GB2126344A - Timer - Google Patents

Abstract

A timer for giving an accumulative indication of the time an electrical appliance has been drawing electrical current includes a digital watch circuit (26) driven through an electrical circuit (21- not shown) in response to the volt drop across a fuse (14) in a plug (11) for connecting the appliance to the mains. The watch will only be driven while the appliance is drawing current. <IMAGE>

Description

SPECIFICATION Timer This invention relates to a timer for giving an accumulative indication of the time an electrical appliance has been drawing electrical current, and according to the present invention, timing means is embodied in an electric plug for connecting the appliance to the mains.

Although it has been previously suggested that a timer is used to provide such accumulative indication such timers have always been connected between the appliance switch and its motor or other load, being responsive to current flowing through theload.

The present invention involves the novel concept of making the timing means responsive to a volt drop due to load current, and conveniently that volt drop is the drop across the conventional fuse in the electrical plug which fuse is conveniently externally accessable for replacement without opening the plug.

The time means conveniently comprises an electrical watch mechanism for example, a digital watch unit driven electrically by that volt drop after amplification, and the watch display can in one embodiment be seen at any time through a transparent cover, or in another embodiment can be examined from time to time after removal of an opaque cover.

The plug body may have a conventional base with an intermediate block containing the electrical circuit and the watch, and a cover over the part of the block where that circuit and watch are housed.

For such a small voltage as might appear across the plug fuse, for example of 20 millivolts, a high gain amplifier is necessary. Positive and negative power supplies of, say, 1.2 volts can be provided from the main connection to the plug through respective diodes and a volt drop circuit. The amplifier can have very low power consumption, but can produce an output in excess of one volt from an input of a few millivolts.

The timer embodied in the electric plug can be very simple, and accurate and reliable without requiring any access to the plug connections once the appliance leads have been initially connected.

The invention may be carried into practice in various ways, and one embodiment will now be described by way of example, with reference to the accompanying drawings, in which: Figure 1 is a rather diagrammatic exploded view of an electric plug embodying the invention; and Figure 2 is a diagram of the circuit contained in the plug of Figure 1.

The plug has a conventional base 11 with line and neutral pins 12 and 13 but it incorporates a fuse 14 which is accessable from the exposed face of the base 11 so that it can be replaced without unscrewing the plug. Leads 17 to an appliance 18 are taken from the usual internal contacts 15 and 16.

Alternatively the leads 17 can lead to a floating socket into which the standard plug on the appliance can be plugged.

Positioned over the base 11 is an insulating moulded block 19 embodying a circuit shown generally at 21 in Figure 2 and contained on a printed circuit board 20 in a recess in the block 14 with input connections 22, 23, and 24 connected respectively to the neutral contact 16the fuse contact 15, and a contact on the line 12. The contacts could be for example stainless steel spring loaded pointed contacts, or beryllium copper strips, or indeed any contact that can be easily made without a danger of corrosion or drop in voltage across the contact. The circuit incudes a digital watch mechanism 26, and the block 19 is covered by a cover 27 which may be opaque if the watch mechanism is not to be capable of being read at any time, or transparent if it is.

Screws 29 for holding the plug components together pass through countersunk holes 31 in the base 11 into threaded inserts (not shown), in the block 19 which are themselves threaded into sockets 32 embedded in the cover 27 which is thus not normally easily removable from the block 19. Once the leads 17 from the contacts 15 and 16 to the appliance 18 have been connected, it may not be necessary to have access to the interior of the plug again, and insulating stops 33 can be fitted over the heads of the screws 29 once the plug has been completed assembled. The fuse 14 can be replaced if necessary from the exposed face ofthe base 11 so that replacement of a fuse does not require removal of the screws 29.

The digital watch mechanism 26 embodies a small battery but that will require replacement very rarely and the cover 27 can be removed for that purpose only.

The circuit of Figure 2 embodies 4 amplifiers, 43, 44,45,46 connected to have very low power consumption but a high gain. The input is taken across the fuse 14 at the connections 23 and 24, and may, depending upon the rating of the appliance, be as low as 20 millivolts, although of course there will be no input to the amplifiers when -the appliance is switched off.

The amplifiers require positive and negative low voltage power supplies of 1.2 volts and these are derived from the mains through the connections 22 and 24 by way of a rectifying volt drop circuit 35, to contacts numbered 4 and 11 on the board 20.

A back-to-back pair of diodes 39 eliminate peak voltages appearing in the input to the amplifier 43.

The amplifier output is rectified at 41 and applied to the base of a transistor 42 which controls energisation of the watch mechanism 26, so that the watch mechanism only runs while there is an output from the amplifier and thus while the appliance is drawing load current to produce a volt drop across the fuse 14.

The display of the mechanism 26 will normally be blank and when it is desired to take a reading a pair of contacts 37 connected respectively to the base and the emitter of the transistor 42 can be shorted to cause the display to appear. One way of doing that is to connect an encapsulatd reed switch across the contacts 37, so that the display can be made to appear by applying a magnet to that switch.

The sensitivity of the device may be limited if the fuse 14 is a large fuse, of say, 13 amps rating, and it may be possible to increase the sensitivity by replacing that with a fuse with a smaller rating. A special arrangement may be needed for a device which only needs a few watts to run, but needs a 13 amp switch for starting.

It will therefore be appreciated that the plug constitutes an interval timer for timing progressively the period over which the appliance 18 is actually switched on by virtue of the fact that the timer is responsive to the volt drop produced across the fuse by the load current. In spite of the fact that that voltage is very small, a high gain amplifier is used to provide sufficient voltage to control the transistor 42 whereas the power supply to the amplifier is derived from the mains through an appropriate voltage dropping device.

If the appliance did not need a fuse, a low value resistor could be used instead in its place.

For industrial use the plug could be a 3 phase plug.

Figures 3 and 4 show further details of the plug.

Claims (9)

1. A timer for giving accumulative information of the time an electrical appliance has been drawing current, including timing means embodied in an electric plug for connecting the appliance to the mains.

2. Atimer as claimed in Claim 1 in which the timing means is driven by an input responsive to the volt drop across a component in the plug due to load current flowing in the component.

3. A timer as claimed in Claim 2 in which the volt drop is the drop across a fuse in the electric plug.

4. A timer as claimed in Claim 3 in which the fuse is externally accessable for replacement without opening the plug.

5. A timer as claimed in any of the preceding claims in which the timing means is an electrical switch mechanism mounted in the plug.

6. A timer as claimed in Claim 5 in which the watch has a display which can be seen at anytime through a transparent cover.

7. A timer as claimed in Claim 5 in which the watch has a display which can be examined after removal of an opaque cover.

8. A timer as claimed in any of the preceding claims in which the plug comprises a conventional plug base, an intermediate block containing an electrical circuit and a watch mechanism, and a cover over the block.

9. Atimerforgiving accumulative information of the time an electrical appliance has been drawing current constructed and arranged substantially as herein specifically described with reference to the accompanying drawings.