AN6 Theuse of Zetex E-Line transistors in motor and solenoid driver functionswithin printers - effective logic to high current load interfacing.

Category: Logic
Manufacture: Zetex Inc.
Datasheet: Download this application note

Application Note 6 Issue 2 November 1995
The Use of Zetex E-Line Transistors in Motor and Solenoid Functions within Printers
Effective Logic to High Current Load Interfacing Neil Chadderton
T h e m a j o r i t y o f mo d er n p ri n te rs u se stepper motors and solenoids for driving the various transport and print mechanisms. These applic ations usually involve using drive signals directly from the control logic; and handling high-pulsed currents and voltages. In the past this has led to TO220 and similar devices being used, but an alternative is now available. Devices from the Super E-Line range of transistors ar e direct r e p l a c e me nts fo r the afo re menti one d d e v i c e s in m edi um pow er s i tua tion s. T h i s A p p li c ati on No te outl ine s a few examples that are applicable and is by no means comprehensive. voltage, while the current flowing in the on-state follows a ramp-like appearance.
S te pper mot ors are used for many printer functions, such as platten roller a n d p r i n t h e ad pos ition dr ive rs , a nd dais y wheel drivers. A typical circuit used in many printers is s h o w n c o n c e p t u a l l y in Fi gur e 1. The driver transistors Q1-Q4 are switched in an ordered sequence to energise the motor' s coi ls to produce the desired action. As the transistor load is inductive, the voltage present on switching off easily exceeds the supply
Figure 1 Typical Stepper Motor Drive C ircuit (diagramatic). These conditions are illustrated in Figure 2, which shows the voltage and current experienced by the transistor in a typical c yc le. (In this c as e, a centre drain MOSFET, the ZVN4206C has been employed).
Application Note 6 Issue 2 November 1995
The negative current pulses due to the dr iv ing c onf ig ur a tion, a nd th e high current involved (greater than 3A), make this an ideal situation for a high performance E-Line Darlington t r a n s i s t o r . U s e d w i t h a n i n e x pe ns iv e externally-wired diode, this offers up to a four-times cost-saving on the expensive TO220 alternative.
Figure 2 Typical Waveforms on a 4-Phase Stepper Motor Driver Transistor (ZVN4206C). Upper Trace: Collector Current - 0.5A/div. Lower Trace: Collector Voltage - 20V/div. (26V supply) For circuit simplicity and ease of driving, Darlington devices are a favourite in this applic ation, but a consequence of driving inductive loads such as these is t h e n e e d t o p r ot e c t t h e d e v ic e fr om negative current pulses. In many of the expensively packaged Darlington transistors, this is effected by an integral collector-emitter diode. An alternative is to use MOSFET transistors, which exhibit all the elec trical requ irements in a smaller pack age and at much reduced cost, and possess the necessary protection diode due to the inherent nature of MOSFET technology. Even in very demanding driving applications for Iarger prin te r motors such as illustrated in Figure 3, E-Line transistors can be used to obtain a large cost advantage.
Figure 3 W a v e f o r m s o n S t e pper Mot or D rive r Transistor for a Large Printer (FXT605). Upper Trace: Collector Current - 1A/div. Lower Trace: 20V/div.
Dot Matrix Head Drivers
For th e dot matrix form of printer, solenoids are usually used to force a pin to strike the ribbon and thereby produce a dot on the paper. A column of these pins is driven as required to match the matrix representation of printable characters. A s i n t h e m o t o r d r i v e r e x a m p l e , the solenoid also presents an inductive load and Figure 4 typifies the circuit used to drive it. A control pulse from the logic turns the transistor on to energise the solenoid coil.

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