GB2215493A - Electronic temperature control apparatus - Google Patents

Abstract

To control the temperature of, for example, a hot melt glue gun having a thermistor and calibration resistor R5 providing a signal representing actual temperature, a reference voltage for comparison therewith in IC1 is provided by a plug-in resistance module 16. The module 16 can be exchanged for another simular module to provide a different reference temperature, e.g. for a different glue, thereby avoiding recalibration of the glue gun. The result of the comparison controls a heater 18 via a triac 22. If no module 16 is present the glue gun is set at its lowest working temperature. <IMAGE>

Description

ELECTRONIC TEMPERATURE CONTROL APPARATUS This invention relates to electronic temperature control apparatus.

Glue melting and dispensing apparatus is well known. The apparatus is usually in the form of a pistol and such pistols are known as hot melt glue guns. The known hot melt glue guns are fed with an adhesive stick which is known as a glue stick. The glue guns have heater means for melting the glue sticks. Different materials to be stuck together often require different types of glue stick. For example, in the plastics industry, if polystyrene needs to be stuck, then only a relatively low temperature needs to be employed and this is reflected in the type of glue stick utilised. In contrast, in the car industry, adhesives usually need to be such as to be able to withstand higher temperatures so that glue sticks need to be employed which melt at relatively high temperatures.The holt melt glue guns are generally produced for specific applications to melt specific types of glue sticks. Due to the electronic temperature control apparatus employed in the known glue guns, each and every operating temperature required for a particular application must be set for each individual glue gun by direct measurement of the glue gun chamber temperature and adjustment of a temperature setting resistor. This is a specialised operation and glue gun proprietors who wish to use their glue guns for different applications invariably have to return their glue guns to their supplier for the glue guns to be re-set. This is inconvenient for the glue gun proprietors and is often inconvenient for the glue gun supplier.

It is an aim of the present invention to provide electronic temperature control apparatus which can avoid the above mentioned problem in glue guns and which can also be used to control the temperature in other apparatus.

Accordingly, in one non-limiting embodiment of the invention which is given by way of example only, there is provided electronic temperature control apparatus comprising first resistance means which is such that its resistance varies with temperature and which is such that during operation of the electronic control apparatus it causes the generation of a sense voltage, second resistance means which calibrates the sense voltage at one single predetermined temperature to equate to a chosen reference voltage at the predetermined temperature, third resistance means for giving the chosen reference voltage, and comparator means for comparing the sense voltage and the reference voltage, the electronic temperature control apparatus being such that the third resistance means is in the form of a plug-in module whereby the electronic temperature control apparatus is settable at a plurality of defined temperature controllevels by changing the plug-in module to one which will give the required defined temperature control level.

Usually, the first resistance means will be such that its resistance decreases with temperature. Preferably, the first resistance means is a thermistor.

Advantageously, the electronic temperature control apparatus is such that the reference-voltage is caused to rise if the plug-in module is removed, whereby the control temperature falls to a predeterminied minimum operating temperature. This provides a safety feature during operation of the electronic temperature control apparatus.

The third resistance means may be a three pin plug-in module which allows the electronic control apparatus to be set up at temperatures below the predetermined minimum operating temperature.

The present invention also extends to apparatus when provided with the electronic temperature control apparatus.

The apparatus is preferably glue melting and dispensing apparatus such for example as a hot melt glue gun, but other types of apparatus may be provided with the electronic temperature control apparatus.

In another non-limiting embodiment of the invention which is given by way of example only, the present invention also provides apparatus comprising heater means and a plug-in module for varying the temperature at which the apparatus operates, the apparatus being such that it will operate. at one of a plurality of pre-determined temperatures consequent upon receiving the plug-in module for that predetermined temperature.

The said apparatus is preferably glue melting and dispensing apparatus as indicated above but it may be other equipment if desired.

Embodiments of the invention will now be described solely by way of example and with reference to the accompanying drawings in which: Figure 1 shows a known electronic temperature control circuit as used in known glue guns; Figure 2 shows an electronic temperature control circuit of the invention; Figure 3 shows in detail an electronic control circuit as applied to a glue gun; and Figure 4 shows a modification to the control circuit shown in Figure 3.

Referring to Figure 1, there is shown electronic temperature control apparatus 2 which utilises a thermistor 4 to sense glue reservoir temperature in a glue gun (not shown). The thermistor 4 is such that its resistance decreases with increasing temperature. As shown, the thermistor 4 is used in a voltage comparator circuit which includes voltage comparator means 6, to. produce a temperature dependent voltage which is compared with a fixed reference voltage.

When the sense voltage is below the reference voltage,the voltage comparator means 6 provides no output signal.

This occurs when the temperature of the thermistor 4 is greater than the required temperature. However, when the sense voltage is greater than the reference voltage, the voltage comparator means 6 provides an output signal that via suitable amplifiers (not shown) energises a heater (not shown) of the glue gun. This occurs when the temperature of the thermistor 4 is less than the required temperature.

The required temperature, i.e. the set temperature, is achieved by adjustment of a variable resistor 8. Due to the very wide production tolerances of the thermistors 4 and the variable resistors 8, there is never a fixed correlation between the angle set on the variable resistor 8 and the resulting control temperature of the glue gun. The net result is that each and every operating temperature required for a particular application must be set for each individual glue gun by direct measurement of the glue gun chamber temperature and adjustment of the temperature setting variable resistor 8.

The majority of customers do not have the available equipment or the time to effect the required direct adjustment and so the glue guns need to be returned to the suppliers, and this is time consuming and inconvenient.

Referring now to Figure 2, similar parts as in Figure 1 have been given the same reference numerals for each of comparison and understanding. In Figure 2, the electronic temperature control apparatus 2 uses a simple and novel technique whereby the effect of components tolerances is eliminated, thereby providing a unique correlation between a desired operating temperature and the setting resistance required to achieve this temperature.

Instead of using one fixed reference voltage for all temperatures as in Figure 1, Figure 2 illustrates how different reference voltages may be provided to exist across a required span of temperatures. These pre-determined voltages may be standard and hence identical for all manufactured examples of a particular glue gun. More specifically, in Figure 2, the thermistor 4 constitutes first resistance means which is such that its resistance varies with temperature and which is such that during operation of the electronic control apparatus 2, it causes the generation of a sense voltage. The illustrated thermistor 1O is a replaceable resistor 10 which has a unique value for each value of set temperature for all glue guns. The resistor 10 constitutes third resistance A means.

Second resistance means in the form of a resistor 12 is employed to calibrate the sense voltage at one single predetermined temperature to equate to a chosen reference voltage at the predetermined temperature. More specifically, the sense voltage generated by the thermistor 4 is calibrated at one and one only temperature to equate to the chosen reference voltage at that temperature. The resistor 12 acts as a calibration resistor which standardises all thermistors 4 down to a common value.

Referring now to Figure 3, there is shown a circuit for electronic temperature control apparatus 2 in the form of a practical embodiment as might be installed in a glue gun. More specifically, the circuit*-shown in Figure 3 demonstrates the technique of the invention as applied to a CA3059 or a CA3079 comparator and triac power driver integrated circuit.

In the integrated circuit 14, the reference voltage at pin 9 and hence the desired temperature, is set up by the potential division action of resistor R6 and the parallel combination of resistors R2 and R1.

The resistor R1 constitutes third resistance means in the form of a plug-in temperature module 16.

If for any reason the plug-in module 16 is removed, the reference voltage rises due to the potential division of R6 and R2, and the glue gun temperature falls to the lowest possible operating temperature. This represents a significant safety feature.

In Figure 3, the temperature variable sense voltage at pin 13 is generated by the potential division action of the parallel combination of the resistors R4 and R5 and the resistance of the thermistor 4. The manufacturing tolerance of the thermistor 4 is cancelled out, on testing of the assembly, by the adjustment of the resistor R5 which is the calibration resistor and which corresponds to the resistor 12 in Figure 2. The resistor R5 is adjusted to give a specific sense voltage at the pin 13 at one particular temperature of the thermistor 4.

In the arrangement shown in Figure 3, the following plug-in resistances will set any glue gun to the temperature shown.

Plug in resistance Set temperature value K ohm DeRree Celcius NONE 160 24 180 12 195 5.6 220 Since close tolerance resistors (1%) are cost effectively available, then any temperature within the designed band can accurately be defined.

Figure 3 shows somewhat schematically how the thermistor 4 is arranged to operate a heater 18 in a glue chamber 20 of the glue gun. The apparatus 2 shown in Figure 3 also includes a triac 22 and a capacitor C1 which are connected as shown.

In a modification of the invention as shown in Figure 4, a further feature may be included to allow the glue gun to be set up at temperatures below the operating band. Normally, with no plug-in module 16 fitted, the greatest reference voltage and hence the lowest temperature is selected by the potential division action of the resistors R6 and R2, since the transistor TR1 is fully conducting (saturated) as a result of the current provided through the resistor R3. However, if a 3 pin plug-in module 16 is used as shown in Figure 4, pins 2 and 3 of the plug-in module 16 divert the current from the base of the transistor TR1, thereby causing the transistor TRI to switch off and isolate the resistor R2.The reference voltage is now generated by the potential division action of the resistors R6 and R1 only, and can therefore now be higher than the normal maximum value. The set temperature can now be set lower than the normal minimum value that is normally achieved when the plug-in module 16 shown in Figure 3 is not present.

It will be apparent from the description provided above with reference to the accompanying drawing that the illustrated electronic temperature control apparatus 2 is able to maintain the temperature of hot melt glue in the glue chamber 18 at the optimum wording temperature of the glue. The invention allows the temperature to be set to any number of defined levels by the use of the plug-in module 16. The invention entirely eliminates the need to physically measure the actual temperature of the glue in order to reset its operating temperature. Any glue gun may be set to a desired operating temperature by inserting a standard plug-in module 16.

It is to be appreciated that the embodiments of the invention described above with reference to the accompanying drawings have been given by way of example only and that modifications may be effected. Thus, for example, all the techniques described can be applied to circuits using basic integrated circuit comparators, combined comparators and driver devices (as shown in the drawings) or as discrete transistor implementations of voltage comparison. The electronic temperature control apparatus 2 can also be used to control the temperature of apparatus other than glue guns.

Claims (11)

1. Electronic temperature control apparatus comprising first resistance means which is such that its resistance varies with temperature and which is such that during operation of the electronic control apparatus it causes the generation of a sense voltage, second resistance means which calibrates the sense voltage at one single predetermined temperature to equate to a chosen reference voltage at the predetermined temperature, third resistance means for giving the chosen reference voltage, and comparator means for comparing the sense voltage and the reference voltage, the electronic temperature control apparatus being such that the third resistance means is in the form of a plug-in module whereby the electronic temperature control apparatus is settable at a plurality of defined temperature control levels by changing the plug-in module to one which will give the required defined temperature control level.

2. Electronic temperature control apparatus according to claim 1 in which the first resistance means is such that its resistance decreases with temperature.

3. Electronic temperature control apparatus according to claim 2 in which the first resistance means is a thermistor.

4. Electronic temperature control apparatus according to any one of the preceding claims in which the reference voltage is caused to rise if the plug-in module is removed, whereby the control temperature falls to a predetermined minimum operating temperature.

5. Electronic temperature control apparatus according to any one of the preceding claims in which the third resistance means is a three pin plug-in module which allows the electronic temperature control apparatus to be set up at temperatures below the predetermined minimum operating temperature.

6. Electronic temperature control apparatus substantially as herein described with reference to the accompanying drawings.

7. Apparatus when provided with the electronic temperature control apparatus as claimed in any one of the preceding claims.

8. Apparatus according to claim 7 and which is in the form of glue melting and dispensing apparatus.

9.- Apparatus according to claim 8 in which the glue melting and dispensing apparatus is a hot melt glue gun.

10. Apparatus comprising heater means and a plug-in module for varying the temperature at which the apparatus operates, the apparatus being such that it will operate at one of a plurality of pre-determined temperatures consequent upon receiving the plug-in module for that predetermined temperature.

11. Apparatus according to claim 10 and which is in the form of glue melting and dispensing apparatus.