Voltage inverter power conservation circuit

A power conservation circuit for a battery-powered voltage inverter is capable of conserving battery power. In a first embodiment, the circuit includes a switch, connected between the input terminals and the output terminals, for switching the circuit between an ON state wherein AC voltage is connected to the output terminals, and an OFF state wherein the inverter is disconnected from the battery. A first sensor senses in the ON state the absence of a load connected to the output terminals and switches the circuit from the ON state to the OFF state. A second sensor senses in the OFF state the presence of a load connected to the output terminals and switches the circuit from the OFF state to the ON state. The switch includes a relay having a pole terminal connected to the output terminals, a first contact terminal connected to the AC output of the inverter, and a second contact terminal connected to battery voltage. In a second embodiment, in an OFF state, the switch connects to the output terminals an AC signal voltage having a frequency measurably different from the frequency of the AC voltage from the inverter. In a third embodiment, the switch switches the base of an inverter power transistor. In a fourth embodiment, the voltage inverter includes a plurality of switched voltage inverter modules.

TECHNICAL FIELD 
The invention relates generally to control circuits for battery-powered 
inverters and more specifically to circuits for minimizing inverter power 
consumption. 
BACKGROUND OF THE INVENTION 
The following references describe relevant background art in that they 
disclose control circuits for controlling battery-powered inverters used 
for AC power supply backup. 
U.S. Pat. No. 4,508,974 to Henderson discloses a logic controlled 
battery-powered inverter for use with a motor generator set that is used 
to provide clean, noise-free power to a load such as a computer. The 
inverter operates only when normal utility power fails, and employs the 
ability of the motor generator set to handle brief power outages to power 
the load for the brief time needed to decouple the main line power supply 
and couple the inverter to the motor generator. 
U.S. Pat. No. 4,667,116 to Honjo et al. discloses an invertor control 
circuit for switching a load from an inverter to a commercial power source 
uninterruptedly by means of a thyristor when access to the synchronization 
circuit of the inverter is required for repair or servicing. 
U.S. Pat. No. 4,946,096 to Ballard et al. discloses a method and apparatus 
for operating a furnace from a 12 V DC battery. A furnace which normally 
operates from a residential AC power source is adapted to operate on 
auxiliary DC power by changing the normal control function to prevent the 
furnace from operating in the high heat mode during periods of auxiliary 
power use. 
U.S. Pat. Nos. 4,956,563 and 5,055,703 to Schornack, the Schornack '563 and 
the Schornack '703 references, disclose a battery operated standby 
inverter power supply using an electromechanical relay switching network 
to connect and disconnect line power between input and output terminals. 
An overcurrent relay pulser rapidly opens the switching system relay 
contacts upon transition to battery mode, and a breakover diode network 
accelerates field collapse in the relay coil attendant to reversion of the 
relay to its normally closed condition attendant to each battery to line 
transition. A fault detector establishes acceptable line voltage 
thresholds, the exceeding of either of which triggers operation from line 
to battery mode. 
U.S. Pat. No. 4,977,351 to Bavaro discloses an emergency lighting system 
which permits at least one gas discharge lamp to be operated from an AC 
power source when AC current is present and from a battery when AC signal 
is not present. To conserve battery power in the DC mode, controls are 
also provided for turning off the light or reducing its output level in 
response to such control inputs as an ambient light detector or manually 
operated dimmer control. 
U.S. Pat. No. 5,001,623 to Magid discloses a power supply which 
automatically adapts to different input power sources. A double pole, 
double throw, latching relay switches the primaries of the rectifier 
transformer to a series or to a parallel configuration depending upon the 
output voltage of the rectifier, thus adapting the circuit to either a 
nominal 220 V AC source or a nominal 120 V AC source. When a 24 V DC 
source is connected to the DC input, it is also connected to the rectifier 
output through a diode, so that power is furnished to the load by 
whichever source has the higher voltage. In this configuration, a battery 
pack can be connected to the DC input to supply automatic backup power. 
U.S. Pat. No. 5,010,469 to Bobry discloses an uninterruptible power supply 
having an inverter that operates in either high voltage DC supplied by a 
line-powered power supply or on low voltage DC batteries. With line power, 
the inverter operates as an H-bridge inverter. With battery power, the 
battery voltage is applied to a smaller portion of the inverter's 
transformer primary. The changeover from line to battery power is 
determined by when an intermediate voltage tapped from the transformer 
drops below the battery voltage. 
U.S. Pat. No. 5,017,800 to Divan discloses an apparatus for AC to DC to AC 
power conversion. It includes a rectifier bridge connected to an AC 
source, the rectifier bridge providing a DC output to DC bus lines. A full 
bridge of active switching devices is connected across the DC bus lines. 
The output of the bridge can provide AC output power to the load through a 
transformer. By providing an external DC power source such as a battery, 
uninterrupted power can be supplied to the load when the AC power source 
fails. 
None of the above-mentioned references attempts to minimize inverter power 
consumption when no appliance is connected to the load terminals. 
SUMMARY OF THE INVENTION 
A power conservation circuit for a battery-powered voltage inverter is 
provided capable of minimizing power consumption under no-load and 
low-load conditions. In a first embodiment, the circuit includes a switch, 
connected between the input terminals and the output terminals, for 
switching the circuit between an ON state wherein AC voltage from the 
inverter is connected to the output terminals, and an OFF state wherein 
the inverter is disconnected from the battery. A first sensor senses in 
the ON state the absence of a load connected to the output terminals and 
provides a first signal indicative of the absence of a load. A second 
sensor senses in the OFF state the presence of a load connected to the 
output terminals and provides a second signal indicative of the presence 
of a load. On receiving the first signal, the switch switches from the ON 
state to the OFF state. On receiving the second signal, the switch 
switches from the OFF state to the ON state. The switch connects battery 
voltage to the output terminals in the OFF state. The second sensor senses 
current from the battery flowing through the output terminals in the OFF 
state. The first sensor senses AC current flowing through the output 
terminals in the ON state. The switch includes a relay having a pole 
terminal connected to the output terminals, a first contact terminal 
connected to the AC output of the inverter, and a second contact terminal 
connected to battery voltage. 
In a second embodiment, in the OFF state, the switch connects to the output 
terminals an AC signal voltage having a frequency measurably different 
from the frequency of the AC voltage. The second sensor senses AC current 
at the frequency of the AC signal voltage passing through the output 
terminals. 
In a third embodiment, a voltage inverter is provided, having a power 
transistor and a switch for switching the inverter between an ON state 
wherein the AC voltage is connected to the output terminals and an OFF 
state wherein battery voltage is connected to the output terminals. In 
this embodiment the switch switches the base of the power transistor. The 
switch may be a relay or a transistor. 
In a fourth embodiment, a battery-powered voltage inverter is provided 
having a plurality of voltage inverter modules. Each module has a powered 
state and a power-conservation state and each module is connected to the 
output terminals. A quantity related to the AC current flowing through the 
output terminals is measured to provide a signal indicative of the AC 
current. This signal is used to switch just sufficient of the voltage 
inverter modules to a powered state to supply the measured current.

DESCRIPTION OF SPECIFIC EMBODIMENTS 
The invention provides a power conservation circuit for minimizing the 
power consumption of a battery-powered voltage inverter under no-load and 
low-load conditions. 
In a preferred embodiment, the power conservation circuit has an ON state 
in which battery voltage (12 V DC) powers the inverter and inverter output 
voltage (120 V AC) is applied to a load via a pair of output terminals, 
and an OFF state in which battery voltage is disconnected from the 
inverter and applied to the output terminals. 
The power conservation circuit remains in the OFF state as long as there is 
no load across the output terminals. When a load appears across the output 
terminals and the circuit is an OFF state, a small current flows, powered 
by the 12 V DC. This current is used to switch the power conservation 
circuit from the OFF state to the ON state. 
The power conservation circuit remains in the ON state as long as load is 
maintained across the output terminals. The presence of load is sensed by 
the flow of alternating current. The cessation of alternating current is 
used to switch the power conservation circuit from the ON state to the OFF 
state. 
FIG. 1 shows the power conservation circuit 12, having circuit output 
terminals c-c, of a preferred embodiment of the present invention. In the 
ON state, relays 7 and 4 are both energized so that contact 10 is closed 
(relay 7) and terminals c-c are connected across a-a (relay 4). Thus, in 
the ON state, (i) battery 2 powers inverter 1 via switch contact 10 and 
connection d-d and (ii) the inverter supplies 120 V AC power via contacts 
a-a to circuit output terminals c-c and thence to load 3. In the OFF 
state, relays 7 and 4 are both de-energized so that (i) the inverter is 
disconnected from the battery and from the load and (ii) battery 2 
supplies 12 V DC via resistor 5 and contacts b-b to terminals c-c. As long 
as there is no load across terminals c-c the control circuit remains in 
the OFF state. When a load is applied (i.e., load 3 becomes capable of 
passing current), a small current from battery 2 flows through resistor 5, 
through load 3 via contacts b-b to the base of power transistor 6, 
switching on the transistor. Transistor 6 may be an NTE 392 NPN power 
transistor. Switching on transistor 6 energizes double-pole double-throw 
(DPDT) relay 4 and single-pole, single-throw (SPST) relay 7. Relay 4 
disconnects the load from the battery and connects the load via contacts 
a-a to the 120 V AC output of inverter 1. Relay 7 connects the battery to 
the coil of relay 4 thereby holding relay 4 energized via transistor 6. 
Relay 7 via (closed) contact 10 and connection d-d reconnects battery 
power to inverter 1, turning the inverter on. 
When the circuit is in the ON state, supplying 120 V AC power via contacts 
a-a to load 3, load current flows through coil 11 inducing a current in an 
inductive pickup coil 8. This induced current, applied to the input of 
operational amplifier 9, holds power transistor 6 ON. Operational 
amplifier 9 may be a Radio Shack LM234N Quad Op-Amp. 
When load is removed (i.e., load 3 stops passing current) the induced 
current falls to zero, transistor 6 switches OFF, relays 4 and 7 
de-energize, load 3 is disconnected from the inverter output, contact 10 
disconnects the inverter from the battery, and contacts a-a connect the 
battery to the load via output terminals c-c. 
In another embodiment the inverter is never disconnected from the battery 
but the base of each power transistor in the inverter is switched by a 
contact of relay 7 from the inverter power conservation circuit as shown 
in FIG. 2. Alternatively, a transistor or other semiconductor switch could 
be used. 
An important aspect of the invention is that it provides a power 
conservation circuit, having load terminals, for a battery-powered 
inverter, the circuit having an ON state in which the load terminals are 
connected to inverter AC output voltage and an OFF state in which the load 
terminals are connected to DC battery voltage for load sensing. The 
battery not only provides power to the inverter when in an "ON" condition 
but which also supplies battery voltage to the load terminals in an OFF 
condition so as to detect the change in load when an appliance is 
connected across the load terminals. 
Capacitive and Inductive Loads 
With the power conservation circuit of FIG. 1, when the load is primarily 
capacitive, as is the case with some types of fluorescent lights, the load 
may draw insufficient current from the 12 V DC source for the control 
circuit to operate properly. In an improved embodiment of the present 
invention, an alternating or pulsating DC signal from a wave form 
generating integrated circuit 31 is connected to the load end of resistor 
5 in FIG. 1. The wave form generating circuit may be a Texas Instruments 
555 timer integrated circuit. FIG. 3 shows a portion of the circuit of 
FIG. 1 with wave form generating integrated circuit 31 connected to the 
load end of resistor 5. The AC signal, having a frequency distinguishably 
greater than 60 Hz and preferably between 100 Hz and 1 k Hz passes through 
the capacitive load and switches the power conservation circuit. FIG. 3 
shows an AC voltage component 32 riding on a DC offset voltage component 
33. The DC component is necessary for inductive loads. For example, a 
motor winding acting as a high inductance filter may block AC signals. To 
enable the power conservation control circuit to work with loads that may 
have a combination of resistive, capacitive and inductive characteristics, 
a load-detect voltage having a 12 volt DC component with an 2 volt AC 
component riding on top of it is provided. 
Varying Loads 
To minimize power consumption when supplying a given load, the capacity of 
the inverter should match the load. An inverter that is required to power 
a varying load is usually sized to supply the largest load. However, when 
the load decreases, the inverter power consumption does not decrease. A 
further improvement of the present invention minimizes the power 
consumption of inverter which is connected to a varying load by 
dynamically adjusting the capacity of the inverter to match the size of 
load. The inverter includes several pairs of output transistors (Q1 and Q2 
of FIG. 2 repeated as Q3 and Q4; Q5 and Q6; etc.). These pairs of 
transistors are connected in parallel to deliver power to the primary 
winding of the output transformer. By switching on only the number of 
transistor pairs necessary to supply the required amount of power to the 
load (e.g., just Q1 and Q2) and leaving the rest of the transistor pairs 
switched off, less power is delivered to the primary winding of the output 
transformer of the inverter. Whenever a larger load is applied, more 
transistor pairs are switched on to supply the increased power (e.g., Q1 
and Q2; Q3 and Q4; and Q5 and Q6). 
A measurement of load current is used to control the switching on of the 
appropriate number of transistor pairs. Load current passing through coil 
11 induces a voltage in inductive pickup device 8 of FIG. 1 that is 
proportional to the load. This voltage is applied to the analog input of 
analog to digital (A/D) converter 41 of FIG. 4. The A/D converter produces 
a digital signal indicative of the reciprocal of the load impedance. 
Digital signals from the A/D converter are used to switch on and off pairs 
of transistors (Q1 and Q2; Q3 and Q4; Q5 and Q6; etc.) so as to supply 
sufficient current to the load while minimizing power consumption. Digital 
output 1 of the A/D converter shown in FIG. 4 is connected to the base of 
transistor Q1 and to the base of transistor Q2. Likewise, digital output 2 
is connected to the base of transistors Q3 and Q4, digital output 3 to the 
base of transistors Q5 and Q6, and so on.