GB2245436A

GB2245436A - Solar-powered fluorescent lamp-drive circuit

Info

Publication number: GB2245436A
Application number: GB9012019A
Authority: GB
Inventors: Ram Shalvi
Original assignee: Solar Wide Industrial Ltd
Current assignee: Solar Wide Industrial Ltd
Priority date: 1990-05-30
Filing date: 1990-05-30
Publication date: 1992-01-02
Also published as: JP2600712Y2; JPH05506539A; WO1991019412A1; ATE128595T1; AU7952491A; HK1007232A1; EP0531438A1; DE69113461D1; DE69113461T2; GB9012019D0; JPH1153U; EP0531438B1

Abstract

A solar lamp drive circuit is arranged todelay start-up while a current is supplied to the lamp filament to warm up the filament. During operation, the back emf induced in the lamp is sampled, and if operating conditions deteriorate the current to the filament is reapplied automatically and the power supply to the lamp increased.

Description

Solar Lamp The invention relates to solar lamps.

The invention relates more particularly to a solar lamp drive circuit for using in controlling a lamp driven by solar cells. Such lamps are used for all forms of lighting applications especially exterior light around dwelling places. The lamps are arranged to be driven by re-chargeable batteries provided with solar panels so that they rely mainly or solely on receiving charging current generated by the solar cells during day light hours.

Commonly a fluorescent tube or lamp is used and driven via an electronic ballast. Battery energy is used to provide a high frequency high voltage power source which is applied between the terminals of the lamp. The ballast consists of a multi-phase transformer with a free-running oscillator.

At present the solar lamps that are known or have been proposed are relatively inefficient in terms of power consumption and the working life of the lamp tends to be limited due to poor control of the supply to the lamp.

According to the invention there is provided a solar lamp drive circuit for connection between a battery power supply and terminals of a fluorescent lamp including an oscillator and a step-up transformer arranged to supply high frequency power across the terminals, a switch connected to control power supply to one of the filaments of the lamp, and a timer for controlling the switch, the circuit being arranged to delay the application of power from the transformer to turn on the lamp at start up to allow the filament of the lamp to warm up.

The timer may be arranged to control the switch to remain closed until the lamp has been turned on for at least a few seconds.

The circuit may include a sampling circuit for monitoring the operating condition of the lamp arranged control the switch to turn on if the lamp operating condition deteriorates to a predetermined condition.

The oscillator output may be arranged to be reduced from a high starting level to a predetermined working level once the lamp has been ON for a few seconds.

The circuit may include a manually adjustable current regulator for altering the brightness of the lamp in use.

The sampling circuit may be arranged to increase the oscillator output if the lamp operating condition deteriorates to the predetermined condition.

A solar lamp drive circuit according to the invention will now be described with reference to the accompanying drawings which shows the circuit diagram.

Referring to the drawing, the circuit is shown consisting of blocks A, B, C and D. Broadly, block A is a battery condition detecting circuit which is arranged to control application of power to a lamp in a manner to prevent using the battery if its charge is too low. Block B is a circuit which monitors the outside light conditions to influence the application of power accordingly. Block C is the main part of the lamp drive circuit and block D is basically a charging circuit but shows the battery and as well as a solar panel.

Dealing with the circuits in more detail, block A includes a voltage comparator U1B with a resistor R20 connected to its output to hold up its output voltage. A latch is formed by gates U3C and U3D.

The output of the latch at U3C is set to high by an output from block B (see below). A battery voltage sampling network consists of resistors R21 and R22, diode Dll and capacitor Cll. The input threshhold of the sampling network is reduced after turn on by a feedback resistor R24. A master reference network consists of resistor R23 and a light emitting diode LED1 (which has a negative temperature coefficient).

A diode Dll compensates for thermal drift of LED1 and a capacitor Cll acts to decouple noise.

If the sampled voltage is below the master reference, the output of the comparator U1B goes low and sets the latch output to low. As a result the output of NAND-gate U3A goes high which, as seen below, causes the lamp to turn off. The resistor 24 acts to increase the sensitivity of the battery sampling network.

A hold circuit consists of diodes D4 and D7, a resistor R34 and a capacitor C3. When diode D4 is forward biassed and supplied from the block C (see below), the capacitor C3 fully charges almost instantaneously. Diode D7 will become forward biassed in turn and applies a voltage to raise an input of the comparator U1B. As a result the output of the comparator U1B is held high. In other words, the comparator U1B is disabled. It will remain disabled until the supply from block C is removed.

Capacitor C3 then discharges through the resistor 34 until the diode D7 is cut off and the comparator U1B is then again released to work normally. The hold circuit is designed to disenable the comparator U1B before the lamp becomes stable and for approximately 0.5 seconds thereafter.

In block B, the circuit is arranged to monitor the light intensity of the environment and cause the lamp to turn off when that intensity is above a certain level. A voltage comparator U1B with an hysteresis loop of a resistor R30 has its input connected between resistors R25 and R28. These resistors divide the output of diode LED1 to provide a smaller reference voltage for the comparator U1A. The voltage across a solar cell (see block D) is proportional to light intensity and this voltage is applied across a resistor R29 via a diode D9. The diode D9 will always be forward biassed unless it is very dark. If the voltage across the resistor R29 is higher than the reference voltage at the comparator U1A its output goes low. Capacitor C8 then discharges gradually through a resistor Rl9 until the latch circuit resets.As a result the output of NAND-gate U3A goes high and causes the lamp to be turned OFF.

By contrast, when the voltage across the resistor R29 is lower than the reference voltage, at the junction between the resistor R25 and R28, the output of the comparator U1A goes high, the capacitor C8 is quickly charged through resistor R13 and diode D12. As a result, the input connected to block B goes high. If the output of the latch circuit is also high, the output of the NAND gate U3A goes low, so that the lamp will be turned ON.

It will be noted that the time delay provided by the resistor Rl9 and capacitor C8 prevents the circuit responding to transient light changes as may be due to car headlights or torches in the vicinity of the lamp. It will also be noted that the circuit responds to the voltage across the solar cell and does not require or use a separate light intensity metering device or similar.

Referring to block C, gate U3B receives start signals, from block A, to switch on the lamp. A start delay circuit consists of a resistor R9, a capacitor C4 and a diode D2. Typically a 10 second start delay is provided by the delay circuit.

However, the lamp can be turned off instantly by including the diode D2. A retriggerable timer consists of a resistor R16 and a capacitor C5. On receipt of a start signal, the output of an invertor U2B goes low and the output of an invertor U2D turns on a transistor Q2. At the same time, capacitor C3 begins to charge up via diode D4 (see block 4) and capacitor C5 charges up via resistor R16. As a result current passes via the transistor Q2, which acts as a switch to turn the current ON and OFF, through one of the lamp elements to warm up the element. The output of inverter U2C is acting at this time to inhibit the battery condition detecting circuit (block A) and increase the power that can be supplied by the lamp drive circuit to a maximum.The inhibition is timed to be about double the start delay so that the filament current, through the transistor Q2, is applied while the lamp is OFF and lasts for sufficient time until the lamp is working at high capacity once it has been turned ON. At the end of the time period, the transistor Q2 is turned OFF and the power supplied to the lamp is adjusted to an optimum setting below a starting setting.

A sampling network consisting of resistors R4 and R5 monitors the back e.m.f. induced in the lamp. A low back e.m.f. indicates the lamp is stably at secondary breakdown region. A high back e.m.f. indicates the lamp is operating unstably. An invertor U2E is connected to the junction between the resistors R4 and R5. If the voltage at the junction rises above a predetermined threshhold, corresponding to non-stable lamp operationthat is, corresponding to an operating condition of the lamp deteriorating to a predetermined condition, the output of invertor U2E goes low and so the retriggerable timer and the transistor Q2 are turned ON immediately. Thus, a restart condition is repeated, increasing power to the lamp and applying a current to the one of the lamp filaments.

The sampling network thus automatically attempts to maintain the lamp in a stable working secondary breakdown condition; if the lamp is allowed to work under unstable conditions it will considerably shorten the life of the lamp.

The lamp is supplied with its main power by a close loop oscillator consisting basically of amplifiers U1D and U1C and invertor U2F, a mosfet Q1 and a resistor R18. The oscillator converts the battery d.c. supply to a high energy high frequency current through a step-up pulse transformer T1. Oscillations can be stopped by supply through an inventor U2A and a biassing network consisting of resistors R1, RlO and R6 and a capacitor C2. The duty cycles are determined by the d.c. input level of amplifier U1D supplied via a network consisting of resistors R31, R32, R7 and R8 and a capacitor C6. The time constant of the oscillator is determined by a filter consisting of a resistor R17 and a capacitor C10.A high d.c. input level or a greater capacitance for capacitor C10 results in a longer ON cycle, or vice versa. The OFF duty cycle is determined by the time taken for a capacitor C1 to charge from VSS to 1/3 VDD in the network including resistors R2 and R3.

A manual switch for altering the intensity of the lamp is provided across a resistor R31. When the switch is closed, the d.c. input level of the amplifier U1D is increased. When diode D3 is forward biassed as a result of the output of invertor U2C being high, as explained this condition applies during start up and when unstable operating conditions are detected, this results in an increase of three time normal. When the switch is positioned to short out the resistor R31 an increase of about 20% is provided.

The voltage across the resistor R18 is proportional to the mosfet source current which is transferred to the lamp by the transformer Tl. This is passed to a low pass filter formed by a resistor R17 and a capacitor C10 to decouple the high frequency component in the switching current. The output of the low pass filter provides an input to the amplifier U1D to maintain the ON cycles.

The start of operation of the oscillator is as follows: Once the start delay circuit and retriggerable timer have timed out, the output of invertor U2A goes low.

The voltage across the resistor R18 is initially zero which is feed to the amplifier U1D. The output of amplifier U1D is then at 1/2 VDD; the voltage across resistor R1 and R2 are equal and the output of amplifier U1C is low. Thus, the output of invertor U2F is high and so turns on the mosfet Q1. The current then applied to the resistor R18 is therefore increasing due to the inductance of the transformer T1. When the voltage across the resistor R18 is greater than the non-inverting value of the amplifier U1D, its output goes low and capacitor C1 discharges, the output of the amplifier U1C goes high, the output of the invertor U2F goes low and the mosfet Q1 turns off.The current to the resistor R18 is cut-off and so the output of the amplifier U1D goes high. The capacitor C1 charges up until it reaches 1/3 VDD, to above the non-inverting input level of the amplifier U1C and so the output of the amplifier U1C goes low.

Thus, the mosfet turns ON again and the cycle is repeated until the non-inverting input level of the amplifier U1C rises to 2/3 VDD. The oscillator is designed to reach stable oscillations at between about 24 to 33 kHz.

A mosfet is used to ensure fast switching time for the current applied to the lamp. To ensure rapid switching the supply to the invertor U2F is separated from the battery voltage by a diode D6. At the beginning of oscillators, the diode D6 is forward biassed and the voltage applied to the invertor U2F is o.6 volts less than the voltage applied to the invertor U2C. However, capacitor C7 gradually charges up through a diode D5 and a resistor Rll due to the back e.m.f. induced in the transformer T1. As a result the voltage applied to the invertor U2F will be approximately 2 volts higher than the voltage applied to the amplifier U1C. Higher VDD level applied to the mosfet Q1 tends to speed up the response time and hence improve the efficiency in working with the high frequency pulse transformer.

It will be seen that any high frequency spikes which are generated as the lamp is operating, especially during early stages after start up, and occurring in the back e.m.f. are used to charge up the capacitor C7 to improve the working efficiency.

In block D, a Schottkey diode D8 allows the solar cell BT2 to charge a battery BT1 at minimum loss due to its forward volt drop characteristics. The diode D8 blocks any battery discharge towards the light detector intensity circuit of block B.

An external charging circuit formed of resistors R14 and R15, a diode D10 and a zener diode ZD1 enable the solar lamp to be recharged from an external power adapter. Resistors R74 and R15 limit the charging current and the zener diode ZD1 protects the battery from over-charging. The diode 10 prevents the battery discharging via the external adapter.

The described arrangement therefore provides at turn on, a delay to allow the lamp filament to warm up. A filament current may be applied again during operations well after turn on if the lamp operating condition deteriorates to prevent the lamp operating at too low a current or unstably for too long which would result in shortening the i life of the lamp.

Although the described circuit is more sophisticated than currently used comparable solar lamp drive circuits and therefore somewhat more costly, the use of the circuit not only leads to much longer lamp operating life but also a greater efficiency in power consumption. The lamp can be run at an optimum power rating where secondary breakdown is stably occurring, and because the lamp operating conditions are continually monitored automatically, and those conditions automatically adjusted when required, no continuous extra power is needed to take account of unforeseen variations. In other words, the lamp need not be over-powered simply to cope with the worst predictable operating conditions. A cruder or simpler drive circuit is usually adjusted to avoid under supply in all environments, at the invariable cost of always supplying more power than is actually required and using up the battery charge more quickly than necessary.

Claims

1. A solar lamp drive circuit for connection between a battery power supply and terminals of a fluorescent lamp including an oscillator and a step-up transformer arranged to supply high frequency power across the terminals, a switch connected to control power supply to one of the filaments of the lamp, and a timer for controlling the switch, the circuit being arranged to delay the application of power from the transformer to turn on the lamp at start up to allow the filament of the lamp to warm up.

2. A circuit according to claim 1, in which the timer is arranged to control the switch to remain closed until the lamp has been turned on for at least a few seconds.

3. A circuit according to claim 1 or 2, including a sampling circuit for monitoring the operating condition of the lamp arranged control the switch to turn on if the lamp operating condition deteriorates to a predetermined condition.

4. A circuit according to any one of claim 1 to 3, in which the oscillator output is arranged to be reduced from a high starting level to a predetermined working level once the lamp has been ON for a few seconds.

5. A circuit according to claim 3 in which the sampling circuit is arranged to increase the oscillator output if the lamp operating condition deteriorates to the predetermined condition.

6. A circuit according to any one of claims 1 to 5, including a manually adjustable current regulator for altering the brightness of the lamp in use.

7. A solar lamp drive circuit substantially as herein described with reference to the accompanying drawing.