The Research Of Digital Active Resistance Box

As a kind of basic components in electronic circuit, resistor is widely used in all kinds of circuit system. But different circuitry has different requirement of the resistance value, resistor power, resistance accuracy and the range of the resistor. In circuit design, we often need to adjust the value of the resistor, in some test circuit, even need to replace the resistor according to conditions frequently. Traditional resistance box, mechanical potentiometer, digital potentiometer, though give the possibility to change the resistance value, but the traditional resistance boxes now used all need to change the value manually, it’s hard to adjust the resistance value automatically, and they usually have the defects of small resistance range,low resolution, large size, and so on. Mechanical potentiometer though the advantages of resistance value continuous adjustable, economic and practical, but after it connected to the circuit, we can’t know its value when adjust it again, it is also easy to aging and reduce accuracy; Digital potentiometer though can achieve digital adjustment of the resistance value, but still exists the problem such as: single resistance range, low resolution. So, a digital resistance box that can provide wide resistance value range, with relatively high resistance accuracy, can satisfy certain resistance power requirement, resistance value can be adjusted automatically is really in need, so that it can bring convenience for circuit system design and testing.The main purpose of this paper is to research and make a digital active resistance box, the research method is to design an analog hardware circuit to produce active resistance. Digital control part uses ARM7 MCU LPC2138 system to control the hardware circuit to change its resistance value automatically. It uses 4×4 matrix keyboard unit and Nokia5110 LCD unit as human-computer interaction devices to realize resistance value setting and synchronous display.The analog circuit part, active resistance circuit is mainly based on ohm’s law: R = U/I, and the single port network electronic circuit model, which is made of active devices such as op-amp. By accurately controlling the current of the single port network, make it in proportion to the port voltage, achieve the active resistance. Innovation of this research is to use three 1024 position digital potentiometer—the AD5231BRU100,together with a potentiometer to form voltage divider circuit. By accurately digital controlling of the voltage division factor of the voltage divider circuit to realize 1.001—100.1 times precision conversion of the benchmark sampling resistor, when the conversion multiples among 1.001—10.000, the resolution is 0.001; when among 10.01—100.1, the resolution is 0.01. By choosing three precise resistance that with high precision, high stability, low temperature drift coefficient and meet a certain power demand(10 ?, 10 W, 0.01%; 1 k ?, 0.25 W, 0.01%; 100 k ?, 0.25 W, 0.01%) as benchmark sample resistor, the active resistance box can achieve all resistance value among 10.01?—10.01 M?. Its resistance value accuracy is ±0.46%, its resistance power is greater than 2W.The active resistance box has the advantage of simple, economical, practical, small volume and wide application prospect. But, since this equipment is based on the principle that controlling the input supply current I, make it proportional to the input voltage, so it can’t be used as load of a constant current source.