WO1991004601A1

WO1991004601A1 - Automatic control circuit for backup voltage supply

Info

Publication number: WO1991004601A1
Application number: PCT/US1989/004117
Authority: WO
Inventors: Jeffrey K. Yard; Robert J. Fehn
Original assignee: Yard Jeffrey K; Fehn Robert J
Priority date: 1989-09-20
Filing date: 1989-09-20
Publication date: 1991-04-04

Abstract

An automatic control circuit for sensing a voltage level from a main voltage supply (V1) where a backup voltage (Vcc) is switched in when the main voltage (V1) drops below a first reference level and is delivered as output of the control circuit (1) throughout a window range of main supply voltages, and the main supply voltage (V1) is switched and compared to a first reference level by an input sensor (11) that provides an output signal to activate a solid state circuit (TR1-TR2) energizes a mechanical relay (RL-1) to switch the backup voltage (Vcc) in and out of the system depending upon the level of the main voltage. An RC feedback network (R8-C2) stabilizes the instantaneous switchover so that no loss of voltage occurs to an external load, and the feedback establishes a window voltage range over which the backup voltage stays switched in as the usable output voltage of the control circuit (1). The automatic control not only provides emergency power in the event of a battery or electrical system failure but is also suited in many fields using portable electronic systems.

Description

AUTOMATIC CONTROL CIRCUIT FOR BACKUP VOLTAGE SUPPLY

FIELD OF THE INVENTION

The present invention relates to automatic control circuits for supplying a backup voltage to electrical, e.g. battery-powered, equipment when the main voltage supply fails.

BACKGROUND OF THE INVENTION

Portable, battery-powered devices have become commonplace in recent years in both business and personal equipment. More particularly, products such as portable cellular telephones, marine equipment such as VHF radios, marine telephones, and LORAN navigational receivers, as well as many types of emergency communication devices all require the use of highly reliable, portable voltage supplies, typically employing one or more batteries. Frequently it is required, and in most instances it is desired, to provide a backup voltage supply in the event of a failure of the main voltage supply where such failure might be due, for example, to discharge of the battery voltage, defects in the wiring, or problems in the internal battery structure. In general, such backup voltage supply is provided in combination with a control circuit for automatically connecting the backup voltage supply into the system when the main voltage of the system fails.

It is known, as disclosed in U.S. Patent 3,577,003 (Behr), to have a main voltage automatically switched off, and a backup voltage switched on, when the main voltage falls below a predetermined level. It is also known to provide, in connection with a backup voltage supply system, a parallel connection between a main voltage supply and a backup voltage supply, with a blocking component such as a diode connected between the two voltage supplies to prevent a discharge of one into the other. For example, U.S. Patent 4,307,789 (Bertot) discloses such a system where the voltage supply is switched over to a backup supply in connection with the opening of doors and windows of an automobile. Another patent of possible interest in the general field is U.S. Patent 4,559,456 which discloses a backup battery supply for use with electrical appliances, such as electric shavers, where a voltage detection circuit recognizes a failure of the main battery and automatically switches on a reserve battery.

While prior art backup voltage systems of the types described above are satisfactory for many applications, they do not, in general, provide the various advantages of the invention described below.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an automatic control circuit that automatically switches in a backup voltage when the main or primary voltage has dropped below a first reference level as would occur, for example, in the event of the discharge of a battery providing the main voltage, and then automatically switches back to the main voltage supply once the failure has been corrected and predetermined conditions have been met, i.e. when the main voltage has returned to a higher second reference level, e.g., after recharging of the main battery.

It is another object of the invention to have the backup voltage remain switched in as the usable output of the control circuit throughout a range of main supply voltages, and not to switch back to the main supply until a high end of a window voltage range is reached or exceeded by the main voltage supply.

It is an additional object of the invention to provide for adjustable settings of both the first reference level at which the backup voltage switches in, and the second reference level at which the backup voltage drops out and the main voltage is switched back in.

According to a preferred embodiment of the present invention, an input sensor detects a voltage of the main voltage supply and compares this voltage with a first reference level, and outputs either a high or a low signal depending on whether the main voltage is above or below the first reference level. The output of the sensor is inputted to a switching network comprising solid state switching components and a mechanical relay. The relay is normally in a nonenergized state, and the main supply voltage is connected as output of the control circuit, when the sensed main voltage is above a first reference level. When the main voltage falls below this reference level, the changed output from the sensor circuit activates the switching network which energizes the relay to connect the backup voltage supply in a parallel connection with the main voltage as the usable output voltage of the control circuit.

An important feature of the control device concerns the provision of a stablizing compacitor and a feedback resistor connected from the switching network to the input sensor. This provides a reference voltage for establishing a "window" range of main voltages during which the backup voltage is connected as the usable output voltage for powering an external load, such as a portable cellular telephone, connected to the control circuit. A solid state switch circuit is used in conjunction with the mechanical relay to provide the means for switching between the switched and unswitched voltage conditions. Protective diodes isolate the main voltage and the backup voltage supplies. In one embodiment, the control device can be operated with a single output voltage providing either the unswitched (main voltage supply) or the switched (backup voltage supply in parallel with the main voltage supply) as the output voltage. In another embodiment, the control device can provide a switched second voltage output through, e.g., an ignition switch of an automobile, to furnish two separate power supplies to equipment requiring such dual voltage sources.

These, and other advantages of the invention will be apparent from the detailed description and appended drawing Figures which follow.

BRIEF DESCRIPTION OF THE DRAWING

Fig. 1 shows a block diagram of a control circuit according to the invention.

Fig. 2 shows a schematic of a control circuit according to the invention.

Fig. 3 shows a flow chart representing steps of the method according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Fig. 1 is a block diagram that depicts the basic components of the control circuit 1 of the invention. An input sensor 11 senses voltage V-^ and provides an output signal to switching network 12 which comprises feedback means that feeds a voltage back to input sensor 11. Main voltage V_. ^anc backup voltage V_cc are inputted to switching network 12 which operates so that either only V_lf or the parallel combination of V-! and V_cc, appears as output at terminal 9. Backup voltage V_cc is also delivered to output terminal 10 when relay RL-1 is energized so that dual parallel voltage sources are outputted from the control circuit to power, for example, two separate pieces of equipment, such as a mobile telephone and a portable scanner. A more detailed description is seen in Fig. 2.

Fig. 2 shows a schematic diagram of the control circuit according to the invention. The circuitry can be broken down into basically input sensor 11 and switching circuit with feedback means 12, as shown in Fig. 1, which are indicated by broken lines 11, 12 in Fig. 2.

Input sensor 11 is built around a comparator intergrated circuit chip IC-1, which could be, for example, the LM339 as indicated in the schematic circuit diagram of Figure 2. The input voltage to IC-1 is determined by the setting of potentiometer R-l which is connected to main voltage V^ . Bypass capacitor C-l is used to prevent undesired oscillations at the input of IC-1. V-_j^ could be, for example, a standard 12-volt automobile battery used to power a mobile cellular telephone, with the control device of the invention serving to provide a backup voltage supply to the mobile telephone in the event of a failure in the automobile's main 12 VDC main battery.

A reference voltage at pin 4 of IC-1 is determined by the voltage divider of resistors R-3 and R-4, and the feedback voltage across resistor R-8. The bias of the output logic signal at pin 2 of IC-1, which is either in a HIGH or LOW state, is set by resistor R-2. Capacitor C-2 provides a time delay of about two seconds to prevent a falsing by momentary voltage drop-outs at the V_ref input, pin 4 of IC-1. Both input sensor 11 and the switching network 12 are powered by backup voltage V_cc which receives a continuous trickle charge from main voltage V-^ through diode D4 and resistor R-6.

Switching network 12 receives as its input signal the output from pin 2 of IC-1 of input sensor 11. Transistors TR-1 and TR-2, (which as indicated in Figure 2 can be a 2N3904 and a ECG-188, respectively) provide an output current when transistor TR-2 is turned on that energizes mechanical relay RL-1. Resistor R-5 establishes the DC voltage bias at the base of TR-2, and resistor R-8 determines the amount of voltage feedback to control the level of V_ref of comparator IC-1. Diode D-l absorbs voltage spikes from relay R-l to prevent damage to transistor TR-2. Diode D4 and resistor R-6 maintain the trickle charge on backup voltage V_cc at all times when V_cc is not switched into the circuit as the usable supply voltage.

In Fig. 2, switch arms 20, 21 of relay RL-1 show by the solid lines the relay in its energized state. This delivers both V_. and V_cc in parallel at each of output terminals 9, 10. Blocking diodes D2 and D3 maintain the proper polarity for outputting the parallel combination of the two voltage sources. The dashed lines of switch arms 20, 21 show relay RL-1 when it is not energized, i.e. when main voltage V-^ is sufficiently high. When not energized, the relay causes output terminal 9 to deliver main voltage V-^ and, in a preferred embodiment, terminal 10 to deliver voltage Ace, which is the vehicle's switched (ignition switch) 12 volt source.

Various modes of operation are possible, with the control device wired in at the time of installation according to the operating mode desired. For example, one mode would be to have a switched and unswitched output available at terminal 10, as indicated in Fig. 2 where the Ace voltage is the vehicle battery V_1# as switched on and off, for example, by the vehicle's ignition switch. In this mode, when the ignition switch is turned on, and relay RL-1 is energized, the parallel combination of V_cc and V-_j^ appears at terminal 10; with RL-1 not energized, diode D4 blocks voltage V_cc and vehicle battery V^ appears at terminal 10 if the ignition switch is on, and no output (zero volts) is at terminal 10 with the ignition switch off. Another mode of operation would be, for example, to have only a switched output available, and a third mode could be having a constant, or unswitched, output available. Appro¬ priate wiring at the time of installation would accommodate the operating mode for the exact output desired.

The backup supply voltage in this case is shown as a 12 volt, "Gel Cell" battery 13, which typically has a 4 ampere hour capacity, with its positive output connected through a 10 amp fuse F-l to the control device as voltage V_cc. This backup voltage supply could be mounted with the control device attached to its terminals in the trunk of an automobile, for example. Turning now to a typical operation of the control device as used in a preferred embodiment of supplying a backup voltage to a mobile cellular telephone in an automobile, input terminals V-^ and V_cc of the device are connected to the automobile main 12 VDC battery and to a backup voltage "gel cell" battery, respectively, as discussed above. Potentiometer R-l is adjusted to provide a 12 volt DC input at pin 5 of IC-1, while V_ref is set, for example, for 10.5 volts by means of resistors R-3 and R-4. The output voltage of comparator IC-1 at pin 2 is HIGH until the voltage at pin 5 drops below the 10.5 volt reference voltage at pin 4 of IC-1. Resistor R-2 functions as a pull up resistor to maintain a high state at the base of transistor TR-1, since TR-1 will conduct as long as pin 2 of IC-1 remains above ground. The collector of TR-1 is normally at essentially ground potential which keeps TR-2 turned off, therefore TR-2 does not output any current from its emitter and relay RL-1 is not energized. When the input voltage at pin 5 drops below 10.5 volts, the output of IC-1 at pin 2 goes LOW, turning off shunt transistor TR-1, which increases the voltage at the base of TR-2, allowing that transistor to conduct and energized relay RL-1 to connect the backup Gel Cell battery to the external load. The external load in the embodiment being described is a mobile cellular car telephone, but it will, of course, be understood that the load could be any other external load, such as a two way radio, emergency flasher lights, or equipment in marine applications, e.g. emergency navigational equipment.

When transistor TR-2 conducts, in addition to energizing relay RL-1, a voltage is also established across feedback resistor R-8 based in part on the resistor divider network R-8 and R-4. This voltage feedback to the reference terminal of comparator IC-1 has the effect of increasing the reference voltage at pin 4, thereby increasing the input voltage needed to turn IC-1 off. In this example, a 33 Kohm value of R-8 has the effect of increasing the voltage at pin 4 from 10.5 to 12.5 volts, thereby keeping the backup voltage in the circuit as the usable output voltage from the control circuit until main supply V _^ is fully recharged, replaced, or the problem otherwise corrected. When the main voltage supply returns to the level of 12.5 voltage or above, this changes the output state of comparator IC-1 of the input sensor so that the comparator output signal at pin 2 goes from a LOW state back to a HIGH state, thereby turning on transistor TR-1, which turns off transistor TR-2 by reducing the voltage at the base of TR-2 to essential ground potential, and relay RL-1 is de-energized with no current flowing through it. Also, no feedback voltage is developed across resistor R-8, so the reference voltage at pin 4 of IC-1 drops from 12.5 volts back to its initial setting of 10.5 volts.

In an alternative embodiment, the control circuit could be supplied with its own nickle-cadmium rechargeable battery supply, packaged as a single unit to provide a source for emergency backup voltage.

Although the embodiment described above is concerned with providing emergency power in the event of a battery or electrical system failure, other applications in many fields using portable electronic systems are equally suitable for the device according to the invention. Illustrative of such applications would be in powering equipment in the emergency medical fields, such as providing backup power for ambulance radios, Advanced Life Support communications, and also for satellite telemetry systems in connection, for example, with a solar cell array voltage supply.

The terms and expressions which have been employed herein are used as terms of description and not of limitation, and no limitation is intended by the use of such terms and expressions, it being recognized that various modifications, including equivalents of the features shown and described, are possible within the scope of the invention claimed.

Claims

We Claim:

1. An automatic control circuit for supplying a backup voltage when a main voltage falls below a predetermined reference level, said circuit comprising:

(a) comparator means, having a first input connected to a main voltage, a second reference input and an output for producing a first output signal when said main voltage falls below a first reference level at said reference input and for producing a second ouput signal when said main voltage rises above a second reference level at said reference input, where said second reference level is higher than said first reference level;

(b) switching means, connected to the output of said comparator means and comprising at least one transistor and a relay for switching a control circuit output from said main voltage to a backup voltage, responsive to said first output signal from said comparator means, and for switching the control circuit output of the control circuit back from the backup to the main voltage response to said second output signal.

2. An automatic control circuit in accordance with Claim 1, wherein the first reference level and the second reference level establish a window range of main voltage during which the backup voltage is connected as output of the control circuit.

3. An automatic control circuit in accordance with Claim 2, wherein said switching means comprises activation means to keep said backup voltage switched in over a predetermined window range of said first and second reference levels.

4. An automatic control circuit in accordance with Claim 3, wherein said activation means comprises a feedback resistor connected from said switching circuit to said comparator second reference input.

5. An automatic control circuit in accordance with Claim 4, wherein said switching network comprises voltage protection means using a time delay capacitor connected between said comparator means output and said second reference input to prevent transitory voltage changes at said second reference input.

6. An automatic control circuit in accordance with Claim 5, wherein said switching means comprises means for automatically maintaining said backup voltage in a trickle charge mode when said backup voltage is not connected as output of the control circuit.

7. An automatic control circuit in accordance with Claim 6, wherein said switching means comprises means for supplying voltage as output of the control device from a parallel connection of both said main voltage and said backup voltage.

8. An automatic control circuit in accordance with Claim 7, wherein said control circuit comprises a plurality of output terminals for supplying the main voltage as an output and the backup voltage as an output at the same time from different output terminals of the control circuit.

9. An automatic control circuit for providing a backup voltage comprising:

(a) a comparator having a first input connected to a main voltage, and a second input connected to a reference voltage, wherein said comparator provides an output signal as a function of said main voltage and of said reference voltage; (b) a backup voltage supply consisting of an internal battery connected to said control device; and

(c) switching means having a first input connected to said comparator output, a second input connected to said main voltage, and a third input connected to said internal battery, wherein said switching means is responsive to said comparator output signal in connecting either said main voltage or said internal battery to a control circuit output terminal.

10. A method for automatically providing a backup voltage in the event of a power failure or other malfunction of a main voltage, comprising the steps of:

(a) sensing an input voltage with a control circuit having the main voltage connected as an first input, a backup voltage connected as a second input, and comprising a switch network to connect either the main voltage or the backup voltage as a useable output voltage of the control circuit;

(b) comparing the input voltage with a reference voltage, and outputting to the switch network one of two possible signals depending on whether the input voltage is higher or lower than a reference voltage;

(c) switching the main voltage as the useable voltage output of the control device when the input voltage is greater than a first reference voltage, and switching the backup voltage as the useable voltage ouput of the control circuit when the input voltage is less than a second reference voltage; and

(d) feeding back from a switching means to an input reference voltage terminal of a comparator a voltage that varies the reference voltage to establish a window range of input voltages over which the backup voltage will be delivered as the useable output voltage of the control circuit.

11. An automatic control device for providing a backup voltage when a main voltage has dropped below a predetermined level, comprising: (a) an input sensor that compares a main voltage to a reference voltage and provides an output level as a function of the main voltage; and (b) a switching circuit comprising at least one transistor that receive as input the output level of the input sensor and causes a relay to operate when there is a change in said output level to deliver either the main voltage or the backup voltage as a usable output voltage of the control device, wherein the switching network and the input sensor are connected by a feedback resistor that establishes a window voltage range over which the backup voltage stays switched in as said usable output voltage.