JPH07266599A - Thermal transfer print device and method of compensating voltage drop of thermal transfer print device - Google Patents

Abstract

PURPOSE: To compensate a voltage drop corresponding to the position of heating resistors in the thermal head of a thermal transfer printer by adjusting the continuing time of a current supplied to the selected heating resistor as the positional function of the heating resistor. CONSTITUTION: A current is selectively supplied to heating resistors 44-1,...44-N from a power supply line 42 during the continuation of enable pulses EN. 256 sets of data bit rows controlling the heating resistors 44-1,...44-N are continuously outputted to a shift register 52 and, by this constitution, the print density of each of the pixels of a printing line is determined by the data code to each of the pixels stored in a control circuit and the continuing time of enable pulses EN.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermal transfer printer, and more particularly to an electric circuit for supplying electric power to a heating resistor of a thermal transfer printer.

[0002]

As is well known in the art, a thermal transfer printer is formed with an array of heating resistors arranged at narrow intervals, and each heating resistor records data by making a hard copy. Powered selectively to power. The data is stored as digital information about text, bar codes, images, or graphics. During operation, the heating resistor of the thermal head is supplied with power from the power supply via the drive circuit according to the stored digital information. The amount of heat generated by the energized heating resistor may be applied directly to the heat-sensitive material, or it may be applied to the color-coated web to transfer the color to paper or other recording media. is there.

The amount of heat generated by each heating resistor is determined from a number of factors, including the voltage applied to the target heating element, the heat state of the target heating element,
And the thermal state of the heating element around the heating element of interest. The thermal head is configured as described above, and an undesired voltage drop occurs between the power source and the heating resistor. The occurrence of this voltage drop is mainly due to the characteristics of the power supply,
This is caused by the resistance of the lead wire and the connector that connect the power supply and the heating resistor, and the resistance of the bus inside the thermal head.

So-called parasitic resistance
The voltage drop due to V.sub.2 changes as a function of the number of heating resistors energized simultaneously. When a large number of heating resistors are simultaneously energized, the effective voltage applied to the heating resistors is reduced, which reduces the gradation of the printed image. Therefore, the gray level for each pixel is changed by the number of heating resistors simultaneously energized and the corresponding parasitic resistance of the system.

Due to the variation in gradation due to such parasitic resistance, a serious image defect is formed. Further, when an excessive amount of current is supplied by energizing a large number of heating resistors at the same time, the voltage drop itself of the effective voltage in the heating resistors fluctuates during the energization period, and further image defects are formed. It In high-speed line printers, for example, the time interval between energy pulses is quite short, so if a voltage drop occurs, the voltage will recover to a specified value before the next pulse is output. There are times when you don't.

Many different techniques have been developed to control the factors that determine the image quality of a printer. United States Patent No. 4,724,336 (Takash on February 9, 1988)
uIchikawa et al.) discloses a power circuit for a thermal head. In this power circuit, the resistance value of the thermal head is stored, and the reference voltage of the power source for the thermal head is selected from the memory according to the resistance value of each heating resistor in the thermal head.
In this way, fluctuations according to the individual resistance values of the heating resistors of the thermal head are compensated.

United States Patent No. 4,531,134 (198
Granted to Frank J. Horlan-der on July 23, 5) discloses a voltage adjusting circuit for a thermal head of a thermal transfer printer. In this circuit, the voltage of one electrode in each heating resistor is checked, the lowest voltage is fed back, and the current value in the resistive ribbon printer is determined via the differential amplification control circuit. To be done. In this way, the power supplied to the heating resistor is maintained at or above the predetermined minimum value. Also, U.S. Pat. No. 4,434,356 (issued to Timothy P. Craig et al. On February 28, 1984) discloses a current drive circuit for a thermal transfer ribbon printer. In this circuit, the voltage for each resistance is examined, and this value is used as an input value for controlling a voltage adjusting circuit that supplies a voltage for heating the resistance of the thermal head.

United States Patent No. 4,774,528 (198
Nobuhisa Kato et al. (September 27, 1996) discloses a recording apparatus for thermal transfer. In this device, the density relating to the gradation of each pixel recorded in the recording device is compared with the reference density. Then, as a result of the comparison, the number of pixels belonging to a certain density range is counted by the counter. This count number is used to adjust the pulse width of the energy pulse, thereby compensating for voltage fluctuations due to the number of heating resistors simultaneously energized.

United States Patent 5,053, jointly filed
Issue 790 (Stanley W. Stephenso on October 1, 1991)
(Given to Mr. N et al.), a thermal transfer printing device is specified. In this device, a current is supplied from the power supply to the thermal head, and under the control of the data bit string, the current is caused to flow only to some heating resistors selected from the plurality of heating resistors. The number of selected heating resistors is derived by generating a signal representing the amount of current supplied from the power supply to the thermal head or by counting the data bit string applied to the thermal head. Then, the voltage applied to the thermal head is controlled according to the number of the selected heating resistors, whereby the voltage applied to the selected heating resistor is irrespective of the number of the selected heating resistors. The voltage value is maintained at a predetermined value, and the voltage value is maintained substantially constant.

United States Patent No. 4,442,342 (198
Assigned to Shigeo Yoneda on April 10, 4) discloses a voltage-frequency converter that outputs different frequencies depending on the voltage value applied to the thermal head.
In this device, the drive voltage of the thermal head is applied to the thermal head until the count number of the output from the voltage-frequency converter reaches a predetermined value.

[0011]

The above patents present various solutions to the problem of dissipating the power provided to the heating resistors, depending on the number of heating resistors being energized at the same time. ing. However, these patents do not present the problem of voltage drop along the direction of extension of the thermal head. The voltage drop along the extending direction of the thermal head changes according to the position of the heating resistor. As shown in FIG. 1, the voltage drop of the heating resistor at the center of the thermal head is larger than the voltage drop of the heating resistor located at the end of the thermal head. Therefore, the prior art must be said to be an approximate method of correcting a voltage drop according to the number of heating resistors that are simultaneously energized, and relates to the position of the heating resistors along the extending direction of the thermal head. I can't solve the problem.

Therefore, it is an object of the present invention to compensate a voltage drop depending on the position of a heating resistor in a thermal head of a thermal transfer printer.

Further, according to the present invention, a current is supplied from the power source to the thermal head, and in the compensation of the voltage drop depending on the position of the heating resistor, some heating resistors selected from a plurality of heating resistors are selected. An object of the present invention is to provide a thermal transfer printing apparatus that supplies an electric current only to the body.

[0014]

In the preferred embodiment of the present invention, the duration of the current supplied to a selected heating resistor is adjusted as a function of the position of the heating resistor on the thermal head. It

In another preferred embodiment of the invention, the duration of the current supplied to the selected heating resistor is
It is adjusted as a function of (i) the position of the heating resistors on the thermal head, and (ii) the number of heating resistors energized simultaneously.

In another preferred embodiment of the present invention, the duration of the current supplied to the selected heating resistor is (i) the position of the heating resistor on the thermal head,
(Ii) the number of heating resistors that are simultaneously energized, and (i
ii) Adjusted as a function of the position of the energized heating resistor.

Preferably, for each printed line, a histogram is formed which shows the respective frequencies at which the currents supplied to the respective heating resistors have different durations, which histograms correspond to the positions. Used to derive the corresponding correction factor. Further, the array of heating resistors is divided based on the physical position on the thermal head and divided into a plurality of groups. The duration of the current supplied to the selected heating resistor is then adjusted as a function of the position of that group, and the position and energization of other groups.

[0018]

EXAMPLES In order that the present invention may be better understood, the present invention will be described in detail below. A heating resistor forming part of or directly related to the present invention will be primarily described herein. It should be noted that the heating resistor takes various forms, which are not explicitly shown here but are well known to those skilled in the art.

FIG. 2 shows a color material transfer type thermal transfer printing apparatus 10 to which the present invention is applied. The thermal transfer printing apparatus 10 includes a rotary drum 12, a sheet-shaped recording medium 14, drive mechanisms 22 and 24, and a color material carrier 1 that is formed as a sheet or preferably a web.
6, a supply roller 20, a winding roller 18, a thermal head 26, and a thermal head control circuit 28. The control circuit of the thermal head has a power source, an image data supply unit, and a control pulse generator. The drive mechanism 22 includes a motor (not shown) mechanically connected to the winding roller 18. The carrier 16 is arranged between the supply roller 20 and the take-up roller 18, and passes between the thermal head 26 and the recording medium 14. Drive mechanism 24
Is configured with a motor (not shown) mechanically connected to the rotary drum 12. In addition, the recording medium 14
Are fixed to the drum 12. Thermal head 26
Is composed of a plurality of heating resistors. The control circuit 28 of the thermal head is connected to the thermal head 26 via a lead wire 30.

The thermal head 26 is rotatably installed, and the heating resistor of the thermal head is normally pressed against the web-shaped carrier 16. The drive mechanisms 22 and 24 rotate the take-up roller 18 and the drum 12, thereby advancing the web-shaped carrier 16 and the recording medium 14. During driving, the drum 12 and the take-up roller 18 are continuously rotated,
The heating resistor of the thermal head 26 is selectively energized according to the data output from the control circuit 28. As a result, the image composed of the data output from the control circuit 28 is transferred onto the recording medium 14. The configuration of the thermal transfer printer shown in FIG. 2 is the same as that of US Pat. No. 4,786,91.
Issue 7 (Edward A. Hauschild on November 22, 1988)
It is similar to the configuration of the thermal transfer printer shown in Mr.

FIG. 3 shows a block diagram of the thermal head 26 and the power supply section 32 of the control circuit 28 according to the prior art shown in FIG. In FIG. 3, a thermal head 26, a power source 32, a power supply bus 34,
A power return bus 36 and power supply connection terminals 38, 40 are shown. The thermal head 26 has a power supply line 42.
And heating resistors 44-1, 44-2, ..., 44-N
, Switches 46-1, 46-2, ..., 46-N
, Power return line 48, and latches 50-1, 50-
2, ..., 50-N, N-stage shift register 52, and enable pulse generator 55
It has an enable line 54 extending therefrom, a latch line 56, a data line 58, and a clock line 60.

In operation, the m-bit data code (eg m = 8) corresponding to the image to be printed is
It is stored in the control circuit 28 shown in FIG. This data code is transmitted from the control circuit 28 to the thermal head 26 so that the heating resistors 44-1, ..., 44-N generate heat.
It is used to generate the data bitstream that is sent to. Each heating resistor is heated by the number of data bits needed to produce the required print density for the corresponding pixel. For an 8-bit data code, the number of data bits varies from 0 to 255.

The data consisting of these data bits is
It is input to the shift register 52 via the data line 58 shown in FIG. A clock pulse generator (not shown) of control circuit 28, well known in the art, controls signal C on line 60 to control the shifting of the data bits into the shift register at a predetermined rate.
The LK is output to the shift register 52. Once N bits of data have been accommodated in shift register 52, those data bits are latched by latch pulse LA from line 56 in a manner well known in the art.
1, ..., 50-N. Switch 46-1,
, 46-N are closed in response to the data bit of the corresponding latch and the enable pulse EN at the enable terminals of the latches 50-1, ... 50-N.
As a result, while the enable pulse EN continues, the heating resistors 44-1 ,.
.., 44-N are selectively supplied with current. The shift register 52 includes heating resistors 44-1 ... 44-N.
256 data bit sequences for controlling each pixel are continuously output so that the print density of each pixel of the print line is determined by the data code stored in the control circuit 28 for each pixel, and the duration of the enable pulse EN. Determined by

Since the data bits latched in the latches 50-1, ..., 50-N and the enable pulse EN respectively control the heat generation amount in the heating resistors 44-1, ..., 44-N, This determines the density of each pixel on the print line. For example, the switch 46-1 is controlled by the latch 50-1, and a pulse having a predetermined width corresponding to the data code stored in the control circuit 28 is supplied to the control terminal of the switch 46-1. The switch 46-1 is closed by the enable pulse EN and the state of the latch 50-1. When the data bit in the latch is 1, the switch 46-1 is closed by the enable pulse EN having a predetermined width. In this way, power is supplied from the power source 32 to the heating resistor 44-1 in accordance with the pixel density determined by the data code stored in the control circuit 28 and the width of the enable pulse EN. Similarly, the latches 50-2, ...
2, ..., 46-N are controlled, and the heating resistor 44-
2, ..., 44-N generate heat according to the control by the switches.

The waveform of the pulse (voltage) is shown in FIG. 4 as a function of time (microseconds). In the figure, the data bits sent to the shift register 52, the clock signal used to put the data bits into the shift register 52, and the data bits are latched 50-1, ..., 5
Latch pulse used to drive 0-N, and data bits latched into control switch 46-
The enable pulses used to send 1, ..., 46-N are shown. The amplitudes of the pulses are unified for the sake of clarity. Waveform 62 shows a clock pulse CLK that controls the input of data bits to shift register 52. Waveform 63 shows one set of data bits out of the 256 on line 58 that is sent to shift register 52 for one print line. A waveform 64 shows a latch pulse used for inputting the set of data bit strings shown in the waveform 63 to the latches 50-1, ..., 50-N. Wave 65
Indicates an enable pulse EN for outputting the data bits of the latches 50-1, ..., 50-N as control inputs of the switches 46-1, ..., 46-N.

A latch pulse is generated after the data bit string of each set is input to the shift register 52. The data bit string on line 58, shown by waveform 63, is shifted to shift register 52 by clock signal CLK, shown by waveform 62, so that each data bit is arranged to control the corresponding heating resistor. It The data bit is represented by a binary value of "0" (L level) or "1" (H level). The heating resistors 44-1, ..., 44-N are energized by the data bit of "1". For example, the data bit numbered 1 in the waveform 63 (the data bit of "1")
Latches 50-1 when N data bit strings for a print line are arranged in the shift register 52.
Sent to. Also, the data bit ("
1 "data bit) is sent to the latch 50-N.
That is, by the latch pulse LA on the line 56, the 1st data bit of "1" is sent from the shift register 52 to the latch 50-1, and the Nth data bit of "1" is sent to the latch 50-N. To be Then, the switches 46-1, ... Corresponding to the latch in which the data bit of "1" is fixed,
.., 46-N is provided with an enable pulse EN having a predetermined width as a control input of each switch. The data bits of the latches 50-1 and 50-N enable the switch 46-1 corresponding to the pulse having a predetermined width.
And 46-N, the corresponding heating resistor 4
The exotherms at 4-1 and 44-N are accurately controlled.

As described above, the number of heating resistors selected in each print line changes according to the data output to the thermal head 26. In FIG. 3, all or some of the heating resistors 4
4-1, ..., 44-N are simultaneously selected. The selected heating resistors include the power supply bus 34, the connection terminal 38, the power supply line 42, the power return line 48,
The power supply 3 through the connection terminal 40 and the power return bus 36
Connected to 2.

Therefore, the more pulses are applied to the heating resistor during the printing of one line of the image, the higher density can be obtained in the corresponding pixel. If 255 pulses can be used for one heating resistor, the number of heating resistors simultaneously energized during printing one line can be changed 255 times.

FIG. 5 shows a block diagram of an apparatus for calculating the correction coefficient and correcting each pulse number given to the thermal head by the correction coefficient. Here, two types of correction are performed. The first correction is an overall correction determined by the number of heating resistors that are energized at the same time. This correction takes into account factors that equally affect all heating resistors, such as, for example, voltage adjustment of the power supply and factors resulting from the conductors and connectors that connect the power supply to the thermal head.
The second correction is a partial correction for compensating for the voltage drop in the thermal head due to the position of the image data and the heating resistor.

The above device functions as follows. The number of pulses for each of the 2560 heating resistors corresponding to one line of the image is the line buffer (lin
ebuffer) 70. This line buffer is divided into a plurality of sections. In the illustrated embodiment, eight sections are set up,
The number of sections can be more or less than this. In this case, each section corresponds to the physical position of the 320 heating resistors in the thermal head.

In each print line, a histogram is shown for each of the eight sections of the line buffer 70 showing the frequency with which different numbers of pulses are applied to the heating resistor. An example of such a histogram for one section is shown in FIG. From such a histogram, the cumulative histogram as shown in FIG. 7 is constructed through the operations shown by the following mathematical expressions.

[0032]

[Equation 1]

Here, i represents the pulse order at that stage, and N _{i, s} represents the number of heating resistors simultaneously energized in the segment s of the thermal head in the pulse stage i. Further, F _{j, s} represents the frequency of occurrence at the pulse number j. The calculated values in the N _{i, s} histogram are eight segment registers 72a, ..., 72h, respectively.
Memorized in.

The cumulative histograms in the eight segments are then combined to form a single histogram for the entire line. This histogram is then stored in the histogram register 74 for the entire line. The pulse stage sequencer 76 cycles through the stages corresponding to the individual pulse stages in time for one print line. For each pulse stage, the number of energized heating resistors is input to the reference table 78 for pulse number correction, and the overall correction coefficient for each stage that is common to all heating resistors is output. It Accumulator and divider 80
ivider) sums up the correction factors of each stage according to the following formula, and divides this total value by the number of stages to calculate the correction factor for each pulse number n given to the heating resistor. .

[0035]

[Equation 2]

Here, G _n represents an overall correction coefficient for the number of pulses n, and K [N _i ] represents a correction coefficient in each pulse stage which is determined by the number of heating resistors simultaneously energized.

The register 82 stores a single correction coefficient for each pulse number given to the heating resistor, regardless of the position of the heating resistor. Then, these total correction coefficients are input to the total correction coefficient unit 84.

The correction coefficient corresponding to the position is calculated from the histogram stored in the segment registers 72a, ..., 72h. At each pulse stage instructed by the sequencer 76, the address calculator 8
8 (address calculator) compares the number of heating resistors that are simultaneously energized with a threshold value. In the illustrated embodiment according to the invention, the number of energized heating resistors is half the number of heating resistors available in one segment of the thermal head (160 in this case).
Compared to. This threshold value changes depending on the configuration of the thermal head, and may change depending on the position of the segment on the assumption of average correction.

When the number of heating resistors equal to or more than the threshold value (half) is turned on, "1" is set as the address bit. When only half or less of the heating resistors are energized, "0" is set as the address bit. Then, in the address calculator 88, the address bits of all the segments are collected. At this time, each 1 bit of the address represented by 8 bits corresponds to one segment. This address is input to the look-up table 90 and is set to 2
One of the 56 power load states is represented. The reference table 90 stores a correction coefficient having a predetermined value obtained from the electrical network model of the thermal head. Each segment of the thermal head predicts the working voltage in each segment according to the given state, which is one of 256 states determined by "1" or "0". To be done. Then, by these used voltages, the electric power used for printing is derived from the following formula.

[0040]

[Equation 3]

Here, P _{i, s} represents the power value consumed in the segment s of the thermal head. Further, V _{i, s} represents a voltage value for the segment, and R _{i, s} represents a net resistance value of the plurality of heating resistors in the segment. Then, the multiple correction coefficient L _i, s for each pulse stage i and segment s is derived from the relative power loss compared to the reference power value in each segment using the following equation.

[0042]

[Equation 4]

Here, P _ref indicates a reference power value. Therefore, for each address of the lookup table 90, the coefficient L _{i, s} is output for each of the eight segments. These correction factors are stored in the accumulator and divider 9
2 is averaged by the number of pulses n given to the heating resistor as shown in the following equation.

[0044]

[Equation 5]

Here, C _{n, s} represents a correction coefficient corresponding to the position of each segment, corresponding to the number of pulses n given to the heating resistor and the segment number s of the segment. The C _{n, s} coefficient is stored in the register 94. Before these correction coefficients are input to the total correction coefficient unit 84, the correction coefficients corresponding to eight positions are interpolated in the register 96 to form 16 effective segments, so that a more gradual correction can be performed. Becomes

In the total correction coefficient unit 84, the overall correction coefficient output from the register 82 and the correction coefficient corresponding to the position output from the register 96 are multiplied,
A single correction factor is calculated that is derived as a function of the number of pulses applied to the heating resistor of interest and the position of that heating resistor. Then, in the multiplier 98, the pulse number data of the line buffer 70 is multiplied by the corresponding correction coefficient, and this product is stored in the output buffer 100.

As described above, the present invention has been described in detail centering on the preferred embodiments, but it will be understood that effective variations and modifications are possible within the scope of the present invention. For example, a thin line in a printed image that occurs in response to a change in the correction coefficient is, for example, output buffer 1
Well-known multi-step dithering technique such as adding noise to 00
(dithering technique). Alternatively, Bayer masking or error diffusion method can be used. In addition, it is possible to perform calculations with greater arithmetic accuracy than image data or thermal head electrical drive data in order to minimize image defects in the printed image.

[Brief description of drawings]

FIG. 1 is a function of the position of a heating resistor as a function of the relative power consumption of each heating resistor when all heating resistors are simultaneously energized in an electrical model of a thermal head of a commercially available thermal transfer printer. Is a graph shown as.

FIG. 2 is a schematic view showing a thermal transfer printer to which the present invention is applied.

FIG. 3 is a block diagram of the thermal transfer printer shown in FIG.

FIG. 4 is a timing diagram showing waveforms of data bits in the circuit shown in FIG.

FIG. 5 is a block diagram illustrating operation of a thermal transfer printer according to the present invention.

FIG. 6 is a histogram showing the frequency of the number of pulses in one segment of the thermal head.

7 is a cumulative histogram of the number of pulses calculated based on the histogram of FIG.

FIG. 8 is a graph showing an overall correction coefficient of a pulse stage with respect to the number of energized heating resistors in the present invention.

[Explanation of symbols]

 10 Thermal Transfer Printing Device 26 Thermal Head 28 Control Circuit (Control Means) 32 Power Supply 34 Power Supply Bus (Electrical Connection Means) 36 Power Return Bus (Electrical Connection Means) 38 Power Supply Terminal (Electrical Connection Means) 40 Power Supply Terminals (Electricity) Connection means) 42 power supply line (electrical connection means) 44 heating resistor 48 power return line (electrical connection means) 55 enable pulse generator (enable means)

Claims (8)

[Claims] 1. A thermal head (26) having a plurality of heating resistors (44), a power supply (32), and a power supply and the thermal head for supplying current to the selected heating resistors. An electrical connection means (34, 36, 34, 36, 36, 36b, 36b, 36b, 36b, 36b, 36b, 36b, 36b, 36b, 36b, 36b provided between which the electrical connection efficiency varies as a function of the position of the heating resistor on the thermal head.
38, 40, 42, 48), a control means (28) connected to the plurality of heat generating resistors and selecting a heat generating resistor for supplying a current from the power source, and a supply means for supplying the selected heat generating resistors. A thermal transfer printing apparatus (10) comprising: an enabling unit (55) for determining a duration of a current to be supplied, the correction unit relating to the enabling unit is provided, and the correction unit selected by the correcting unit. A thermal transfer printing apparatus characterized in that the duration of the current supplied to the heating resistor is adjusted as a function of the position of the heating resistor on the thermal head. 2. The thermal transfer printing apparatus according to claim 1, further comprising: a means for deriving the number of the heating resistors that are simultaneously energized, wherein the correction means adjusts the current supplied to the selected heating resistors. A thermal transfer printing apparatus characterized in that the duration is adjusted as a function of (i) the position of the heating resistor on the thermal head and (ii) the number of heating resistors energized simultaneously. 3. The thermal transfer printing apparatus according to claim 2, wherein a duration of a current supplied to the heating resistor selected by the correcting unit is a function of a position of the heating resistor simultaneously energized. A thermal transfer printing apparatus characterized by being adjusted as follows. 4. The thermal transfer printing apparatus according to claim 1, wherein an assembly composed of a plurality of adjacent heating resistors is formed. 5. The thermal transfer printing apparatus according to claim 1, wherein the correction unit generates a histogram for each print line, the histogram indicating the frequency of occurrence of different durations of the supply current, and the correction unit according to the position. A thermal transfer printing apparatus comprising: a means for generating a correction coefficient. 6. A thermal head having a plurality of heating resistors, a power source, and an electrical connection means installed between the power source and the thermal head for supplying a current to the selected heating resistor. A method of compensating for a voltage drop caused by the position of the heating resistor in a thermal transfer printing apparatus including: a step of selectively supplying a current from the power source to the heating resistor; Adjusting the duration of the current supplied to the resistor, adjusting the duration of the current supplied to the selected heating resistor as a function of the position of the heating resistor on the thermal head. A method of compensating for a voltage drop, comprising: 7. The method according to claim 6, further comprising the step of deriving the number of the heating resistors that are simultaneously energized, wherein the duration of the current supplied to the selected heating resistors is (i). A method of compensating for a voltage drop, comprising adjusting as a function of the position of the heating resistors on the thermal head, and (ii) the number of heating resistors energized simultaneously. 8. The method of claim 7, wherein the duration of the current supplied to the selected heating resistor is further adjusted as a function of the position of the heating resistor being energized at the same time. How to compensate for the voltage drop.