Bruin Research Bullerin, Vol. 4, pp. 139-140.
Printedin the
U.S.A.
Electrically Quiet Temperature Controller STEPHEN Department
of Environmental
YEANDLE
Biosciences, Naval Bethesda, MD 20014
(Received
Medical
Research
Institute
14 August 1978)
YEANDLE, S. Electrically quiet temperature confroller. BRAIN RES. BULL. 4(l) 139140, 1979.-A circuit for an electrically quiet temperature controller is described that is suitable for controlling the temperature of a neurophysiological preparation above ambient temperature. By not generating spurious electrical signals it facilitates the recording of low level biological potential changes when temperature control is required. The device, constructed of standard electronic components, uses a thermistor as a sensing element and an ordinary heat lamp or heating strip as a heating element. Temperature
controller
Neurophysiology
FREQUENTLY the electrophysiologist wishes to control the temperature of a preparation at some temperature above ambient while recording bioelectric potentials in the microvolt range. For example, such a need might arise while recording extracellularly nerve impulses from mammalian small axons. Most commercial temperature controllers operate on a switching principle, whereby the proportion of time a heater is on is proportional to a thermistor bridge unbalance. The switching acting often generates high frequency signals that interfere with the recording of low level bioelectric potentials. The circuit shown in Fig. 1, which has operated successfully for over a year, does not cause such interference, controls the temperature of a preparation to within 0.1°C and is made of easily obtainable components. Briefly, it is a proportional controller using a thermistor as the sensing element and either a 150 Wheat lamp or a heating strip as the heating element. The circuit operates as follows. All op amps are powered by a standard 2 15 V supply. The output voltage of op amp 2 is proportional to the difference between the conductances of the thermistor and R,,, the variable resistor used to set the value of the temperature to be controlled. This op amp must have an input bias current much less than the thermistor current which is kept low to minimize thermistor self heating. For ease of adjusting Rh the thermistor should have a resistance less than but in the same range as the maximum of Rh. Presently a Yellow Spring Instrument 507 probe is being
Copyright 0 1979 ANKHO
used as the thermistor. The output voltage of op amp 2 is amplified and rectified by the components between op amp 2 and switch SW and applied to the input of the power amplifier containing op amp 6 and Sylvania transistor ECG 124 and ECG 281. One hundred twenty volts is supplied to point B by a Standard Power, Model 5OOB 120 unregulated power supply. This supply consists of a transformer, rectifier bridge, and RC filter made of a 1300 PF capacitor paralleled by a 5K resistor. Variable feedback resistor R,, by determining the gain of the power amplifier, practically eliminates any voltage fluctuation across the lamp due to power supply fluctuations. The 420 PF capacitor shorts to ground any high frequency parasitic oscillation that might develop in the power amplifier. The gain of the system, controlled by resistors R,, R,, R:, and R,, is set to be sufftciently great to achieve adequate control but sufficiently small to prevent oscillation of the heater output. Switch SW allows the control feedback loop to be broken and the heater to be controlled manually with bias pot R, and op amp 5, if so desired. ACKNOWLEDGEMENT This work was supported by the Naval Medical Research and Development Command, National Naval Medical Center, Department of the Navy, Research Task No. ZF51.524.013.1018. The opinions and statements contained herein are the private ones of the author and are not to be construed as official or reflecting the views of the Navy Department or the Naval Service at large.
International Inc.-0361-9230/79/010139-02$00.70/O
YEANDLE
140
R3=50K51
0.1 /P R2 = 100 Kn
l-H---l
20 KR 25Os1
=
xick 1021
pot.
B
lOKS1
420~ F
1
12OV
I Heating element
R4=25
-I_
KS1
-15v
+ 747
’
FIG. 1. Circuit diagram of temperature controller described in text.
R5=IOOK
I
=
Printedin the
U.S.A.
Electrically Quiet Temperature Controller STEPHEN Department
of Environmental
YEANDLE
Biosciences, Naval Bethesda, MD 20014
(Received
Medical
Research
Institute
14 August 1978)
YEANDLE, S. Electrically quiet temperature confroller. BRAIN RES. BULL. 4(l) 139140, 1979.-A circuit for an electrically quiet temperature controller is described that is suitable for controlling the temperature of a neurophysiological preparation above ambient temperature. By not generating spurious electrical signals it facilitates the recording of low level biological potential changes when temperature control is required. The device, constructed of standard electronic components, uses a thermistor as a sensing element and an ordinary heat lamp or heating strip as a heating element. Temperature
controller
Neurophysiology
FREQUENTLY the electrophysiologist wishes to control the temperature of a preparation at some temperature above ambient while recording bioelectric potentials in the microvolt range. For example, such a need might arise while recording extracellularly nerve impulses from mammalian small axons. Most commercial temperature controllers operate on a switching principle, whereby the proportion of time a heater is on is proportional to a thermistor bridge unbalance. The switching acting often generates high frequency signals that interfere with the recording of low level bioelectric potentials. The circuit shown in Fig. 1, which has operated successfully for over a year, does not cause such interference, controls the temperature of a preparation to within 0.1°C and is made of easily obtainable components. Briefly, it is a proportional controller using a thermistor as the sensing element and either a 150 Wheat lamp or a heating strip as the heating element. The circuit operates as follows. All op amps are powered by a standard 2 15 V supply. The output voltage of op amp 2 is proportional to the difference between the conductances of the thermistor and R,,, the variable resistor used to set the value of the temperature to be controlled. This op amp must have an input bias current much less than the thermistor current which is kept low to minimize thermistor self heating. For ease of adjusting Rh the thermistor should have a resistance less than but in the same range as the maximum of Rh. Presently a Yellow Spring Instrument 507 probe is being
Copyright 0 1979 ANKHO
used as the thermistor. The output voltage of op amp 2 is amplified and rectified by the components between op amp 2 and switch SW and applied to the input of the power amplifier containing op amp 6 and Sylvania transistor ECG 124 and ECG 281. One hundred twenty volts is supplied to point B by a Standard Power, Model 5OOB 120 unregulated power supply. This supply consists of a transformer, rectifier bridge, and RC filter made of a 1300 PF capacitor paralleled by a 5K resistor. Variable feedback resistor R,, by determining the gain of the power amplifier, practically eliminates any voltage fluctuation across the lamp due to power supply fluctuations. The 420 PF capacitor shorts to ground any high frequency parasitic oscillation that might develop in the power amplifier. The gain of the system, controlled by resistors R,, R,, R:, and R,, is set to be sufftciently great to achieve adequate control but sufficiently small to prevent oscillation of the heater output. Switch SW allows the control feedback loop to be broken and the heater to be controlled manually with bias pot R, and op amp 5, if so desired. ACKNOWLEDGEMENT This work was supported by the Naval Medical Research and Development Command, National Naval Medical Center, Department of the Navy, Research Task No. ZF51.524.013.1018. The opinions and statements contained herein are the private ones of the author and are not to be construed as official or reflecting the views of the Navy Department or the Naval Service at large.
International Inc.-0361-9230/79/010139-02$00.70/O
YEANDLE
140
R3=50K51
0.1 /P R2 = 100 Kn
l-H---l
20 KR 25Os1
=
xick 1021
pot.
B
lOKS1
420~ F
1
12OV
I Heating element
R4=25
-I_
KS1
-15v
+ 747
’
FIG. 1. Circuit diagram of temperature controller described in text.
R5=IOOK
I
=
