Battery Operation

 Background

A thermal printer must supply a heater voltage (VH) to the thermal printhead as a power source for the heaters themselves. A thermal printer must also supply a logic voltage (VDD) to operate the logic within the driver ICs. VDD is specified as 5.0 volts (4.75~5.25) for most Kyocera thermal printheads.

VH varies across all models of thermal printheads. The driver IC breakdown voltage is 27V for most Kyocera printheads. VH must always be designed to be less than 27V in order to provide a margin for surges when heaters are switched off or on. This leads to the standard specification for low cost printheads to operate with a fixed VH=24V. Higher performance printer designs often adjust VH to compensate for the variation in printhead resistance, sometimes leading to slightly higher VH. Many printer designs choose to use a lower heater voltage because of system or power supply considerations. Kyocera can easily manufacture semi-custom printheads to have a lower average resistance for operation at a lower VH. If such a printhead is operated above the specified VH, it can easily fail from an excess of energy to its heater elements, even though the IC breakdown voltage is never exceeded. Kyocera does specify an IC breakdown voltage, but does not specify a maximum heater voltage. Instead, Kyocera specifies a maximum energy, by which heater voltage, pulse width and actual average resistance can be traded-off against each other, as described in the Maximum Operating Conditions document.

Certain Kyocera driver ICs are designed with a breakdown voltage of 8.5V specifically for use in battery-powered thermal printheads. Related IC characteristics translate into a three-fold reduction in the voltage drop (VL) within the IC and a three-fold increase in the maximum current available to each heater.


 Energy and Current

The sensitivity of the thermal paper imposes an absolute requirement because the printer must provide enough energy to darken a dot on the media. Batteries typically supply less voltage than wall-powered power supplies. Lower voltage means much less power. Kyocera can lower the average resistance to partially offset this loss of power, but at the expense of higher current. To provide the same energy to the media at a lower voltage, a battery-powered printhead will draw more current and the printer will still have to provide a longer pulse width.

Higher current makes common voltage drop losses even worse. The limit on the number of simultaneous dots that can print is more severe on battery-powered printheads due to the higher current per dot. Printing a horizontal line is difficult with most printheads and even more difficult with battery-powered printheads. To deal with this limitation, Kyocera provides more strobe pins than on conventional printheads. This allows the printer controller greater control over the maximum current. The combination of longer individual pulse widths and multiple strobe groups means that battery-powered thermal printing will be slower than wall powered printing. Speed can be improved by designing the application to intrinsically limit the number of simultaneous dots by avoiding ladder bar codes or other horizontal lines and by using only san-serif fonts.


 Special Printheads for Battery Operations

Kyocera is producing some thermal printheads specifically for low-voltage battery operations, which required design changes to the glaze shape and to the driver ICs. For glaze shapes, it is desirable to have a thicker, more massive glaze which retains heat. It is also desirable to have a smaller radius of curvature which produces higher contact pressure on the heater element, which in turn leads to better heat flow into the media. These extreme glaze shapes are possible because battery powered printheads do not operate at high speeds. For a highly efficient, more massive glaze, Kyocera recommends Double Partial Glaze which is like a small radius partial glaze on top of a large, flat glaze base. In 2001, Kyocera introduced a Small Radius Partial Glaze, which has the same height as partial glaze but with a narrower base.

Since 1998, certain Kyocera driver ICs have been produced with a breakdown voltage of 8.5V specifically for use in battery-powered thermal printheads. Related IC characteristics translate into a three-fold reduction in the voltage drop (VL) within the IC and a three-fold increase in the maximum current available to each heater. Since 2001, a higher efficiency driver IC has been available with an even smaller VL and a VDD of 3.3V.

 


 3.3 Volt

3.3 volts is now a standard for logic voltage on laptop PCs and other devices. The benefits of the new driver IC with reduced voltage drop and smaller radius glaze are especially noticeable in this voltage range.

heat efficiency of battery operation printhead


Conditions: VH=3.6V, VDD=3.3V, 64 Dots Printing
The above results show gains in efficiency of over 30% relative to prior technology used in printheads.


 5.0 Volt

5 volts is convenient because it is widely used as the logic supply voltage. At this voltage Kyocera offers 115 ohm average resistance, which yields 0.204 w/dot, and recommends a maximum of 64 simultaneous dots.


 7.2 Volt

7.2 volts is convenient for Ni-Cd batteries and it is the maximum voltage for Kyocera's driver IC for battery-powered printheads. At this voltage, Kyocera offers 145 ohm average resistance which yields 0.340 w/dot power and recommends maximum 64 simultaneous dots.


 12 Volt

For 12V operation, Kyocera offers 24V printheads with reduced average resistance. To handle the higher current, high-speed printheads should be chosen. For example, this table shows how the high-speed KPA series is modified to achieve standard speed at 12 volt battery power.  

  KPA series KYT series KPA series
Application High speed Std speed Battery
VH (volt) 24 24 12
RAV (ohm/dot) 660 1500 350
Power (w/dot) 0.841 0.370 0.384
Cycle time (ms) 1.0 1.75 1.64
Paper speed (ips) 5 3 3
Max pulse width (ms) 0.30 0.80 0.78
Max energy (mJ/dot) 0.252 0.296 0.300

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