OPERATOR'S MANUAL MAINTENANCE

Section C - Electrics
Basic System Operation
Pulse Width Modulation (PWM)
C-14 C-14
9803/9880-3
Pulse Width Modulation (PWM)
Ohms law states that, the amount of current flow in a circuit
is determined by the voltage, and the resistance. A 24v
circuit with a resistance of 6 ohms, would draw a current of
4 amps. This would be the case for a standard solenoid,
which is either on or off.
Once the voltage is applied to the circuit, it is present 100%
of the time. This would be known as a 100% duty cycle.
Therefore the circuit will draw 4 amps constantly.
C007260GB-2
Fig 15.
A proportional solenoid requires differing amounts of
current, depending on its condition. As the coil has a fixed
resistance, changing the current rating can be done in
either of two ways,
1 Having lots of different resistors switched in and out
of the circuit at different times to change the current
flowing.
2 To change the duty cycle of the solenoid.
It is easier, more economical, and more reliable to change
the duty cycle of the circuit, especially using today's
computer/ controller technology.
The duty cycle is the amount of time a component is
switched on compared to the time it is switched off. If a
solenoid is on for three seconds, then off for one second,
on for three, off for one etc. this would be a 75% duty cycle.
The graph A although unstable would give a current rating
of three amps. To stabilize the current in the circuit, the
frequency would need to be increased. If the time scale on
the graph A was one second, the frequency would be 4Hz
(Hertz (cycles per second)). The graph B shows the same
duty cycle, but at a higher frequency of 32 Hz. The
proportional solenoids fitted to JS machines operate at a
frequency of 75 Hz.
C007270GB-2
Fig 16.