PRACTICAL METHODS FOR TEMPERATURE CONTROL 



^-■^ — Q- 



O- 



■o- 



1 



1^. 



©= 



Ex 



Figure 29.18 Essential layout for control by circulating cold liquid. The 

 refrigerator (R) controls the temperature of the exchanger tank {Ex) from 

 which circulating fluid is pumped (P) to the controlled tanks (T^-T^ in 

 series. At each tank a solenoid-operated valve {V.^-V^ can switch the 

 liquid either through the tank or through the by-pass. Each solenoid is operated 

 via a relay and the thermostat in the appropriate tank; tank T^ will run coolest 



250-0-250 



(a) 



(b) 



9V 



(c) 

 Figure 29.19 Electronic relays for use with very sensitive contact thermostats; 

 in all cases the sense of the relay must be chosen {normally open or closed) in 

 relation to the control of heat or coolth. {a) A large tetrode operated from 

 100-0-100 V d.c. mains; the relay {rdc) and its series resistance R^ must be 

 selected according to the valve, but for an 807 valve, a 32 mA, 50 V relay is 

 suitable, {b) Similar circuit using unrectified power supply; the power available 

 at the relay will be only about i that of a corresponding d.c. plate voltage and 

 the relay must be selected for anti-chatter, (c) Transistor operated relay 

 designed for a P.O. relay. Almost any small Junction transistor is suitable 



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