At the same time, VT3 and VT1 get the base current and turn on. At this time, VT2 is cut off, and the voltage of C pole of VT2 rises to +5V, consolidating the low-level output state of LM709. When it drops to 0V, Schmidt reverses and the output voltage goes low (-15V). The +15V power supply charges Cx through R1 and R2, and the output voltage of the LM741 gradually decreases. When LM709 outputs a high level, VT2 is saturated and turned on, and the voltage of C pole of VT2 is 0V. LM709 forms a positive feedback through R7 and VT2 to the negative input to form a Schmidt trigger.

If R1 and R2 use 1KΩ resistance, the measurement time can be shortened to 1/10.Ĭircuit output U. The capacitance can be read out by connecting a external counter.įigure 1.

When the capacitor Cx is not connected, A2 oscillates with a delay of about 20 μS, and Cx can be calculated to compensate for this. The measurement time is 10S when the Cx capacitance is 1000μF. The absolute values of positive and negative supply voltages need to be equal. The range of power supply voltage is ☑3 to ☑5V. However, due to the limitation of A2 differential input voltage and the constant of transistor driving circuit, it is not allowed to change the supply voltage greatly. In principle, the oscillation period should not be affected by the supply voltage. The charging current of Cx is determined by the supply voltage and R2, and the discharge current is determined by the supply voltage and R1 + R2. Its amplitude is determined by R4 and R3, equal to 1 / 3 of the supply voltage. A Schmidt circuit consists of A2 and the Schmidt circuit forms positive feedback and generates oscillation. A Miller integrating circuit consists of A1. And the capacitance can be measured by the oscillation period of the triangular wave. CatalogĪs Figure 1 shows, the measuring principle of this circuit is: a triangular wave is formed from the process of charging and discharging of measured capacitor Cx. This article will introduce the circuit of capacitance measurement composed of LM741. The high gain and wide range of operating voltage provide superior performance in integrator, summing amplifier, and general feedback applications. It is intended for a wide range of analogue applications. The LM741 series are general-purpose operational amplifiers.

Lm741 pinout download free