GB2076997A - Control circuit for electric heater elements - Google Patents

Abstract

A control circuit for electric heater elements 12, 13 e.g. of a fan heater includes triacs 14, 15 in series with the respective elements 12, 13. A known logic device 16 generates, at its terminal 4, trains of control pulses, the duration of each pulse train being dependent on the amount by which a temperature sensed by a thermistor 17 is less than a desired temperature set led by a resistor 20. The triac 14 is responsive to the pulse trains, and a delay circuit 34, 35, 36, 37 generates a control signal to switch on the triac 15 only after the aforesaid pulse trains have become continuous, which occurs when the difference between the desired and sensed temperatures exceeds a predetermined value. <IMAGE>

Description

SPECIFICATION control circuit for electric heaters This invention relates to control circuits for electric heaters, and in particular to control circuits for electric fan heaters having a plurality of heater elements.

It is known to provide that where fan heaters are used for room heating, the control circuit is arranged so as respond to temperature at an air inlet of the heater rather than to the heater air outlet temperature. A known type of control circuit for the elements of a fan heater includes a logic unit which is responsive to an input voltage which corresponds to a difference between a sensed and a desired air temperature, and the logic circuit generates a ramp voltage of a predetermined interval and amplitude. The ramp voltage and the input voltage are compared and used to generate control signals, for a switching device for the heater elements, whereby the elements are switched on for a period in each cycle of the ramp voltage, the period depending on the difference between desired and sensed air temperatures.It has been proposed, as disclosed in patent application 8008625, to arrange that the ramp frequency is between 1 and 2 Hz.

In known arrangements all of the heater elements may be switched off for part of the ramp cycle, and even with a ramp cycle frequency of between 1 and 2Hz the changes in the temperature of air delivered by the heater may cause discomfort to a user. It has been found disadvantageous to switch off the fan at the same time as the heaters, since in these circumstance heat can be transferred from the elements to a temperature sensing device which may therefore not properly respond to room temperature and extend the switched-off time of the heater.

It is accordingly an object of the invention to provide a heater control circuit, and in particular a control circuit for a fan heater, in which the foregoing disadvantages are overcome.

According to the invention there is provided a control circuit for the elements of an electric heater having a plurality of such elements, said circuit including means for generating an input voltage which corresponds to a difference between a desired and a sensed temperature, means for generating a ramp voltage, means for generating a first control signal for a part of each cycle of said ramp voltage in which said input signal has a predetermined difference from said ramp voltage, a first switching device responsive to said first control signal for controlling current supply to one of said heater elements, means responsive to said first control signal for providing a second control signal after a predetermined delay which is longer than the cycle time of said ramp voltage, and a second switching device responsive to said second control signal for controlling current supply to another of said heater elements.

Preferably there is provided means for adjusting the length of said predetermined delay.

As shown in the drawings a fan heater includes a fan motor 10 which is energisable by a mains voltage through a switch 11, together with a one kW heater element 1 2 and a two kW heater element 1 3. Triacs 14, 1 5 are connected in series with the respective elements 12, 1 3 across the mains supply, the two series arrangements being in parallel with the motor 10.

A logic device 1 6 is of the type marketed by Plessey Semi-Conductors Limited under the designation SL 441 C, and includes a ramp voltage generator as aforesaid. The device 1 6 also includes a known type of zero voltage control, whereby output signal pulses from a terminal 4 of the device 1 6 occur at zero voltages of the mains cycle.

A 47k ohm thermistor 1 7 is connected between terminals 5 and 8 of the device 1 6 and is located adjacent an air inlet of a fan driven by the motor 10. A 3.3nF capacitor 18 is connected between terminal 8 and the mains neutral terminal N. In parallel with the capacitor 1 8 is a series arrangement of a 47k ohm resistor 1 9 and a 22k ohm variable resistor 20, these being operable to set a control voltage corresponding to a desired air temperature. A 22nF capacitor 21 is connected between the neutral terminal N and terminal 6 of the device 16, and provides a pulse delay associated with the zero voltage switching feature of the device 1 6.

Terminal 1 of the device 1 6 is connected directly to the mains neutral terminal N, and the mains line terminal L is connected to terminal 2 of the device 1 6 through a 56k ohm resistor 22 and a line 23. The alternating supply on line 23 provides a synchronising signal for zero voltage switching and is also applied to terminal 3 of the device 14 through an internal diode.

A 220of smoothing capacitor 24 is connected between terminal 3 and the mains neutral N to maintain a d.c. voltage at terminal 3. A diode pump circuit comprising a 120 ohm resistor 25, a 471of capacitor 26 and diodes 27, 28 is connected between the line and neutral terminals L, N and the terminal 3 of device 1 6 to provide a d.c. supply to the device 1 6. Between terminal 3 of the device 1 6 and the mains neutral terminal N is a series arrangement of a 1 M ohm resistor 29 and a 4'71LF capacitor 30.The junction of resistor 29 and capacitor 30 is connected through the terminal of the device 1 6 to a ramp generator circuit therein, and the frequency of the ramp cycle is thus dependant on the values of the components 29, 30. In the present example these values are selected to give a ramp frequency of 33Hz.

Terminal 4 of the device 1 6 is connected through a 68 ohm resistor 31 to the gate of the triac 14, and also to the neutral terminal N through a diode 32, a 8 2K ohm resistor 33 and a 1 001if capacitor 34 which are arranged in series. A 100k ohm resistor 35 is connected in parallel with a capacitor 34. The junction of resistor 33 and capacitor 34 is connected through a 2 2M ohm resistor 36 to one input terminal 3 of an operational ampli fier 37 of the type supplied by National Semi Conductors under the designation LM 741.

The supply terminals 4 and 7 of the amplifier 37 are respectively connected to the neutral terminal N and to the d.c. supply terminal 3 of the device 16. The input terminal 2 of the amplifier 37 is connected to the moving con tact of a potentiometer 38 which is connected between the neutral terminal N and the d.c.

terminal 3 of device 1 6.

The terminal 4 of device 1 6 is connected to the gate of triac 1 5 through a thyristor 40 and a 68 ohm resistor 41. The junction of the thyristor 40 and resistor 41 is connected to the output terminal 6 of the amplifier 37 through a 1 K ohm resistor 42 and an 8 2k ohm resistor 43 arranged in series. The gate of thyristor 40 is connected to the junctions between resistors 42 and 43. The gain of the amplifier 37 is selected by a suitable resistor 44 connected between its input terminal 3 and its output terminal 6.

In use, resistor 22 acts to reduce the volt age applied to the a.c. terminal 2 of the device 16, and the diode pump circuit formed by components 24, 25, 26, 27, 28 provides a d.c. supply to terminal 3 of the device 1 6.

As previously described the ramp voltage fre quency of 33Hz is set by resistor 29 and capacitor 30. The temperature at which a control signal is provided at terminal 4 of the device 1 6 is dependent on the voltage at the terminal 8 thereof, as determined by the ther mistor 1 7 and resistor 20. When the tempera tur sensed by the thermistor 17 is less than that selected by the resistor 20, the control signal at terminal 4 is in the form of a continuous train of pulses whose timing is such as to cause the triacs 14, 15 to switch on at, or about, the zero voltage cross-over points of the main voltage. The amplifier 37 provides a further control signal only when the capacitor 34 has charged so that the voltage at terminal 3 of amplifier 37 is equal to that set at its terminal 2 by the potentio meter 38.The time to reach this condition may thus be set by the potentiometer 38, and is arranged to be longer than the cycle time of the ramp voltage of the device 1 6. Typically the delay imposed by the amplifier 37 and its associated components is 8 seconds.

When the sensed temperature is substantially equal to that selected, the control signal is no longer present at terminal 4 of the device 1 6 and the gate signals are thus red moved from both triacs 14, 1 5, which switch off. The subsequent fall in sensed temperature causes the signal to reappear at terminal 4 of device 1 6 for a part of each ramp cycle of approximately 3 seconds, to effect tempera ture control. Since the duration of the control signals at terminal 4 of the device 1 6 is less than the 8 seconds delay imposed by the amplifier 37 and its associated components, the amplifier 37 provides no output and the triac 1 5 is not switched on through the thyristor 40.

The device 1 6 is such that it provides an output on only part of its ramp cycle when the sensed temperature is within 1 C below the desired temperature. In this temperature band, therefore, temperature control is ef fected solely by switching the 1 kW element 1 2. If the sensed temperature is more than 1 C below the desired temperature, a continuous pulse train is supplied at terminal 4 of device 16, and when this pulse train has existed for more than 8 seconds both heater elements 12, 1 3 will be energised until the desired temperature is reached, whereupon switching of the element 1 2 only restarts.

During the parts of the ramp cycle in which no control signal is provided at terminal 4 of the device 16, the capacitor 34 discharges through the resistor 35, so that the 8 second delay is re-imposed at each ramp cycle.

Use of the 1 kW element only for temperature regulation in the aforesaid 1"C range has the effect that this element is required to be energised for a longer period of each ramp cycle than would be the case if the 2kW -element was also energised. Fluctuations in the temperature of the air delivered by the fan heater as a whole are thus smaller, and less likely to be troublesome to a user. Additionally, it has been found that the more even temperature obtainable with the foregoing arrangement results in a more constant density of the air flowing through the fan, and this is reflected in a reduction in changes in the pitch of the noise of the fan.

Claims (5)

1. A control circuit for the elements of an electric heater having a plurality of such elements, said circuit including means for generating an input voltage which corresponds to a difference between a desired and a sensed temperature, means for generating a ramp voltage, and for generating a first control signal for a part of each cycle of said ramp voltage in which said input signal has a predetermined difference from said rarnp voltage, a first switching device responsive to said first control signal for controlling current supply to one of said heater elements, means responsive to said first control signal for providing a second control signal after a predeter mined delay which is longer than the cycle time of said ramp voltage, and a second switching device responsive to said second control signa! for controlling current supply to another of said heater elements.

2. A circuit as claimed in claim 1 in which said first control signal comprises trains of discrete pulses, the duration of each train corresponding to the duration of each said part of a ramp cycle, said pulses being timed to operate said switching devices at zero volt conditions of an alternating supply voltage.

3. A circuit as claimed in claim 1 or claim 2 in which said means for generating the ramp voltage is operable to provide said first control signal continuously when said desired temperature is more than a predetermined amount less than said sensed temperature, said means for providing the second control signal being responsive to the continuous existance of said first signal for the period of said delay.

4. A circuit as claimed in any preceding claim in which the means for providing the second control signal includes a capacitance which is chargeable by said first signal, and means for discharging said capacitance when said first signal is not present.

5. A control circuit for the elements of an electric heater, substantially as hereinbefore described with reference to the accompanying drawings.