ENCAD 42e, Service Manual

Page: 35 / 162

Theory of Operation 35
THEORY OF OPERA
TION
Novajet PROe Series Service Manual
Each circuit contains four main blocks (see Figure 2-7):
1. Reference waveform generator
The microprocessor uses a D/A (digital to analog) converter to set
the desired level for the current in the stepper motor winding.
The output of the D/A converter varies in time to create a refer-
ence waveform. This reference waveform is centered around
10V.
2. Motor current sense
The voltage across a series current sense resistor is measured
and level shifted so that it is centered around 5V.
3. Comparator
This portion divides the output of the reference waveform gen-
erator by two and compares it to the output of the motor current
sensor. Logic inside the gate array generates the control signals
for the power driver that applies voltage across the motor wind-
ing in order to make the actual current match the reference
waveform.
4. Power driver
An H-bridge allows the supply voltage to be applied across the
winding in either polarity to drive the current to the desired
value.