Resistance of Thermal Printheads and Resistance Variation

Thermal printheads print by applying an electric current to heating elements on a substrate to generate Joule heat, which reacts with thermally reactive materials such as thermal paper or thermal transfer ribbons. A number of heating elements are arranged on the substrate and the resistance values of these elements are an important factor that determines the heat generation properties of the printhead.

Three Resistance Values That Affect Printing

The resistance in thermal printheads is considered to consist of three resistance values: the average resistance (R_ave) of the heating elements, a common resistance (R_com), and a driver-on resistance (R_ic).
To obtain good printing quality, these resistances must be understood and set appropriately.
The details of each resistance are explained below.

1. Average Resistance (R_ave)

The resistance values of a heating element are the combination of the heating element resistance (R_res) and the lead (wiring) resistance (R_lead). Average resistance (R_ave) is the average of these resistance values for all heating elements in the same printhead. The smaller the variation in resistance values within the printhead, the more evenly the heating elements are heated and the less frequent the occurrence of uneven printing density. If R_ave is set properly, the heating elements are heated stably, ensuring uniform printing quality and efficiency. Depending on the resistance of the heating elements, a tolerance (%) is set for the thermal printhead used, which represents the permissible variation of the R_ave average resistance.

R_ave = R_res + R_lead: Average resistance
R_res: Heating element resistance
R_lead: Lead resistance

If a printhead has N heating elements, R_ave can be expressed by the formula below. Here, the i-th heating element is dot number #i, and the heating element resistance is defined as R_resi and the lead (individual wiring) resistance as R_leadi.

Resistance of a heating element at dot number #i:

\(R_i=R_{\mathrm{res}i}+R_{\mathrm{lead}i}\)

R_ave of a thermal printhead having N heating elements:

\(R_{\mathrm{ave}}=\displaystyle \frac{1}{N}\sum_{i=1}^{N}R_i\)

The average resistance values given in the specifications are the values of R_ave based on the above calculations. Generally, the lead resistance is small in relation to the heating element resistance, and therefore its influence is also small. In actuality, there are variations in the values of R_resi and R_leadi between individual heating elements, but as it is not possible to separate individual R_resi and R_leadi in a thermal printhead, the individual resistances are not taken into account.

Thermal printheads are manufactured so that the heating element resistance R_i of dot number #i satisfies a certain standard value for the average resistance value R_ave.

2. Common Resistance (R_com)

Common resistance (R_com) is the resistance located in the wiring of the common electrode section (common line) from the print voltage input section (VH) to the heating elements in the thermal printhead. This resistance is used to control the current and to stabilize the overall current in the common circuit section to which all heating elements are connected (the heating element resistances and lead resistances are excluded).
R_com varies depending on the product specifications. Strictly speaking, the value varies with each dot, but the R_com of the center dot, which has the highest common resistance value in the head, is set as the common resistance value.

3. Driver-On Resistance (R_ic)

Driver-on resistance (R_ic) is the resistance that occurs when the driver circuit used in the thermal printhead is in the on state, i.e., the internal resistance that results when the switch to control each thermal element is turned on. R_ic depends on the driver IC used.

If R_ic is too large, print density will be insufficient, while if it is too small, excessive current will flow, which may result in uneven print density. Therefore, to ensure even print quality, it is important to set an appropriate R_ic according to the application.

In selecting switching elements such as transistors, one should use elements with the lowest possible R_ic to minimize the voltage drop and maximize the current supply to the heating elements.

Since R_ic is set on a dot-by-dot basis, the actual resistance value will vary from dot to dot. However, considering that the variation is small and that the value varies depending on the logic voltage, R_ic is treated as a uniform value in calculations.

Relationship of Resistance to Print Voltage and Print Speed

The resistance values of a thermal printhead are closely related to the print voltage and print speed, which should also be considered when determining the resistance values.

Print Voltage (VH)

The print voltage is the voltage applied to the heating elements. The higher the print voltage, the more power is supplied to the heating elements and the higher the temperature. To prevent damage due to high temperatures, the heating element resistance is set according to the voltage range used.

Print Speed

To achieve high-quality printing even at high speed, the heating elements must reach the required temperature in a short time and also cool down rapidly. On the other hand, low-speed printheads are designed to dissipate heat at a slower rate. It is therefore necessary to ensure that the heating elements do not generate excessive heat. For this reason, it is common to set a lower resistance value for high-speed printing and a higher resistance value for low-speed printing.

The print speed is determined by the print cycle, or scanning line time (SLT), which is the printing time per dot line.

We can suggest resistance values to suit your application, such as for mobile, POS and barcode applications, so please feel free to contact us for more information.