Abstract: In an automatic recording instrument having a thermal recording-pen to record on a heat sensitive recording-paper, an operational amplifier output supplies the heating current to the thermal pen through a resistor. A non-linear transfer element receives the output voltage of the amplifier and delivers a voltage having a predetermined non-linear relation to its input. The voltage difference between the output of the non-linear transfer element and the junction point of the thermal pen to the resistor is feedback connected to the input of the amplifier. When the relative velocity between the pen and the paper is increased, the input voltage of the amplifier is increased to increase the output voltage, and when the output voltage is increased, the resistance of the thermal pen is increased to balance to the output of the non-linear transfer element for reducing the input to the amplifier.

Abstract: In a multi-channel recording instrument, the reference signal of each channel is floating from the ground level. Therefore, one each isolated power supply means must be provided for each channel. However, when the error signal for each channel is transmitted through a photo-coupler, the circuit after the photo-coupler can be isolated from the biasing voltage of the reference signal, and a common power supply can be used for all the stages after the photo-couplers, including servo-motor drivers, of all the channels. This reduces the size and cost of the power supplies as a whole. The non-linear character of the photo-coupler does not cause a positioning error, since the photo-coupler is in the servo-loop having a sufficiently large gain.