Modelling a constant current circuit using a BC548 transistor and LM1085 Voltage Regulator for 4-wire resistance measurement

[afzal94]

I have modelled a constant current circuit that can be switched using a digital input to the BC548 base. The application of the circuit is to measure milli ohms resistance. A 5V vcc is supplied to the BC548 collector, and the emitter is used to input to the LM1085 voltage regulator, which is used to produce a constant current source of 100 mA (Vref: 1.25 V across 12.4 Ohms resistor). The constant current is passed through a resistor of 1 mOhms, and the potential drop across it is amplified by a gain of 1000, using the TLC2652 ACP chopper-stabilized op amp.

I am using am 'Interactive Digital Constant' to switch the transistor. A 1 uOhms resistos is added in series to the digital constant to convert to 5V. The issue is that if I start with a value of 1, I get the expected amplified potential drop of 100 mV. However, when I use the interactive constant to switch the circuit off, the potential drop seems to decrease very slowly. Conversely, if I start off with 0, the potential drop across the 1 mOhm resistor increases very slowly.

Please find attached my Multisim model.

[edjones93]

Hello,

Based on the description of your issue, I believe it is the simulation time you are seeing cause this slow behaviour. Essentially a large algorithm in the background is computing using Finite Element Method the circuit. This will require significant computation time so is the cause of the simulation speed. You can configure the simulation speed as discussed in the below linked forum page:

http://forums.ni.com/t5/Multisim-and-Ultiboard/Speed-up-the-Simulation-time/td-p/809724

However keep in mind that as with the Finite Element Method, if you increase the time step drastically, there is a threshold where the simulation with diverge and error so only increase the time step incrementally and in small quantities to the power of 10.

Best regards,

Ed