Variable candela strobe

The invention pertains to strobe devices of a type used in alarm systems. More particularly, the invention pertains to strobe devices with selectable candela outputs.

Strobe devices require the application of a relatively high voltage across a flash tube in order to produce a gaseous discharge in the tube. In known devices, this high voltage is achieved by using a charge pump to transfer energy to an internal capacitor from an external energy source. The external source typically can be nominally 12 volts or 24 volts.

The capacitor is coupled in parallel with the flash tube and provides the energy for the flash. The amount of light from the flash tube is directly proportional to the energy stored in the capacitor that is discharged into the flash tube.

A single capacitor can be charged to various voltages in order to provide a multi-candela (multi-intensity) unit. However, there are limitations on the range of candela (intensity) that can be reliably achieved. One problem is that to flash the tube requires that the voltage across it be greater than a predetermined threshold amount (e.g. 180 volts) for reliable operation.

Present designs for multi-candela strobes include a range of 15 candela to 100 candela. To achieve such outputs, the capacitor needs to be charged to 240 volts for the 100 candela, but will only need to be charged to 120 volts for the 15 candela output. The 120 volts is, however, below the exemplary 180 volts needed for reliable operation.

In order to overcome this low voltage problem, known designs incorporate a voltage booster circuit to increase the voltage across the flash tube. One type of a voltage booster circuit is a voltage doubler circuit. One known voltage doubler design is disclosed in a “flashtubes” EG&G Heimann Optoelectronics Catalog, pg. 7, 1991. This document discloses a voltage doubler circuit to be used with a flash tube.

A prior art strobe unit with a known doubler is illustrated in FIG. 1. A capacitor C 3 stores the energy that is going to determine the candela of the flash. It is coupled across a series combination of a flash tube LP 1 and a diode Dl 3 .

Capacitor C 13 is the doubler capacitor. It is charged through resistor RI 5 to the same voltage VC as capacitor C 3 is charged. The polarities of the voltages on the capacitors C 3 and Cl 3 are the same. Capacitor C 4 is used for the trigger function and is charged to the same voltage and polarity as is capacitor Cl 3 .

When the unit is triggered, by a signal from the trigger circuit, SCR Q 8 will conduct and pull node A low. This causes C 4 to discharge through Q 8 and the primary winding of TR 2 , the trigger transformer.

Until the flash tube is triggered by the voltage out of the secondary winding of Tr 2 , C 13 and C 3 cannot discharge. However, the voltage across the flash tube at this time is double the voltage VC of C 3 (far exceeding the minimum required voltage). When the tube flashes, it first discharges capacitor C 13 , then capacitor C 3 . The energy stored in capacitor C 3 provides the preselected candela output from tube LP 1 .

While known devices provide a selectable candela output, the use of a voltage doubler does have some disadvantages. At high output intensities, the voltage across the tube LPI is substantially equal to 2VC which can be quite high. This high voltage requires the use of components rated therefore. In addition, in compact units with the circuitry implemented on a printed circuit board, arcing is a potentially problem.

There thus continues to be a need for multi-candela strobe units which provide reliable, triggerable light of a selected intensity. Preferably such reliability could be achieved in compact, high density packaging, without the necessity of high voltage components. It would also be preferable if operational reliability could be achieved while simultaneously eliminating arcing during normal operation.

FIG. 1 is a schematic diagram of a prior art strobe alarm;

FIG. 2A is a schematic diagram of a first embodiment of the invention;

FIG. 2B is a schematic diagram of a second embodiment of the invention; and

FIG. 3 is a schematic of another embodiment of the invention.

While embodiments of this invention can take many different forms, specific embodiments thereof are shown in the drawings and will be described herein in detail with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the specific embodiment illustrated.

The disadvantages of the prior art above can be overcome with a power supply in accordance with the invention. A more controlled voltage is achieved across the flash tube using a voltage booster circuit that is not a “doubler” but rather an “adder” type circuit. The present circuit operates significantly differently than the prior art circuits.

FIGS. 2A , 2 B illustrate two different embodiments 10, 10-1 of strobe alarms in accordance with the invention. In both FIGS. 2A , 2 B the maximum voltage capable of being applied across tube LP 12 is the sum of VC, the voltage across capacitor C 3 plus the voltage across an off-set circuit 12 , FIG. 2A or 12 - 1 , FIG. 2 B. This total voltage, as discussed below, will be substantially less than 2VC but great enough to produce reliable, repeatable flashing of tube LP 1 .

In FIG. 2A , a power supply PS provides energy for circuit 10 , from a remote source. Supply PS is coupled to controller CC, and charge pump circuit CP as well as other circuitry as would be known to those of skill in the art. The output of charge pump CP couples energy to capacitor C 3 and other components of circuit 10 as discussed subsequently. Trigger circuit IC provides trigger gate signals to SCR Q 8 .

Candela selecting switches S or S′ can be coupled to either controller CC or charge pump CP without limitation. Switch settings can be established manually, electronically or both without limitation.

In FIG. 2A , an off-set circuit 12 includes a Zener diode D 11 which is coupled, in parallel, with the capacitor C 13 . Diode D 11 causes the voltage of node A′, Q 8 , C 4 ′, and C 13 ′ to be clamped to a known, predetermined voltage (e.g. 100 volts). This voltage is limited by the off-set circuit 12 , for example by the voltage across D 11 . It will not vary with the different charging voltages of C 3 used to obtain the different candela rating. Alternate sources, such as a power supply could be used instead of capacitor C 13 without limitation.

When the capacitors have been changed, the voltage across the flash tube is VFT=VC+VD 11 . The addition of the diode D 11 limits the voltage variations across tube LP 1 . It is possible to control the total voltage across the flash tube LP 1 to tighter limits than is achieved by a prior art voltage doubler The tighter voltage range is demonstrated in Table 1 which compares a voltage limiter as in FIG. 2A to a doubler, as in FIG. 1 .

In the embodiment of FIG. 2A , the flash tube LP 1 requires 180 volts minimum to flash reliably. Table 1 illustrates that both the circuits of FIG. 1 and FIG. 2A create voltages greater than 180 volts to satisfy this requirement. However, at the highest candela rating, the circuit of FIG. 2A applies 140 volts less to tube LP 1 than does the circuit of FIG. 1 . This is a significant difference.

The limiter circuit 12 couples a smaller range of voltages across the flash tube LP 1 while operating from 15 candela to 100 candela than do the prior art doubler-circuits. The lower over-all voltage translates to reduced breakdown voltage specifications for the various components (less expensive) and possibly to a more compact spacing, higher density of circuit board points, without arcing, than is the case with the voltage doubler configuration of FIG. 1 .

It will be understood that the specific minimum threshold voltage to flash the tube LP 1 reliably may vary as a result of tube geometry, gas and the like without limitation. Such variations come within the spirit and scope of the invention.

Table 1 illustrates that the voltage across the flash tube varies less across a range of different candela outputs with the limiter circuit 12 than in the prior art. Both capacitor values and the value of the constant voltage used in the limiter circuit of FIG. 2A can be changed to different values without departing from the scope of this invention.

FIG. 2B illustrates an alternate embodiment 10-1. Components common to those in FIG. 2A have been designated with the same identification numerals. In FIG. 2B , a limiting circuit 12 - 1 includes capacitor C 13 and Zener diode D 11 , between node A″ and ground. Circuit 12 - 1 limits the total voltage across LP 1 prior to a trigger event. In circuit 12 - 1 , current through Zener D 11 advantageously does not flow through diode D 13 .

The voltage across the flash tube LP 1 can also be established by the use of a non-booster type circuit design. In this embodiment, two capacitors are still used. They are coupled in parallel, not in series with the flash tube as in the prior art. FIG. 3 illustrates an embodiment 10-2 in accordance with this configuration.

In the embodiment of FIG. 3 , circuit 10 - 2 incorporates power supply PS′ which receives electrical energy from an outside source and couples same to a controller circuit CC′ as well as a charge pump circuit PC′ and other components of circuit 10 - 2 as would be understood by those of skill in the art. Controller circuit CC′ is coupled to trigger circuit TC′ whose output provides gate drive voltage to SCR Q 8 . The output from the charge pump circuit CP′ is available to couple energy to capacitors in circuit 10 - 2 including capacitor C 3 - 1 , C 4 - 1 and Cl 3 - 1 as discussed below.

In circuit 10 - 2 , capacitor C 13 - 1 will be charged, via charge pump circuit CP′, through diode D 14 - 1 . This establishes a voltage across the ionizationable output device LP 1 having a value VC′.

Capacitor C 3 - 1 is charged to a voltage VC″ by the charge pump circuit CP′ through diode D 15 - 1 , resistor R 26 - 1 and Zener diode D 16 - 1 . The voltage VC″ will be less than the voltage across capacitor C 13 - 1 when both capacitors are fully charged. In this condition, diode D 13 - 1 is reversed biased.

In the circuit 10 - 2 , the capacitor C 4 - 1 is charged to substantially the same voltage as is capacitor C 3 - 1 . Capacitor C 4 - 1 provides triggering energy via transformer TR 2 to the trigger electrode of the gas filled tube LP 1 .

When the capacitors C 3 - 1 , C 4 - 1 and C 13 - 1 are fully charged, control circuit CC′ can initiate a flash or an optical output by causing trigger circuit TC′ to trigger SCR Q 8 . This causes the voltage at the anode of SCR Q 8 , node A′″ to drop to a voltage close to ground, the anode-cathode voltage of Q 8 , which in turn couples a pulse via transformer TR 2 to trigger the gas filled tube LP 1 . Capacitor C 13 - 1 , which has been charged to the higher voltage VC′, will discharge first through the anode/cathode of tube LP 1 . This will in turn cause initial ignition, ionization, in the gas filled tube LPI which in turn will continue to discharge capacitor C 13 - 11 .

When diode D 13 - 1 becomes forward biased, capacitor C 3 - 1 , with a value that is much larger than capacitor C 13 - 1 , will discharge through tube LP 1 .

The amount of energy required to initiate ionization and to start the tube LP 1 to flash is relatively small compared to the amount of energy needed for the selected candela output. As a result, capacitor C 3 - 1 , which is substantially larger than capacitor C 13 - 1 provides the primary stored energy for producing the desired level of illumination from the gas tube LP 1 .

When capacitor C 3 - 1 has discharged, ignition ceases. Charge pump circuit CP′ is then able to recharge the capacitors of circuit 10 - 2 for the next optical output cycle.

The selected intensity or candela output level can be selected in circuit 10 - 2 using switches S- 1 or S- 2 coupled respectively to controller circuit CC′or charge pump circuit CP′, all without limitation. Such switches can be manually or electronically settable.

In summary, the embodiments 10, 10-1 and 10-2 all illustrate driving circuits which provide alternates to voltage doubler circuitry for purposes of generating selectable candela output levels in a strobe alarm. In all instances, a voltage substantially less than twice the voltage on the major illumination providing storage element is added to that voltage to initiate ignition of an ionization gas discharge tube. Subsequently, the energy stored in the primary storage capacitor is used to provide the selected candela output level for the circuit.

From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope of the invention. It is to be understood that no limitation with respect to the specific apparatus illustrated herein is intended or should be inferred. It is, of course, intended to cover by the appended claims all such modifications as fall within the scope of the claims.