GB2192997A - A portable presettable electrical power meter - Google Patents

Abstract

A portable electrical power metering unit in a casing (10) with input and output power cables (16, 18) includes integrated circuits (ICE,ICF) in the form of a presettable counter/register, the metering unit operating as an A.C. integrating ammeter and allowing a predetermined amount only of electrical energy to be supplied to a load via a relay (RL1) controlled (TR3, TR4) by the metering unit. The voltage across a series resistor (R1) is sensed (ICB,ICC) in a voltage to frequency converter arrangement (TR1, TR2, ICD). <IMAGE>

Description

SPECIFICATION A portable electrical power metering unit The present invention relates to a portable electrical power metering unit of the kind used to record the quantity of power delivered to a customer.

It is particularly applicable for use on boating marinas, caravans or camping sites at which electricity is available to the customers who intermittently use such sites. In the case of the boating marina, customers may use the marina for periods of time ranging from a weekend to perhaps six months or so a year depending upon the individuals specific requirements.

While the several methods at present in use may have a wide variety of applications, most of the present methods in use are vulnerable to abuse, and consequently there is much fraud with some of the common applications of this type of meter. The type of meter herein described provides a means of achieving greater fairness in the distribution of electricity, and also provides a margin of user convenience with its physical characteristics of small size, weight, and ease of application.

With respect to abuse, with known types of meters which are located at individual mooring sites on the pontoons around the marine harbour, it is easy enough for another yachtsman or unauthorized person to moor his boat alongside and connect himself up to another persons power point and thus effectively steal another person's electricity. Also with boats and yachts which frequently use different berths, one has to make a note of the meter readings at the time of arrival and leaving a particular berth, and this is all extra work for the harbour master and his staff, as they are having to continually check readings and make entries against their numerous yachting customers.

Jt is therefore an object of the present invention to overcome partially or wholly the above disadvantages of the present metering system in operation at most boating marinas around the country.

According to the present invention, there is provided a portable electrical power metering unit including: (a) a casing within which the electrical circuitry of the metering unit is housed; (b) input and output power leads for connection to the mails supply and a load to be supplied; (c) presettable means which enable a predetermined quantity of electrical power to be supplied to the load; and (d) means for enabling the quantity of power consumed to be readily read and reset when required.

Preferably the casing is of a cylindrical shape, with the input and output power leads articulating coaxially.

Preferably the casing is designed such that it can be an integral part of one or both power leads, so that it can be suspended on a semi taut cable, or left on the ground.

Preferably the casing is sealed in order to prevent the ingress of moisture and contaminants.

The present invention will now be described in greater detail by way of example with reference to the accompanying drawings, wherein: Figure 1 is a plan view of the housing for a preferred form of portable electrical power metering unit according to the present invention; Figure 2 is a side sectional elevation view of the housing showing the printed circuit boards on which the electrical components of the power metering unit are located together with the power cables entering and leaving the housing; and Figure 3 is an electrical circuit diagram of one preferred form of power metering unit.

Referring first to Figures 1 and 2, a casing 10 of generally cylindrical shape has tapered end sections 12 and 14. The input and output ends 16 and 18 of a power cable are clamped to the respective end sections 12 and 14 by means of clamping nuts 20 and 22. A removable water-tight cover 24 is provided in the central cylindrical part of the casing 10. Thus cover 24 can be removed by the customer, harbour master or other authorized person in order to read or reset the meter.

Supported within the central cylindrical part of the casing 10 are two printed circuit boards 26 and 28 one above the other. Electrical components 30 are soldered into the printed circuit boards 26 and 28. A mains transformer 32 is located towards the input end of the casing 10.

In an alternative form (not shown) the casing 10 may be formed as an integral part of either or both the input and output power leads 16 and 18.

Referring now to Figure 3, the electrical power metering unit includes the following components: (a) a transformer TRF1 (referenced 32 in Figure 2); (b) integrated circuits ICA to ICF; (c) transistors TR1 to TR4; (d) resistors R1 to R32; (e) capacitors C1 to C13; (f) diodes D1 to D12; (g) potentiometers RV1 to RV3; (h) a relay RL1/1; and (i) a battery B having a potential of 3.6 volts.

Out of the six integrated circuits shown, three, ICB, ICC and ICD are in the form of operational amplifiers having inverting and noninverting inputs.

Out of the thirteen diodes, D5, D8 and D13 are zener diodes.

The power for the unit is obtained by the transformer TRF1 which, in conjunction with standard rectifier, smoothing and regulating circuits, comprising diodes D1 and D2, resistor R30, capacitors C1, C2 and C6 and zener diode D5, provides plus and minus 15 volts DC. The positive supply is stabilized by an IC type regulator PICA, so that the positive rail remains stable for reasonable temperature and mains voltage fluctuations. It is used as a reference supply.

The unit operates as an A.C. type integrating ammeter. Two FET operational amplifiers ICB and ICC with good stability, and gain/bandwidth product are configured to operate, in conjunction with the transistor TRI and associated circuitry as a precision full wave rectifier, constant current sink and D.C. amplifier gain block. The signal developed across the series resistor R1, appears as an amplified, positive going signal at the base of the transistor Tor 1. This transistor circuit provides a voltage to current conversion. Feedback is applied around the front end circuitry in such a way that one feedback loop takes out the system errors. The use of wide band amplifiers, of the FET, type provides high conditional stability, and ensures that the circuitry is immune from local sources of R.F. interference.

The resistor R7 and the diode D8 clamps the base of the transistor TR1 in the event of overload, and ensures that the circuitry is free from latch up effects. The capacitor C7 is charged from the constant current source output from the transistor Tor 1. The operational amplifier ICD is configured to act as a comparator circuit with hysterisis.

When the capacitor has C7 charged to a specific negative value, the the transistor TR2 rapidly discharges it, the resistor R11 limiting the current to a safe value. The cycle thus repeats.

The frequency of the charge/discharge cycle is directly proportional to the value of constant current delivered by the transistor Tor 1.

The useful frequency range of this circuit lies between 500 Hz and 50 KHz; corresponding to a dynamic range of 100 to 1.

Operation at frequencies significantly in excess of 50 KHz cause excess non linearity, while frequencies much below 500 Hz cause poor resolution; considering the frequency of the mains being sampled is 50 Hz.

The potentiometers RV1 & RV2 are adjusted so that with no current passed through the series resistor R1, the outputs of the two operational amplifiers ICB and ICC are negative by about 10 volts, with respect to the 0 volt rail. The RV3 takes out the system error, adjustment is carried out with a known current passed through the series resistor R1, to produce a predtermined repetition rate. A quick check at two other known current values, one higher the other lower than the reference current are used to check the linearity.

The output from the comparator stage ICD is coupled through the resistor R15, this limits the current to a few microamperes when the mains is disconnected, to prolong battery life.

The integrated circuits ICE & ICF are counter ICs. They are configured to provide a variable count, this is determined by linking the appropriate output from the counter ICF.

The mains is connected to the load through the small power relay RL1. The transistors TR3 & TR4 are connected to drive the single pole relay unit RL1. The relay is turned on when the transistor TR3 is off. When the counter ICF produces the appropriate output, the transistor TR3 is turned on, starving the transistor TR4 of base bias, so turning the relay off and removing power from the load.

Under these conditions the unit is still powered from the mains; the battery B is only used as a memory backup when the mains is completely removed.

The final five outputs of the counter ICF are connected to a parallel resistor network comprising resistors R22 to R26, the special feature of this network is that it provides a ramp output (steadily increasing output) as the stored count increases. To maintain safety, the summing output from the resistor network and the Ov return are connected through 1M ohm resistors to the reader port. This limits the leakage current to a few microamperes.

Likewise, the resetting function is also accomplished through a 1M ohm resistor.

This is a satisfactory way of limiting leakage current for safety reasons, but is is of tantamount importance that the unit is connected to the mains and load with the live, neutral and earth wire connections checked, to ensure correct polarisation.

If this is done, the user only makes connection through the 1M ohm resistors to the neutral side of the mains and the system is then entirely safe.

The battery B is a lithium battery which provides power for the circuitry to memorize the energy consumed when the metering unit is disconnected from the mains.

The above described portable electrical power metering unit together with its associated cable can thus be used to supply metered power to a boat or yacht moored at a marina. The casing containing the metering unit is positioned at a convenient position on the deck of the boat or yacht, and the input end which is provided with an appropriate plug can readily be connected to a power socket, there being one power socket provided for every berth or mooring point around the marina.

In one preferred form of the portable electrical power metering unit, an impulse counter can be provided so that the user can readily check the power consumed on a daily or weekly basis. Moreover, for the purpose of charging the customer for the electricity con sumed, the harbour master or authorized person can read the meter at periodic intervals.

It will be appreciated from the above description that the electrical metering unit in fact only measures the current and not V1 cos 0 as in the case of a domestic watt meter. However, since it may normally be assumed that the mains voltage is constant within a few percent and that the electrical load of the boat or yacht is mainly resistive rather than inductive, so that cos (p will be approximately unity, measurement of the current is a very good approximation to the actual power consumed.

By using the integrated circuit counter ICF in conjunction with the relay RL1, the user is able to set for a predetermined amount of power to be supplied to his boat or yacht over a given period of absence. This ensures safety in the event of equipment failure or unauthorized access to the interior of the boat or yacht. Moreover, it can be set to seven different output ratings, can be reset to zero, and can be interrogated whilst operating to establish how much power has been consumed.

Accordingly, the above described electrical power metering unit has the following advantages: (a) It is reasonably accurate.

(b) It is cheap to manufacture.

(c) It is easy to install and use in practice.

(d) It is substantially fraud proof and is not susceptible to misreading due to radio or electro-magnetic radiation.

(e) It is capable of withstanding severe mechanical shock, and reasonable temperature cycling, without undue drift in reading accuracy.

(f) It is capable of remembering the stored value of power delivered to a user, for long periods (e.g. up to a year) with no mains power applied.

(g) It presents a minimum burden on the mains supply, to minimize internal heating, reduce the cost of running, and reduce the size of memory battery.

(h) It presents an easy means of reading, and resetting the unit to zero count, for dispensing a new quota of power.

(i) Finally, it is capable of operating with loads having nonlinear voltage/current characteristics, the circuitry being capable of taking this into account and still retain accuracy. In this respect, one of the worst load types would be that of half wave rectified battery charging equipment.

Claims (11)

1. A portable electrical power metering unit including: (a) a casing within which the electrical circuitry of the metering unit is housed; (b) input and output power leads for connection to the mains supply and a load to be supplied; (c) presettable means which enable a predetermined quantity of electrical power to be supplied to the load; and (d) means for enabling the quantity of power consumed to be readily read and reset when required.

2. A portable electrical power metering unit according to claim 1, wherein the casing is of a cylindrical shape, with the input and output power leads articulating coaxially.

3. A portable electrical power metering unit according to claim 1 or 2, wherein the casing is formed as an integral part of one or both power leads.

4. A portable electrical power metering unit according to any one of the preceding claims, wherein the casing is sealed against the ingress of moisture and contaminants.

5. A portable electrical power metering unit according to any one of the preceding claims, wherein the casing includes a removable watertight cover.

6. A portable electrical power metering unit according to any one of the preceding claims, wherein the casing houses printed circuit boards, electrical components soldered to said boards, and a mains transformer.

7. A portable electrical power metering unit according to claim 6, wherein said electrical components include integrated circuits and discrete elements.

8. A portable electrical power metering unit according to claim 7, wherein said integrated circuits include operational amplifiers, a regulator, and counters, arranged such that said metering unit operates as an A.C. type integrating ammeter.

9. A portable elecrical power metering unit according to claim 8, additionally including an impulse counter provided to enable the user to check the power consumption at intervals of time.

10. A portable electrical power metering unit according to claim 8, including a relay which used in conjunction with one of the integrated circuit counters enables a predetermined amount of power to be supplied over a predetermined period of time.

11. A portable electrical power metering unit constructed substantially as herein described with reference to and as illustrated in the accompanying drawings.