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Programmable Print Hammer Driver Circuit for Daisy Wheel Printer

IP.com Disclosure Number: IPCOM000048633D
Original Publication Date: 1982-Feb-01
Included in the Prior Art Database: 2005-Feb-09
Document File: 4 page(s) / 87K

Publishing Venue

IBM

Related People

Hallman, BL: AUTHOR

Abstract

The purpose of the circuit described below and illustrated in the drawings is to provide a constant energy pulse to a print hammer coil of a daisy wheel printer. Further, it allows the user to program any pulse shape or duration in order to maximize energy utilization. The circuit's on-board power dissipation is significantly lower than conventional circuits, resulting in lower heat generation. Finally, the circuit described below allows the implementation of improved current-chopping techniques, requires no regulated power supply, allows complete current pulse shaping, and provides a self-generated clock.

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Programmable Print Hammer Driver Circuit for Daisy Wheel Printer

The purpose of the circuit described below and illustrated in the drawings is to provide a constant energy pulse to a print hammer coil of a daisy wheel printer. Further, it allows the user to program any pulse shape or duration in order to maximize energy utilization. The circuit's on-board power dissipation is significantly lower than conventional circuits, resulting in lower heat generation. Finally, the circuit described below allows the implementation of improved current-chopping techniques, requires no regulated power supply, allows complete current pulse shaping, and provides a self-generated clock.

Referring first to Fig. 1, the heart of the circuit's logic section is a digital-to- analog converter (DAC) 10 which is driven by an external microprocessor (not shown). The output of the DAC is amplified by amplifier 12 and presented to the input of a comparator 13 as the reference voltage. When the DAC is active, transistors Q1 and Q2A and Q2B become conditioned and will turn on with the occurrence of a TRIGGER pulse (which is generated by the microprocessor) through latch 14. The trigger pulse may be a 20 KHz signal with a 20 percent duty cycle. When the TRIGGER pulse occurs, transistors Q1 and Q2 are forced into an active state which turns on transistors Q3 and Q4. The minimum on-time equals the duration of the TRIGGER pulse (10 Mus). Both transistors Q3 and Q4 operate in saturation; hence, Q4's power dissipation is significantly lower than state of the art circuits presently employed. When transistor Q4's emitter current generates a voltage V3 across the sense resistor which is greater than the reference voltage V4, the comparator 13 turns off transistors Q1 and Q2 (by resetting latch 14), and transistors Q3 and Q4 are then forced off. When transistors Q3 and Q4 are turned off, the magnetic field in the hammer coil 15 collapses, and the resulting current passes from ground to the power supply via diodes D3 and D4.

In a typical application, zener diodes are used to provide a constant voltage across the hammer coil. This is done in order to generate a fixed rate of current- rise. If the zener diodes were not used, the rate of current-rise would vary with the applied voltage. The applied voltage may vary over a large range (on the order of +15 percent to -25 percent). Thus, for current pulses of fixed duration, the area under the current time curve would vary with the applied voltage. The area under this curve describes the magnitude of the energy pulse delivered to the hammer system. In other words, the energy pulse will vary with voltage when the rate of current-rise is not fixed.

In the circuit disclosed in Fig. 1, the DAC 10 is used to provide the same function as the zener diodes described above. A microprocessor outputs a sequence of increasing counts to the DAC. The DAC, in turn, converts the digital sequence into its analog form.

The analog signal represe...