The low resistance meausring circuit explained below can be used for measuring all resistances below 1 ohm with extreme accuracy. The resistance to be measured can be as low as 0.01 ohm.
The output of the circuit converts the resistance value to exactly equivalent volts, which means the output of the circuit could be hooked up with DMM voltmeter range for getting the low resistance values in terms of voltage with extreme precision.
Accuracy and Resolution
The majority of digital multimeters might correctly measure resistance values as low as five ohms only.
Below 5 ohms, you immediately start facing the digital multimeter resolution issues and start seeing resistance values that are rubbish.
We say rubbish, because of the following reason: Normally, when we try measuring a 0.1 ohm resistance value on a digital multimeter, we need to rotate the selector switch to the meter's lowest range (which can be usually the 200 ohm range).
For almost all standard DMM's, the resolution specs is provided as ±1 digit. Put simply, when the meter display shows 0.1 ohm, the true resistance value may be anywhere from 0 to 0.3 ohm. This equals to an accuracy of ±100%, which is not really very helpful for the majority of applications.
Likewise, in case you try measuring a 1 ohm resistor over a 200 ohm range of a DMM, the most accurate results that you may anticipate is a measurement display of 1.0 ±1 digit; That means, the most effective accuracy is ±10%. Therefore, the meter resolution significantly decreases the reliability of the measurement, although you may find most DMM's are accurate within ±1% only if we measure any parameter that may be higher than lowest available meter range.
However you will find numerous scenarios where measuring low-ohm resistance precisely becomes crucial. These may include evaluating meter shunt resistances, building loudspeaker crossover networks and amplifier output stages, and testing or repairing power supplies or any some other circuitry which involve serious use of low value resistors.
The circuit for measuring low value resistance below 1 ohms presented below eliminates the resolution limitations of the standard DMMs. You are able to plug in the circuit directly to the probe slots of the DMM and measure small value resistances as low as 0.01 Ohms.
However, the low resistance measuring circuit has one limitation. As the resistance value to be measured decreases below 0.01 Ohms, issues due to contact resistance of the probes, and connecting wire resistances wires starts developing causing discrepancies in the end result.
The low ohm measuring circuit as indicated in diagram below includes a 5 volt regulator stage, a constant-current source stage using diodes D, D2, and transistor Q1, and an op amp gain control stage (U1).
The circuit is powered from a 9 V PP3 battery. This 9 V output is regulated to +5 volts (DC) by a 78L05 regulator. The regulation enables a stabilized power supply for the constant current source stage and the opamp.
The balance of the circuit only gets linked with the battery as soon as test-switch S1 is pressed. The current is used from the battery only during the time the resistance measurement is being tested, which ensures a prolonged battery life.
Constant-current source stage is built using the parts D1, D2, and transistor Q1 along with a 1k resistor R1.
Transistor Q1 is configured in the form of an emitter-follower stage. Its emitter side terminal follows the voltage applied to its base, with a reduction of around 0.6 volt due to the inherent base-emitter voltage drop. The Series diodes D1 and D2 maintain the Q1 base at a constant 1.2 volts below the +5 V DC supply line. This ensures that the Q1 emitter is constantly 0.6 volt lower than the + 5 DC line. Resistor R1 fixes the current at 5 mA via the two diodes D1 and D2.
This 0.6 V DC generated across one of the multi-turn trimmer potentiometers, R2 or R3, as per the selection by the switch S2-a. The 0.6 V fixes the current by means of Q1 and the resistor under test, Rx.
In case R2 is selected, the test current becomes 1 mA; with the selection of R3, the test current turns into 10 mA. Across the a pair of ranges (x 1 and 10) at the bottom, the voltage across the resistance under test, Rx, is executed right to the DMM terminals through the banana plugs.
On thecouple of ranges from the top, the op-amp gain stage (U1) gets switched ON enabling the DMM to read the voltage across the opamp output (pin 6) and provide the measured date for the test resistor, Rx.
The op amp U1, is configured in the form of a non-inverting op amp stage having a constant gain of 1 + 10,000/100 = 101. Since we would like to have a gain of exactly 100, we determine the voltage between the op amp output and the voltage across Rx.
Therefore, if switch S2 is moved to the position 3 (x 100), the current established through the constant-current source turns 1 mA; the multiplying element for Rx will be x100. When S2 is turned to the position 4 (x1000), the current will be 10 mA and the multiplying aspect will be 100 x 10 = 1000. Multi-turn trimmer-potentiometer R6 modifies the offset parameter of the op-amp to ensure that, when there's zero voltage across Rx (meaning, when the measurement probes are short circuited), the output also turns to zero.
The complete circuit low Ohms Adapter circuit can be enclosed inside a tiny plastic box. On the box's front panel can be a couple of multi-way binding post terminals fixed, on which the resistor to be measured (Rx) could be hooked up.
Additionally there will be a rotary switch with 4-way range (x1, x10, x100, and x1000) as well as a TEST push-button. A pair of banana plugs may be used protruding out at a right angle from the backside of the box; which may be positioned some distance apart so that it allows the entire low resistance circuit to be easily plugged-in into practically any standard digital multimeter or DMM terminal holes.
The low resistance measuring circuit's output generates a voltage which is directly equivalent to the low resistance that is measured. Practically, the circuit is calibrated to ensure that 1 ohm generates an output of 1 millivolt multiplied by the calibration provided on the the range-switch setting. For instance, on the x1000 range, 1 ohm would be corresponding to 1 mV x 1000 = 1 volt. On the x10 range, 1 ohm would be similar to 10 mV, and so on.
How to Calibrate
Switch on power supply by pushing button S1. Verify that the regulator (U2) produces the required +5V at its output, and about 3.8 V DC is produced across the 1K resistor (R1) in series with diodes D1 and D2.
Next, hook up your DMM across the Rx test terminals and set it to the DC 2mA scale. Adjust the switch S2 to the x1 position and set R2 for getting a display of 1 mA. Once these are accomplished, adjust the DMM to the DC 20 mA scale, set up S2 to the x10 position and adjust R3 to get a reading display of 10mA.
After these steps the calibration could be accomplished by fine-tuning the offset voltage. To get this done, remove the meter from the above discussed position, and set it up to the DC 200 mV range.
After doing this, adjust S2 switch of the circuit to the x100 position, short circuit the Rx terminals with a copper wire, and next push the banana plugs of our Low Ohms measuring circuit into the COM and VDC terminal inputs of your DMM.
Start rotating the potentiometer R6 for ensuring a starting reading of slightly above 0 mV on the DMM display….immediately after this rotate the R6 back to get a reading of exactly 0 mV on the DMM display.
This finishes the calibration procedure.