Milliohm Meter Adapter
Measure low resistance values and locate short circuits.
There are several situations where you may need to measure low resistance values, down to the milliohm range. One example is locating a short circuit on a PCB, but it’s also useful for testing electrical contacts, wires, and winding resistances. The adapter presented below allows you to do this together with the multimeter you already have.
At first, I imagined an all-in-one meter with a display and advanced features, but while researching the topic, I came across an Analog Devices application note. This simple circuit uses a multimeter as the display, significantly simplifying the design. Digging further, I eventually found an Elektor project from 1992, which may very well have been the main inspiration for the 1998 application note from Dallas Semiconductor (which was acquired by Maxim in 2001, and later by Analog Devices in 2021). That settled it. Instead of designing something entirely from scratch, and in the spirit of standing on the shoulders of giants, I decided to simply modernize the application note circuit.
Switch S2 is the range selection switch. This 2P3T-type (or DP3T) slide switch can be found online as a hair-dryer spare part. Make sure to check the dimensions before ordering it as there appear to be several variants around carrying the same reference. An alternative is the L203011MS02 from C&K, which also fits on the PCB.
The current is applied to the resistance-under-test—such as a length of cable, a switch contact, a PCB trace, or even a resistor. This current creates a voltage drop across the resistance, which can be measured using a multimeter set to its millivolt range. In a way, it serves as a practical demonstration of Ohm’s law.
Note that the milliohmmeter adapter is suitable for occasional use. If you find you using it frequently, you are probably better off investing in a dedicated milliohmmeter.
IC2 should have low input offset voltage. The schematic shows an MCP6401 which has a specified input offset voltage of 0.8 mV. I also tried the MAX4238 (0.1 µV) and LTC2054 (0.5 µV) but didn’t notice any difference in performance, only in price. These op-amps all come in a SOT-23 package (6-pin for the MAX4238, 5 pins for the others) and therefore can be mounted on the PCB.
For purely ergonomic reasons there are two Test pushbuttons, S1 and S101, placed on either end of the board. It is up to you to decide which position you prefer.
The board includes multiple holes for securing the BAT1 holder with a nut and bolt. Properly fastening it is highly recommended, as any movement can lead to a poor connection over time.
Calibration of the two other ranges is a bit more involved. A good way is to solder R8 and R11 first. Connect a resistor decade box or a trimmer in parallel to R8 and set S2 to the 100-mA position (middle). Connect the test leads to calibration resistor R15 and connect a multimeter to measure the voltage drop over it. Press the Test button and adjust the decade box or trimmer to obtain a reading of 100 mV. Measure the resistance of the trimmer or decade box and replace it by a parallel combination of two fixed resistors on positions R9 and R10. The 10-mA range is calibrated in the same way but now using R16, R12, and R13.
Another option is to only calibrate the 1-A range. Then, for the other two ranges, first measure a known resistor and divide the real value by the measured value. This is the correction factor. Next, measure the unknown resistance and multiply it by the correction factor. Finally, apply the range multiplier. Example:
Suppose the value of the known reference resistor is 18 Ω. Measured in the 100 m Ω /mV position, we find 169.8 mV, which corresponds to 16.98 Ω. The correction factor therefore is 18/16.98 = 1.06. Next, we measure an unknown resistance and find a value of 258.58 mV. Multiplied by the correction factor, we get 274.09 mV, and, after multiplying by the scale factor, we obtain 27.41 Ω.
The good news is that you only have to fully calibrate the adapter if you want to take precise resistance measurements. But then you must also use an accurate multimeter. For finding shortcuts or testing switch contacts, and many other applications, the absolute value is less important than the relative value. When looking for a short, you just want to find the lowest value, no matter what it is. Here is a tutorial on how to find short circuits on a PCB.
Set the multimeter to the millivolt range to ensure that the resistance value will be displayed in millivolts. Multiply the readings by 1, 10 or 100 depending on the position of the slide switch to convert the values into milliohms. As an example, if a reading of 123 mV is obtained with the switch in the 10 mΩ/mV position, the resistance is 1230 mΩ, or 1.23 Ω. If the switch was in the 100 mΩ/mV position, the resistance is 12300 mΩ, or 12.3 Ω.
At first, I imagined an all-in-one meter with a display and advanced features, but while researching the topic, I came across an Analog Devices application note. This simple circuit uses a multimeter as the display, significantly simplifying the design. Digging further, I eventually found an Elektor project from 1992, which may very well have been the main inspiration for the 1998 application note from Dallas Semiconductor (which was acquired by Maxim in 2001, and later by Analog Devices in 2021). That settled it. Instead of designing something entirely from scratch, and in the spirit of standing on the shoulders of giants, I decided to simply modernize the application note circuit.
The Circuit
The resulting schematic (see KiCad project in the download section) is quite simple (and not particularly original). Essentially, it is a programmable current source with three selectable settings: 1 A, 100 mA, and 10 mA. This provides three corresponding measurement ranges: 1 mΩ/mV, 10 mΩ/mV, and 100 mΩ/mV.Switch S2 is the range selection switch. This 2P3T-type (or DP3T) slide switch can be found online as a hair-dryer spare part. Make sure to check the dimensions before ordering it as there appear to be several variants around carrying the same reference. An alternative is the L203011MS02 from C&K, which also fits on the PCB.
The current is applied to the resistance-under-test—such as a length of cable, a switch contact, a PCB trace, or even a resistor. This current creates a voltage drop across the resistance, which can be measured using a multimeter set to its millivolt range. In a way, it serves as a practical demonstration of Ohm’s law.
Note that the milliohmmeter adapter is suitable for occasional use. If you find you using it frequently, you are probably better off investing in a dedicated milliohmmeter.
Programmable Current Source
The current source is a classic circuit built around IC2 and T1. The resistance from the source of T1 to GND determines the current. The voltage on the source of T1 should be set to 100 mV to obtain a 10x mΩ/mV scale (with x an integer). IC1 provides the reference voltage that can be adjusted with P1.IC2 should have low input offset voltage. The schematic shows an MCP6401 which has a specified input offset voltage of 0.8 mV. I also tried the MAX4238 (0.1 µV) and LTC2054 (0.5 µV) but didn’t notice any difference in performance, only in price. These op-amps all come in a SOT-23 package (6-pin for the MAX4238, 5 pins for the others) and therefore can be mounted on the PCB.
Battery Powered
The milliohmmeter adapter is powered from two 1.5-V AA cells. As the power supply is only applied when one of the Test pushbuttons is pressed, they will live a long and happy life. Life is a bit tougher on BAT2, especially in the 1 mΩ/mV position when it must deliver a current of 1 A. However, in most cases, a test never takes longer than a few seconds, and so it will not suffer that much. I didn’t change it once during my numerous experiments. A D-type cell will provide a longer lifespan, but it won’t fit on the PCB.For purely ergonomic reasons there are two Test pushbuttons, S1 and S101, placed on either end of the board. It is up to you to decide which position you prefer.
Assembling the Adapter
As mentioned several times above, a printed circuit board (PCB) has been designed to hold all the components, including the batteries. It fits within one half of a low-cost plastic Hammond 1593N-type enclosure. This design protects the bottom-mounted components on the PCB while keeping the batteries, range slide switch, and test buttons accessible.The board includes multiple holes for securing the BAT1 holder with a nut and bolt. Properly fastening it is highly recommended, as any movement can lead to a poor connection over time.
Calibration
The PCB has room for three calibration resistors, one for each range, R14, R15 and R16 in the schematic. These can be had as 0.1% types, allowing for precise calibration of the adapter. The first part of the calibration procedure is simple. Slide switch S2 to the 1 mΩ/mV position (1 A), connect short test leads to the 0.1-Ω calibration resistor R14, and connect a multimeter to measure the voltage drop over the resistor. Press the Test button and adjust P1 to obtain a reading of 100 mV.Calibration of the two other ranges is a bit more involved. A good way is to solder R8 and R11 first. Connect a resistor decade box or a trimmer in parallel to R8 and set S2 to the 100-mA position (middle). Connect the test leads to calibration resistor R15 and connect a multimeter to measure the voltage drop over it. Press the Test button and adjust the decade box or trimmer to obtain a reading of 100 mV. Measure the resistance of the trimmer or decade box and replace it by a parallel combination of two fixed resistors on positions R9 and R10. The 10-mA range is calibrated in the same way but now using R16, R12, and R13.
Another option is to only calibrate the 1-A range. Then, for the other two ranges, first measure a known resistor and divide the real value by the measured value. This is the correction factor. Next, measure the unknown resistance and multiply it by the correction factor. Finally, apply the range multiplier. Example:
Suppose the value of the known reference resistor is 18 Ω. Measured in the 100 m Ω /mV position, we find 169.8 mV, which corresponds to 16.98 Ω. The correction factor therefore is 18/16.98 = 1.06. Next, we measure an unknown resistance and find a value of 258.58 mV. Multiplied by the correction factor, we get 274.09 mV, and, after multiplying by the scale factor, we obtain 27.41 Ω.
The good news is that you only have to fully calibrate the adapter if you want to take precise resistance measurements. But then you must also use an accurate multimeter. For finding shortcuts or testing switch contacts, and many other applications, the absolute value is less important than the relative value. When looking for a short, you just want to find the lowest value, no matter what it is. Here is a tutorial on how to find short circuits on a PCB.
Using the Milliohmmeter Adapter
The milliohmmeter adapter uses a 4-wire (Kelvin wire) approach to measure resistance. The device injects the current into the resistance-under-test and then the voltage drop over the resistance is measured using a multimeter. The advantage of this technique is that it eliminates the resistance of the multimeter leads, which results in more accurate readings. The multimeter leads must be as close as possible to the contacts of the resistance-under-test to also eliminate component lead resistance. The placement of the adapter leads is not critical. However, they must remain as short as possible. Long test leads with banana plugs will very likely make the adapter oscillate and the obtained readings will be useless. If you find unexpected values, check the leads.Set the multimeter to the millivolt range to ensure that the resistance value will be displayed in millivolts. Multiply the readings by 1, 10 or 100 depending on the position of the slide switch to convert the values into milliohms. As an example, if a reading of 123 mV is obtained with the switch in the 10 mΩ/mV position, the resistance is 1230 mΩ, or 1.23 Ω. If the switch was in the 100 mΩ/mV position, the resistance is 12300 mΩ, or 12.3 Ω.
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