Abstract:
An electronic module comprising a power supply conductor coupled to one or more electromechanical actuators to supply operable power to the electromechanical actuators. The electronic module also includes a moisture sensor positioned to sense moisture intrusion into the module. Further, the electronic module comprises a first transistor responsively coupled to the moisture sensor, the first transistor coupled to draw current from the power supply conductor when the first transistor is in a conductive state. In addition, the electronic module includes a second transistor responsively coupled to the first transistor and in turn controllingly coupled to the first transistor.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to moisture protection circuitry for electronic systems. 
     2. Description of the Related Art 
     Electronic control of electromechanical actuators is common. In some applications, there is the potential for the control electronics to become wet to due moisture intrusion from the surrounding environment. Such moisture intrusion may cause the control electronics to inadvertently actuate the actuators they control, due to unintended conductivity in various parts of the circuitry caused by the intruding moisture. 
     In motor vehicles, electronic control of power seats is becoming increasingly common, in order to provide features such as “memory” power seats. The most efficient place to package the control module for the power seats is frequently beneath the seats themselves. However, the floor of a motor vehicle can occasionally get wet. Intrusion of moisture into the control module, and the possibility that the moisture intrusion will cause the control electronics to inadvertently move the power seats, is thus an issue. 
     Therefore, in the motor vehicle power seat environment and in other environments as well, a system which will prevent inadvertent actuation due to moisture intrusion into control electronics will prove advantageous. 
     SUMMARY OF THE INVENTION 
     The present invention provides an electronic module comprising a power supply conductor coupled to one or more electromechanical actuators to supply operable power to the electromechanical actuators. The electronic module also includes a moisture sensor positioned to sense moisture intrusion into the module. Further, the electronic module comprises a first transistor responsively coupled to the moisture sensor, the first transistor coupled to draw current from the power supply conductor when the first transistor is in a conductive state. In addition, the electronic module includes a second transistor responsively coupled to the first transistor and in turn controllingly coupled to the first transistor. 
     By providing moisture protection which can prevent inadvertent actuation of electromechanical actuators controlled by electronic control circuitry, the present invention provides advantages over alternative systems. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a power seat control module  10 . 
     FIG. 2 is a bottom view of control module  10  of FIG.  1 . 
     FIG. 2A is an enlarged view of the area labelled “ 2 A” in FIG.  2 . 
     FIG. 3 is a schematic of the electrical components within control module  10 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Refer first to FIG.  1 . Illustrated there is a control module  10  for a power seat system of a motor vehicle. Such a module takes inputs from various control switches and seat position sensors and drives the motors which position the power seats. Frequently, it is desirable to package control module  10  under a seat in the motor vehicle, for maximum packaging efficiency. Control module  10  preferably has a molded plastic housing within which is housed a circuit board containing the control circuitry which controls the power seat motors. 
     Several holes  20 ,  22 ,  24  and  26  are provided in the bottom of the housing of control module  10  (see FIGS.  2  and  2 A). Holes  20 ,  22 ,  24  and  26  are located directly opposite capacitors  30 ,  32 ,  34  and  36 , which are located on the circuit board within control module  10 . 
     Refer now additionally to FIG. 3 for a description of the circuitry within control module  10 . The circuitry includes control circuitry  40  which performs control of the power seat motors. Control circuitry  40  preferably includes a microprocessor. Control circuitry  40  controls one or more relays  44  via control of relay coils  46  and  48 . Relay contacts  50  and  52  are coupled to a power seat motor  54  to effect bi-directional control of power seat motor  54 . Additional relays and power seat motors are also controlled by control circuitry  40  as appropriate. 
     Power for relay coils  46  and  48  is designated V CP  in FIG.  3 . V CP  is provided via a fuse F 1  from V BAT , the battery voltage in the vehicle. Fuse F 1  is included in a fuse holder having terminals  56  and  57 . 
     Control module  10  also includes moisture protection circuitry  58 . Moisture protection circuitry  58  includes a plurality of moisture sensing locations  60 ,  62 ,  64  and  66 , which are located on the circuit board within control module  10 . (Preferably, the circuitry of FIG. 3 is of “surface mount” technology.) Each moisture sensing location includes two solder pads (for example, solder pads  60 A and  60 B of moisture sensing location  60 ). Also, for noise immunity, each moisture sensing location includes a capacitor coupled across its solder pads (for example, capacitor  30  coupled across solder pads  60 A and  60 B). It is noted that any insulating coating which may be applied to the circuit board is not applied to solder pads  60 A,  60 B,  62 A,  62 B,  64 A,  64 B,  66 A and  66 B; it is through these that moisture intrusion into control module  10  is sensed. 
     Moisture sensing locations  60 ,  62 ,  64 ,  66  are coupled to a pull-down resistor R 4  and an R-C low-pass filter comprising resistor R 3  and capacitor C 5 . The output of this filter is coupled to gate  70  of field-effect transistor (FET) Q 1 . Drain  72  of Q 1  is coupled via a resistor R 1  to fuse F 1 . FET Q 1  and resistor R 1  are selected such that when FET Q 1  is fully “ON” (that is, fully “enhanced”), the current drawn through resistor R 1  and FET Q 1  is sufficient to “blow” fuse F 1 . 
     Drain  72  of FET Q 1  is also coupled, via resistor R 2 , to the base  80  of p-n-p bipolar transistor Q 2 . Emitter  82  of transistor Q 2  is coupled to V CP , and collector  84  of transistor Q 2  is coupled to the outputs of moisture sensing locations  60 ,  62 ,  64  and  66 . Capacitor C 6 , along with resistor R 2 , form a low-pass filter which improves the noise immunity of the response of transistor Q 2  to actuation of FET Q 1 . 
     Zener diode Z 1  is provided to protect gate  70  of FET Q 1  from voltage transients which may exist, for example, in an automotive environment. 
     The moisture protection circuitry  58  of FIG. 3 operates as follows. If sensing locations  60 ,  62 ,  64  and  66  become sufficiently dampened by moisture, the conductivity between their respective solder pads (e.g., solder pads  60 A and  60 B of moisture sensing location  60 ) increases. The voltage across pull-down resistor R 4  thus increases as well, increasing the voltage at gate  70  of FET Q 1 . With a sufficient voltage rise at gate  70 , the resistance between drain  72  and source  90  of FET Q 1  will decrease, causing significant current to flow through resistor R 1 . If the voltage at gate  70  of FET Q 1  is large enough, the current flowing through R 1  will be large enough to cause fuse F 1  to “blow”. Power for coils  46  and  48  of relay  44  (and coils of other relays also controlled by control module  10 ) will thus be removed. Inadvertent operation of power seat motor  54  due to moisture infiltration into control circuitry  40  of control module  10  will thus be prevented. 
     Transistor Q 2  has two major purposes. First, if the voltage at gate  70  of FET Q 1  has risen only to the point that FET Q 1  is only marginally conductive enough to blow fuse F 1 , transistor Q 2  will ensure that the gate voltage of FET Q 1  rises enough to blow fuse F 1 . This is accomplished because, with a modest current flowing through resistor R 1 , the emitter-base junction of transistor Q 2  will become forward biased. This will cause current to flow from emitter to collector of transistor Q 2 , increasing the gate voltage of FET Q 1 . This positive feedback, wherein FET Q 1  beginning to turn “ON” turns “ON” transistor Q 2 , which in turn more fully turns FET Q 1  “ON”, ensures that fuse F 1  will blow when moisture is sensed by moisture-sensing locations  60 ,  62 ,  64  and  66 . 
     The second function of transistor Q 2  is to ensure that the voltage V CP  is held low even if conductivity between terminals  56  and  57  exists after fuse F 1  is “blown”. It has been observed that in the presence of fluids with relatively substantial conductivity, such as salt water, significant current can flow between terminals  56  and  57  even after F 1  has blown. Such current may even be sufficient enough to power coils  46  and  48  of relay  44 . However, if V CP  remains at a significant voltage even after fuse F 1  has blown, the emitter-base junction of transistor Q 2  will remain forward biased. Transistor Q 2  will thus continue to conduct, ensuring that FET Q 1  has sufficient gate voltage to heavily load V CP , which would have a very high source impedance. Under these circumstances, V CP  would not be sufficient to power coils  46  and  48  of relay  44 . 
     Two other design considerations should be noted. First, several components (most notably resistors R 4  and R 1  and FET Q 1 ) can be selected to adjust the sensitivity of moisture protection circuitry  58  as appropriate. Second, Q 1 &#39;s selection as an FET, as opposed to a different type of semiconductor device, is advantageous because the typical failure mode of FETs due to thermal failure is to fail conductive. That is, if FET Q 1  fails due to thermal “overload”, it will tend to continue drawing current through resistor R 1 . This will provide continue to load down V CP , providing a “fail safe” situation. 
     Various other modifications and variations will no doubt occur to those skilled in the arts to which this invention pertains. Such variations which generally rely on the teachings through which this disclosure has advanced the art are properly considered within the scope of this invention. This disclosure should thus be considered illustrative, not limiting; the scope of the invention is instead defined by the following claims.