Low leakage, solid state a-c power contact

A power contact with low off current for switching a-c loads in response to a logic signal includes a rectifier bridge circuit in a series with the load and the a-c power source, a MOSFET connected across the d-c terminals of the bridge, and a series RC filter circuit in parallel with the MOSFET. A metal oxide varistor connected across the d-c side of the bridge protects the MOSFET from transients in the a-c circuit. While this power contact is capable of switching sizable a-c currents, the leakage current with the MOSFET off is less than 1/10 of a milliamp so that it can also be used to switch loads which draw very low level currents. The MOSFET is electrically isolated from the logic signal by an opto-isolator which has a unique switchable output which permits the power contact to be readily adapted for use with normally energized or normally deenergized loads.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to solid state power contacts for switching a-c 
power in response to logic signals and particularly to such power contacts 
having low leakage characteristics, yet also being capable of switching 
sizable currents. The invention further relates to power contacts in which 
the logic circuit controlling the switch is electrically isolated from the 
a-c circuit being switched and in which the contact is protected from 
surges in the a-c circuit. 
2. Prior Art 
It is common today to use low level d-c logic signals to control a-c power 
circuits. For instance, in many applications the output of a digital 
computer is used to control components energized by an a-c power source. 
These components can include a wide range of devices from highly inductive 
loads such as motors which draw a substantial amount of current to 
indicators such as neon bulbs which only draw 1 to 2 milliamperes of 
current. It is highly desirable to have to provide only one type of power 
contact to accommodate all of these types of loads. 
However, these various loads impose their own sometimes opposing 
limitations on the power contact. The highly inductive loads generate 
switching transients which must be suppressed to protect the contacts. The 
load devices operated by very low currents require that the power contacts 
have very low leakage current when in the off state in order to deenergize 
these devices. Unfortunately, the devices normally used to suppress the 
transients caused by the inductive loads add to the leakage current. The 
whole problem is further compounded in instrumentation circuits which must 
meet IEEE surge protection specifications. 
Relays are one type of device used as power contacts in computer 
interfaces. In addition to being comparatively slow, consuming substantial 
power and not being as dependable as solid state switches, relays require 
sizeable RC snubber circuits across the contacts to suppress the 
transients generated by inductive loads. In one application where the 
leakage current through the snubber was substantial enough to cause a 
stepper motor being controlled to skip positions, a rectifier bridge 
circuit was inserted between the a-c circuit and the relay so that the 
contacts switched d-c current to control the a-c current. 
Solid state switches such as back to back thyristors and triacs are also 
used to control a-c circuits with d-c logic signals. These devices also 
require RC snubbers to suppress switching transients in the a-c circuit 
which also generate a-c leakage currents unacceptably high for the very 
low current loads such as neon indicator bulbs. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a solid state power contact for 
switching an a-c power source in response to a digital logic signal 
utilizes a MOSFET as the switching device. The MOSFET is inserted into the 
a-c circuit through a rectifier bridge circuit connected in series with 
the a-c source and the load. A series resistance capacitor filter circuit 
connected across the MOSFET at the d-c terminals of the rectifier bridge, 
provides the constant d-c voltage for operation of the MOSFET. 
The MOSFET is protected from transients in the a-c current by surge 
suppression means, such as a metal oxide varistor, connected across the 
d-c terminals of the rectifier bridge which limits the voltage applied by 
the transient to the MOSFET to a value between the peak voltage of the a-c 
waveform and the rated voltage of the switching device. In the preferred 
embodiment of the invention where the MOSFET is electrically isolated from 
the digital logic signal and is referenced to a floating ground, 
additional surge suppression means are connected between the MOSFET source 
electrode and earth ground. 
The leakage current, that is the current with the MOSFET turned off, is 
determined by the components on the a-c side of the bridge, namely the 
surge suppression means and the MOSFET. Thus, the invention avoids the 
sizable a-c leakage currents which have plagued most prior art a-c power 
contacts. The leakage current of the subject invention is below that which 
would sustain energization of even load devices which operate at very low 
current levels, and in the exemplary embodiment of the invention, the 
leakage current is less than about 1/10 milliamp. Since the MOSFET is 
capable of handling large currents also, the power contact of the 
invention is useful for switching a-c power to a variety of loads having 
widely varying current requirements. 
In accordance with another aspect of the invention, the digital logic 
signal is applied to the gate electrode of the MOSFET through a drive 
circuit which selectively provides for the MOSFET to be either turned on 
or off in response to an active logic signal. This feature is provided, 
together with electrical isolation of the MOSFET, by an opto-isolator 
having its output transistor connected across two output terminals. The 
first output terminal is connected to a floating power supply through a 
first output resistor and the second is connected to a floating ground 
through a second output resistor. By connecting a jumper between one 
output terminal and the gate of the MOSFET and shunting the output 
resistor connected to the other output terminal, a selection can easily be 
made between a drive signal for the MOSFET which either has the same sense 
or is complimentary to the logic signal. Thus, the power contact of the 
invention is easily adapted for use in controlling normally engergized or 
normally deenergized loads with a digital logic signal.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
As shown in the drawing, the power contact 1 of the invention controls the 
energization of a load 3 by an a-c power source 5 in response to a logic 
signal 7. The a-c power source 5 typically provides 120 volt, 60 hertz 
power to the a-c circuit 9. The a-c terminals 11 of a rectifier bridge 
circuit 13 are connected in series with the load 3 and the a-c source 5. 
The d-c terminals 15 of the rectifier bridge circuit 13 are connected to 
the source (S) and the drain (D) electrodes of a MOSFET 17. A suitable 
MOSFET is the IRF 451 n-channel enhancement mode device manufactured by 
International Rectifier Corporation which has a drain source voltage 
rating of 450 volts and can handle up to 13 amperes of current with a 
forward resistance of about 1/4 ohm. A filter circuit 19 comprising series 
connected capacitor 21 and resistor 23 is connected in parallel with the 
MOSFET 17 across the d-c terminals of the rectifier bridge circuit 13. 
This filter circuit 19 smooths out the half waves of the d-c signal 
produced by the rectifier bridge circuit so that a constant d-c voltage 
equal to the peak voltage of the a-c waveform, which is approximately 170 
volts, is applied to the MOSFET 17. Suitable values for these components 
in the exemplary filter are 1 to 4 microfarads for the capacitor 21 and 20 
ohms for the resistor 23. 
The MOSFET 17 is controlled by a drive circuit 25 connected to the gate 
electrode by a lead 27. The drive circuit includes an opto-isolator 29 
such as a 6N139. Such an opto-isolator includes an LED 31 at its input 
which, when energized, turns on a photo transistor (not shown) to provide 
an output which is electrically isolated from the input as is well known. 
The output of the opto-isolator 29 includes a transistor 33 having its 
collector connected to a first output terminal 35 and its emitter 
connected to a second output terminal 37 in a selector circuit 39 within 
the drive circuit 25. The collector is also connected, through a first 
output resistor 41 to a floating +15 volt d-c supply. In a similar 
fashion, the emitter is connected through a second output resistor 43 to a 
floating ground 45. The source electrode of the MOSFET 17 is connected to 
the same floating ground 45. 
The anode of the LED 31 at the input to the optoisolator 29 is connected to 
a +5 volt supply while the cathode is connected through a current limiting 
resistor 47 to the open collector of a transistor 49 in the output stage 
of a logic circuit 51 which generates the logic signal 7. With the 
transistor 49 turned off, the LED is deenergized so that the transistor 33 
is also turned off. With transistor 49 turned on, the LED 31 conducts and 
transistor 33 is turned on. 
The selector circuit 39 provides the capability of generating a gate signal 
for the MOSFET 17 which has the same sense as the logic signal 7 or which 
is complimentary to it. For instance, by connecting the first output 
terminal 35 to the lead 27 with a jumper J1 and shunting the resistor 43 
with a jumper J2 as shown in FIG. 1, a signal is applied to the gate of 
MOSFET 17 which is complimentary to the logic signal 7, since with the 
logical signal inactive, the transistor 33 is off and the +15 volts of the 
floating supply is applied to the gate. With the logic signal 7 active, 
the transistor 33 is turned on so that the gate of the MOSFET 17 is at 
virtually the potential of the floating ground. On the other hand, with 
the jumpers J1 and J2 removed and a jumper J3 connecting the lead 27 to 
terminal 37 and a jumper J4 shunting resistor 41 as shown in FIG. 2, the 
gate of the MOSFET 17 is at almost 15 volts when the logic signal 7 is 
active and the transistor 33 is on, while the gate is at the potential of 
the floating ground when the logic signal is off. 
With the 15 volt signal from the floating power supply applied to lead 27, 
the MOSFET 17 is turned on to energize the load 3 from the a-c source 5. 
Where the power contact of the invention is used in an application which 
calls for safe failure modes such as the protection system for a nuclear 
reactor, the logic signal 7 can be used as a trip signal for initiating 
automatic responses to an abnormal condition. Under these conditions, 
logic signal 7 can be generated as an active signal under normal 
conditions and an inactive signal under trip conditions so that failure of 
the logic signal generating circuit will result in a trip signal. For a 
normally energized load then, that is a load which is energized in the 
absence of a trip signal and deenergized by a trip, jumpers J3 and J4 
should be used so that MOSFET 17 is turned on under normal conditions and 
is turned off by a trip. For the normally deenergized load, jumpers J1 and 
J2 should be used so that the MOSFET 17 is turned on when a trip signal is 
generated. 
In order to protect the MOSFET 17 from transients in the a-c circuit 9, a 
surge suppressor in the form of a metal oxide varistor (MOV) 53 is placed 
in parallel with the MOSFET 17 across the d-c terminals 15 of the 
rectifier bridge circuit 13. A suitable MOV is a GE 180ZA1 manufactured by 
the General Electric Company which has a normal breakdown voltage of 180 
volts which is just above the 170 volts d-c generated by the rectifier 
bridge circuit in the absence of transients. This particular varistor will 
limit the voltage appearing across the MOSFET 17 to 300 to 350 volts in 
response to the 3,000 volt, 1 megahertz transient which decays 
exponentially by 50% in 6 microseconds called for by IEEE instrumentation 
surge protection Standard 472-1974. This peak voltage during the transient 
is well below the 450 volt drain to source voltage rating of the MOSFET. 
Transorbs also could provide the surge protection required for the MOSFET 
in the subject power contact. Whatever device is used for surge 
protection, it must also have very low leakage current below the breakdown 
voltage for reasons to be discussed below. 
A zener diode 55 operates in cooperation with a resistor 57 to clamp the 
gate to source voltage of the MOSFET 17 to 15 volts to protect the 
transistor from transients on the lead 27. Additional surge protection is 
provided for the power contact by a MOV 59 connected between the source 
electrode of MOSFET 17, which is at floating ground potential, and earth 
ground. Another MOV, 61, blocks transients from the load 3 or its 
associated wiring from being transmitted to other circuits similar to that 
shown in the drawing which are served by the a-c source 5. Ferrite beads 
63 and capacitor 65 filter out any RF noise that might be generated in the 
load circuit. 
As can be appreciated from the above discussion, the disclosed power 
contact can handle loads which draw sizable a-c currents and is well 
protected from transients. The purpose of the invention, however, is to 
provide a versatile power contact that can be used with a wide variety of 
loads including loads that draw a small amount of a-c current in their 
energized state. Thus, in making a power contact robust enough to handle 
sizable currents and, in protecting it from transients, the ability to 
switch low level currents must not be sacrificed. The difficulty arises if 
the leakage current with the switch open is of sufficient magnitude to 
sustain energization of the load. Some loads such as neon indicator bulbs 
draw only about 1 milliamp of current from a 120 volt supply. The snubbers 
connected around the contacts of a-c switches can easily draw this much 
a-c leakage current. With the rectifier bridge circuit in the power 
contact circuit, it is leakage current on the d-c side of the bridge that 
must be controlled. This is basically a matter of the off drain to source 
resistance of the MOSFET 17 and the resistance of the varistor 53. Since 
the filter 19 is on the d-c side of the bridge, there is no a-c leakage 
current through it as would be the case with snubbers across a-c contacts. 
The combined resistance of the MOSFET 17 when it is off and the varistor 
53 using the components noted above is approximately 2 megohms. Thus, at 
the operating voltage of 170 volts, the leakage current is less than 1/10 
of a milliamp which is significantly less than that required to sustain 
energization of a neon bulb. 
While a specific embodiment of the invention has been described in detail, 
it will be appreciated by those skilled in the art that various 
modifications and alternatives to those details could be developed in 
light of the overall teachings of the disclosure. Accordingly, the 
particular arrangement disclosed is meant to be illustrative only and not 
limiting as to the scope of the invention which is to be given the full 
breadth of the appended claims and any and all equivalents thereof.