Abstract:
An apparatus and method for suppressing voltage fluctuations across a relay coil is disclosed. The method includes the steps of monitoring a voltage drop across a relay coil by a difference amplifier; providing an output of a reference source and an output of the difference amplifier to an integrator amplifier; providing an output of the integrator amplifier to a transistor; and driving the relay coil by controlling an output of the transistor based on the output of the integrator amplifier, wherein the output of the reference source is selectively applied to the integrator amplifier in response to a monitored undesired voltage fluctuations across the relay coil.

Description:
BACKGROUND 
       [0001]    This disclosure relates generally to the field of electronics and, more specifically, to systems and methods for suppressing transient voltages across a relay coil. 
         [0002]    Power conditioning units (PCU&#39;s) use airborne aircraft +28 Vdc bus to power relay coils. These coils are normally rated for +29 Vdc maximum, with a few rated for +32 Vdc maximum. The +28 Vdc power specification is 22 to 29 Vdc, with an additional 1.5 V of ripple. In addition, a 50 V transient voltage may also be present. 
         [0003]    To solve transients and over voltage conditions on the +28 Vdc bus, past attempts have included connecting a zener diode or a transient suppressor across the bus, or by simply doing nothing. Zener diodes and transient suppressors suffer from the limitation that they will most likely burn up after only one over voltage condition. What is needed is an apparatus and method that handles such transient voltage conditions without destroying components in a PCU. 
       SUMMARY 
       [0004]    In accordance with various embodiments, a method of suppressing voltage fluctuations across a relay coil is disclosed. The method comprises monitoring a voltage drop across a relay coil by a difference amplifier; providing an output of a reference source and an output of the difference amplifier to an integrator amplifier; providing an output of the integrator amplifier to a transistor; and driving the relay coil by controlling an output of the transistor based on the output of the integrator amplifier, wherein the output of the reference source is selectively applied to the integrator amplifier in response to a monitored undesired voltage fluctuations across the relay coil. 
         [0005]    In accordance with various embodiments of this disclosure, an apparatus that suppresses voltage fluctuations across a relay coil is disclosed. The apparatus comprises a difference amplifier configured to monitor a voltage drop across the relay coil; an integrator amplifier configured to provide an output responsive to an input from a reference source and the output of the difference amplifier; a transistor arranged in series with the relay coil and configured to be controlled by the output of the integrator; and a controller configured to control the reference source so as to drive the relay coil by controlling an output of the transistor so as to suppress voltage fluctuations across the relay coil. 
         [0006]    In accordance with various embodiments of this disclosure, an apparatus for suppressing voltage fluctuations in a power conditioner unit that powers a power relay coil is disclosed. The apparatus comprises an active feedback loop configured to monitor a voltage drop across the power relay coil to apply power to the power relay coil so as to suppress voltage fluctuations associated therewith. 
         [0007]    These and other features and characteristics, as well as the methods of operation and functions of the related elements of structure and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various Figures. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of claims. As used in the specification and in the claims, the singular form of “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  shows a conventional design to drive a relay coil. 
           [0009]      FIG. 2  shows a block diagram of a design to drive relay coil in accordance with an embodiment. 
           [0010]      FIG. 3  shows an exemplary circuit diagram configured to drive a relay coil in accordance with one or more embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    In the description that follows, like components have been given the same reference numerals, regardless of whether they are shown in different embodiments. To illustrate embodiments of the present disclosure in a clear and concise manner, the drawings may not necessarily be to scale and certain features may be shown in somewhat schematic form. Features that are described and/or illustrated with respect to one embodiment may be used in the same way or in a similar way in one or more other embodiments and/or in combination with or instead of the features of the other embodiments. 
         [0012]    This disclosure monitors the voltage across a relay coil and provides feedback to an on/off circuit or an integrator. The integrator may be configured to maintain a predetermined voltage across the relay coil by driving a transistor, e.g., a field effect transistor (FET). The relay coil voltage rating is thereby not exceeded, regardless of the transient performance of the +28 Vdc bus. 
         [0013]    In an embodiment, the +28 Vdc aircraft bus characteristics may be defined by MIL-STD-704, which states that the aircraft steady state voltage will be between 22 to 29 Vdc, with a ripple voltage of 1.5 V. This ripple voltage is not included in steady state limits. Therefore, in this embodiment, the aircraft voltage can be as high as 30.5 V. In addition to the steady state values, transients to 50 V for 12.5 ms can occur and then decay to 32 V for 75 ms. 
         [0014]    Three power relays are generally used in PCU&#39;s. They are the power relay to switch 400 Hz prime power, in-rush relay to switch in current limiting resistors and discharge relay (high voltage type) to switch in resistors to discharge large output capacitors. 
         [0015]    These relays have the following contact and coil characteristics as detailed in Table 1. 
         [0000]    
       
         
               
             
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Typical relay contact and coil characteristics 
               
             
          
           
               
                   
                 COIL VOLTAGE 
               
             
          
           
               
                 RELAY 
                 VENDOR 
                 CONTACT LIFE 
                 TYPICAL 
                 MAXIMUM 
               
               
                   
               
               
                 Power 
                 Leach 
                 100k cycles min.* 
                 +28 Vdc 
                 +29 Vdc 
               
               
                 In-rush 
                 Leach 
                 200k cycles min.* 
                 +28 Vdc 
                 +29 Vdc 
               
               
                 Discharge 
                 Cii Tech 
                 100k cycles 
                 +26.5 Vdc   
                 +32 Vdc 
               
               
                   
               
               
                 *Contact life at 25% rated load 
               
             
          
         
       
     
         [0016]    Previous designs have used zener diodes or transient suppressors across the +28 Vdc aircraft bus in an attempt to limit the transient voltage. A typical circuit configuration  100  is shown in  FIG. 1 . As shown in the Figure, transient suppressor  110 , such as a zener diode, is used to across +28 Vdc aircraft bus  105  in an attempt to limit transient voltages. Relay coil  115  are controlled by driver  120  and field-effect transistor  125  arranged in series. When activated, relay coil  115  controls switch  130 . Both an +1.5 V reference signal and an on/off signal are provided from field programmable gate array (not shown) and are transmitted to driver  120 . An output of driver  120  is supplied to field-effect transistor  125 , which is then used to control relay coil  115 . 
         [0017]    For example, the F-18 aircraft uses a RUG PCU having 500 watt peak pulse transient suppressor (part number 1N6120A) and the B-2 aircraft uses a RMP PCU having 1500 watt peak pulse transient suppressor (part number 1N6156A), which is from the same family as the F-18 RUG part. The only difference is the peak power capability. Subsequent analysis showed that the B-2 RMP part was insufficient in handling more than one voltage transient. As a result of this analysis, the part was removed from the circuit to prevent it from failing and causing (possible) board damage. 
         [0018]      FIG. 2  shows a simplified design to drive relay coil in accordance with an aspect of the present disclosure.  FIG. 3  shows an exemplary circuit diagram in accordance with  FIG. 2 . The design, indicated generally by  200 , includes relay coil  205  that is powered by bus  210 . In some embodiments, bus  210  may have a voltage of +28 V, which is suitable for aircraft usage. Other bus voltages may be used that are in accordance with bus characteristics defined by MIL-STD-704, including a steady state voltage of about 22 to 29 Vdc, with a ripple voltage of 1.5 V. Active feedback loop  215  is configured to monitor the voltage across relay coil  205  and to suppress transient voltage or voltage spikes by turning power off to relay coil  205 . Thus, preventing damage from occurring to relay coil  205 . When activated, relay coil  205  controls switch  240 . 
         [0019]    Active feedback loop  215  may include difference amplifier  220 , integrator amplifier  225 , reference source  230 , and transistor  235 . Voltage across relay coil  205  is measured by difference amplifier  220 . In some embodiments, output from difference amplifier  220  is scaled down to +5 V or +3.3 V, depending upon the type of reference source used. The measured voltage difference from difference amplifier  220  is provided as an input to integrator amplifier  225 . By way of a non-limiting example, difference amplifier  220  and integrator amplifier  225  may both be an integrated circuit (IC), such as, for example model number LM124, which is a low power quad operational amplifier manufactured by National Semiconductor. A reference signal is provided from reference source  230  to another input of integrator amplifier  225 . Reference source  230  is provided with an on/off signal  240  from controller (not shown). In some embodiments, controller may be a field programmable gate array. Integrator amplifier  225  provides an output voltage based on the two inputs and supplies the output voltage to transistor  235 . By way of a non-limiting example, when an overvoltage occurs on bus  210 , excess voltage, as measured by difference amplifier  220  and integrator amplifier  225 , is dissipated across transistor  235 . In some embodiments, transistor  235  may be a field-effect transistor. Controller (not shown) is configured to control enable pin of reference source  230 , which allows integrator amplifier  225  to turn on or off power to relay coil  205 . 
         [0020]    Regulation is achieved by setting the output of difference amplifier  220 . By way of a non-limiting example, if +28 V is the desired voltage across relay coil  205 , the difference amplifier gain is set to yield an output of +5 V. In this case, reference source  230  output is +5 V. Integrator amplifier  225  is configured to drive transistor  235  to yield +28 V across relay coil  205 . If bus  210  is at 30 V, transistor  235  will drop 2 V, with the remaining 28 V dropped across relay coil  205 . If bus  210  has a transient of 50 V, transistor  235  will drop 22 V. 
         [0021]    By way of another non-limiting example, in the case of a lower voltage on bus  210 , such as 22 V, transistor  235  will drop a very small amount of voltage (approximately 0.1 V), with the vast majority of the 22 V dropped across relay coil  205 . 
         [0022]    In the event that relay coil  205  must be turned off, the controller (not shown), such as a field programmable gate array, will turn off reference source  230  via enable pin (not shown). The output of reference source  230  will then drop to zero volts and the output of integrator amplifier  225  will be very close to zero volts. This will turn off transistor  235  and all of the bus voltage will be dropped across transistor  235 . 
         [0023]    This design will be able to turn relay coil  205  on and off and that no more than 28 V will appear across relay coil  205 . Relay coil  205  will be able to operate with the correct coil voltage, as per the manufacturer&#39;s specifications. 
         [0024]    Although the above disclosure discusses what is currently considered to be a variety of useful embodiments, it is to be understood that such detail is solely for that purpose, and that the appended claims are not limited to the disclosed embodiments, but, on the contrary, is intended to cover modifications and equivalent arrangements that are within the spirit and scope of the appended claims.