
i
0.6
i
0.8
i
1.0
Position
Fig. 3. Linearized lamp intensity and corresponding bulb voltage. The addition of a shunt resistor (R~) to a simple voltage divider between the wiper and ground linearizes the intensity curve. Depending on the wattage of the lamp, a ratio of between 5 and 8 for the trimpot resistance to the shunt resistance will straighten the curve. The simplest way to determine the appropriate R~ is to measure the voltage across the bulb when the trimpot is set to 50c~ and adjust R~ until the lamp voltage is 9.814 V.
sired (Rummreich, 1988). Placing R, between the wiper and ground and using a ratio of 7.5 between the trimpot resistance and R~ provided the voltage and intensity curves shown in Fig. 3. The intensity approximates a linear function over the range of the trimpot, with only slight deviations near full intensity. Due to variations in resistance between different wattage bulbs, it is sometimes necessary to adjust the value of R~ when the circuit is used with different lamps. This controller has been used with 20-W and 50-W bulbs, and the ratio of trimpot to shunt resistors has been between five and eight. The simplest way to determine R~ is to measure the voltage across the bulb when the trimpot is set to 50% and adjust R~ until the lamp voltage is 9.814 V. The choice of the M O S F E T is dependent on the maximum lamp wattage and the drain current. The MTH50N05E (Motorola Inc.) is an
inexpensive M O S F E T which has sufficient drain current (50 A) and power dissipation (125 W) ratings for lamps with wattages up to 100 W. in addition, the rj,, of 28 mg~ minimizes the voltage drop across the F E T during conduction. The F E T must have an adequate heat sink since the feedback amplifier will attempt to maintain control even during thermal runaway. The OP-90 (Precision Monolithics Inc.) operational amplifier was chosen because it operates from a single supply, draws only 12-p,A quiescent current, and accepts inputs down to negative rail. The op-amp must be able to handle this zero-volt input condition because maximum brightness is obtained when the input is at zero, and normal op-amps tend to lose control as they approach the rail. To indicate the lamp's intensity a voltmeter may be placed across its terminals; however, this will follow the same non-linear relationship shown in Fig. 2. A better approximation of intensity can be obtained by using a ganged trimpot on the input. While the first trimpot provides the control signal for the lamp, the second trimpot is a simple linear voltage divider and approximates the lamp intensity. If finer control is desired, a l()-turn potentiometer with a turns counting dial may be used to show the percentage of available intensity. Unlike the previous two methods, it will not indicate how much the battery voltage has decreased with use. Standard lead-acid storage cells have a discharge rate which follows a relatively flat timecourse over the period of their A - h rating (Salkind et al., 1984). Even though this discharge is minimal during most experiments, it is desirable to recharge the battery after use to maintain its charge and service life. A charging circuit based on a design from National Instruments is also shown in Fig. 1 (Hoffart, 1988). After the lamp controller is switched off the battery is recharged by supplying a constant voltage to the cells until the current to the battery through the 0.14/ resistor drops below 180 mA. When this occurs, the op-amp output goes high, reducing the charging voltage and 'floating' the battery at a lower voltage which is adequate to offset for its self-discharge. This modified constant potential technique helps to prolong battery life by reduc-
206 ing grid corrosion and water loss which arise when the battery is overcharged. The circuit uses an LM317 adjustable voltage regulator (maximum current 2 A) to set the charging voltage at 13.9 V, and the float level at 13 V. This ratio of charge to float voltage is set by the 100-S2 resistor in the adjust arm of the regulator and the 1-k J2 resistor in the collector of the transistor. When the battery is disconnected from the charger, the 2-k J2 trimpot should be adjusted so that the output is 13 V. The 2 LEDs indicate the status of the battery and charger. When the red L E D is lit, the battery is charging, and when the green L E D is lit, the battery is fully charged. Since the input to the charger is a rectified line voltage, it is best to either keep the circuit outside the Faraday cage during an experiment or unplug the A C connection and then reconnect it at the end of an experiment. The circuit described provides linear control of lamp intensity and is useful in optical experiments where continuous control of reduced light intensity is required. By using a DC supply the circuit removes a sometimes troublesome source of noise from the environment. The power dissipation of the control circuit is negligible, and its construction requires only a few inexpensive components. Although the circuit was designed with
microscope lamps in mind, it can also be useful for other high-current loads where a non-linear control signal is required.
Acknowledgments This work was supported by a fellowship from the Anesthesia Foundation of the University of California at San Diego. Special thanks to J.R. Galbraith for his technical advice, and R.R. Myers for laboratory facilities.
References Hamill, O.P., Marty, A., Neher, E., Sakmann, B. and Sigworth~ F.J. (1981) Improved patch-clamp techniques for high resolution current recording from cells and cell-free membrane patches. Pfliigers Arch., 391: 85-100. Hoffart, F. (1988) Charger extends lead-acid-battery life. EDN. 33(15A): 22. Kaufman, J. (Ed.). (1984) IES Lighting Handbook. Illuminating Engineering Society of North America, New York. Rummreich, M. (1988) Resistors provide non-linear pot tapers. EDN 33(15A): 69-70. Salkind, A., Mayer, G. and" Linden, D. (1984) Lead-acid batteries. In: D. Linden (Ed.), Handbook of Batteries and Fuel Cells. McGraw-Hill, New York.