Frank w. Darrow
Ithaca,
lthaca College New York 14850
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I
A Sensitive Solid State Thermoregulator Relay
Mercury-in-glass tl~ermoregulator.; arc capable of senvtlvities of *0.002'C bnt they demand very small currents to minimize arcing at. t.he mercury surface. A sensitive relay device must be int,erposed betweeri the regulator and the heater. Conventional devices either terminate in a large thyratron tube, which is expensive, or in a mechanical relay, which is subject t,o mechanical failure. The devices described here can control currents of sevcr:d amperes in the load circuit less t,han a microampere flowing in t,he cont,rol circuit,. They are all solid state, thus having virtually unlimit,ed life with no mechanical wear (and complctcly silent operation) and they are inexpensive. The heart of each cont,rol is a silicon cont,rolled switch (GF: t,ype 3N81)'. An SCS is a P N P N st,ruct,ure with all four regions accessible rather than only three as is the case v.ith a silicon controlled rectifier (SCR). These circuits maim use of the high t,riggeringsensitivit,y of the SCS; a current of less than one microampere in the cat,hode gate circuit will trigger the device. Whcn the t~hermoregulatoris open currcnt flows into the SCS cathode gate causing the SCS to conduct and this in turn directs a few milliamperes of current into the SCR gate causing it t,o conduct and the heat,cr to t,ur~ion. When the thermoregulator closes the SCS turns off and this turns off the SCIt and the heater. Both devices will only conduct during the positive half of the ac cycle. Control A (Fig. 1) is connected directly to the ar line and the current t,hrough the heater is only about one half of the full wave value. This control is useful when t,he life of t,he heating device can be lengthened by not operat,irig a t maximum power (an incaudescent lamp is such a heater). Control B (Fig. 2) is powered by the full wave rectified out,put of a bridge rectifier. Since the load is in the rectifier input it is operated a t nearly full power; this is a useful control for large resistive heaters which do not operate a t incandescent temperat,ures. Both devices have been in use for several months in t,his laborat,ory. A well stirred and irisulated oil bath is brought t,o a few tenths of a degree below the desired operating temperature by a "bucking" system of simult,aneous cooling with a water cooled coil arid heating with a Variac controlled 125 W elect,ric heater. A 40 W showcase bulb is immersed in the bath to supply the remaining heat and operated t,hrongh the cont~roldeJ. F. C m m u , Edilor, "GE Transistor Manual," 7th ed., General Electric Company, Syracuse, New Yark, 1964, p. 301. 2Allied Radio Corporation, Allied Industrid Eleetmnier Catalog, 100 N. Western Avenue, Chicago, Illinois 60fiXO.
vice and a "Micro-set" thermoregulator. This system maintains the temperature to within a few thousandths of a degree Celsius for unlimited time periods. The parts for A will coyt about $15 and B will cost about $202. Standard construction practices should be
Figure 1.
Control A.
I Figure 2.
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DII-UII.TO&
Control 8.
Ports List for Controls A
and B
800 ohm, 10 W 66 megohm, W 22 megohm, I / * W 180 kiluhm, I/* W heater, resistance greater than 20 ohm SPST switch 10 amp R amp iuse, type 3AG '1, amp f o x , type 3AG 6 amp fuse, type 3AG 120 T- thynxtrn (CX 611820SP4B4 or equivalent) 200 V, 7 amp SCll (GE CZOR or equivalent) SCS (GE 3NR1) silieau rectifier, (IR 5A4 or equivalent) 10 amp bridge ~.ectifier(I'AHO 1N4436/S or eqoivalent)
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followed, and the entire unit can be housed in a small, well-ventilated cabinet. To reduce possible shock hazards the cabinet should be grounded. A t,hree conductor utility cord may be used to obt,ain the indicated polarity. I n cont,rol B the thermoregulator is electrically live and must be carefully insulated from
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Journal o f Chemical Education
ground and possible human contact. Good air circulation and thus cooling of the serniconduct,ors is very important; excessive heat will cause errat,ic behavior and possible permanent damage. The maximum load for each coutrol as given here is a heater equivalent to 660 W a t 110 V.
