Electrical transmission or interconnection systems – Switching systems – Condition responsive
Reexamination Certificate
1999-09-30
2001-09-11
Jackson, Stephen W. (Department: 2836)
Electrical transmission or interconnection systems
Switching systems
Condition responsive
C307S039000, C307S126000
Reexamination Certificate
active
06288458
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to low energy switches. The invention relates specifically to a mechanically actuated, power stealing, solid state switch providing a drop-in replacement for mechanical or electromechanical low energy switches.
Mechanical and/or electromechanical low energy switches, i.e., switches carrying low current at low voltage, designed for no-spark applications, e.g., 10 milliamperes (mA) at 5 volts (V) which may be used for small indicator lights, digital logic, or the like, suffer from corrosion build-up, oxidation, or contamination at their electrical contact points because there is not sufficient energy carried by the switch to create arcing or spark at the contacts in order to burn off the accumulation of contaminating material when the switch is actuated. Snap springs, armatures and like physical members are also subject to fatigue, assembly problems and packaging difficulties. Solid-state switches do not suffer from these problems.
Power stealing switches, which steal a small portion of power supplied to an actuated device and boost this stolen power to a voltage level sufficient to operate the power stealing switch, are known in certain contexts. The primary motivation for power stealing is to avoid rewiring of existing circuitry. In the art, there are power stealing switches, or parts thereof, in the context of thermostat control typically using twenty-four volt alternating current (AC) or ten-volt direct current (DC) at about 0.5 ampere. These switches steal power at the zero crossing points on the AC waveform. This opens the circuit for a short time and produces voltages that exceed the minimum digital logic low level, typically 0.5 volt. In the DC mode and with the switch on (ON) rather than off (OFF), the voltage is also greater than 0.5 volt. The related art has one or more voltage diode drops involved in the power stealing circuits and therefore does not allow operation in circuits with voltages as low as 1.5 volts.
It would therefore be desirable to provide a low power, solid state, two- or three-terminal switch with power stealing. Such a switch is further desirably connectable in any circuit configuration of current polarities and voltages ranging from 1.5 volts to 30 volts. A practical low power realization of a manually actuated, direct current, solid state switch of this type does not appear to be in the art.
SUMMARY OF THE INVENTION
Accordingly it is an object of the present invention to disclose a power stealing, solid state, manually actuated, drop-in replacement, low power switch for existing electromechanical low power switches. The switch design is preferably executed with a single IC substrate or die to be practical and economical.
In general the present invention preferably has an actuator assembly for mechanical operation of the switch, a power stealing section for supplying power to run the switch, a logic section for control of the switch functions, and output transistors acting to open and close the switch.
The actuator may comprise a plunger mechanically moving a variable capacitor, which is sensed and amplified with a built-in hysteresis to prevent ON/OFF cycling chatter. The power stealing circuit has little or no diode drop and has very low leakage and/or operating current in the OFF state, e.g., less than 50 microamperes, for operating a steering or control circuit. This circuit automatically steals power from the terminal with the highest voltage of the two or three terminals, and returns the current to the lowest voltage terminal or the lowest voltage at the IC (CMOS) substrate voltage level or ground. This thereby allows a direction-insensitive connection of the attached device, i.e., the switch's power supply, to the switch terminals. That is, the terminals are not dedicated, and the switch will work however it is connected to the device.
All functional groups of the present embodiment are designed to minimize power consumption of the solid state switch while providing a switch that can handle 1.5 to 30 volts DC and a 250 microampere (&mgr;A) to 100 mA load in the embodiment, or, generally, the maximum power permitted by the substrate or IC used. A 1.0 ampere ten-times inrush, nonrepetitive current capability for powering start-up of cold filament indicator lights is further provided. The maximum current can be increased by using larger output field effect transistors (FETs); however, this may necessitate an increase in the die size.
The switch circuitry detects and operates in the two-wire or three-wire mode. In the three-wire mode, the switch steals current in microamperes from the open terminal and provides an ON resistance in the milliohm range between the two remaining terminals. In the two-wire mode, the output of the device is clamped to less than 0.5 volt, or less than the digital logic low level for TTL or CMOS logic. This clamped ON voltage is then charge-pumped up to voltages sufficient to operate the internal circuitry of the switch.
The present embodiment details a combination two/three-terminal, single pole double throw (SPDT) style switch which steals only 0.5 VDC (ON state), and 50 &mgr;A maximum (OFF state) for operation. It can function as a two-terminal single pole, single throw style switch. The switch is easily shielded and sealed, and insensitive to current direction and voltage hookup to the terminals.
A solid state switch that has the same form, fit, and function as an electromechanical SPDT switch and offers high reliability in low energy applications. The solid state switch uses power stealing for circuit operation to perform all the functions of a SPDT switch including bi-directional current flow from any of the SPDT terminals. The three terminals of the SPDT switch can be connected in ten combinations of polarity (four combinations of polarity for the two wire single pole single throw (SPST) style switch having normally opened or normally closed contacts, and six combinations of polarity for the three-wire SPDT switch configurations). The switch employs efficient power stealing with no diode drops and therefore can work down to three volts. The present circuit can be implemented using a single die of conventional bulk CMOS.
The SPST is used with a two-wire operation. A charge pump that operates continuously on only 0.5 volts input and has an output of 2.5 volts (five-times charge pump). The input is less than a typical FET threshold voltage (1.0 volt). If the switch is commanded to be “ON” during the initial power up there is a delay of less than 500 &mgr;s to allow the charge pump to start. This time is less than the typical switching time of the equivalent mechanical switch (1 ms). Once the charge pump is running, the output voltage (2.5 volts) is used internally to run itself. At this time the input voltage can be clamped to 0.5 volts and still maintain the internal voltage of 2.5 volts.
The constant ON state voltage control is independent of load current. The ON state voltage is below the worst case digital logic low level.
There is on-chip polarity sensing for bi-directional operation. FETs operating in a reverse mode allow for polarity independent operation.
The SPDT can be used for three-wire operation. The three-terminal (SPDT) power stealing circuit is independent of polarity. This smart power stealing circuit seeks the most positive and most negative of the three terminals. The most negative terminal potential is assigned to the substrate potential voltage. Therefore, this switch can be implemented in conventional bulk CMOS without the use of silicon-on-insulator (SOI) technology or two dies to isolate the three terminals.
There is efficient power stealing with no diode drops. If all three terminals have different voltages by greater than the FET threshold voltage there are no diode drops. If the two input terminals have the same voltage, the output is reduced by only one diode drop. If no diode drops are needed, two of the two-terminal power stealing circuits can be used by switching them during the break before make time.
Break-before-make lo
Abeyta Andrew A.
Honeywell International , Inc.
Jackson Stephen W.
Polk Sharon
Shudy, Jr. John G.
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