GB2248319A

GB2248319A - Transportable ac power supply apparatus.

Info

Publication number: GB2248319A
Application number: GB9021150A
Authority: GB
Inventors: Graeme William Younger
Original assignee: Individual
Current assignee: Individual
Priority date: 1990-09-28
Filing date: 1990-09-28
Publication date: 1992-04-01
Also published as: GB9021150D0; AU8541491A; WO1992006531A1

Abstract

A power supply for generating a facsimile of the ac mains is powered by a dc battery in a portable unit. The invention combines a triple dependence upon the role of a flyback-connected transformer (a) in introducing dc stabilization control by energy packaging in the buffer transfer (b) in using the hysteresis feature as a barrier obstructing reverse energy flow on overload and its retardation effect to allow time for sensing and correction of overload and (c) in using the electronic control of a kilohertz pulsing action of the transformer to form the ac output wave.

Description

TRANSPORTABLE AC POWER SUPPLY APPARATUS FIELD OF INVENTION This invention relates to transportable ac power supply apparatus in which the primary power is a dc source, typically comprising precharged electric batteries, and the output is of ac form, typically a facsimile version of the standard industrial or domestic ac mains power supply.

In one form the invention relates to a portable unit in which a heavy duty electric storage battery is connected by solid-state electronic circuitry, including an inverter, to produce ac at mains voltage and frequency, thereby allowing hand tools normally powered by connection to the mains supply to be used in circumstances where a wired connection to a mains supply is not feasible.

BACKGROUND OF INVENTION Portable power units for generating ac from battery sources have hitherto been subject to certain operational difficulties, arising from the problems of controlling what can be high dc current demands on the supply system. Several known products which have reached the market place and have high power specifications have proved unreliable due to unpredictable load conditions, which arise particularly where several handtools or electrical appliances are in use simultaneously.

Attempts to make the system sufficiently robust to withstand higher load conditions adds unwanted weight and cost. However, by astute and innovative design as provided by this invention, it has been found possible to provide apparatus which has a very high reliability in coping with adverse load conditions and preventing consequential failure.

The applicant is unaware of any prior art disclosure dealing with the problems of overload detection in the ways covered by this invention.

BRIEF DESCRIPTION OF INVENTION According to the invention, transportable ac power supply apparatus comprises a dc storage battery as a source of power, an oscillator determining the ac supply frequency, main power driver means controlled by a regulator circuit for admitting a regulated current to flow from the battery and forming, with the oscillator, inverter circuit means for converting the regulated current flow into an ac power supply at the oscillator frequency, characterized in that the regulator circuit incorporates a pulse generator controlling regulating power driver means to permit a sequence of current pulses to flow from the storage battery to the primary winding of a flybackconnected transformer, the secondary winding of the flyback-connected transformer being connected to feed power into a rectifier-filter circuit to supply a regulated de power output which is connected to the main power driver means and is operative in conjunction with the oscillator to determine the ac power delivered by the apparatus.

Typically the pulse generator may operate at between 10 kHz and 20 kHz in supplying the flyback-connected transformer.

According to a feature of the invention, the flyback-connected transformer is connected to feed power into a rectifier-filter circuit to supply a regulated de power output which is connected to provide a stabilized power source for electronic circuit components of the apparatus.

According to another feature of the invention, the oscillator determining the ac supply frequency is a two-phase oscillator operative to produce two square wave signals in antiphase, said main power driver means comprising two drivers which are respectively controlled by these antiphase signals and connected to permit current flow in separate halves of a centre-tapped primary winding on a power transformer during different half cycle periods of operation at the ac power supply frequency, the secondary of which transformer supplies the output ac power, whereby to allow current to be drawn from the dc storage battery via a connection to the transformer primary winding at the centre-tapped position, and the apparatus is characterized by having overload control circuitry which includes driver interrupt means, preset signal reference means and a current sensor responsive to electric current drawn from the battery to produce a signal which is compared with a reference determined by the preset signal reference means and which, if sensed as excessive, activates the driver interrupt means to switch off, for the remainder of the related half cycle period, the driver then controlling the supply of current from the battery.

According to yet another feature of the invention, the regulator circuit comprises a fixed frequency sawtooth oscillator which determines the pulse frequency and a controlled voltage reference source connected to a pulse-width modulator and operative to produce width-modulated pulses which control the regulating power driver.

According to a further feature of the invention, the controlled voltage reference source serves to monitor the state of the dc storage battery and operates to inhibit output power supply should the battery voltage be too low, the apparatus further comprising a timer circuit, a comparator circuit and latch circuit, the timer circuit being connected to start upon closure of a battery switch which establishes the battery supply and initiates circuit operation, the timer circuit being also operative to hold the latch in its reset state for a predetermined period during which period the comparator circuit compares the battery voltage with that of the voltage reference source to set the latch circuit at the end of the predetermined period if the battery voltage is then below the reference voltage, the power output supply being inhibited during the predetermined period and thereafter if the latch is in its set state.

According to another feature of the invention, the apparatus further comprises a clamp circuit operative to protect against power feedback from a load circuit connected to receive the ac power supplied by the apparatus, this clamp circuit including a voltage reference circuit, a summing amplifier circuit, and a power amplifier circuit which in its power-on state acts as a clamp limiting the voltage on the drivers, the operation of the clamp circuit being dependent upon push-pull antiphase excitation of the separate halves of the centre-tapped primary winding of the power transformer, whereby, during the halfcycle periods when a driver carries no current, there is an induced voltage between the centre-tapped position and the end connection between the primary winding and that driver, this induced voltage being normally an antiphase version of the normal driver voltage but being abnormally related to excessive reverse voltage on the secondary winding of the power transformer, the controlling voltage signal from such an end connection and a voltage reference signal from the voltage reference circuit providing inputs to the summing amplifier circuit and the latter circuit operating to hold the power amplifier in an off state when the controlling voltage signal is less than the voltage reference signal, but in the alternative event for an excessive controlling voltage signal causing the power amplifier to be in its power on state.

According to an alternative feature of the invention, the apparatus further comprises a clamp circuit operative to protect against power feedback from a load circuit connected to receive the ac power supplied by the apparatus, this clamp circuit including a zener diode, a resistor network and a HEXFET field effect transistor which in its power-on state acts as a clamp limiting the voltage on the drivers, the operation of the clamp circuit being dependent upon push-pull antiphase excitation of the separate halves of the centre-tapped primary winding of the power transformer, whereby, during the halfcycle periods when a driver carries no current, there is an induced voltage between the centre-tapped position and the end connection between the primary winding and that driver, this induced voltage being normally an antiphase version of the normal driver voltage but being abnormally related to excessive reverse voltage on the secondary winding of the power transformer, the controlling voltage signal from such an end connection being connected across the HEXFET transistor as bridged by a series connection of a series gate resistor of the transistor and the zener diode, with the intermediate voltage of such series connection providing the gate control voltage of the HEXFET transistor, whereby in the event that the controlling voltage signal rises sufficiently to overcome the zener breakdown voltage the gate voltage of the HEXFET transistor rises to turn on the transistor and thereby clamp the controlling voltage to the level set by the zener diode and the current gain of the HEXFET transistor.

According to yet a further feature of the invention, the main power driver means comprise two drivers each of which is a circuit including a low impedance power driver feeding series gate resistors of a plurality of parallel-connected HEXFET transistors, whereby, if an HEXFET transistor develops a gate drain short to cause its gate voltage to rise excessively, the adverse effects are restricted to that transistor.

According to an aspect of the invention, transportable power supply apparatus comprises a dc storage battery as a source of power, a pulse generator determining a pulse frequency and controlling the generation of the rate at which current pulses flow from the storage battery, a flyback-connected transformer having its primary winding supplied by said pulses and its secondary winding feeding power into a rectifierfilter circuit to control the supply of output power from the apparatus at a regulated voltage.

Apparatus according to this aspect of the invention may comprise a dc storage battery as a source of power, a first oscillator determining an ac supply frequency, a second oscillator determining a pulse frequency and controlling the generation of a multiplicity of current pulses flowing from the storage battery during each half-cycle period of the ac supply frequency, and a flyback-connected transformer having its primary winding supplied by said pulses and its secondary winding feeding power into a rectifier-filter circuit to supply a regulated dc power output connected to determine the ac power delivered by the apparatus under the control of the first oscillator.

The apparatus may further include main power driver means controlled by a regulator circuit for admitting a regulated current to flow from the battery and forming, with the first oscillator, inverter circuit means for converting the regulated current flow into an ac power supply at the oscillator frequency, wherein the first oscillator determining the ac supply frequency is a two-phase oscillator operative to produce two signal waveforms in antiphase, said main power driver means comprising two drivers which are respectively controlled by these antiphase signals and connected to permit current flow in separate halves of a centre-tapped primary winding on a power transformer during different half cycle periods of operation at the ac power supply frequency, the secondary winding of which transformer supplies the output ac power, whereby to allow current to be drawn from the dc storage battery via a connection to the transformer primary winding at the centre-tapped position, the apparatus being characterized in that overload control circuitry includes driver interrupt means, preset signal reference means and a current sensor responsive to electric current drawn from the battery to produce a signal which is compared with a reference determined by the preset signal reference means and which, if sensed as excessive, activates the driver interrupt means to switch off, for the remainder of the related half cycle period, the driver then controlling the supply of current from the battery.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 and Fig. 2, in combination, show a schematic circuit of apparatus incorporating the invention.

Fig. 3 shows in greater detail a component circuit of Fig. 1.

Fig. 4 shows a clamp circuit which can be used with the circuit of Fig. 2.

Fig. 5 shows an alternative heavy duty version of a clamp circuit which can be used with the circuit of Fig. 2.

DETAILED DESCRIPTION OF INVENTION Referring to Fig. 1, an electric storage battery 1 is connected via a switch 2 to supply power to an inverter circuit 3, which is the block component shown in more detail in Fig. 3. The output from inverter circuit 3, supplied along leads X-X provides the supply service input to the electronics of the system as a whole. Thus there are connections (not shown) by which the supply from X-X is fed to the components of the circuits depicted in the drawings. The dc supply service input fed by leads X-X is a regulated supply generated from the battery power by the inverter circuit 3. The main battery supply along leads Z-Z provides the main input to the circuit depicted in Fig. 2, the latter providing the eventual output of the apparatus from the secondary winding of power transformer 4.Note also that the supply along leads Z-Z constitutes the dc input denoted by the plus and minus symbols in Figs. 4 and 5.

Collectively, these Figs. 1, 2 and 3 show a circuit embodiment of the invention in a transportable ac power supply apparatus. In operation, when the system is turned on by closing battery switch 2, the inverter circuit 3 supplies the systems power input along leads X-X, which is a smoothed version of a pulse-width modulated signal generated by inverting the main dc supply from the battery. An inversion frequency of between 10 and 20 kHz generates pulses which are smoothed to provide the dc system power. Once the system supplies are activated in this way the output signal along the leads X-X initiates a timing action by timer circuit 5, which in turn holds a latch circuit 6 in its reset position until the time-out condition of the timer circuit occurs.Meanwhile a comparator circuit 7, responsive to the incoming supply direct from battery 1 via switch 2, compares the voltage of the battery with that produced by a voltage reference circuit 8, as predetermined by an adjustable setting. This comparator circuit 7 is connected to control the latch circuit 6 and operates to set this latch circuit in the eventuality that the battery voltage is below the predetermined level once the time-out condition is reached.

The outputs supplied by latch circuit 6 and timer circuit 5, denoted Y-Y are supplied to both logic blocks 9 in Fig. 2 (see the two arrowed lines on each block) and operate to inhibit the function of these blocks if either the latch circuit 6 is in the set position or the timer circuit 5 has not reached the time-out condition.

As so far described the apparatus is protected under start-up conditions when the load on the battery is large. This normally causes a drop in voltage which would, in turn, cause the power devices to draw excessive power if the drive stage signals are not maintained during this initial period. The circuit described by reference to Fig. 1 defines the minimum voltage as its first priority before allowing the circuit to deliver current to any load. The system is therefore monitored by the voltage setting of voltage reference circuit 8. If the state of the battery is not adequate to meet the performance requirements, then the system senses this discrepancy and locks off so that it signals a problem and may be reset after the problem is cured.

A similar safeguard feature comes into operation if the load is excessive. In the apparatus being described this excessive load condition is detected by a current sensor 10, which is shown in Fig. 2 and is a high speed Hall effect sensor, positioned to measure the current in the primary winding of power transformer 4. The battery supply leads Z-Z are connected separately, one such connection being via this sensor 10 to the centre-tap position on the transformer primary winding and the other connection being via the power lines feeding two power driver circuits 11. These are regulated in pushpull fashion to supply currents in antiphase to the separate halves of the primary winding of transformer 4.

The ac power supply frequency of the apparatus derived from the secondary winding of transformer 4 is determined by an oscillator circuit 12, which is set to operate at the required power frequency, such as 50 Hz or 60 Hz, depending upon the usual standard mains frequency locally applicable. This oscillator circuit is a two-phase oscillator producing square wave signals in antiphase, each phase of which provides the controlling input to a respective one of two logic blocks 9. These, in turn, control the power driver circuits 11.

These logic blocks may be inhibited by the signal inputs on leads Y-Y, as already explained, and also by a control asserted via the current sensor 10. The signal produced by this sensor, which is a measure of the current being fed into the transformer 4, is compared against a preset reference by comparator circuit 13. The reference circuit 14 is preset to produce a signal corresponding to the required reference current value.

By using the high speed Hall effect sensor the current can be sensed and the circuit protected before a full half cycle period of the output is completed. As the circuit operates in push-pull mode, this means that the next half cycle starts from normal and will also be discontinued if the current is excessive. An added feature not shown in the drawings can be the provision of a circuit which responds to the sensing of rapid rise in current, indicative of a short-circuit.

This latter event can be signalled by a visible light emitting diode display.

Conventional systems usually operate by relying on controlling the entire oscillator from the power input. Compared with such prior art systems, the subject invention has the advantage of providing apparatus which not only responds faster under fault conditions, but performs diagnostic checks on the new status of the output as part of the start-up operation.

In the circuit shown in Fig. 2, in the absence of a signal from comparator circuit 13, the outputs from the oscillator 12, which are square waves produced in antiphase, pass unmodified to the driver circuits 11. The outputs from the driver circuits are on-off currents, also in antiphase, and these power the power transformer 4 to cause the secondary, via the turns ratio of the transformer windings, to develop an output, at the 50 Hz or 60 Hz frequency and at a standard voltage, such as 240 V or 120 V.

For one phase, or one half-cycle period, at the start of the driver-on period, the current in one half of the transformer winding rises at a rate determined by a load (not shown) connected to the secondary winding. As already explained, if this load current is excessive then the Hall effect sensor will come into operation to protect the system.

The current in the related half of the primary winding of transformer is powered by a voltage and, although during this half-cycle period, the other half of the primary winding is not receiving current from its driver circuit 11, there will be a similar voltage signal of opposite polarity induced in this other section of the winding. This induced voltage can be used to provide further security against adverse overload conditions, as will be described later in this specification.

Referring now to Fig. 3, the inverter circuit 3 is shown to comprise a fixed frequency sawtooth oscillator 15 which provides input to a pulse width modulator 16, the latter being subject to control by a voltage reference circuit 17 and feedback from one of the output lines denoted by leads X-X. The pulse width modulator circuit 16 controls a power driver circuit 18 connected to supply dc pulses drawing on current from the battery 1 of Fig. 1, which pulses are fed to the primary winding of a flyback-connected transformer 19. The secondary winding of transformer 19 feeds its output to the rectifier-filter circuit 20 to produce the internal dc supply supplied as output along leads X-X.

In operation, the pulse train produced by this device injects, for each pulse, a controlled amount of energy into the core of the transformer and the energy stored in the core is released as output as each pulse subsides. The advantage of this conversion of d.c. from the battery, first by inversion to pulse form and then back to d.c. by rectification and filtering, before that new d.c. form is used by the system electronics. This means that a wide range of load and battery conditions can be accommodated, without adversely affecting the quality of the a.c. output generated by the circuit of Fig. 2.

The circuit of Fig. 3 operates by responding to the feedback in such a way that if the output voltage drops below its preset level, as determined by the setting of circuit 17, the width of the pulses is increased. In turn, this increases the amount of energy transferred via the transformer for each pulse and so restores the output voltage to its preset level.

A particular advantange of this circuit is that the battery supply voltage can be greater or smaller than the d.c. voltage supplied via the leads X-X without altering circuit connections, a feature which adds to the versatility and the reliability of the apparatus as a whole.

The remainder of this description concerns features which protect the apparatus against failure or excessive damage in the event of surge effects from load conditions when several power devices are being operated from the system in parallel and there is a load fault.

Failure of one such device can trigger heavy currents, surges and voltage spikes and can cause overloads affecting both the supply apparatus and the other power load devices if there are no special safeguards.

The subject invention lends itself to the incorporation of a particularly advantageous clamp circuit feature, which applies to Figs. 4 and 5. The common feature is the centre-tapped primary winding on the power transformer 4 and the fact that the two halves of the winding carry current in different half-cycle periods of the ac power supply frequency. In this description only the circuit associated with one phase is illustrated.

The special advantage of the clamp feature to be described is that the two separate phases of the power input to this transformer can serve to allow the dormant phase to be tested for adverse voltage conditions during the half-cycle period when the other phase is activated. The separate control of voltage and current eliminates the tendency found in some prior art systems for the apparatus to go into a runaway state under certain unpredictable load conditions. The voltage induced in the half-section of the primary winding in the dormant phase is normally an antiphase version of the normal operating voltage supplied via the active driver, but under adverse load conditions the induced voltage is more representative of the voltage surge on the loaded secondary winding of the transformer. It is this which can be sensed for the adverse overload condition.

To avoid substantial power being fed back into the transformer 4 from the load, so damaging the driver circuits, it is necessary to provide some form of voltage limiting protection.

Given that the supply voltage on the primary side of the transformer 4 is that across one half of the winding, the total voltage across the whole primary winding is double this, owing to induction in that half not in the current-on state. This square wave double-voltage signal appears between the end connections of the primary winding of transformer 4. The voltage induced between the centre-tap position of the primary winding and the end connection shown in Fig. 4 is fed back as input to a summing amplifier 21. A separate input is from the voltage reference circuit 22. When the feedback voltage is less than the reference voltage the output of the summing amplifier 21 is 'off' and the power amplifier 23 to which such output is supplied is in a high-impedance 'off' state.If the primary voltage rises above that of the reference the output impedance of power amplifier 23 rapidly lowers to absorb the power feedback from the load and so hold the primary voltage at the reference level.

Referring to Fig. 5, an alternative circuit feature is shown. This replaces the power amplifier 23 by the HEXFET transistor 24 with its gate resistor 25 and replaces the voltage reference circuit 22 by the zener diode 26. When the voltage at the end connection of the centretapped primary winding of transformer 4 rises above a threshold level the zener diode voltage rises and turns on the HEXFET transistor 24.

The result is that the voltage of the transformer is clamped to the level determined by the zener diode and the current gain of the HEXFET transistor.

To enhance the reliability of the circuit it is desirable to incorporate a plurality of such clamp circuit arrangements connected in parallel in the system. This is depicted by the showing of a second combination of HEXFET transistor, gate resistor and zener diode in Fig. 5. The principal advantage is that such a parallel connection of a multiplicity of clamp circuit provides a heavy duty clamp and so renders the system more immune to overload transients which could feed power back into the system.

Fig. 5 also shows how the power drivers 11 in Fig. 2 may be replaced by a circuit supplied via a low impedance power driver 27 feeding a plurality of HEXFET transistors 28, each having a gate series transistor, such as 29. In this case the protective features of the the operation become active if one HEXFET transistor 28 develops a gate drain short. The gate voltage then rises to the primary voltage on the transformer 4. This is a low impedance source and, as a result, the full voltage drop appears across resistor 29. Under this fault condition the associated HEXFET 28 burns out due to the excessive power dissipated within its case, but this leaves the remaining devices in the circuit intact, so that the system can remain operational.

Claims

1. Transportable ac power supply apparatus comprising a dc storage battery as a source of power, an oscillator determining the ac supply frequency, main power driver means controlled by a regulator circuit for admitting a regulated current to flow from the battery and forming, with the oscillator, inverter circuit means for converting the regulated current flow into an ac power supply at the oscillator frequency, characterized in that the regulator circuit incorporates a pulse generator controlling regulating power driver means to permit a sequence of current pulses to flow from the storage battery to the primary winding of a flyback-connected transformer, the secondary winding of the flyback-connected transformer being connected to feed power into a rectifier-filter circuit to supply a regulated dc power output which is connected to the main power driver means and is operative in conjunction with the oscillator to determine the ac power delivered by the apparatus.

2. Transportable ac power supply apparatus according to claim 1, wherein the flyback-connected transformer is connected to feed power into a rectifier-filter circuit to supply a regulated dc power output which is connected to provide a stabilized power source for electronic circuit components of the apparatus.

3. Portable ac power supply apparatus according to claim 1, wherein the oscillator determining the ac supply frequency is a two phase oscillator operative to produce two square wave signals in antiphase, said main power driver means comprising two drivers which are respectively controlled by these antiphase signals and connected to permit current flow in separate halves of a centre tapped primary winding on a power transformer during different half cycle periods of operation at the ac power supply frequency, the secondary of which transformer supplies the output ac power, whereby to allow current to be drawn from the dc storage battery via a connection to the transformer primary winding at the centre-tapped position, characterized in that overload control circuitry includes driver interrupt means, preset signal reference means and a current sensor responsive to electric current drawn from the battery to produce a signal which is compared with a reference determined by the preset signal reference means and which, if sensed as excessive, activates the driver interrupt means to switch off, for the remainder of the related half cycle period, the driver then controlling the supply of current from the battery.

4 Transportable ac power supply apparatus according to claim 1, wherein the regulator circuit comprises a fixed frequency sawtooth oscillator which determines the pulse frequency and a controlled voltage reference source connected to a pulse-width modulator and operative to produce width-modulated pulses which control the regulating power driver.

5. Transportable ac power supply apparatus according to claim 4, wherein the controlled voltage reference source serves to monitor the state of the dc storage battery and operates to inhibit output power supply should the battery voltage be too low, the apparatus further comprising a timer circuit, a comparator circuit and latch circuit, the timer circuit being connected to start upon closure of a battery switch which establishes the battery supply and initiates circuit operation, the timer circuit being also operative to hold the latch in its reset state for a predetermined period during which period the comparator circuit compares the battery voltage with that of the voltage reference source to set the latch circuit at the end of the predetermined period if the battery voltage is then below the reference voltage, the power output supply being inhibited during the predetermined period and thereafter if the latch is in its set state.

6. Transportable ac power supply apparatus according to claim 1, wherein the pulse generator operates at a frequency between 10 kHz and 20 kHz.

7. Portable ac power supply apparatus according to claim 3, further comprising a clamp circuit operative to protect against power feedback from a load circuit connected to receive the ac power supplied by the apparatus, this clamp circuit including a voltage reference circuit, a summing amplifier circuit, and a power amplifier circuit which in its power-on state acts as a clamp limiting the voltage on the drivers, the operation of the clamp circuit being dependent upon push-pull antiphase excitation of the separate halves of the centre-tapped primary winding of the power transformer, whereby, during the half-cycle periods when a driver carries no current, there is an induced voltage between the centre-tapped position and the end connection between the primary winding and that driver, this induced voltage being normally an antiphase version of the normal driver voltage but being abnormally related to excessive reverse voltage on the secondary winding of the power transformer, the controlling voltage signal from such an end connection and a voltage reference signal from the voltage reference circuit providing inputs to the summing amplifier circuit and the latter circuit operating to hold the power amplifier in an off state when the controlling voltage signal is less than the voltage reference signal, but in the alternative event for an excessive controlling voltage signal causing the power amplifier to be in its power on state.

8. Portable ac power supply apparatus according to claim 3, further comprising a clamp circuit operative to protect against power feedback from a load circuit connected to receive the ac power supplied by the apparatus, this clamp circuit including a zener diode, a resistor network and a HEXFET field effect transistor which in its power-on state acts as a clamp limiting the voltage on the drivers, the operation of the clamp circuit being dependent upon push-pull antiphase excitation of the separate halves of the centre-tapped primary winding of.the power transformer, whereby, during the half-cycle periods when a driver carries no current, there is an induced voltage between the centre-tapped position and the end connection between the primary winding and that driver, this induced voltage being normally an antiphase version of the normal driver voltage but being abnormally related to excessive reverse voltage on the secondary winding of the power transformer, the controlling voltage signal from such an end connection being connected across the HEXFET transistor as bridged by a series connection of a series gate resistor of the transistor and the zener diode, with the intermediate voltage of such series connection providing the gate control voltage of the HEXFET transistor, whereby in the event that the controlling voltage signal rises sufficiently to overcome the zener breakdown voltage the gate voltage of the HEXFET transistor rises to turn on the transistor and thereby clamp the controlling voltage to the level set by the zener diode and the current gain of the HEXFET transistor.

9. Portable ac power supply apparatus according to claim 3, wherein the main power driver means comprise two drivers each of which is a circuit including a low impedance power driver feeding series gate resistors of a plurality of parallel-connected HEXFET transistors, whereby, if an HEXFET transistor develops a gate drain short to cause its gate voltage to rise excessively, the adverse effects are restricted to that transistor.

10. Transportable power supply apparatus comprising a dc storage battery as a source of power, a pulse generator determining a pulse frequency and controlling the generation of the rate at which current pulses flow from the storage battery, a flyback connected transformer having its primary winding supplied by said pulses and its secondary winding feeding power into a rectifier-filter circuit to control the supply of output power from the apparatus at a regulated voltage.

11. Transportable power supply apparatus according to claim 10, comprising a dc storage battery as a source of power, a first oscillator determining an ac supply frequency, a second oscillator determining a pulse frequency and controlling the generation of a multiplicity of current pulses flowing from the storage battery during each half-cycle period of the ac supply frequency, and a flyback-connected transformer having its primary winding supplied by said pulses and its secondary winding feeding power into a rectifier-filter circuit to supply a regulated dc power output connected to determine the ac power delivered by the apparatus under the control of the first oscillator.

12. Portable power supply apparatus according to claim 11, including main power driver means controlled by a regulator circuit for admitting a regulated current to flow from the battery and forming, with the first oscillator, inverter circuit means for converting the regulated current flow into an ac power supply at the oscillator frequency, wherein the first oscillator determining the ac supply frequency is a two-phase oscillator operative to produce two signal waveforms in antiphase, said main power driver means comprising two drivers which are respectively controlled by these antiphase signals and connected to permit current flow in separate halves of a centre-tapped primary winding on a power transformer during different half cycle periods of operation at the ac power supply frequency, the secondary winding of which transformer supplies the output ac power, whereby to allow current to be drawn from the dc storage battery via a connection to the transformer primary winding at the centre-tapped position, the apparatus being characterized in that overload control circuitry includes driver interrupt means, preset signal reference means and a current sensor responsive to electric current drawn from the battery to produce a signal which is compared with a reference determined by the preset signal reference means and which, if sensed as excessive, activates the driver interrupt means to switch off, for the remainder of the related half cycle period, the driver then controlling the supply of current from the battery.