Abstract:
A voltage clamping circuit comprises a first high current gain circuit adapted to receive current from the first line; and a first switching circuit that turns on the first high current gain circuit to flow current away from the first line when the first switching circuit senses a first voltage from the first line above a clamping voltage, and turns off the first high current gain circuit when the first switching circuit senses the first voltage below the clamping voltage.

Description:
BACKGROUND OF THE INVENTION 
   The present invention is directed to bicycles and, more particularly, to various inventive features of a circuit used with bicycle dynamos. 
   Bicycles are equipped with dynamos for the purpose of illuminating headlamps, powering electrical components, and so on. The voltage generated by a dynamo is typically proportional to the speed of the bicycle which, in turn, is determined by the rate of rotation of the wheels. At high speed, the voltage can exceed 100 V in some instances. It is therefore necessary to design electrical components powered by the voltage generated by the dynamo to be able to withstand such high voltage. Unfortunately, components designed to withstand high voltages lack general application and tend to be expensive as well. 
   Another difficulty arising from the generation of voltages from a dynamo is fluctuation in the electrical load connected to a dynamo which, in turn, can result in bursts of extremely high voltage, termed “surge voltage”. Thus, a circuit for clamping voltage is needed to enable the use of standard electrical components and the like, as well as to protect components from extremely high voltage. 
   Conventional clamping circuits proposed to date include a circuit like that illustrated in  FIG. 1 . This conventional circuit is a bidirectional voltage clamping circuit having two Zener diodes. When positive voltage is output at the positive (+) terminal of dynamo GE, then element DZ 1  functions as a Zener diode, and element DZ 2  functions as a normal rectifier diode. Similarly, when positive voltage is output at the negative (−) terminal of dynamo GE, then element DZ 2  functions as a Zener diode, and element DZ 1  functions as a normal rectifier diode. 
   With the circuit illustrated in  FIG. 1 ,
 
 Vc 1 =Vz 1 +Vf 2  (1)
 
where Vc 1  is the clamping voltage for the dynamo voltage, Vz 1  is the Zener voltage of element Dz 1 , and Vf 2  is the forward voltage of element Dz 2 . When positive voltage is output at the negative (−) terminal of dynamo GE, the clamping voltage Vc 2  for dynamo voltage is given by
 
 Vc 2 =Vz 2 +Vf 1.
 
   Such a conventional clamping circuit has the advantage of relatively few parts. However, diodes Dz 1  and Dz 2  tend to generate heat, which can lead to problems in degraded characteristics. Assume, for example, that clamping voltage is set to Vc 1 =Vc 2 =10 V; semiconductor junction temperature Tj prior to clamping is 25° C.; current flow to elements Dz 1  and Dz 2  during clamping is constant; and semiconductor junction temperature at thermal equilibrium after commencing clamping is 100° C. When Vz 1 =9.1 and Vf 2 =0.9, at the instant of clamping, Equation (1) gives:
 
 Vc 1=9.1+0.9=10 ( V )
 
whereas at thermal equilibrium, where the temperature coefficient αT=5 (mV/° C.),
 
 Vc 1=9.1+(α T/ 1000)×(100−25)+0.9=10.375 ( V ).
 
Thus, there is a significant variation in the clamped voltage.
 
   To reduce degradation in characteristics due to heat generation, it is possible to design a circuit like that illustrated in  FIG. 2 . In this circuit, current flow is sensed by a current sensor element A. Loss P occurring in elements DZ 1  and DZ 2  is calculated from the current value Iz and clamping voltage Vc as follows:
 
 P=Vc×Iz.  
 
If loss P increases, then a switching element SW is opened to limit current flowing to the circuit.
 
   However, such a circuit requires elements with high withstand voltage for the switch SW and sensor A controlling it. Another drawback is the increased number of components required. Furthermore, where a device is charged by the dynamo, failure to charge adequately may result from the operation of the switch. 
   Another possibility is a circuit like that shown in  FIG. 3 . In this circuit, the voltage Vty across the anode and cathode of a thyristor Th is sensed, and a trigger pulse is applied to the gate at the instant the voltage Vty exceeds a preset voltage. Application of a trigger pulse produces shorting (conduction) across the anode and cathode of the thyristor so that voltage Vty drops to around 0 V. Shorting continues until the current across the anode and cathode falls below the characteristic holding current of the thyristor. 
   However, this circuit design has problems, particularly when the signal produced by the dynamo is used to generate pulses that indicate bicycle speed. For example,  FIG. 4(   a ) shows the waveforms generated during operation of a typical dynamo and speed sensing circuit;  FIG. 4(   b ) shows the waveforms generated during operation of a dynamo and speed sensing circuit constructed in accordance with  FIGS. 1 and 2 ; and  FIG. 4(   c ) shows the waveforms generated during operation of a dynamo and speed sensing circuit constructed in accordance with the clamping circuit shown in  FIG. 3 . In each figure, Vs is the decision voltage for producing a speed sensing pulse, and the resultant pulse will have the shape shown at the bottom of each figure. The pulses produced by a dynamo and speed sensing circuit constructed in accordance with  FIGS. 1 and 2  are the same as the pulses generated during the operation of a typical dynamo and speed sensing circuit. Thus, speed can be sensed with no particular problems using these pulses. However, when a clamping circuit has been designed with a thyristor-shorted circuit as shown in  FIG. 4 , clamping will produce a waveform like that shown at the top of  FIG. 4(   c ) due to the tendency to drop to 0 V when clamped by the clamping thyristor. As a result, the pulse produced by decision voltage Vs will have a disturbed waveform like that shown at the bottom  FIG. 4(   c ), thus making it impossible to sense speed accurately. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to various inventive features of a circuit used with bicycle dynamos. In one inventive feature, the voltage clamping circuit comprises a first high current gain circuit adapted to receive current from the first line; and a first switching circuit that turns on the first high current gain circuit to flow current away from the first line when the first switching circuit senses a first voltage from the first line above a clamping voltage, and turns off the first high current gain circuit when the first switching circuit senses the first voltage below the clamping voltage. 
   Among other things, there is now provided an inexpensive circuit having minimal thermally-induced degradation of characteristics. Where speed pulses are generated using voltage generated by the dynamo, speed can be sensed accurately. Additional inventive features will become apparent from the description below, and such features alone or in combination with the above features may form the basis of further inventions as recited in the claims and their equivalents. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a conventional voltage clamping circuit; 
       FIG. 2  is a schematic diagram of an improvement to the circuit of  FIG. 1 ; 
       FIG. 3  is a schematic of another conventional voltage clamping circuit; 
       FIGS. 4(   a )- 4 ( c ) show waveforms of speed sensing pulses generated from dynamo outputs; 
       FIG. 5  is a schematic diagram of an embodiment of a voltage clamping circuit with inventive features; 
       FIG. 6  is a schematic diagram of another embodiment of a voltage clamping circuit with inventive features; 
       FIG. 7  is a schematic diagram of another embodiment of a voltage clamping circuit with inventive features; 
       FIG. 8  is a partial schematic diagram of a circuit according to the embodiment shown in  FIGS. 5 and 6 ; 
       FIG. 9  is a partial schematic diagram of a circuit according to the embodiment shown in  FIG. 7 ; 
       FIG. 10  is a schematic diagram of another embodiment of a voltage a clamping circuit with inventive features; 
       FIG. 11  is an alternative physical embodiment of a voltage clamping circuit constructed for a bicycle dynamo; 
       FIG. 12  is a physical embodiment of the voltage clamping circuit shown in  FIG. 11  mounted to a bicycle; and 
       FIG. 13  is an alternative physical embodiment of a voltage clamping circuit constructed for a bicycle dynamo. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
     FIG. 5  is a schematic diagram of an embodiment of a voltage a clamping circuit  1  with inventive features. This voltage clamping circuit  1  comprises a bridge circuit Db connected to a dynamo GE, and a load L such as a headlamp. Dynamo GE may be a hub dynamo integrally provided in a hub of the bicycle. Bridge circuit Db has four diodes connected to provide full wave rectification of the output of dynamo GE. Clamping circuit  1  clamps the output voltage of bridge circuit Db to a predetermined voltage (clamping voltage) and supplies the clamping voltage to the load L. Clamping circuit  1  includes a power transistor circuit TR 1 . A Zener diode DZ 1  and a resistor R 1  are provided for controlling the on/off operation of transistor circuit TR 1 . 
   Transistor circuit TR 1  includes two NPN bipolar transistors t 1 , t 2  connected in series (a Darlington connection in this example). Transistor circuit TR 1  also is connected in parallel with headlamp L. In this example, the collector of transistor t 1  is connected to the positive terminal of bridge circuit Db, and the emitter of transistor t 2  is connected to the negative terminal of bridge circuit Db. 
   The Zener diode DZ 1  functions as a switching element that operates at a Zener voltage Vz 1 , and it is connected so as to apply reverse bias across the positive terminal of bridge circuit Db and to the base of transistor t 1  during normal unclamped operation. During operation, the power output of dynamo GE is rectified by bridge circuit Db and then applied to transistor circuit TR 1  and headlamp L. As long as the generated voltage is below a predetermined voltage, the Zener diode DZ 1  blocks the flow of current to the base of transistor t 1 , and transistor circuit TR 1  remains off. As a result, unregulated voltage is applied to headlamp L. 
   If, on the other hand, the generated voltage goes above the predetermined voltage, current flows through Zener diode DZ 1  so that a forward biasing voltage is applied across the base and emitter of transistor t 11 . As a result, transistors t 1  and t 2  become conductive, and the entire transistor circuit TR 1  is switched on. Current flows through transistor circuit TR 1  so that the predetermined (clamping) voltage is applied to headlamp L. 
   Clamping voltage Vc is given by
 
 Vc= 2 ×Vfd 1+ Vz 1 +Vbe  
 
In this equation, Vfd 1  is the forward voltage of an individual diode in bridge circuit Db, and Vbe is the voltage across the base and emitter of transistor circuit TR 1  when switched on. Equivalent impedance across the base and emitter of transistor circuit TR 1  varies so as to maintain this clamping voltage Vc.
 
   In this embodiment, by constructing the transistor circuit TR 1  of two Darlington connected transistors, gain may be increased, and current flow through the Zener diode may be minimized. This, in turn, reduces the amount of generated heat and holds to a minimum the degradation in characteristics due to heat and current fluctuations. Also, the clamping voltage is relatively stable. Since the dynamo waveform is resistant to disturbance, speed pulses also may be accurately generated to calculate the speed of the bicycle. 
     FIG. 6  is a schematic diagram of another voltage a clamping circuit  2  with inventive features. Voltage clamping circuit  2  employs PNP bipolar transistors for a transistor circuit TR 1 ′. While the polarity is different, the basic arrangement is the same as that of the circuit shown in  FIG. 5 . More specifically, transistor circuit TR 1 ′ includes two Darlington connected PNP bipolar transistors t 1 ′, t 2 ′, arranged with the emitter of transistor t 1 ′ connected to the positive terminal of bridge circuit Db and the collector of transistor t 2 ′ connected to the negative terminal of bridge circuit Db. A Zener diode DZ 1  serving as the switching element is connected so as to apply reverse bias across the base of transistor t 2 ′ of transistor circuit TR 1 ′ and the negative terminal of bridge circuit Db during unclamped operation. The circuit operates in the same manner as the circuit shown in  FIG. 5   
     FIG. 7  is a schematic diagram of another voltage clamping circuit  10  with inventive features. Voltage clamping circuit  10  is connected in parallel with a dynamo GE and with a headlamp or other load L. Clamping circuit  10  comprises first and second power transistor circuits TR 1  and TR 2 ; first and second Zener diodes DZ 1  and DZ 2 ; first and second resistors R 1 , R 2  connected to the Zener diodes DZ 1  and DZ 2 , respectively; and first and second diodes D 1  and D 2  serving as rectifier elements. As in the preceding embodiments, dynamo GE may be a hub dynamo integrally provided to a hub of the bicycle. 
   First power transistor circuit TR 1  includes two Darlington connected NPN bipolar transistors t 11  and t 12  that are connected in parallel with respect to headlamp L. More specifically, the collector of transistor t 11  is connected to the positive terminal of dynamo GE, and the emitter of transistor t 12  is connected via second diode D 2  to the negative terminal of dynamo GE. 
   First Zener diode DZ 1  operates at Zener voltage Vz 1 , and it is connected so as to apply reverse bias across the positive terminal of dynamo GE and the base of transistor t 11 . First diode D 1  is connected in parallel with first transistor circuit TR 1  and so as to apply forward bias when positive voltage is output at the negative terminal of dynamo GE. 
   Second power transistor circuit TR 2  includes two Darlington connected NPN bipolar transistors t 21  and t 22  that are connected in parallel with respect to headlamp L. More specifically, the collector of transistor t 21  is connected to the negative terminal of dynamo GE, and the emitter of transistor t 22  is connected via first diode D 1  to the positive terminal of dynamo GE. 
   Second Zener diode DZ 2  operates at Zener voltage Vz 2 , and it is connected so as to apply reverse bias across the negative terminal of dynamo GE and the base of transistor t 21 . Second diode D 2  is connected in parallel with second transistor circuit TR 2  and so as to apply forward bias when positive voltage is output at the positive terminal of dynamo GE. 
   With this circuit arrangement, as long as positive voltage is output at the positive terminal of dynamo GE and the generated voltage is below a predetermined voltage, the flow of current to the base of transistor t 11  is blocked by the Zener diode DZ 1 , so first transistor circuit TR 1  remains off. Thus, substantially all of the generated voltage (excepting some circuit loss in the components) is applied to headlamp L. 
   If the generated voltage goes above the predetermined voltage, then current flows through first Zener diode DZ 1  so that a forward biasing voltage is applied across the base and emitter of transistor t 11  of first transistor circuit TR 1 . Transistors t 11  and t 12  become conductive, and current flows along the path: dynamo GE→first transistor circuit TR 1 →second diode D 2 →dynamo GE, and the predetermined voltage (clamping voltage) is applied to headlamp L. In this instance, the second transistor circuit TR 2  does not function. 
   In this case, the clamping voltage Vc is given by
 
 Vc=Vfd 2+ Vz 1+ Vbe 1
 
where Vfd 2  is the forward voltage of the second diode D 2 , and Vbe 1  is the voltage across the base and emitter with first transistor circuit TR 1  in the conductive state.
 
   If, on the other hand, positive voltage is output at the negative terminal of dynamo GE and the generated voltage is below a predetermined voltage, the flow of current is blocked by Zener diode DZ 2 , and transistor circuit TR 2  is off. Thus, substantially all of the generated voltage (excepting some circuit loss in the components) is applied to headlamp L. 
   If the generated voltage subsequently goes above the predetermined voltage, current flows through second Zener diode DZ 2  so that forward biasing voltage is applied across the base and emitter of transistor t 21  of second transistor circuit TR 2 . Transistors t 21  and t 22  become conductive, and current flows along the path: dynamo GE→second transistor circuit TR 2 →first diode D 1 →dynamo GE, and clamping voltage is applied to headlamp L. In this instance, the first transistor circuit TR 1  does not function. The clamping voltage is analogous to that described previously. 
   Darlington connected transistor power circuits typically incorporate diode elements to prevent reverse electromotive force from being applied to transistors. This embodiment employs such diode elements (D 1  and D 2 ), thus allowing the circuit to be constructed more cheaply. By virtue of being provided with first and second diodes D 1 , D 2 , the circuit of this embodiment offers, in addition to similar advantages to the embodiment shown in  FIG. 5 , the further advantage of obviating the need for the bridge circuit provided as a rectifier circuit in  FIG. 5 . In this embodiment, of the factors determining clamping voltage, there is only one diode forward voltage, thus affording more consistently accurate clamping voltage. Additionally, since one of the half-wave components of the dynamo output is handled by the first transistor circuit TR 1  and the other half-wave component is handled by the second transistor circuit TR 2 , the amount of heat per unit of time produced by a single transistor is half that in the embodiment shown in  FIG. 1 , thus allowing for greater thermal resistance from the transistor junction to the air. 
   More specifically, let it be assumed that loss P occurring in the circuit shown in  FIG. 5  is 5 W, and similarly that loss P generated in the circuit shown in  FIG. 7  is 5 W. Clamping voltage in each circuit is 10 V, and forward voltage drop at each diode is 0.6 V. 
   If the loss j occurring in transistor circuit TR 1  and diode D 1  in the circuit shown in  FIG. 5  are respectively denoted as Ptr and Pdi, then
 
 Pdi =( P/Vc )×0.6×2=(5/10)×0.6×2=0.6 (W), and
 
 Ptr=P−Pdi= 4.4 (W).
 
   The losses occurring in (TR 1 +D 1 ) and (TR 1 +D 2 ) in the circuit shown in  FIG. 7  (denoted as Ptr 1  and Ptr 2 ), are:
 
 Ptr 1= P /2=5/2=2.5 (W), and
 
 Ptr 2= P /2=5/2=2.5 (W).
 
   The transistors used in the/circuit shown in  FIG. 5  and the circuit shown in  FIG. 7  are each assumed to have:
         an upper limit of junction withstand temperature Tj(max)=150 (° C.);   a thermal resistance between junction and package Rth (j−c)=3.125 (° C./W);   a thermal resistance between the package and radiator junction Rth (c)=1.5 (° C./W); and   a radiator having an infinite surface area Tf=30° C.       

   The circuits shown in  FIGS. 5 and 7  are modeled in  FIGS. 8 and 9 , respectively. 
   Thermal resistance R between the transistor junction J and the radiator  8  shown in given by:
 
 R=Rth  ( j−c )+ Rth  ( c )=3.125+1.5=4.625 (° C./W),
 
and junction J temperature is given by:
 
 Tf+R×Ptr= 30+4.625×4.4=50.35 (° C.)  {circle around (1)}
 
   Thermal resistance R between the transistor junction J and the radiator  8  shown in given by:
 
 Tf+R×Ptr 1=30+4.625×2.5=41.56 (° C.)  {circle around (2)}
 
   From the results of equations {circle around (1)} and {circle around (2)} it will be apparent that with the above parameters, the circuit shown in  FIG. 7  has greater latitude in terms of transistor junction temperature. Conversely, the circuit shown in  FIG. 7  allows for higher package thermal resistance. Consequently, an inexpensive transistor with a small package can be used. This also has the advantage that two medium sized transistors have relatively lower thermal resistance with the radiator than does one large sized transistor. 
   A voltage clamping circuit according to a fourth embodiment is illustrated in  FIG. 10 . The circuit shown in this embodiment employs PNP bipolar transistors for first and second transistor circuits TR 1 , TR 2 . While the polarity is different, the basic arrangement is the same as that of the circuit shown in  FIG. 7 . 
   More specifically, first and second transistor circuits TR 1 ′, TR 2 ′ each include two Darlington connected PNP bipolar transistors t 11 ′, t 12 ′, t 21 ′, t 22 ′, with the emitter of transistor t 11 ′ of the first transistor circuit TR 1 ′ connected to the positive terminal of dynamo GE, and the collector of transistor t 12 ′ connected via a second diode D 2  to the negative terminal of dynamo GE. The emitter of transistor t 21 ′ of the second transistor circuit TR 2 ′ is connected to the negative terminal of dynamo GE, and the collector of transistor t 22 ′ is connected via a first diode D 1  to the positive terminal of generator GE. A first Zener diode DZ 1  is connected so as to apply reverse bias across the base of transistor t 12 ′ of first transistor circuit TR 1 ′ and the negative terminal of dynamo GE, and a second Zener diode DZ 2  is connected so as to apply reverse bias across the base of transistor t 22 ′ of second transistor circuit TR 2 ′ and the positive terminal of dynamo GE. The first and second diodes D 1 , D 2  are the same as those in the embodiment shown in  FIG. 7 , and the operation of this embodiment is analogous to the embodiment shown in  FIG. 7 . 
   Exemplary installations of a voltage clamping circuit pertaining to any of the preceding embodiments are illustrated in  FIGS. 11-13 . In the example shown in  FIG. 11 , a box  6  separate from the hub dynamo  5  is disposed between hub dynamo  5  and headlamp L, with the voltage clamping circuit being housed within this box  6 . The hub dynamo  5  and voltage clamping circuit within the box are connected by electrical wire  7 . A radiator fin  8  is provided to the box  6  housing the voltage clamping circuit. The box  6  provided with the radiator fin  8  may be secured, for example, to the upper end of the front fork  9 , as shown in  FIG. 12 . In the example shown in  FIG. 13 , the voltage clamping circuit is housed within a hub dynamo  5 ′, and the hub dynamo  5 ′ may be provided with a fin  8 ′. 
   While the above is a description of various embodiments of inventive features, further modifications may be employed without departing from the spirit and scope of the present invention. For example, the transistor circuits may be fabricated using FETs rather than bipolar transistors, although the turn-on voltage of bipolar transistors is more stable. A transistor circuit may be fabricated from a single transistor element or some other circuit element, provided that the desired gain is achieved (e.g., current gain of 50-200 per transistor in some embodiments). 
   The size, shape, location or orientation of the various components may be changed as desired. Components that are shown directly connected or contacting each other may have intermediate structures disposed between them. The functions of one element may be performed by two, and vice versa. The structures and functions of one embodiment may be adopted in another embodiment. It is not necessary for all advantages to be present in a particular embodiment at the same time. Every feature which is unique from the prior art, alone or in combination with other features, also should be considered a separate description of further inventions by the applicant, including the structural and/or functional concepts embodied by such feature(s). Thus, the scope of the invention should not be limited by the specific structures disclosed or the apparent initial focus on a particular structure or feature.