6.4 TONER DENSITY SENSOR
A toner density sensor (the TD sensor) is installed under the development
unit. It works in conjuction with the ID sensor to control the amount of toner
in the developer mixture.
The toner density sensor has two basic functions. First, it controls the toner
supply when the toner density is greater than the upper limit or less than the
lower limit. The ID sensor input is ignored. If the amount of toner in the
developer is greater than the upper limit (in this case, if the toner density
sensor output, Vt, is less than 1.5V), toner supply is prohibited. Second, it
takes over toner supply control completely when TD sensor supply mode is
selected with SP2-208-001.
The toner density sensor receives 12 volts from CN122-11 of the main
board. The sensor’s sensitivity is set by the control signal applied at
CN122-10. The input signal from the sensor comes in at CN122-12. This is
an analog signal. When the toner density is within the standard range, the
analog signal is within the range given below. In this model, the ID sensor
has more priority over toner supply control than the TD sensor, so the TD
sensor does not have to be controlled so precisely. As a result, the standard
range is wider than in previous models.
Standard Range: 1.5V ≤ Vt ≤ [Initial Setting (2.5 ± 0.1) + 1.0] V
The active sensing element is a very small transformer with three coils.
When iron ferrite (carrier) is near the sensing element, the inductance of the
coils changes, causing the current through the transformer to change. As the
amount of toner in the developer increases, the effect of the carrier particles
decreases and the voltage applied to CN122-12 decreases. Conversely,
when sensor coils increases and the voltage at CN122-12 also increases.
A156/A160/A162 2-56 STM
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