Abstract:
A fastener driving device such as a tacker, including a silicon-controlled rectifier (SCR) actuating circuit for producing a power stroke for substantially a complete half cycle of an AC potential supply and which occurs following the actuation of a switch actuable at random times with respect to the cycles of the AC supply. The circuit features three functional stages. A firing synchronizer circuit is provided to inhibit firing at other than the beginning or end of a half wave cycle of rectified AC potential. A memory supporting circuit provides a high degree of switch bounce immunity. A protecting circuit prevents an accidental and undesirable firing of the SCR that might otherwise automatically occur when a sudden increase of voltage occurs at its anode such as might occur when power is first applied to the actuating circuit.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates in general to fastener driving devices, and more specifically, to control circuits for electrically operated fastener driving devices such as for example electric tackers. 
     An example of an electrically operated tacker is shown in the commonly assigned U.S. patent application of Bernecki et al, Ser. No. 294,422, filed Aug. 19, 1981, now U.S. Pat. No. 4,417,681. That teachings of that application are hereby incorporated by reference into this application. 
     In order to positively prevent unwanted double firing, it is appropriate to energize the tacker&#39;s solenoid for no more than a single one-half cycle of commercially available AC power. Various circuits are known and used for this purpose, such as, for example, U.S. Pat. No. 3,215,864--Doyle et al (Nov. 2, 1965), the teachings thereof being incorporated herein by reference. In the Doyle et al circuit, a unidirectional controlled conduction device, such as a silicon controlled rectifier (SCR) is used to close a power circuit so that current will flow in an electrical load such as the tacker&#39;s solenoid winding. Current flow through the solenoid winding causes a mechanical power stroke to be produced for effecting a tacking operation. 
     Known control circuits generate a gate signal for triggering the SCR on the first properly poled half cycle of an applied AC voltage after the randomly timed actuation of a switch by a user. The control circuit also includes means for preventing the application of a further gate signal to the SCR until the switch is released and then re-actuated by the user. 
     These known actuating circuits use various schemes for generating the gate signal for triggering the SCR to &#34;fire&#34; the solenoid. For example, in the Doyle U.S. Pat. No. 3,215,864, a charge is built up on a capacitor. When the switch is actuated this charge is used to generate the gate signal. This is a rather typical approach to generating the firing signal. Such known circuits are, however, susceptible to misfirings (unintended firings) and are generally not immune to switch bounce, i.e., proper firing is inhibited when the actuating switch does not make a clean firing closure but instead, its switch contacts bounce open and close again one or more times before remaining closed. 
     SUMMARY OF THE INVENTION 
     The present invention provides a fastener driving device featuring a three stage firing circuit for generating the firing signal. It includes a firing synchronizer for inhibiting firing at other than the beginning or end of a half wave rectified AC input power sinewave; a memory supporting circuit providing switch bounce immunity and a protection circuit for preventing undesirable firings that might occur with sudden voltage increases at the anode of a silicon controlled rectifier in the firing circuit. 
     The firing synchronizer allows the firing of a main SCR (SCR2) only at the beginning or end of a half wave rectified sinewave when the instantaneous voltage drops below 6 V. This is accomplished by turning on a transistor (TR) providing a shorting path for the SCR&#39;s gate when the line voltage is higher than 6 V. 
     The memory supporting circuit is, in essence, a feedback circuit for maintaining a firing signal on the SCR&#39;s gate even upon the occurrence of switch bounce. The memory supporting circuit includes a capacitor (C2) charged through a diode (D5) to activate an additional SCR (SCR1). The memory supporting circuit capacitor discharges through a resistor (R6) the gate of the additional SCR (SCR1) and an additional resistor (R10) for about 70 milliseconds. When the additional SCR (SCR1) is activated, it momentarily (about 0.2 milliseconds) discharges a main firing capacitor (C1) through a small valve resistor (R4). If the switch bounces thereby reducing the anode current of SCR1 to zero (which would normally turn off SCR1), the current from C2 keeps SCR1 turned on longer than the time it takes for C1 to be completely discharged. 
     The SCR protecting circuit includes a series resistor-capacitor circuit in parallel with the main firing SCR (SCR2) to prevent its accidental firing responsive to a fast rise of anode voltage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An exemplary embodiment of the present invention will be described in detail with reference to the drawings. The described embodiment is the presently contemplated best mode for carrying out the invention although it is to be considered a non-limitative example. In the drawings, 
     FIG. 1 is a vertical sectional view of a fastener driving device shown as an example of a device to which the present invention applies; and 
     FIG. 2 is a schematic diagram of the actuating circuit according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1 there is shown a vertical sectional view of an exemplary embodiment of a fastener driving device to which the present invention applies. The fastener driving device includes a housing 10 defining a drive track 12. A magazine 14 is provided for feeding fasteners to drive track 12 so that they can be driven one at a time by driver 16. Driver 16 is powered through a drive stroke in drive track 12 by power derived from a solenoid 18. To initiate a fastening operation, a user pulls on a trigger 22 which actuates an electrical contact switch 20. Switch 20 forms a part of an actuating circuit embodying the principles of the present invention since the invention is particularly concerned with the circuit and its function in connection with the basic components of the fastener driving device thus far described, the detailed construction of the basic components will not be described, it being understood that reference may be made to the aforesaid Bernecki et al application for any detailed understanding required. 
     Referring now to FIG. 2, there is shown a detailed schematic diagram of the actuating circuit according to the present invention. As previously stated, the circuit is intended to provide a firing signal to energize solenoid 18 over a single AC half wave for each actuation of a switch 20. 
     The heart of the actuating circuit is a series connection of a diode D1, solenoid 18, a silicon controlled rectifier SCR 2 constituting a controlled conduction device and resistor R8 across terminals 32 intended for coupling to a commercial AC line power mains. Whenever SCR 2 is fired, this circuit including solenoid 18 is completed and the solenoid is actuated to initiate a fastening operation. This occurs each time trigger 22 is actuated to move switch 20 from its &#34;off&#34; position to its &#34;fire&#34; position. The circuit is supplied by AC line voltage coupled to terminals 32. The voltage at terminals 32 is rectified by a diode D2 and then reduced in voltage by a voltage divider including resistors R1 and R2 to produce a voltage of about 16 volts peak. This voltage is applied to a capacitor C1 through a diode D3 when switch 20 is in its &#34;off&#34; position. Diode D3 prevents capacitor C1 from being discharged when the instantaneous voltage level at node 40 drops to zero. 
     From node 40 of the voltage divider, a &#34;firing synchronizer&#34; circuit is supplied. The firing synchronizer circuit includes a resistor R3 and a transistor TR. The firing synchronizer allows for the firing of SCR 2 only at the beginning or end of a half wave rectified sinewave when the instantaneous voltage drops below 6 volts. This function is achieved by turning transistor TR &#34;on&#34; to ground node 42 at the collector of the transistor when the line voltage is higher than 6 volts. Grounding node 42 prevents any voltage from being supplied to the gate of SCR 2 regardless of the position of switch 20. 
     When switch 20 is moved from the &#34;off&#34; to the &#34;fire&#34; position, the voltage on fully charged capacitor C1 is applied to the &#34;fire&#34; side of the switch. Voltage at node 44 is almost the same as that at the &#34;fire&#34; terminal of switch 20 because SCR 1 is &#34;off&#34; and resistor R4 is very small. The voltage at node 42 depends on the phase of rectified voltage. Assuming that switch S20 was moved to the &#34;fire&#34; position during a period between half sinewaves and the voltage at node 40 was zero. At this time, transistor TR in the firing synchronizer circuit is &#34;off&#34; and the voltage at the gate of SCR 2 is a function of the voltage on capacitor C1 divided by resistors R4, R5 and R7. This voltage is sufficient to fire SCR 2. 
     Whenever the voltage at the anode of SCR 2 reaches a sufficient level, then current begins to flow through SCR 2. After the rectified line voltage exceeds 6 volts, transistor TR turns on, thereby reducing the voltage at node 42 to the collector saturation level of the transistor. This is generally in the range of 0.1 to 0.3 volt. The voltage at the collector of transistor TR is additionally reduced by the forward voltage drop of a diode D4 having an anode coupled to the transistor&#39;s collector. The voltage from the collector of transistor TR is coupled via diode D4 to the gate of SCR 2. SCR 2 is already &#34;on&#34; and the gate voltage reduction caused by the grounding of node 42 does not affect its operation. Increasing the line voltage and current flowing through diode D1 and SCR 2 and solenoid 18 activates solenoid. 
     During the firing period of SCR 2 which is about 7-7 1/2 milliseconds, a high current flows through resistor R8 and produces a voltage pulse at the cathode of SCR 2. This voltage pulse has a peak of about 4 volts and lasts for about 7 milliseconds. This voltage pulse is utilized by a memory supporting circuit to eliminate any possibility of switch bounce malfunction. 
     The memory supporting circuit includes a diode D5, capacitor C2, resistor R10, resistor R6 and SCR 1. The voltage at the cathode of SCR 2 charges capacitor C2 through diode D5 and activates SCR 1. Capacitor C2 discharges through resistor R6, the gate of SCR 1 and resistor R10 for about 70 milliseconds. When SCR 1 is activated, it momentarily discharges capacitor C1 through resistor R4. This takes approximately 0.2 milliseconds. If there are some contact bounces of switch 20 which temporarily reduce the anode current of SCR 1 to zero, then the gate current from discharging capacitor C2 keeps SCR 1 turned on longer than the time required for capacitor C1 to be completely discharged. This provides switch bounce immunity. Diode D5 prevents capacitor C2 from being rapidly discharged through the small value resistor R8. Resistor R10 discharges capacitor C2 after the voltage on the gate of SCR 1 drops below the gate turn on level. 
     Assume now that switch S20 was closed when a rectified line voltage was higher than 6 volts and was rising with rising slope of the half sinewave form. At that moment, transistor TR in the firing synchronizer is &#34;on&#34; and the potential at node 42 is low. This prevents the firing of SCR 2. Transistor TR1 is &#34;on&#34; until the rectified line voltage drops below 6 volts on the falling slope of the half sinewave form. After the voltage drops below 6 volts, transistor TR goes &#34;off&#34; thereby increasing the potential at node 42 and the gate of SCR 2 thereby activating SCR 2. Since the line voltage is only 6  volts and is still dropping, the current flowing through SCR 2 is very low and lasts only for a short period of time. This time period is about 0.3 milliseconds. This low current is not able to activate the solenoid or turn on SCR 1. During the time interval between two half sinewave forms, transistor TR is &#34;off&#34; and sufficient voltage is supplied to the gate of SCR 2 to turn it &#34;on&#34;. The time constant of the circuit including capacitor C1 and resistors R4, R5 and R7 is selected such that sufficient voltage is supplied to the gate of SCR 2 to keep it &#34;on&#34; for about 30 milliseconds. After the line voltage starts rising, firing can take place as discussed above. 
     When the circuit is connected to AC line voltage via terminals 32 at a moment when the peak voltage is high, there will be a sudden increase in voltage at the anode of SCR 2. There is a high rate of change of voltage with respect to time at this point. This sudden change of voltage at the anode of SCR 2 might activate it. This would be an unintended actuation not caused by the switching of switch 20. To prevent this undesirable actuation, there is provided an SCR protecting circuit including resistor R9 and capacitor C3. The time constant of this circuit prevents the automatic and unintended actuation of SCR 2. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims which scope is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures.