April 1, 1969 W. B. BURKETT ET AL 3,436,639 VOLTAGE CUT-OFF CIRCUITS Filed March 6, 1967 Sheet / of 4 AfA. | | | | -- /7 H A aw H A. l-l -- -- // -------- Af 3/ --- -- / 22 - l -- M A. A | | | | | | 7 -- - A --- M -- i a 2 - - 2 - - v A. a f, A4 227 22 A/zza2%22zz 77A/7//WZZ/2Z2W27- - aay M AaaavAM BY - 47722/16 April 1, 1969 W. B. BURKETT ET All- 3,436,639 WOLTAGE CUT-OFF CIRCUITS Filed March 6, 1967 Sheet 2 of 4 M A 1 / A 22 21 2 aa 27 27 27 77%. M/ 622M/22 A. Af 4. 7 /2 A Sn N s S 7 S 4 4. N s s S 4. a N S 4 2 S 2 M a 211 TTTV AH A. M / 2. 2 4° 4 A 7 a 2 /7 A // /22 INVENTORS, A/Zazaz AA2A277 422 azer A Azeszw 22 4% af7727A3 April 1, 1969 W. B. BURKETT. ET All 3,436,639 VOLTAGE CUT-OFF CIRCUITs Filed March 6, 1967 Sheet 3 of 4 2 INVENTORS. aaayA/2a262 A aAzagal/ 42a27/ 2.7%ay722A7% April 1, 1969 W. B. BURKETT ET All- 3,436,639 WOLTAGE CUT-OFF CIRCUITS Filed March 6, 1967 Sheet 1 of 4 . A/Zaza2AA%//I NWENTOR6. 624%r W.Azasza/ 4.4%aV/221AA-12 ul United States Patent Office 3,436,639 Patented Apr. 1, 1969 1. 2 3,436,639 VOLTAGE CUT-OFF CIRCUITS brationremoval inequipment. power lawn care equipment and power Snow Wilford B. Burkett, Pacific Palisades, and Robert W. Jackson, Los Angeles, Calif., assignors to McCul This transient voltage can cause a false indication of loch Corporation, Los Angeles, Calif., a corpora the condition of the battery that is being charged, with a tion of Wisconsin resultant premature removal of the charging current. It can also cause a surge current through the control Continuation-in-part of application Ser. No. 530,129, circuits with the possibility of damage to the elements in Feb.No. 62,01425, 1966. This application Mar. 6, 1967, Ser. the control circuits. Int, C. H02 7/10 U.S. C. 320-27 3 Claims 10 SUMMARY OF THE INVENTION In accordance with the present invention there is pro vided a voltage-responsive circuit for cutting off the charg ABSTRACT OF THE DISCLOSURE ing current between a variable voltage source, such as a A voltage responsive cut-off circuit cuts off the charg DC generator, and a battery. The generator is subject ing current supplied to a battery by a variable direct-cur 5 to being driven at widely varying speeds and includes a rent source when the terminal voltage of the battery shunt field winding. A rectifier is connected in series with reaches a preselected value. The cut-off circuit is voltage the battery, the rectifier being so poled as to permit responsive and includes circuitry for limiting the current charging of the battery when the voltage across the gen flow therethrough to prevent burning out the elements erator exceeds the battery voltage. There is further pro and further includes circuitry for bypassing the transient 20 vided a switch such, for example, as a silicon controlled voltages so that the cut-off circuit will be responsive to rectifier,the generator. which is connected across the shunt winding of the true direct-current voltage level across the battery. This switch is adapted to short-circuit the shunt wind ing when triggered. The triggering is effected by control 25 circuit means coupled to the switch and responsive to CROSS-REFERENCE TO RELATED the voltage across the battery. This circuit means may APPLICATION consist of a voltage divider which may, for example, in This application is a continuation-in-part of our earlier clude a Zener or breakdown diode. application Ser. No. 530,129, filed Feb. 25, 1966. Additionally, in accordance with the present invention, 30 a current limiting means is connected in the current path BACKGROUND OF THE INVENTION of the element that short circuits the shunt winding in Field of the invention order to reduce current surges therethrough. This invention relates generally to voltage-responsive Further, the voltage-responsive control circuit includes circuits, and more particularly relates to an electronic a means for filtering out the transient voltages so that circuit for cutting off the charging current between a the circuit will be responsive only to the true direct variable, direct-current voltage source and a battery. current voltage level across the battery. The voltage-responsive circuit of the present invention is particularly designed to cut off the charging current BRIEF DESCRIPTION OF THE DRAWING which charges a battery of secondary cells from a DC 40 The novel features that are considered characteristic (direct-current) generator driven by an internal com of this invention are set forth with particularity in the bustion engine with a widely varying speed. For example, appended claims. The invention itself, however, both as the circuit of the present invention is particularly de to its organization and method of operation, as well as signed for use in connection with a chain saw driven by additional objects and advantages thereof, will best be a single cylinder, two-cycle, internal combustion engine. understood from the following description when read in An electric generator having series and shunt field wind connection with the accompanying drawings, in which: ings may be mechanically coupled to the engine and may FIG. 1 is a circuit diagram of a circuit for charging be utilized as the starter for the engine. The battery in a battery from a direct-current generator; turn consists of a plurality of secondary cells such as FIG. 2 is a graph plotting the current as a function of nickel-cadmium cells that are rechargeable. Thus, by driv 50 time and showing the current through the switching means ing the DC motor from the battery, the engine can be included in the circuit of FIG. 1; started. After the engine has been started, the battery FIG. 3 is a graph showing the charging current as a must be recharged. To this end the generator may be function of time required to recharge the battery after directly driven by the engine to supply charging current a typical starting discharge; to the battery. 55 FIG. 4 is another graph showing the voltage as well However, the charging voltage must be closely con as the current as a function of the number of revolutions trolled to avoid damage to the secondary batteries. To of the engine driving the generator and indicating the this end it may be assumed that the voltage across the characteristics of the generator output voltage and charg battery is an indication of the state of charge of the ing current; battery. 60 FIG. 5 is a schematic circuit diagram of a preferred Chain saws driven by a single cylinder, two-cycle, in voltage-responsive circuit having a current limiting ele ternal combustion engine are subject to a large amount ment and a transient voltage filtering circuit in accord of vibration. This vibration affects the generator that is ance with the present invention; and being driven by the engine and causes brush bounce at FIGS. 6-8 are circuit diagrams of other embodiments the commutator. This brush bounce creates an undesir 65 ofinvention. the voltage-responsive cut-off circuits of the present generator.able high voltage transient on the output voltage of the This condition also exists in other tools that have an DESCRIPTION OF THE PREFERRED electric starter and a battery thereof that is recharged EMBODIMENTS by a generator driven by the engine of the tool. For 70 Referring now to the drawings, wherein like elements example, there is a similar transient voltage creating vi ae designated by the same reference characters, in FIG. 1 there is illustrated a prime mover generally indicated at 3,436,639 4. 3 put terminals 20 exceeds the forward voltage drop of 10 such, for example, as an internal combustion engine. diode 2 and the battery voltage, a charging current flows As previously pointed out, the circuit of the present in through the battery 18. As a general rule and as an ap vention for controlling the charging of a battery from a proximation, the battery voltage indicates the state of generator is particularly designed for use with a chain charge for any secondary cell battery. As a result the saw driven by a single cylinder, two-cycle, internal com battery voltage increases as the percentage of the charge bustion engine. However, in general, it will be understood increases. Thus, the forward voltage drop of the diode 21 that the circuit of the invention is particularly applicable may, for example, be 0.6 volt. It may further be assumed for all types of internal combustion engines where the that the battery 18 consists of ten secondary cells each engine speed varies over a wide range and the engine sub rated at 1.25 volts so that the nominal voltage across the jects the generator to vibrations with resultant brush 0. battery 18 is 12.5 volts and may be as high as 15 volts. bounce and generation of high voltage transients. Exam Accordingly, assuming the flow of a charging current, ples of Such engines are outboard boat engines, engines the battery voltage increases and the generator output for automotive vehicles as well as other engines used for voltage also rises. This generator output voltage appears portable or transportable power tools and even for sta across the two Zener diodes 25, 26 and the resistor 27.