Precise Automatic Temperature Control for Microelectrode Technique

Abstract

An automatic temperature control system was devised for a small volume (-6 cm³) preparation chamber to be used in microelectrode techniques. The principle of the device was a negative feedback control system.
The temperature of the preparation was measured by a bead thermistor with a small tip which was placed closely to the preparation. The temperature was converted to voltage by a wheatstone bridge, one of the arms of which consisted of the thermistor.
In order to cool or warm the preparation, two Peltier plates were used. The plate has the following properties : (1) when DC current is passed through the plate in one direction, its upper surface becomes cool and its lower surface warm. When the direction of current is reversed, the temperature of the surfaces is also inverted. (2) The power of cooling or warming increases according to the increase in DC current. Heat generated on the lower surface of the Peltier plates was deprived by perfusing tap water under the Peltier plates. The temperature of the preparation without passing DC current but with water perfusion is referred to as the environmental temperature.
(I) Keeping the preparation temperature below the environmental temperature.
The subtraction of the output of the thermometer from a reference DC voltage, actual signal, was amplified by a chopper DC amplifier (66 dB). Its output drove a relay switch which operated on a power supply for the cooling (warming) device. When the sign of the actual signal was positive, then the cooling device functioned to cool the preparation. When the sign was negative, the cooling automatically stopped, and thus the temperature was kept at a constant level.
(II) Keeping the preparation temperature above the environmental temperature.
The principle was almost the same as stated in (I). The difference was as follows : (1) First, we reversed the polarity of DC current through the Peltier plates. (2) Second, we reversed the sign of the actual signal of (I), so that a positive signal now produced warming of the upper surface of the plate.
With the aid of this device, the preparation temperature was kept constant at a desired temperature between 5°C and 37°C within an error range of ±0.1°C to ±0.25°C. Alteration of the temperature level was easily accomplished by setting the desired reference voltage. The newly set temperature level was reached within a few minutes.