High Country Tek DVC80 - Manual (Page 158)

P/N: 021-00154, Rev. A.6 - updated for V4.7 Tools Page | 158
COIL
CURRENT
SWITCH
ON
OFF
0 A
FULL
TIME
LOW FREQUENCY PWM
FIG. 6
The amount of dither changes as the average coil current changes. The dither is a maximum at 50% duty cycle
and decreases to zero at 0 and 100 % duty cycles. This may result in too much dither at some current levels
and not enough at others.
The dither current amplitude at a given average current is a function of coil inductance and PWM frequency.
The inductance of coils within a manufacturer's line is generally a function of their rated voltage and wattage.
24volt low wattage coils usually have more inductance (thus less byproduct dither for a given PWM frequency)
than 12volt high wattage coils.
Different spools having a different response to the same dither current further complicate this. Changing the
PWM frequency will allow adjusting the dither, but the amplitude and frequency of the dither cannot be set
independently as may be required.
High frequency PWM
When the PWM frequency is high enough, typically above 5 KHz, the coil current will not have time to change
significantly (FIG. 7). No "byproduct" dither is produced by high frequency PWM.
COIL
CURRENT
SWITCH
ON
OFF
0 A
FULL
TIME
HIGH FREQUENCY PWM
FIG. 7
The use of high frequency PWM with a dither generator (FIG. 8) solves many of the problems with low
frequency PWM dither. The dither waveform is produced deliberately and added to the command input.