Tri-state electrical circuit

An electrical circuit having three states depending upon the magnitude of an electrical signal coupled, in operation, to an input thereof, the circuit having a first stable state when the said magnitude is less than a first, predetermined value, a second, stable state when the said magnitude is greater than a second, predetermined value higher than the first said predetermined value, and a third, unstable state when said magnitude is equal to or between said first and second predetermined values. In a development of the invention, the circuit is incorporated in a control apparatus for a ducted, warm-air, central-heating system in which the gas supply to a burner is supplied continuously when the temperature of a space to be heated is below a predetermined temperature, is cut-off when the temperature of the space is above another, higher predetermined temperature, and is switched on and off at a rate dependent upon the magnitude of the temperature when it is equal to or between the two values.

BACKGROUND OF THE INVENTION 
This invention relates to an electrical circuit having three different 
states depending upon the magnitude of an electrical input signal coupled 
to its input. 
Such a circuit will have many applications, and one application for which 
it is suitable is as part of a control apparatus for a domestic, 
gas-fired, warm-air central heating system in which air is heated by 
passing it over a heat exchanger and then passed along a duct, or ducts, 
to the space or spaces to be heated. 
SUMMARY OF THE INVENTION 
According to the invention there is provided an electrical circuit having 
three states depending upon the magnitude of an electrical quantity 
coupled, in operation, to an input thereof, the circuit having a first 
stable state when the said magnitude is less than a first, predetermined 
value, a second, stable state when the said magnitude is greater than a 
second, predetermined value higher than the first said predetermined 
value, and a third, unstable state when said magnitude is equal to or 
between said first and second predetermined values. 
The circuit may include a semi-conductor device arranged to conduct current 
continuously in one of its stable states, to be substantially 
non-conductive in the other of its stable states and to oscillate between 
a conductive state and a substantially non-conductive state in its third, 
unstable state. 
Preferably the circuit according to the last preceding paragraph is so 
arranged that when it is in the third, unstable state the ratio of the 
time it is in the conductive state to the time it is in the substantially 
non-conductive state is dependent upon the magnitude of the input 
electrical quantity. 
The circuit may have an output for connection to a load, such as a relay, 
arranged to be energised continuously and de-energised when the circuit is 
in one and the other of the first and second states respectively, and to 
be energised intermittently when the circuit is in the third state. 
Preferably the circuit comprises two transistors of opposite conductivity 
type wherein a first transistor has its collector coupled to the base of 
the second transistor and the collector of the second transistor is 
coupled to the base of the first transistor by way of a feedback network 
for determining the period of oscillation of the circuit when it is in the 
third, unstable state. Both of the transistors are arranged to be "on" 
that is conductive in one of the stable states and to be "off", that is 
substantially non-conductive, in the other of the stable states. A load, 
such as a relay, may be coupled between the collector of the second 
transistor and its collector supply voltage. In the case of a relay, or 
like load, the second transistor is substantially non-conductive when the 
magnitude of the current therethrough is insufficient to energise the 
relay, or like load. 
Preferably, the circuit is so arranged that both the transistors are not 
completely saturated when in the conductive state. 
The input of the circuit is preferably the base of the first transistor. 
In an embodiment comprising a first NPN transistor and a second PNP 
transistor and the input signal is a d.c. voltage applied to the base of 
the first transistor by way of a resistor, both transistors will be "on" 
when the magnitude of the input voltage is greater than a first 
predetermined value and both transistors will be "off" when the magnitude 
is less than a second predetermined value lower than the first said value. 
When the magnitude of the input voltage is equal to or between the first 
and second predetermined values the transistors are switched "on" and 
"off" continuously. The feedback network between the collector of the 
seccond transistor and the base of the first transistor may comprise a 
capacitor and resistor in series, and have values which determine the 
period of oscillation in the third state of the circuit. The mark-space 
ratio (ratio of the length of time the transistors are "on" to the length 
of time the transistors are "off") of the circuit in the third state is 
dependent upon the magnitude of the input voltage, increasing as the said 
magnitude increases. The feedback network may comprise a further circuit, 
such as a second capacitance-resistance circuit in parallel with the first 
mentioned capacitance-resistance circuit, to supplement the first circuit 
when the input voltage is equal to or between the first and second 
predetermined values to ensure that the circuit remains in its unstable 
state, at the two extreme ends of the controlled mark/space ratio state of 
oscillation. 
While the input signal is a voltage coupled to the base of the first 
transistor by way of a resistor (that is a current into the base) it can 
be derived from other electrical quantities, such as a variable 
resistance. 
The input signal is preferably applied to the input of the circuit 
according to the invention by way of a buffer amplifier. The sign of the 
input signal will depend upon the design of the circuit. 
A circuit according to the invention may find application, inter alia, as 
part of a control apparatus for controlling a variety of parameters such 
as temperature, and the circuit may be arranged to form part of a servo 
control system. 
Thus according to a development of the invention, there is provided control 
apparatus for controlling the amount of heat generated by a gas heater 
according to the temperature of an object or a space to be heated, the 
apparatus comprising a circuit according to the invention having its 
output coupled to a gas control means for controlling the supply of gas to 
a burner, and its input coupled to means for monitoring the said 
temperature and for providing an input signal of such magnitude that when 
the temperature is less than a first predetermined value, the circuit 
causes the gas control means to supply gas continuously to the burner, 
when the temperature is more than a second predetermined value higher than 
the first said predetermined value, the circuit causes the gas control 
means to stop the supply of gas to the burner, and when the temperature is 
equal to or between the said two predetermined values, the circuit causes 
the gas control means to supply gas to the burner intermittently and at a 
rate dependent upon the magnitude of the temperature. 
The means for monitoring the temperature may be a device, such as a 
thermistor, the resistance of which varies with temperature, or a 
thermocouple, the output voltage of which varies with temperature. 
The control apparatus may be arranged to control the amount of heat 
generated in a central-heating system by monitoring the ambient 
temperature of a space to be heated. 
Advantageously the control apparatus is used with a warm-air 
central-heating system in which air is passed-over a heat exchanger where 
it is heated and then passed through ducts to be discharged into a space 
or spaces to be heated. It is usual to provide a fan to assist the flow of 
warm air, and in a still further development of the invention, the 
operation of the fan is controlled further to improve the effectiveness of 
the central-heating system. 
Control apparatus currently fitted to domestic, gas-fired, warm-air 
central-heating systems may have operating characteristics which do not 
produce optimum comfort to the occupants of a room or rooms to be heated, 
particularly when the warm air is discharged into rooms at a low level. 
When the rooms are at or about a predetermined temperature, selected by a 
room thermostat for example, and the apparatus is operative to maintain 
the temperature constant within predetermined limits, the delivered air 
fluctuates in temperature in response to intermittent burner operation and 
its flow is periodically interrupted. This intermittent operation can give 
rise to discomfort in a heated room because of temperature swings, 
intermittent air currents and intermittent fan noise. 
In the said further development, the control apparatus comprises means for 
controlling the speed of the fan motor to control the volume of air passed 
over a heat exchanger and delivered to the space or spaces to be heated in 
dependence upon the temperature at a predetermined position in the system, 
thereby to maintain the temperature in the space, or spaces substantially 
more constant. 
The fan motor may be controlled by apparatus according to the invention 
disclosed in our United Kingdom application No. 20200/75 entitled 
"Apparatus for controlling the speed of a motor". 
Thus the control apparatus may comprise apparatus for controlling the speed 
of rotation of an electric fan motor in dependence upon the magnitude of a 
temperature at a predetermined position in the system, the apparatus 
comprising an analogue to digital converter having an input for receiving 
an analogue input signal dependent on the temperature and an output 
coupled to an input of a motor control means which is arranged, in 
operation, periodically to decode the digital output of the converter and 
to control the speed of rotation of the motor in dependence thereon. 
Means may be provided for periodically resetting the output of the analogue 
to digital converter to a datum value and for inhibiting change in 
operation of the motor control means during a conversion period. 
The analogue to digital converter may comprise an analogue to time interval 
converter arranged to generate an output signal having a time duration 
dependent upon the magnitude of the input signal, and means coupled to the 
output of the analogue to time interval converter for causing clock pulses 
to be counted by a counter for the duration of said output signal. Thus 
the count in the counter at the termination of the output signal from the 
converter, that is at the end of a conversion period, is dependent upon 
the magnitude of the input signal and therefore the temperature. The count 
is then decoded by the motor control means. 
The analogue to time interval converter may comprise a ramp generator for 
generating a ramp output signal which varies at a substantially constant 
rate during a conversion period and means for comparing the magnitudes of 
the ramp signal and the input signal, or a signal derived from the input 
signal, and for generating a comparison signal when the two compared 
signals balance out, and the means coupled to the output of the analogue 
to time interval converter is arranged to cause pulses to be counted 
during a time interval starting when said ramp signal is equal to a datum 
value and terminating with the generation of the comparison signal. 
Preferably, the ramp generator is a staircase generator. 
The staircase generator may comprise a resistor and capacitor connected in 
series to a source of a.c. potential whereby the potential across the 
capacitor increases in a stepwise fashion as it is charged by alternate 
half cycles of current from the a.c. source, the half cycles being 
obtained by way of a diode. 
Preferably means is provided for discharging the capacitor to a datum level 
at the start of each conversion period. 
The means for comparing may be a bistable circuit arranged to be set to one 
state at the start of a conversion period and reset by the comparison 
signal. 
The clock pulse generator may be a switching device, such as a transistor, 
arranged to be switched on and off at a constant rate when the bistable is 
set. Thus the clock pulse generator will provide one or more pulses and 
the number of pulses generated will be dependent upon the length of time 
the bistable circuit is set. 
Preferably, the switching device is switched on and off by an alternating 
signal, for example a signal alternating at line frequency. 
The pulses may be coupled to the counter by way of pulse shaping means. 
The motor control means may comprise a plurality of latching circuits 
coupled to the outputs of the counter and arranged to vary the 
energisation to the electric motor according to the weighting of its 
associated counter stage. Thus for a four-stage binary counter the four 
outputs thereof would be representative of counts 1, 2, 4 and 8 
respectively and the associated latching circuits could be arranged to 
vary the power supplied to the motor by corresponding amounts. 
In a preferred embodiment of the invention comprising a four-stage binary 
counter and four corresponding latching circuits coupled to the outputs 
thereof, four resistors having weightings R, 2R, 4R and 8R are connected 
in series with the alternating current supply to a motor. Each resistor 
has a normally-open relay contact connected across it, and the relay 
contacts are controlled by relay coils energised by corresponding latching 
circuits, for example the third counter stage is coupled to the third 
latching circuit which controls operation of the relay contact across the 
resistor of value 4R. 
The motor control means may further include means responsive to the input 
signal for inhibiting operation of the motor when the magnitude of the 
input signals is less than a predetermined value. 
Sampling means may be provided for controlling the periodic operations of 
the apparatus, such as the discharge of the capacitor in the staircase 
generator, the decoding operation of the motor control means and the 
resetting of the analogue to digital converter. 
Preferably the sampling means is synchronised to a predetermined point on 
the waveform of the line frequency. 
Preferably the temperature is measured at or near to the heat exchanger. 
The temperature may be measured by a device, such as a thermistor whose 
resistance varies with temperature, or a thermocouple, the output voltage 
of which varies with temperature. 
In another aspect the control apparatus may comprise apparatus for 
controlling the speed of rotation of an electric motor, in dependence upon 
the value of a thermistor connected in the input circuit of an oscillator 
including a unijunction transistor, in which the oscillator is coupled to 
a power supply having an alternating component in such a way that the 
unijunction transistor is fired on a particular part of the alternating 
component of the supply determined by the magnitude of the resistance of 
said thermistor, and the output of the oscillator is coupled to the gate 
of a thyristor or triac having one electrode coupled to one side of an 
alternating current supply and its other electrode coupled through the 
motor to the other side of the supply, whereby the point on the a.c. 
waveform at which the thyristor or triac is fired is determined by the 
output of the oscillator. 
Thus the power supplied to the motor by the thyristor or triac, and hence 
the speed of the fan motor, is dependent upon the magnitude of the said 
resistance, and therefore temperature. 
Preferably the output of the oscillator is coupled to the said gate by way 
of a pulse transformer. 
Thus a control apparatus embodying a circuit according to the invention can 
be designed to provide better comfort by delivering air substantially 
continuously at almost constant temperature and matching its volume to the 
heat load. Thus temperature swings in heated rooms can be much reduced, 
air currents are continuous, satisfactorily slow and always of adequate 
temperature and fan noise is low and continuous. This can be achieved by 
controlling the fuel input in shorter time cycles and by controlling the 
fan speed according to heat exchanger temperature.

DETAILED DESCRIPTION OF THE DRAWINGS 
Referring to FIG. 1, there is shown a control circuit 10 for controlling 
the supply of gas to a gas burner 46. The control circuit 10 comprises a 
circuit shown within a broken line 12 having three states depending upon 
the magnitude of the potential at its input 12a. 
The circuit 12 comprises a first, NPN transistor 14 having its emitter 
connected to a zero volt rail 16 of a power supply shown within a broken 
line 17, and its collector coupled through a resistor 18 to the base of a 
second, PNP transistor 20 and through a resistor 22 to a +30 volt rail 24. 
The transistor 20 has its emitter connected to the +30 volt rail 24 and 
its collector coupled through a relay coil 26 to the zero volt rail 16. 
The collector of transistor 20 is also coupled to the base of transistor 
14 through a basic timing control circuit comprising a capacitor 28 and 
resistor 29 in series and through a secondary timing control circuit 
comprising a capacitor 32 and resistor 34 . The two timing control 
circuits are connected in parallel as shown. 
The base of the transistor 14 is coupled through a capacitor 36 and 
resistor 37 connected in parallel to the zero volt rail 16 and through a 
diode 38 poled as shown and a resistor 40 to the input 12a. 
When the potential at the input 12a is above a first predetermined value 
the resulting current into the base of transistor 14 turns on the 
transistor 14 and the reduction in its collector voltage turns on 
transistor 20 so that the emitter-collector current of transistor 20 
energises relay 26. Relay 26 has a pair of normally-open contacts 26a 
arranged when closed to energise a gas control valve 42 connected in line 
between a gas supply 44 and the gas burner 46. A 24 volt a.c. power supply 
for the gas valve 42 is connected to terminals 48 and 50. 
The values of resistors 18 and 22 are so chosen that transistor 20 is 
caused to operate on a part-linear section of its operating curve to 
ensure that it is not fully saturated when it is on. 
When the potential at the input 12a is less than a second predetermined 
value, lower than the first said predetermined value, the relatively low 
potential at the base of transistor 14 cuts-off the transistor and the 
resulting relatively high potential at its collector cuts-off transistor 
20 as well so that relay 26 is de-energised to cut-off the supply of gas 
to the burner 46. 
When the potential at the input 12a is equal to or between the said first 
and second predetermined values the potential at the base of transistor 14 
is such that the circuit operates as an astable circuit with both 
transistors being alternately on and off together. The mark-space ratio, 
the ratio of the time the transistors are both on, to the time they are 
both off, is determined by the magnitude of the potential applied to the 
input 12a, that is the mark-space ratio increases as the potential 
approaches the said first value. 
The values of the capacitor 28 and resistor 29 are so chosen that in the 
third state the circuit oscillates with a total cycle time (mark + space) 
of about 2.5 minutes, and the circuit 12 is so arranged that it is in the 
third, oscillatory state when the magnitude of the input is between about 
10% and 90% of its dynamic range. The input 12a is coupled to the 
collector of a transistor 48, which collector is also coupled through 
resistors 50, 52 in series to the positive rail 24. The transistor 48 has 
its emitter coupled through a gain-setting resistor 54 to the zero volt 
rail 16 and its base coupled through a resistor 56 to an input terminal 
10a. The base of transistor 48 is also coupled through a capacitor 58 and 
a resistor 60 connected in parallel to the zero volt rail 16. 
Coupled between the junction of resistor 50 with resistor 52 and the zero 
volt rail 16 is a 22 volt zener diode 62, the resistor 52 and zener diode 
62 acting as a voltage regulator. Connected across the zener diode 62 is a 
series circuit consisting of resistors 64 and 66 and a thermistor 68. The 
thermistor 68 provides some temperature compensation for the circuit as 
the ambient temperature for the circuit may vary considerably, for example 
if the central heating system is switched off for long periods during the 
day or night. The junction of resistor 64 with resistor 66 is connected to 
a second input terminal 10b. 
A temperature sensing circuit comprising a thermistor 70 connected in 
parallel with a series arrangement of a resistor 72 and a variable 
resistor 74 is connected to the input terminals 10a and 10b. The 
temperature sensing circuit is situated in a convenient position in a room 
to be heated and the resistor 74 can be adjusted to select the temperature 
of the room. 
The capacitor 58 is provided to de-couple unwanted a.c. signals from the 
base of the transistor 48 and the value of resistor 60 is preset to 
control the temperature control range of the apparatus. 
The capacitor 32 and resistor 34 are not essential but are provided to 
ensure extended on and off conditions of the circuit 12 when the input 
potential is greater than the first predetermined value and less than the 
second predetermined value respectively. The capacitor 36 is provided to 
prevent spurious triggering of the circuit 12 and the diode 38 is provided 
to prevent current flowing from capacitor 28 and resistor 29 to the 
collector circuit of transistor 48 when the collector of transistor 20 is 
more positive than that of transistor 48. 
In operation, when the temperature at the temperature sensing circuit in a 
room to be heated is much lower than the selected control temperature set 
by resistor 74, the resistance of thermistor 70 is high and the potential 
at input terminal 10a is low so that transistor 48 is cut-off. Thus the 
potential at the collector of transistor 48 is greater than a first 
predetermined value and both transistors 14 and 20 are rendered 
conducting. The relay 26 is energised to open the gas valve and the burner 
46 operates continuously. As the temperature in the room increases the 
resistance of the thermistor 70 decreases and the consequent increase in 
current fed to the base of transistor 48 causes it to conduct and the 
potential at its collector (input 12a) drops to a value equal to or less 
than the said first predetermined value when the circuit 12 starts to 
oscillate as described as aforesaid. The circuit 12 oscillates with a 
cycle time of about 2.5 minutes set by capacitor 28 and resistor 29 and 
with a mark-space ratio determined by the magnitude of the potential at 
the input 12a. That is, when the temperature is such that the potential at 
the input 12a is equal to or just greater than the first predetermined 
value the transistors 14 and 20 are on almost continuously but as the 
temperature increases towards a second higher predetermined value the 
resultant reduction in value of the potential at input 12a reduces the 
mark-space ratio accordingly. When the temperature exceeds the second, 
predetermined value the potential at input 12a is such that the 
transistors 14 and 20 are cut-off continuously. Thus when the temperature 
is less than the first predetermined value the relay 26 is energised and 
the burner operates continuously. When the temperature is equal to or 
between the first and second predetermined values set by the temperature 
sensing circuit the relay 26 and the burner are operated intermittently so 
that the amount of heat generated is sufficient to maintain the 
temperature of the room between these two values. If the temperature 
exceeds the second predetermined value the relay is de-energised and the 
burner extinguished. The circuit 12 is so arranged that it is in its 
third, oscillatory, state for between about 10% to 90% of the design heat 
load requirement of the heated space. 
In the circuit as described, if inputs 10a and 10b are connected together 
the transistor 48 will be turned on and gas will be supplied continuously 
to the gas burner 46. 
FIG. 5 shows a part of the circuit of FIG. 1 modified to provide a 
fail-safe feature. In this arrangement the temperature-sensing circuit 
comprises a negative temperature coefficient thermistor 80, a temperature 
set-point control resistor 76, a calibration control resistor 78, an NPN 
transistor 82 and a silicon diode 84 connected as shown. 
With this configuration the transistor 82 operates as an amplifier and 
inverter so that as the ambient temperature around the negative 
temperature-coefficient thermistor 80 rises, the potential at the base of 
the transistor 82 fails tending to turn-off the transistor 82, resulting 
in an increase in its collector voltage. The resulting increase in 
collector potential is applied by way of diode 84 and resistor 56 to the 
base of transistor 48 which decreases the "on" to "off" time of timer 
circuit 12 thus reducing the amount of gas supplied to the gas burner 46 
to maintain the ambient temperature at the required value. 
In FIGS. 2, 3a and 3b of the drawings, like parts are given like 
references. 
Referring now to FIG. 2, there is shown apparatus 110 for controlling the 
speed of rotation of a fan motor 112 in dependence upon the magnitude of 
an input signal applied to an input terminal 114. The apparatus 110 
comprises an analogue to digital (A to D) converter shown within a broken 
line 116, and having an input 118. The input terminal 114 is coupled 
through an amplifier 120 to the input 118. The input current for the 
circuit and applied to terminal 114 is derived from a thermistor (not 
shwon) situated in the air stream of the fan of the central-heating system 
and close to the heat exchanger (not shown). 
The A to D converter 116 is arranged to convert an analogue signal applied 
to input 118 into a digital number representative of its magnitude. 
The output of the A to D converter 116, in the form of a digital count, is 
coupled to a motor control means shown within a broken line 122 arranged 
periodically to decode the digital number and to control the speed of 
rotation of the motor 112 in dependence upon the magnitude thereof. Thus 
the speed of rotation of the motor 112 is dependent upon the magnitude of 
the analogue input signal. 
A sample and reset timer 124 is arranged to generate at outputs 126a and 
126b a sample and reset pulse 100 .mu.s long every 10 secs. The reset 
pulse is used periodically to reset the A to D converter 116 to a datum 
level to start a fresh conversion of the input signal and, indirectly, to 
cause the motor control means to decode the output of the A to D converter 
116 each time it has effected a conversion. 
The A to D converter 116 comprises a staircase generator 128, typically a 
resistor and capacitor (not shown) coupled in series between signal ground 
and a source of alternating current, so that, in operation the potential 
across the capacitor increases in a stepwise fashion. The staircase 
generator 128 has a reset input 128a coupled to the output 126a of the 
sample and reset timer 124 and an output 128b coupled to one input 130a of 
a comparator circuit 130. Thus the generator 128 supplies to the 
comparator 130 a staircase waveform which is reset periodically to a datum 
level. The other input 130b of the comparator 130 is coupled to the input 
118. The comparator 130 is a bistable device having an output set to one 
level when the magnitude of the signal at input 130b is greater than both 
a first predetermined value and the magnitude of the staircase waveform at 
input 130a and is reset to its other level when the magnitude of the 
staircase waveform is equal to or greater than the signal at input 118. 
The output 130c of comparator 130 is coupled to the control input 132a of 
a pulse generator 132 arranged to provide pulses to the input 134a of a 
4-stage binary counter 134 when the comparator output is set to its said 
one level. Each stage of the counter 134 has a corresponding output 134c 
to 134f coupled to the motor control circuit 122. The outputs 134c to 134f 
of counter 134 correspond in known manner to counts of 1, 2, 4 and 8 
respectively, that is the counter can count input pulses up to a maximum 
of 16 (that is, 0 and 1 to 15) and the dynamic range of the input signal 
is such that a full-house count of 15 in counter 134 would correspond to 
at least the maximum value of the input signal, and preferably a greater 
value. 
The staircase generator 128 and counter 134 are reset to datum once every 
10 seconds by a reset pulse applied to inputs 128a and 134b respectively. 
The motor control circuit 122 comprises four identical latching circuits 
136c to 136f coupled to counter outputs 134c to 134f respectively. The 
latching circuits 134c to 134f are arranged to control four normally-open 
switches 138c to 138f respectively which are connected in parallel with 
four corresponding resistors 140c to 140f connected in series between one 
terminal 112a of the motor 112 and the neutral terminal 142 of an 
alternating current supply for the motor 112. Thus closure of a switch 
will have the effect of shunting or short-circuiting its associated 
resistor. The values of the resistors 140c to 140f are binary weighted so 
that if the resistance of resistor 140c in parallel with switch 138c is R 
then the resistance of resistors 140d, 140e and 140f is respectively 2R, 
4R and 8R. Thus the value of resistance in circuit is indirectly 
proportional to the count in the counter 134. 
The other terminal 112b of the motor 122 is coupled through a normally-open 
switch 144 to the line terminal 145 of the alternating supply for the 
motor. Operation of the switch 144 is controlled by the output of the 
amplifier 120 such that when the magnitude of the output of the amplifier 
120 is greater than a second predetermined value (which is typically less 
than the first predetermined value required to set the comparator 130) the 
switch 144 is closed so that an energising current flows in the motor 122. 
The magnitude of the current is determined, externally of the motor 112, 
by the sum of the resistors 140c to 140f in circuit. 
In operation, the sample and reset timer 124 resets the output of the 
staircase generator 128 to its datum level and counter 134 to zero once 
every 10 seconds, and the motor control means 122 is arranged to control 
the speed of the motor 112 during any 10 second period according to the 
count in the counter 134 at the end of the immediately preceding 10 second 
period. 
When the magnitude of an input current applied to terminal 114 is such that 
the output of the amplifier 120 is less than the said second predetermined 
value the motor is not energised. As the magnitude of the input signal is 
increased until the output of the amplifier 120 is equal to the second 
predetermined value the switch 144 is closed and the motor 112 is 
energized with all four resistors 140 in circuit so that the fan rotates 
at its lowest speed. If the magnitude of the input signal remains at a 
value between the second and the first, higher predetermined value the fan 
would continue to operate in this manner. When the input is increased 
still further until the output of amplifier 120 is equal to or greater 
than the first predetermined value and that of the staircase waveform the 
comparator 130 is set, to cause the pulse generator 132 to feed pulses to 
the counter 134. Assuming that the staircase generator 138 and counter 134 
have just been reset, the counter will continue to count pulses until the 
magnitude of the staircase waveform becomes equal to or greater than the 
output of the amplifier 120 when the comparator 130 will be reset and thus 
inhibit the pulse generator. Thus it will be seen that the time interval 
during which the conparator 130 is set and therefore the number of pulses 
counted by the counter 134 will be dependent upon the magnitude of the 
input signal. 
At the end of the analogue to digital conversion period the motor control 
means 122 decodes the count in the counter 134. This is achieved by 
coupling the reset signal from the output of the comparator 130 to an 
input of each of the latch circuits 136c to 136f in such a way that if an 
output stage of the counter 134 has changed state since the immediately 
preceding conversion the corresponding latching circuit is charged 
accordingly to open or close its associated switch 138. For example, if 
the count is six, the counter outputs 134c and f would be at zero and 134d 
and e would be at one so that switches 138c and f would be open and 138d 
and e closed to short out resistors 140d and e and the current through the 
motor would be increased accordingly to increase the speed of the motor 
and therefore the fan. Thus the speed of the fan is dependent upon the 
magnitude of the input signal. 
At the next reset pulse the counter is reset to zero count, and staircase 
generator is reset to datum level. If the magnitude of the signal at the 
input 130b of the comparator 130 is still equal to or greater than the 
first predetermined value the comparator 130 is again set to cause pulses 
to be coupled until the magnitude of the staircase waveform again equals 
or exceeds that at the input 130b to reset the comparator 130. When the 
comparator 130 resets, its output also causes the latching circuits 136 to 
decrease the output of the counter to determine the speed of the motor 112 
during the next succeeding conversion period and so on. 
Thus the fan is stationary until the temperature at the heat exchanger 
reaches a predetermined value and then there is a 16-step control of the 
speed of the fan to maintain the room temperature substantially constant. 
Referring now to FIGS. 3a and b there is shown a circuit diagram of 
apparatus 150 according to the invention for controlling the speed of 
rotation of a motor 112 arranged to drive a fan in a warm-air central 
heating system according to the resistance of a thermistor 152 situated in 
the air stream of the fan and close enough to the heat exchanger (not 
shown) to be responsive to heat radiated or convected therefrom. 
The circuit is somewhat similar to that shown as a block diagram in FIG. 2 
and parts of the circuit which generally correspond to a block in FIG. 2 
are shown in outline by a broken line bearing the same reference although 
it will be appreciated that individual components may form part of more 
than one block. 
The thermistor 152, which has a resistance characteristic which decreases 
with increase in temperature, is connected between input terminals 154a 
and 154b. Terminal 154a is coupled through a resistor 156 to the base of a 
NPN transistor 158. A chain of resistors 160, 162, 164 and 166 in series 
is connected between a +5 volt rail 168 and a zero volt rail 170 and the 
terminal 154b is connected to the junction of resistor 162 with resistor 
164. The transistor 158 has its collector coupled through a relay coil 172 
to a +30 volt rail 174, and its emitter connected to the zero volt rail 
170. The relay coil 172 has a pair of normally-open contacts 172a 
connected between one input of the motor 112 and the line terminal 226 of 
an alternating current supply. A capacitor 176 is connected between the 
emitter and base of the transistor 158 and a variable resistor 177 is 
connected between the zero volt rail 170 and the junction of the capacitor 
176 with the resistor 156. Three diodes 178a, 178b and 178c poled as shown 
are connected in series between the emitter of transistor 158 and the 
junction of resistor 160 with resistor 162 and operate as a voltage 
regulator with a negative temperature coefficient of resistance to 
compensate for changes in ambient temperature. Thus the transistor 158 and 
its associated components can be likened to the amplifier 120 of FIG. 2. 
A sample and reset circuit shown within a broken line 124 is an astable 
multivibrator arranged to generate a pulse about 100 .mu. secs. long every 
10 seconds. The multivibrator 124 comprises two NPN transistors 180 and 
181, resistors 182 to 187, capacitors 188 and 189 and a diode 190 
connected as shown. Pulses of 100 .mu. secs. duration and of opposite 
polarity appear at the collectors of the two transistors 180 and 181. The 
resistor 187 has one end coupled to the base of transistor 180 and through 
resistor 185 to the +5 volt line, and the other end to a 24 volt a.c. rail 
192 thereby to synchronise the operation of the multivibrator 124 to a 
predetermined point on the waveform of the line frequency. 
A staircase generator shown within a broken line 128 comprises a resistor 
194 and a capacitor 196 connected in series between the 24 volt a.c. rail 
192 and the zero volt rail 170. The junction of the resistor 194 and 
capacitor 196 is coupled to the collector of transistor 181 by way of 
diode 198 so that the capacitor is discharged to a datum level 
(substantially zero volts) once every 10 seconds by a reset pulse. The end 
of the resistor 194 remote from the capacitor is connected to the 24 volt 
a.c. rail 192 so that the capacitor is charged in stepwise fashion by the 
positive half cycles of alternating current to generate a staircase output 
waveform. 
The output of the staircase generator 128 is coupled by way of a resistor 
200 to the input of a bistable circuit shown within a broken line 130. The 
bistable circuit 130 comprises two NPN transistors 201, 202 the resistor 
166, resistors 203, 204 and 205 and a capacitor 206, the input of the 
circuit 130 being the base of transistor 201. 
The bias voltage at the junction of resistors 162 and 164 is dependent upon 
the resistance of thermistor 152 and hence temperature of the air stream 
near the heat exchanger. When the resistance of the thermistor 152 is 
high, that is the temperature is low, the bias voltage is such that the 
transistor 201 is ON, that is conducting and transistor 202 is OFF, that 
is substantially non-conducting. When the thermistor resistance falls to a 
first predetermined value or lower, the bias voltage increases in a 
negative direction and, assuming the capacitor 196 is discharged or nearly 
so, the transistor 201 is turned OFF and transistor 202 is turned ON so 
that the bistable is set to one state. Then, as the magnitude of the 
staircase waveform increases it will eventually reach a value at which it 
offsets the change in the bias at the base of transistor 201 due to the 
change in resistance of thermistor 152 so as to turn ON transistor 201 and 
turn OFF transistor 202 to reset the bistable circuit 130. Thus the 
bistable 130 will be set for a time dependent upon the magnitude of the 
change in the bias level due to change in the resistance of the thermistor 
152. The bistable circuit 130 can therefore be likened to the comparator 
130 of FIG. 2. 
A chain of resistors 208, 209 and 210 is coupled between the collector of 
transistor 202 and the 24 volt a.c. rail 192, a capacitor 211 being 
connected in parallel with resistor 210. A NPN transistor 212 has its base 
connected to the junction of resistor 208 with resistor 209, its emitter 
connected to the zero volt rail 170 and its collector coupled through a 
resistor 213 to the +5 volt rail 168. Thus part of the circuit is so 
arranged that when transistor 202 is OFF the bias voltage at the base of 
transistor 212 is sufficient to ensure that transistor 212 is conducting 
continuously, that is, it is ON. When transistor 202 is switched ON the 
bias at the base of transistor 212 is removed and the positive half cycles 
of alternating voltage from line 192 are sufficient to switch transistor 
212 ON and OFF at line frequency. Thus transistor 212 and its associated 
circuitry within a broken line 132 can be regarded as a pulse generator 
operative when the bistable circuit 130 is set. 
The output of the pulse generator 132, the collector of transistor 212, is 
connected to the clock input 134a of an integrated circuit, four stage, 
divide by 16 binary counter 134. The collector of transistor 212 is also 
coupled to the base of transistor 158 by a resistor 216 for a purpose to 
be described hereinafter. 
The counter 134 also has a reset input 134b coupled to the collector 
transistor 180 in the reset timer 124, whereby the counter is reset to 
zero every 10 seconds. 
The outputs of the four stages 134c to 134f corresponding to the first 
stage of the counter 134 are connected to four identical latching circuits 
218 C to F of which only one circuit 218 C is shown in detail. 
Each of the latching circuits 218 includes components like those 
illustrated in circuit 218c which; comprises a NPN transistor 219c having 
its collector coupled through a resistor 220c to the output of its 
corresponding counter stage, and its emitter coupled through a capacitor 
221c to the zero volt line 170 and through a resistor 222c to the base of 
a power NPN transistor 223c. The emitter of transistor 223c is connected 
to the zero volt rail 170 and the collector is coupled through a relay 
coil 224c to the +30 volt rail 174. The base of transistor 219c is coupled 
through a resistor 225c to the collector of transistor 202, and the 
circuit is so arranged that the transistor 219c are maintained 
non-conducting when transistor 202 is ON and pulses are being counted by 
the counter 134. When the bistable circuit 130 is reset so that transistor 
202 is switched OFF, the bias levels at the bases of the transistors 219c 
is such that the transistors 219c can conduct to decode the output 0 to 1 
of their corresponding counter stages and energise of de-energise relay 
224c depending upon the count in the stage. If the count in the counter 
stage is 0 the relay is not energised and if the count is 1 the relay is 
energised and this state is maintained until after the bistable circuit 
130 has been set and reset once more, that is until the next succeeding 
conversion of the analogue input signal to a digital representative in the 
counter. 
One side of the motor 112 is coupled through a normally-open relay contact 
172a, actuated by relay 172, to the line terminal 226 of the alternating 
current mains supply and the other side is coupled through four binary 
weighted resistors 140C to 140F to the neutral terminal 228. The resistors 
140C, 140D, 140E and 140F have resistance values of R, 2R, 4R and 8R 
respectively. Connected in parallel with each of the resistors 140C to 
140F is an associated normally-open relay contact 224C/1 to 224F/1 
respectively, each relay contact being actuated by its corresponding relay 
coil 224. 
The power supply for the circuit 150 is shown within a broken line 230. 
In operation, when the central heating system is switched-on the heat 
exchanger will be cold and the resistance of the thermistor 152 high. The 
fan motor should not be operative as cold air would be circulated to the 
spaces to be heated. 
The staircase generator 128 will operate continuously, generating a 
staircase waveform which is reset to zero once every 10 seconds by a reset 
pulse from the sample and reset timer 124. The bistable circuit 130 will 
be reset with transistor 201 ON and transistor 202 OFF so that the 
generation of pulses to the counter 134 is inhibited, and the count is 
zero. 
When the thermistor 152 is cool, its resistance will be high and the 
potential at the junction of resistors 162 and 164 will be high so that 
sufficient current will flow through resistor 164 to the base of 
transistor 201 to ensure that transistor 201 is maintained ON even when 
the capacitor 196 is discharged to substantially zero volts by timer 124. 
As the temperature at the thermistor increases, its resistance falls until 
the bias level at the base of transistor 158 is sufficient to turn it ON, 
and the resulting collector current energises relay 172 to close the 
normally-open contacts 172a in the power supply to the fan motor 112. As 
the contacts 224 are open all of the resistors 140 will be in circuit and 
the fan motor will rotate at its slowest speed to provide some air flow. 
This occurs when the temperature at the thermistor is about 50.degree. C. 
As the temperature at the thermistor 152 increases the bias voltage at the 
junction of resistors 162 and 164 reduces. When the temperature at the 
thermistor is at a value about 55.degree. C, the bias is reduced to a 
level such that when the capacitor 196 is next discharged by a reset 
pulse, the transistor 201 is turned OFF and transistor 202 is consequently 
turned ON, that is the bistable circuit 130 is SET. The resistor 204 and 
capacitor 206 are "speed-up" components selected to ensure a rapid 
transition between operative states of the circuit 130. 
The bistable circuit 130 remains in the set state until the potential 
across the capacitor 196 has been increased in stepwise fashion by 
positive half cycles of alternating current on rail 192 to a value at 
which it is sufficient to offset the change in bias at the base of 
transistor 201 due to the change of resistance of the thermistor 152. At 
this time the transistor 201 is switched ON again and transistor 202 is 
switched OFF, to reset the circuit 130. Thus the circuit 130 will be set 
for a period dependent upon the magnitude of the change in resistance of 
the thermistor. If the thermistor is heated to a value at which its change 
in resistance is just sufficient to switch OFF transistor 201, then the 
circuit 130 will be set for only a short time. If the thermistor 152 is 
heated to a much higher value then the circuit 130 will be set for a 
correspondingly longer time. Thus the combination of circuits 120, 124, 
128 and 130 can be regarded as an analogue input to time duration 
converter. 
For the duration of the time that the circuit 130 is set, transistor 202 is 
ON and the resulting reduction in potential at its collector is such that 
positive half cycles of alternating voltage on rail 192 can turn 
transistor 212 ON and OFF at the frequency of the a.c. mains supply. The 
circuit 132 including transistor 212 is arranged to shape the waveform 
appearing at the collector to a substantially square wave which is then 
coupled to the clock input 134b of the counter 134. The values of 
resistors 209 and 210 are selected to present a suitable value of 
alternating current to transistor 212 and the capacitor 211 is provided as 
a "speed-up" component to increase the rate of rise of the waveform. 
The resistor 216 is provided to ensure that if a clock pulse is applied to 
counter 134 prior to switch 172a being closed, then amplifier 120 receives 
a small pulse to energise relay coil 172 and therefore close switch 172a. 
At the end of a conversion period when the transistor 202 is turned OFF, 
the count in the counter 134 will be dependent upon the length of time the 
circuit 130 was set and hence on the magnitude of the temperature at the 
thermistor 152. Being a 4-stage converter the number of pulses counted 
during a conversion period can have any one of sixteen values, zero to 
fifteen. 
The outputs 134c, 134d, 134e and 134f of the counter 134, corresponding to 
counts of 1, 2, 4 and 8 respectively are decoded by the latching circuits 
218C to 218F which actuate relay contacts 218C/1 to 218F/1 accordingly to 
adjust the power supplied to the motor 112. 
Each base resistor 225 of the latch circuits 218 is connected to the 
collector of transistor 202 so that when transistor 202 is ON to cause 
clock pulses to be counted, the transistors 219 are cut-off. At the end of 
a conversion, transistor 202 is turned OFF and allows the transistors 219 
to sample their corresponding counter output stages. If a counter stage 
has changed its state in the immediately preceding conversion period its 
associated latch circuit changes state accordingly to energise or 
de-energise its relay as the case may be. The latch circuits 218 are then 
maintained in that state until they take a fresh count sample at the end 
of the next succeeding conversion period. 
By selecting the resistors 140C to 140F to have a binary relationship to 
each other it is thus possible to convert the binary number in the counter 
to a decimal value of resistance and thus to vary the voltage to the fan 
in regular steps accordingly to the count in the counter. The air flow 
caused by the fan will thus be increased or decreased with increase or 
decrease in temperature at the thermistor thus tending to stabilise the 
temperature. 
Connected in parallel with the resistor 200 is a series circuit comprising 
a resistor 232 and three diodes 233, 234, and 235 poled as shown which 
introduce a nonlinearity into the control system which tends to compensate 
the non-linear voltage against speed relationship of the motor. 
Various modifications can, of course, be made to the circuit without 
departing from the scope of the invention. For example, the 
binary-weighted resistors 140 could be replaced by four binary-weighted 
voltages derived from separate windings of a transformer. 
As the whole process is directly geared to a specific point on the supply 
waveform, the switches 224C/1, 224D/1, 224E/1 and 224F/1 will open at, or 
near to, the zero voltage point on the supply waveform to minimise 
sparking (interference) and prolong the life of the switches. 
Referring now to FIG. 4 there is shown a circuit diagram of apparatus 310 
for controlling the speed of an electric motor 312 in accordance with the 
value of resistance of a resistor 314. As described with reference to FIG. 
3 the motor 312 is arranged to drive a fan in a warm-air central heating 
system and the resistor 314 is a thermistor situated in the air stream and 
in a position to receive radiant heat from the heat exchanger. 
The apparatus 310 comprises a PNP transistor 316 having its emitter coupled 
through a preset resistor 318 to a +24 volts unsmoothed line of a power 
supply 320 and its collector coupled through a resistor 322 to base 2 of a 
unijunction transistor 324. A capacitor 326 and resistor 328 are connected 
in parallel between the base and emitter of transistor 316. The base of 
the transistor 316 is coupled to the emitter of the unijunction transistor 
324 by way of the thermistor 314 and a resistor 330, and the emitter of 
the unijunction transistor 324 is further coupled to the zero volt line of 
the power supply 320 through a capacitor 332. The base 1 of the 
unijunction transistor 324 is coupled through the primary winding 334a of 
a pulse transformer 334. The transformer is wound on a ferrite core and 
has a ratio of 1:1. 
One end of the secondary winding 334b of the transformer 334 is coupled 
through the motor 312 to one side of the alternating current supply. The 
other end of the secondary winding 334b of the transformer 334 is coupled 
to the gate electrode of a thyristor or triac 336 which is connected 
between the motor 312 and the other side of the alternating current 
supply. A surge suppression circuit comprising a resistor 338 and a 
capacitor 340 are connected in series across the thyristor or triac 336 to 
protect the thyristor or triac, and reduce radio frequency interference. 
In operation, when the temperature at the thermistor 314 is low, the 
thermistor resistance is high and the circuit is non-oscillatory. As the 
temperature increases so that the thermistor resistance decreases the 
transistor 316 will start to conduct at a predetermined value of the 
thermistor resistance set by resistor 328, and current will flow through 
resistor 322 to base 2 of the unijunction transistor 324. Current will 
also flow by way of thermistor 314 and resistor 330 to the emitter of the 
unijunction transistor 324 so that the transistor 324 starts to oscillate 
at a rate determined by the capacitor 332. The frequency of oscillation is 
about 1KHz. The oscillator waveform is developed across the primary 
winding 334a of the 1:1 transformer 334 which both shapes the pulses and 
provides isolation from the a.c. mains potential at the secondary winding 
334b. The potential developed across the secondary winding 334b is fed to 
the gate of the triac 336. The triac 336 is thus fired by the oscillator 
pulses and, depending upon the point on the 50Hz mains cycle on which it 
is fired, provides a proportional amount of power to energise the motor 
312. For example if the triac 336 is fired on the peaks of the a.c. 
waveform power will be fed to the motor 312 for two quarter cycles in each 
cycle and the motor will be energised effectively at half power. 
The triac can be fired at any point of the 50Hz a.c. waveform and as a 
result the power fed to the motor will be varied proportionately. During 
normal operation, as the resistance of the thermistor varies, the firing 
point of transistor 324 on the a.c. waveform which appears as ripple on 
the +24 volt supply varies, up to a maximum allowed by the value of 
resistor 318. The resistor 318 can be preset to adjust the maximum power 
to and the speed of, the motor 312. The resistor 322 is provided to limit 
the current through transistor 324 in the event that the resistor 318 is 
set to zero ohms. 
The transistor 316 ensures a rapid escalation of power at the commencement 
of oscillation, to ensure that the motor 312 does not receive power at too 
low a level which is inadequate to cause the motor to run. If that happens 
overheating and eventual failure of the motor would occur. 
The resistor 330 is provided to prevent the unijunction transistor 324 from 
"locking-on" in the event that the resistance of the thermistor 314 is 
reduced to a very low value.