A constant current source

Update date:2018-06-02 Source:MAXGE

A constant current source

1, how to design a three-wire constant current source driving circuit

The constant current source driving circuit is responsible for driving the temperature sensor Pt1000, converting its sensed temperature-dependent resistance signal into a measurable voltage signal. In this system, the required constant current source must have a constant output current, good temperature stability, large output resistance, and an output current of less than 0.5 mA (without the upper limit of Pt1000 self-heating effect). The load is grounded at one end and the polarity of the output current can be Changes and other characteristics.

Because the influence of temperature on integrated op amp parameters is not as significant as that of transistor or FET parameters, the constant current source composed of integrated op amp has the advantages of better stability and higher constant current performance. Especially when the load needs to be grounded, it is widely used. Therefore, the dual-amplifier constant current source shown in Figure 2 is used. The amplifier UA1 constitutes an adder, the UA2 constitutes a follower, and the UA1, UA2 use a low-noise, low-offset, high open-loop gain bipolar operational amplifier OP07.

Circuit breaker

Let the potentials of the reference resistor Rref at the upper and lower ends of Fig. 2 be respectively Va and Vb, Va is the output of the in-phase adder UA1. When the resistor R1=R2 and R3=R4 are taken, then Va=VREFx+Vb, so the constant current The output current of the source is:

 

This shows that the dual-operated constant current source has the following significant features:

1) The load can be grounded; 2) When the op amp is powered by dual power supplies, the output current is bipolar; 3) The constant current is achieved by changing the input reference reference VREF or adjusting the reference resistor Rref0, which is easy to get stable Low current and compensation calibration.

Due to the mismatch of the resistors, the voltage across the reference resistor Rref0 will be affected by the terminal voltage Vb of its driving load. At the same time, because it is a constant current source, Vb will certainly change with the change of the load, which will affect the stability of the constant current source. Obviously this is not acceptable for high precision constant current sources. Therefore, the selection rules for the four resistors R1, R2, R3, and R4 are that the mismatch must be as small as possible, and the mismatching direction of each pair of resistors must be the same. In practice, it is possible to screen a large number of precision resistors in the same batch and select four resistors whose resistances are close to each other.

2, switching power supply high voltage constant current source circuit

The development of the instrument requires a constant current source capable of producing a 1mA current in a 0 to 3 MΩ resistor. A UC3845 is used in conjunction with a 12V battery. A transformer is used in a color TV high voltage package, in which L1 is wound around the original high voltage package core with an enameled wire. L3 uses a coil of the original high voltage package and L2 uses the high pressure portion of the high voltage package. L3 and LM393 form a voltage limiting circuit, limiting the output voltage is too high, adjust R10 can adjust the open circuit output voltage.

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