Do you have any idea how to realise it ? Get the BOM. So we want to design our circuit such that the effect of this offset voltage will be minor. The average R is far less important than the matching Rs on Diff-amps, so .5% to 1% F/S on the absolute resistance is fine if they have the same tempco. Also, be aware that the power dissipation of Q1 is proportional to the positive supply voltage, so keeping the input voltage as low as possible will reduce Q1's power dissipation. Thanks for the post! Low reistance measurements are essential for checking out domestic wiring (yes, with the power off) for continuity and making sure the overall resistance is low enough to trip the MCB’s and other protection devices - the lower the conductor resistance, the faster and more reliably the trip will go - could be a life-saver. Polymorph and others, You are correct about the 4 point method, but there are so many crude components here that I don’t think it would matter much. Using the same current and resistance values of 100 ohms and 10K gives voltages of 0.1volts (100 ohms / 0.001amps = 0.1volts) and 10 volts (10000 ohms / 0.001amps = 10 volts). One point i do not agree with Robin: and obviously no significant input current flows into the device. I have 2 question regarding this circuit! The arrangement of Ohmmeter with a typical circuit is shown below. We find the value of the balancing length l 1 , corresponding to a given value of S. Sorry I am confused… Why do we even need the Vee and gnd connection in the two potentiometers? My interest in building this circuit is to measure the primary of automotive single cylinder spark coils.Most are below ten ohms. 100mA through 1mOhm = 0.1mV, which means 1mOhm resolution on the 200mV scale. Thus, a 470 Ω resistor rarely provides exactly 470 Ω of resistance. Touch the meter leads to the two points in the circuit for which you wish to measure resistance. This means that we have to choose a constant-current source that creates a voltage significantly larger than 2mV when the current is passing through a resistance of 0.1Ω.

If it autoranges down to 200mV scale, that would give you 0.1mOhm minimum measurement. A fuse between the collector and VCC wouldn’t be a bad idea. I need to measure temperature via resistor sensor RTD, NTC, PTC etc. Stay below 100v if you do this, otherwise you will need to coat things in corona dope for long term service.

Please do! BTW, If you put another Diff-Amp at the front end (again with precision resistors), you’ll have a nice voltage to current converter. If you can’t do a custom one, then use Perfboard and use a socket for when you fry the Quad. Will this device have enough range? One important consideration is the op-amp's input offset voltage, which is modeled as a voltage source in series with the op-amp's inverting or noninverting input terminal. This voltage drop will then be amplified and fed into a standard multimeter. R1, D1 and D2 form a simple form a simple voltage regulator circuit, which feeds a potential of just over 1.2 volts to the non-inverting input of IC1a. The last stage is an amplifier with gain of 10.

To calibrate an analog meter, touch the two meter leads together.

It would be better to use op-amps that have better characteristics than the LM358 but if on a budget can be OK! Explanation : 100mA through transistors such as a BC548 and 2N3904 cook! If you’re using an analog meter, calibrate it. Then, adjust the meter’s calibration knob until the meter indicates 0 resistance. It is therefore necessary to use a high impedance voltmeter circuit. This, together with the high quality of the constant current source, and the practically non-existent distortion through IC1b due to the high feedback level gives this circuit excellent linearity. By precision, I mean .1% of each other, as in the same package of 4 identical Rs, each the same resistance. S3a gives 5 switched emitter resistances for Q1, and therefore 5 switched emitter currents. Never use breadboards for HV. However, note that the negative rail is used only in the circuit that adds an adjustable offset voltage to the output of the differential amplifier. The full circuit diagram of the Linear Resistance Meter is given in Figure 1.

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