Flat Resistance Profile

 Background
A thermal printhead consists of a row of heater elements. Ideally, these elements should have identical electrical resistance. In actuality, if you look closely enough, the resistance values will be different. These differences in resistance can cause differences in the optical density of the printed dots. If two heater elements are fired with the same common voltage for the same amount of time, the print energy supplied by the elements will be different because the power is inversely proportional to the heater element resistance. The element with the lower resistance will produce more power. In bimodal printing, the media is driven to a saturated black state, so small differences in print energy will not be noticeable. In gray scale printing, the same differences may be noticeable.

 Resistance Variation Defined

A printhead specification contains a nominal average resistance value. Resistance variation is quantified in 3 different ways.

  1. Different printheads of the same model will have different average resistance values, as shown on the label of each printhead. This label average resistance value will be within some percent of the nominal average resistance, as shown in the specification. Better thermal printers will compensate for this variation between heads by adjusting either the heater voltage or the pulse width. This is discussed in detail elsewhere in connection with maximum operating conditions.
  2. The resistance variation among heaters on the same printhead will be limited by the specification. For bimodal printheads, the signed difference between a heater's resistance and the label average resistance, expressed as a signed percentage of the label average resistance, will be within a percentage range. For example
    - 10% < δ R / RAV < +15% where δ R = (R- RAV)
    For gray scale printheads, sometimes the resistance variation among heaters on the same printhead is expressed as
    |RMAX- RMIN| / RAV
    and is specified to be less than a certain percent.
  3. Some specifications also limit the resistance difference between adjacent heaters. This would be expressed as
    |Rn- Rn+1| / RAV and would be specified to be less than a certain percent. Again RAV means the label average resistance on the label.

 AD Process Defined

Kyocera has a proprietary technique, called the AD process, which adjusts the heater resistance values to bring them within a tighter range. The following charts illustrate this. The x-axis is the number of the heater element across the printhead. The y-axis is the resistance value.

resistance profile of AD printhead

 

resistance profile of non AD printhead

 Measurement of Heater Resistance

Before driver ICs are mounted and protective coatings are applied, Kyocera probes each ceramic wafer to measure the resistance of each heater element, then computes the average resistance which is eventually written on the printhead label. If heater element resistances are measured through the connector after assembly, they will be different and their average will not match the label average resistance value, but the shape of the profile will be the same.

  1. The resistance of the driver IC will be combined with the resistance of the heater, tending to make the measured resistance higher.
  2. Thin film heaters have a negative thermal coefficient of resistance. The resistance drops as the heater element gets hot. To measure heater element resistance before assembly, Kyocera uses a constant current power supply at about 1 milliampre for about 1 millisecond. Measurements under other conditions will be different.
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