GB2176665A - Dual voltage power supply - Google Patents

Abstract

The supply arrangement comprises a triac TR for applying a.c. power to a load, such as heating elements 36, and a selector SWa, SWb for conditioning the arrangement for either of two voltage supplies to power the load. The selector determines that the triac TR conducts only on alternate half cycles of the a.c. supply when the arrangement is conditioned for the higher of the two voltages and to conduct on both positive and negative half cycles when the arrangement is conditioned for the lower of the two voltages. The selector SWa, SWb is set in the factory before the unit is closed. The arrangement shown is embodied as a proportional temperature controller for an air-supply filter-heater. The triac TR is burst fired when a thermistor TH1 senses a demand for heat, each burst having a predetermined duration set by a circuit IC1. Heater 36 may be semi- conductor ceramic discs, preferably of doped barium titanate exhibiting a rapid rise in electrical resistance with increase in temperature, above the normal working temperature range. so that they are self-protecting. The discs have a honeycomb structure. <IMAGE>

Description

SPECIFICATION Dual-voltage power supply arrangement This invention relates to a dual-voltage power supply arrangement in or for electrical apparatus, particularly electric heaters in a line supplying compressed air for example.

Electrical apparatus are generally arranged to be powered from a fixed voltage supply, or if they are capable of being powered selectively from either of two voltage supplies the selection arrangements are either complex or bulky and typically will include a transformer.

In accordance with this invention, there is provided a dual-voltage power supply arrangement, in or for an electrical apparatus, the power supply arrangement comprising a triac or other bidirectional power switch for applying a.c. power to a load, and a selector means for conditioning the arrangement for either of two voltage supplies to power the apparatus, the selector means enabling the triac to conduct only on alternate half cycles when conditioned for the higher of the two voltages and to conduct on both positive and negative half cycles when conditioned for the other of the two voltages.

This dual-voltage supply arrangement may in particular be incorporated in a temperaturecontrolled electric heater, especially an electric heater with a proportional temperature control.

In these arrangements the triac is rendered conductive (for either alternate or both half cycles, depending which voltage supply is applied, as just mentioned) when a temperature sensor calls for heat. A particular use of the dual-voltage power supply arrangement is in a filter heater unit to be coupled into a line supplying compressed air for example.

The dual-voltage selection facility is of considerable advantage and is particularly effective and simple in the arrangements in accordance with this invention.

An embodiment of this invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a side elevation of a filter heater unit; Figure 2 is a section on the line ll-ll of Fig.

1; Figure 3 is a plan view of the filter heater unit; and Figure 4 is a circuit diagram of a dual-voltage power supply arrangement incorporated in the filter heater unit.

Referring to Figs. 1 to 3, there is shown a filter heater unit for filtering and heating a sup ply of compressed air. The unit comprises an aluminium head member 10 with an inlet port 12 and an outlet port 14 for the compressed air, on opposite sides of the unit. The lower side of the head 10 receives an aluminium bowl 16, these two members forming a screw coupling between each other and an 0ring seal 17 being provided between them.

The bowl 16 houses a cylindrical filger element 18 and a drain unit 20 at its bottom.

The upper end of the filter element fits, with the interposition of an O-ring 21, into a passage 22 which communicates with the inlet port 12. A tie rod 24 of the filter element is screw-coupled into the head 10 at 25. A Schrader Valve 26 is mounted in a passage through the bottom of the bowl 16.

An upper bowl 28, also of aluminium, is crew-coupled into the upper side of head 10, with an O-ring seal 29 between them. The heater is disposed within this bowl 28 and will now be described. An end cap 30 of the heater is fitted into an annular recess of the head 10, with an O-ring seal 31 interposed. A heater housing 32 seats with its rim on the end cap 30 and the rim of the bowl 28 seats on a radial flange 33 of the heater housing 32: thus when the bowl 28 is screwed home, it urges the end cap 30 against its seat in the head 10, via the heater housing rim. A sintered bronze disc 34 is positioned on the upper side of the end cap 30. The heater comprises three elements 36 and brass electrodes 38 and is held in place between internal ribs 39 of the heater housing and the disc 34, with an interposed O-ring 37.An electrical cable 43 enters the top of the bowl 28 through a gland 40 and passes through the top of the heater housing, where it is sealed by silicone rubber 41. The cable includes power conductors 42 which are connected to electrodes 38, and also conductors which make circuit with a thermistor TH1 positioned adjacent the top of the heater housing.

Compressed air entering the inlet port 12 passes into the centre of the filter element through its upper end, then passes radially through the filter element to the clearance space within the lower bowl 16, around the filter element. Then the air passes through a passage 11 in the head 10 to the end cap 30, through the apertures in this end cap and then through the sintered disc 34 to the heater elements. These elements are each in the form of a disc of semi-conductor ceramic material provided with hexagonal apertures, in the manner of a honeycomb. Thus the air passes upwards through these apertures and through the electrodes, which are ring-shaped, to arrive at the top region of the heater housing.The air then flows through apertures in the heater housing 32 and into the clearance space between this housing and the upper bowl 28, and finally through a passage 13 in the head 10 to the outlet port 14.

The heater elements preferably comprise doped barium titanate exhibiting a rapid rise in electrical resistance, with temperature, above the normal range of temperature for which the unit is to be used. Thus, if the heater ele ments reach an excess temperature they will rapidly become of high electrical resistance and self-limit the current which they draw, and prevent any risk of burning-out.

The filter heater unit is provided with a pair of mounting brackets 44 (Figs. 2 and 3) and a further bracket 45 mounts a control box 46 at the front of the unit. The front face of this control box has a temperature selector knob 47 and L.E.D. indicator lamp L1 and L2.

The control box 46 houses the dual-voltage power supply arrangement and the circuit of this is shown in Fig. 4. Elements 36 are assembled in series but connected in parallel with each other and in series with triac TR.

The indicator lamp L2 is connected in series with a resistor R4 and across the heater elements and a protective diode D3 is connected across the L.E.D. L2. The control circuit for the triac comprises an integrated circuit IC1, which is an SL441C chip marketed by Plessey Semiconductors. A regulated voltage supply is obtained from pin 5 of IC1 and thermistor THI is connected from pin 5 to a variable tapping point on a potentiometer VR1 (under control of selector knob 47) which is in series with a resistor R6. This tapping point is connected to pin 8 of IC1.1C1 includes a ramp generator which produces a voltage which rises linearly for a period of time then drops.

This time period is set by a resistor R3 and a capacitor C4 (which are connected in series from pin 3 to ground with their junction connected to input pin 7). Pin 8 provides an input to a comparator within It 1. When the temperature as sensed by the thermistor is low (as at switch-on), a high potential appears at pin 8 and is compared with the ramp voltage: whilst the thermistor voltage is higher than the ramp voltage, then IC 1 will produce a voltage pulse of predetermined duration on pin 4 to energise the gate of the triac, causing a burst of the a.c. mains to be applied to the heaters 36: successive such bursts will similarly be caused until the temperature (set by knob 47) is reached.IC1 acts to ensure that the mains voltage is connected to the heaters at the zero crossing point so as to avoid the possibility of spurious firing of the triac due to radio frequency interference. L.E.D. L1 is connected in series with a resistor R5 from pin 5 of IC1 and indicates when power to the circuit is switched on.

A double pole switch with contacts SWa, SWb serves to condition the circuit for either a 240 volts or 110 vols a.c. mains supply.

For a 240 volts supply, contact SWb is open and capacitor C1 disconnected: resistor R2 and capacitor C2 provide the correct voltage (14 volts) to pin 3 of IC1. Contact SWa is also open so that the gate drive to the triac (from pin 4 of IC1) is via an adjustable resistor VR2. This adjustable resistor is set in the factory to limit the current such that the triac will only fire (when passing through zero voltage) on positive half cycles of the mains. For a 110 volts supply, contacts SWa and SWb are both closed: capacitor C1 is now in parrallel with capacitor C2 to provide the 14 volts required on pin 3, from the 110 volts supply.

Adjustable resistor VR2 is now short circuited allowing full gate drive to the triac so that this now fires on both positive and negative half cycles of the mains supply.

The circuit provides for a proportional temperature control of the heater elements. The filter heater unit is able to supply compressed air of constant heat (as selected by the control) regardles of air pressure flowrate and the circuit acts with reduced hysterisis and is therefore much improved as compared with known units. The facility to select for either of 240 or 110 volt supply lends particular versatility to the unit and the selection arrangements are simple and effective. The unit will thus be the same in all respects (including the heater elements) and simply the changeover switch SWa, SWb will be set within the factory before the control box 46 is closed and the unit despatched.

Instead of providing a combined filter-heater, the filter maybe provided in a separate unit, the air passing first through the filter unit and then through the heater unit (which then incorporates no filter other than preferably the sintered disc 34).

Claims (9)

1. A dual voltage power supply arrangement, in or for an electrical apparatus, the power supply arrangement comprising a bidirectional power switch for applying a.c. power to a load, and a selector means for conditioning the power supply arrangement for either of two voltage supplies to power the apparatus, the selector means enabling the bidirectional power switch to conduct only on alternate half cycles of the a.c. power when the power supply arrangment is conditioned for the higher of the two voltages and to conduct on both positive and negative half cycles when the power supply arrangement is conditioned for the lower of the two voltages.

2. A dual voltage power supply arrangement as claimed in Claim 1, comprising an electrical circuit fed from the a.c. power, the selector means including a selector switch controlling the feed to said electrical circuirt such that it is independent of whether the power supply arrangement is conditioned for (and receives) the higher or lower of the two supply voltages.

3. A dual voltage power supply arrangement as claimed in Claim 1 or 2, in which the selector means includes a selector switch controlling a current feed to said bidirectional power switch according to whether the power supply arrangement is conditioned for the higher or the lower of the two supply voltages.

4. A dual voltage power supply arrange ment as claimed in any preceding claim, in which the bidirectional power switch comprises a triac.

5. A temperature-controlled electric heater, comprising an electrical heating element and a dual voltage power supply arrangement for the heating element, said power supply arrangement being as claimed in any preceding claim.

6. A temperature-controlled electric heater as claimed in Claim 5, including a temperaturedependent resistance controlling the (or an) electrical circuit which in turn controls said bidirectional power switch according to the temperature sensed by said resistance.

7. A temperature-controlled air supply heater, comprising a housing having an air inlet and an air outlet, and further comprising an electric heater as claimed in Claim 5 or 6, the electrical heating element being disposed within the housing and in an air passage extending between said air inlet and said air outlet.

8. A temperature-controlled air-supply heater as claimed in Claim 7, serving as a filter-heater and comprising an air filter disposed within said housing and air passage.

9. A dual voltage power supply arrangement substantially as herein described with reference to Fig. 4 of the accompanying drawings.