CA1201759A

CA1201759A - Constant illumination circuit

Info

Publication number: CA1201759A
Application number: CA000415990A
Authority: CA
Inventors: Henry R. Mallory
Original assignee: Duracell International Inc
Current assignee: Duracell Inc USA
Priority date: 1981-12-16
Filing date: 1982-11-19
Publication date: 1986-03-11
Also published as: GB2111730A; AU9070882A; AU566782B2; US4499525A; GB2111730B

Abstract

ABSTRACT OF THE DISCLOSURE
A power supply circuit for generating a power pulse train for an incandescent bulb in a flashlight which results in a relatively constant illumination output from the flashlight despite the decline in output voltage from a battery power supply therefor during discharge. The power supply circuit operates to compensate for the lower output voltage from the battery power supply by increasing the duty cycle of the pulse train to provide wider pulse widths therein, and eventually the wider pulse widths merge into a DC power supply near the end of the useful service life of the battery power supply.

Description

~ONSTANT ILL~MINATION FLASHLIGHT
BACKGROUND OF THE INV~NllON
1. Field Of The Invention The present inventiQn relates generally to a flashlight for pro~iding a rela~ively constant illumination output over the u~eful servic~ life of batteries employed ~herein despite the relatively poor voltage maintenance characteristics of ~he batteries during discharge. More particularly the subject inven-tion pertains to a simple and inexpensive circuit for incorpor~-tion in a flashlight which provides a relatively eonstant power supply to an incandescent bulb therein over su~stantially the entire useful service life of the flashlight batt~ries. The circui~
gen~ra~es a pulse train for the incandescent lamp and provides a relatively constant power supply by varying the duty cycle ~hereof by generating pulses having a greater pulse width as the voltage from the power supply batteries declines.

2. Discussion of the Prior Art Traditional ba~teries employed as a power supply for incandescent flashlights have very poor voltage maintenance characteristics over their useful service life. Traditional batteries of the kind discussed herein typically includ~ aqueous electrolyte, manganese dioxide bat~eries such as zinc carbon or 2inc chloride bat~eries or manganese alkaline batteries. As an example, a conventional zinc carbon lantern battery provides only four to four and one half hours of continuous service 7 with an initial illumina~ion output of forty-fîve lumens and a change in illumination over its useful life of approximately five to one, thereby producing an ou~put near the end of , - ~1i~a ,- !
1 its service life f~f s~nly nine lumens. Moreover, as the battery dischar~es the lower ~c: ltage xesults in a subs~an-tial filamer~t colox charl~e in the incandescent :bul~ from an initial white light output ts: a noticeably yell~3w light 5 output, which i~ ccnsiderg~d lto be a negative factor from a consumer acceptanc:e point of view.
Lithium }:a~ed batter.ies now under extensive develop~
ment in ~hP art do not ~xhibit a ~our or more "co c ne ratio decl~ne in output voltage over their useful service life 10 because of voltage mainterlance s:~harac~eristics which a:re vastly superior to zinc car:bon ~ zinc chloride and rnangane~
alkaline ~atteries. In view thereof, it would be desir~le to provide a de~relopment ~uch that ltraditionally ~owered flashlights re~ain competitive with the new~y developlng lithium powered lighting ~roductsO
SU~RY OF THE IN~ENTION
Accordingly, i is a primary object of the present invention to provide ~ power supply circuit operated from a battery powex supply whi~h pxovid~s a relatively constant power output to a load over substantially the entir~ u~eful ~ervice life ~f the battery despite the xelativerly po3r ~oltage maintenance characteristi~s thereof during dischargeO
A further object ~f the subject invention is the provision of a circuit of the aforementioned tyDe desi~3ned ~o power a load in the form of an incandescent bulb for a flashlight such that the result is a relativel~
constant illumination output from the 1ashlight over the useful service life ~f the batteries employed thereiII~
Yet another object ~f ~he present invention as a power ~upply circuit of the subject kind which operates almost entirely in a substantially nondissipative mannerO
thereby resulting in a power supply circuit having very little internal power loss.

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In accordance with the ~ eachi~gs he:rein~ the present invention pr~vides a circuit f~r generating. a p~wer pulse train for a load in the form of ax~ incarldescent bulb in a flashlight which results in a relatively corl~tarlt 5 illumina~:ion ou~put from ~he :~lashlight despite the dec:line in outpu~ voltage frc~m a batt~ry power 3;upply ~:herefor during dischar~e. The poweX E;Uppl y circui~ basic:ally s:~perateEi to c~mp~nsate for the lower output vol~a9 ~rom ~he ba~tery power supply ~y increasi nS3 the dut.y cycle of ~he puï~e train 10 to pro~ide wider pul~e Wid~h6 ~herein, and eventually the wider pulse widths merge in~o a DC power 6upply near the end of ~he useflll servis~e life of the ba~tery power ~upply.
The circui~ in~ludes a drive ~:ransis~or haYing i~s emitter and collec~or coupled in series wi~h ~he incandess:ent bulb and the ba~ery pDWer ~upply. A control circuit i~
coupled to the base of the drive transistor in a r~nner to cause it to genera~e a pulse train in which the du~y cycle thereof is increased by providing pulses having a greater pulse width as the voltage from ~hle battery powe~ supply declines during dischar~eO In this arrangement the rated voltage of the incandescent lamp ils prefera~ly approximately half the ra~ed voltage of ~he battery supply, which is normally an aqueous electr~lyte manganese d.ioxide bat~ery. The drive transistor is selected ~o have a ~:i.gh beta characteristic and is driven in a non~issipa~ive mode in which i~ is either in a fully sonductive, saturate~ s1~te or in a n~nconductive state, such that pow~r dissipa~ion in the drive transistor in partially conductive modes is ~voided~
In a preferred embodimen1;., the control circui~
comprises firs~ and second con~rol transistors coupled

3~

1 together in a complementary ~anner such that only one control transistor is conducting at a time. An output of the first control transistor is coupled to the ~-ase of the drive transistor to dri~e it in a saturated mode when the first control transis~or is conduc~ing. The second ~ran~
sistor functlons a5 an inhibit transistox having an output thereof cou~led to ~he baRe of the first control transi.stor to render it nonconductive when the inhibit transistor is conduc~inq. An ~C timing netwoxk is coupled to the base of the inhibit transi~tor to control the frequency of the pulse train generated by the cixcuito In greater detail~
the drive transis~or comprises an NPN transist~r, and the first and second control transistors are PNP ~ransistors coupled in a co~non e~itter circuit. The co~n ~mitter connection is cou~led directly to the anode of ~he battery while the collector of the fixst control transistor is coupled to the base of the drive transistor. At least one diode functions as a volta~e thre5hold cirucit, and is coupled between the coll~ctor of the drive transistor and the base of the inhibit transistor~ such that when the voltage of the threshold circuit is exceeded, ~he inhibit transistor is turned on, which resul~ in both the fir~t control transistor and the drive transis~or b~ing rendered nonconductive.
BRIEF DESCRIPTION OF THE DR~WINGS
The foregoing object~i and advantages o~ the ~resent invention for a c.ircuit for a c:onstant illumination ~lash-light may be more xeadily understood by ~ne ~killed in the art with refexence being had to th~ following detailed descrip-3 tion of several preferred Pmbocliments thereof, taken inconjunction with the accompanyi.ng drawi~gs~ in which~
~ igure 1 is an elec~riGal schematic ~f a first embodiment o~ a circuit or a consta~t ill~mination flash~
liyht constructed pursuant to t:he teachin~s o~ the prP~en~
invention;

i3~

l Figure 2 illustrates t~o curves of illumination output ~ersus continuous hours of service, the fir~t of which is for a typical prior art ~l~sh~ight and the second of which is for a flashlight incorpora~ing ~hP cixcui~ of 5 Figure 1; and ~ igure 3 illustrates an elec~rical schematic for a second exemplary embodimen~ oi- a circuit for a con~.ant illumination flashlight constructed pursuant to the teaching~
herein.
1~ DETAILED DESCRIPTIOM OP THE DRAWIN~S
Referring to the drawings in detail, Figure 1 is an electrical schematic of a fir8t emb~diment of a circuit desiyned to provide a substantially cons~ant power output to a l~ad in the form of an incandeseent bulb 10, ~5 which in this embodiment is ~ commercially designated PR2 bulb in standard usa~e in two D cell flashligh s. A PR2 bulb is normally rated ~t 2.38 volts and 500 millamps~ The power ~up~ly therefor in this circuit is a 6 volt manganese dioxide battery 12, c~mmercially designation 308l which i5 20 a typical commercially available lantern battery with sPring contacts on top thereof. ~he battexy 12 is connected in ~eries with bulb 10 throuyh an on-off switch 14 and the emitter and collector of an NPN drive or pass transistor 16 commercial designation 92PU01. I~ an arrangement o~ this 25 type, the lamp 10 is 5elected to have approxima~ely one half the rated voltage of the battery, Primarily because operation is i~ a pulsed mode rather than a typical prior art DC mode.
Transistor 16 is ~elected to have a high beta characteristic, and its base current re~uired during con~
30 duction i5 approximately 10% ~f its emitter to coll~ctor cur.rent, which in the ~pPration of the disclosed circuit is iess than 500 milliamps~ Accordingly~ a base current of 50 3~

~ 7~i~

milliamps or less i~ su:Eficien~. ~o cause fully saturated c~nduction of the tr;~nsi~tor 16.
The pulsed conduction OI' transist~r 10 i~
controlled by a circuit includ:ing ~irst and ~ecc~nd cs~trc)l transistors 18 and 20 which are connected in a co~nc~n emitter arxangement, wi~h the co~anorl ~uniters being coupled directly to thç~ posi~ive terminal or anode o~
battery 12. The con~rol ~ansi stoxE; 18 and 20 are connec~ed together in a complementary manner such that c)nly one 10 transistor at a time is conduc~ q. The fir~t control transistor 18 is coupled by its emi~ter to the ba~e of drive transistox 16 ~o turn t~e lat~e:r oll in a :Eully saturated, conductive mode whs~r.~ cc:ntr~ rans~ s~or :1~ is conducting. Transis~or 18 is selected ~o su~ply an ~mitter 15 current of 50 milli~mps or less, to the ba~e o~ drive transis or 16 to drive ~he latter ~n a ful.ly saturated mode. The second control transistor 20 f~mctions as an inhi~it transistor relati~e to ~he first control transistor 18, with the collector of inhibit transistor 20 being coupled to the 20 base of the first oontrcl transistor 18 to render the la~ter nonconductive when inhibit tran.sistor 20 is conducting.
Commexcially designated ~PS37D25 PNP transistors are sui~t-able for the first and second control transistorsl~ and 20.
An RC timing network compri6ing resistor 22 and 25 capacitor 24 is coupled to the base o~ inh~hit transistor 20, and ~heir ~alues control the frequency of ~he pulse train genera~ed by ~he powPr supply circuit. The frequency of the pulse train should be higher than ~he retinal per sistence of ~he human eye such that it appears ~o the user 30 of the flashlight that ~he illumination ~herefxom is constantO

7~1 ~7--1 In general, the requency of the gener~ted p~lse t~ai~
should be greater than 50 her~z, and 5 kilohertz is a good practical frequency or a flashlight power supply circu~t of this type. In the illustxat:ed embodiment resistor 22 5 is rated at 1.8 kilo~ohms while capaci~a~or 24 is rated at 5 microfarads and 3 vGlt~o A voltage thres~old circuit in ~he form of three diodes 26 is cou~led between ~he collector of drive transistor 16 an~ the base of inhibit ~ransis~or 20, and func~ions in a 10 manner to ~urn inhibit ~ransistor 20 on when ~he vol~age at the inhibit transistor 16 exceeds the pxedetermined threshold of the diodes 16. In this circuit each diode 26 has a threshold of .8 volts, and accordingly the cumulative threshold voltage of the three diodes in series is 2.4 volts. In 15 alternative embodiments, other types of thresh~ld circuits could be utilized. For example, each diode could have a lesser or greater threshold vol1tage~ and more or les~ diode5 could be utilized, or in ~ome circuits zener diodes may be suitable.
The resistors 28, 30,, 32 and 34 are selected to establish suitable biasing vol~clges ~hroughout the circuit, and their values in this circui~: are shown in parentheses in ~igure 1. A capacitor 36 has been added ~o ~he circuit as it results in improved perf~rmarlce thereof, but is n~t necessary 25Eor its operation. The constant: power supply circui~ o the present invention is shown in Fi.gure 1 as being ~ncompassed b~ dashed line 3B, while the more conventional CDmpO~ents of a 1ashlight are shown outside the dashed line 3~. However, it should be realized that in a conventional flashlight, a 30PR2 bulb would not be utilized with a lantern battery as this type of bulb is not designed to acceDt the full DC ~ower o:E
~ fully charged lantern battery.

. .

1 The circuit operatec: as ~ollows. Closure 3f switch 14 applies opera~ional voltages to transi~tor 18 which results in its conduction and the application of a conductive voltage and current ~o the base of drive trans 5 istor 16, which xesults in it bein~ switched into a satu-rated conductive state, thereby applying almost the ~ull voltage of battery 12 ~minu~ a small vol~age drop of approxi-ma1tely ~ volt across tran~istor 16) acro~s lamp 10 at the leading edge of a pulse in ~he pulse ~rain. Transi~tor 16 10 continues conducting until ~he vol~age at itscollector exceeds the threshold of 2.4 volts of the three series connected diodes 26, at which t.ime an act~vating ~ltage i~ applied through diodes ~6 and resis~or 28 t~ the ba~e of inhibit transistor 20, thereby aausing ~aturated conduction 15 of transistor 20 and als3 charg:ing of capacitor 24~ Con-duction of transistor 20 aoplies a reverse bia~ to the base of control transistor 18, therPby erminating its conduction and also terminating conduction of drive transistor 16 at the trailing edge of a pulse in the pulse train. Trans-20 istor 24 discharges through resi.stor 22, thereby removingthe forward bias voltage from i~:s base and ~erminating conduction of inhibit transistor 20. T~rmina~ion of con-duction of inhibit transistor 20 allows con~rol transis~or 18 to conduct again, thereby driving transistor 18 into ~5 saturated condition at the leading edge of the next pulse in the pulse train~ etc~
The circuit continues supplyiny a power pulse train to lamp 10 in this manner as the voltage su~plied ~y battery 12 gradually declines because o its constant 30 discharge. A decline in vol~age of ~at~ry 12 resul~s in a --9~

1 decrease in the amplitude of ~he voltagP of each pulse ~as that arnp:l itude is slightly less than the voltage supplied ~y battery 12). However~ a decli:ne in vslt:age of battery 12 also rPsults in a corlcurrent increase in the time dura~ion 5 after initialtiorl o~ the leading edge o~ a pulse~ for ~he collectc>r of trarlsistor 16 to reach the ~hreshold voltage across diodes 26, and hence a concurren increa~e of the pulse width of each pulse in the 1:rairl. The increase in pul~e width increases ~he duty cycle o~ ~:he pulse ~rain and 10 compensates for he lowering o khe s?oltage amplitude of the pulse train, such that a subst an~ially constan~ amount of l~ower is sups~lied to incandescent lamp 10 despite the gradual decline in voltage ~upplied by ba~tery 12 a~ it discharges. The pulse width of each pul~e will ~radually 15 increase until the battery voltage! declilles to a point lat substantially the end of i~s usefLIl service life3 at which the threshold of diodes 26 is not exceeded, thereby :resulting in continuous conduction of drive transi~tor 16~ After reaching this point, the circuit cannot compensate fox a 20 further declin~e in the output voltage of battery 12, thereby resulting in a gradual decrease in t.he power suoplied to lamp 10.
The following Table I supplies technical data on the operation of the circuit of Figure 1. This data 25 was derived from measuring the illumination GUtpUt of bulb 10, the voltage outpu~ of battery 12 and the f.requency of the output Duls~e train. The avlerage current of the pul~e train and the power figures were then computed vn ~he basis o~ these measurements.

TABLE I Lumens per Volt~ MA Watts KhZ Lumen6 Watt ~ .~
6.3 2~8 1.44 S,2 10.~ 7.5 ~.0 235 1.~1 5.2 10.3 7.3 S.~ 245 1.35 5.7 9.3 6.8g 5.0 ~62 1.31 6.1 8.6 ~.56

4.0 310 ~.24 7.2 7.7 3.0 412 1.2~ 6.5 7.9 ~.37 2.5 49~ 1.25 Cont. 8.4 6.72 2.0 442 .83 Cont. 3.6 4.09 Figure 2 illustrates two curves of illumination output of a flashlight bulb versus continuous hours of service.
A first curve 40 shows the illumina~ion output of a typical prior art flashlight wherein a six volt 7inc carbon lan~ern battery is discharged directly through a ~uitable incandes-cent lamp, such as a bulb de~ignated therPfor, A second curve 42 illustrates the illumination output produced by bulb 10 in the circuit of Figure 1. As illustrated by these curve~3, the constant illumination circuit p.roduces bout twice the useful 2n service hours as the prior art i~pproAch. Of course, as also illustrated by the curves this :increase in useful service hours is achieved by sacrificing the :initial high illumination ou~put produced by the prior art approach.
Figure 3 is an electr:ical schematic of a ~econd exem-2S plary em~odiment of a circuit for a constant illumination flash-light designed to produce slightly over twice the illumina~ion output a~ the first embodiment fr~m a larger six volt lantern battery 44. This circuit i~ de~3igned to operate with a higher 7~i~

pswer lamp 46 rated at 2.75 volts and g50 milliamper~s, because of the higher power throughput of t~e circuit. The increased current through the circui~ also requires a higher power drive transistor 48, commercial designation TlP29 or equivalent. The operation of this circuit is subs~antially the same as the operation of the first emhodiment, and accordingly will not be explained separately in de~ai-l herein.
The following Table II supplies technical data on the operation of the circuit of Figure 3. This data was derived from measuring the illumination outplut of lamp 46, ~he voltage output of battery 44, and the frequency of the output pulse train. The average current of the pulse train and the power figures were then derived from the measured parameters.

w w r~ r~) ~ 1 ~n o ~- o ~n o u- 1--~ 8 Z~uo~~ Q z Z 0 Z 5 9 6 Z Z 51 6 S Z
5L S~uc~:~ z5 z IS - Z Z - LI ~ - z 866 S~-L~lo~ SL-Z ~L-Z L~Z g~-~ 5~01 SZ~
L~uo:~ g~ z SB - Z S - 8Z99 ' ~ Sl~01 S - E
IZL9û~1 tLZ LZ-Z S-~Z O~-E 058 0-~
0 8 9~ 1 6 9 Z o v . 6 Z ~, L ~ 8 v ~ 5 ~
Z6-9 80l 89-~ 9~-1 S~ZZ S~ ûS9 ~-S
S~g Z0-~ 6~-Z ~g-l I-~Z 85-~ . 8~5 05~5 ~9-L S~'l TL'Z 85-1 L-~Z Zl' Z~S SL-S
88L OV-I ~L~ ~5-1 9-~Z Zt- OZS 0-9 S~L~M ZH~ SW~ s~ ~ll SL~M ~ S~ l~
s~llq ~n~L ~a - SL~0~ ans ~oa ~3.t3 1 While several embodiments and variati~s of th~
present inYention for a circuit f~r a constant illuminati3n flashlight are describedlin detail herein~ it should be apparent that ~he disclosure and teachings of the present

5 invention will suggest many alternative designs to th~se skilled in the art. For instance d the circuits could be easily modified t~ provide a flashiny illumination ~utput.

3o

Claims

1. A portable flashlight designed to provide a relatively constant illumination output over the useful service life of the battery power supply therefor, despite a relatively poor battery maintenance characteristic during discharge, comprising an incandescent lamp for providing illumination, a circuit for providing relatively constant electrical power to the incandescent lamp over substantially the entire useful life of a least one power supply battery, having a relatively poor voltage maintenance characteristic during discharge, said circuit including a drive transistor having the incandescent lamp coupled in series with the emitter and collector of the drive transistor and further coupled in series with said battery power supply, said circuit further including a control circuit including first and second control transistors coupled in a complementary manner such that only one control transistor is conducting at a time, said first control transistor being coupled to the base of said drive transistor, to drive it into a saturated conductive mode when said first control transistor is conducting, and said second control transistor being an inhibit transistor having a drive output thereof coupled to the base of said first control transistor to turn it off when said inhibit transistor is conducting, wherein said control circuit causes said drive transistor to generate a current pulse train in which the duty cycle thereof is increased, by providing current pulses having a greater pulse width to compensate for a lesser pulse amplitude, as the voltage from the power supply declines during its discharge to provide relatively constant electrical power to the incandescent lamp over substantially, the entire useful life of the battery power supply, wherein said drive transistor has a relatively high beta characteristic and is driven by said control circuit in a non-dissipative manner in substantially either a fully conductive state or a fully non-conductive state to produce the pulse train, whereby power dissipation in the drive transistor during partically conductive modes is substantially eliminated, and where an RC timing network is coupled to the base of said inhibit transistor for controlling the frequency of the pulse train generated by the circuit.

2. A portable flashlight as in claim 1, wherein said drive transistor comprises an NPN transistor, nd said first and scond control transistors comprise PNP transistors coupled in a common emitter circuit, coupled directly to the anode of said at least one battery, with the collector of said first control transistor coupled to the base of said drive transistor.