How to measure low resistance devices

The blog post Measuring Resistances Less Than 1Ohm[1] introduces a simple and accurate method for measuring low resistance devices. Usually, the on-resistance of some devices is very small, such as low-resistance current measuring resistors, wire resistances, switch resistances, fuses, relays, and igniters. Some examples of such low resistance devices are given below.

Commonly used multimeters (whether pointer or digital) are not accurate when measuring resistances below one ohm. Some multimeters even have large fluctuations in resistance readings below 10 ohms. In this case, you need to use a four-wire low-resistance ohmmeter to measure accurately. In fact, as long as your multimeter can accurately measure millivolt DC voltage, it can measure low-resistance resistors quite accurately.

▲ Some low resistance devices

01 Measurement plan

Equipment needed to measure low resistance devices:

● A digital multimeter that can measure V, mV and resistance;

● 220 ohm resistor, or other resistor with similar resistance value;

● 5V regulated power supply (AC adapter, desktop power supply, or 7805 voltage regulator circuit)

● 0.1uF, 10uF capacitors, and a breadboard.

The 5V power supply needs to be kept constant during the measurement process, otherwise it will affect the measurement accuracy. In fact, most power supplies with voltage regulation function can meet the requirements.

● During measurement, the circuit needs to remain stable, with no switch opening or closing;

● Use capacitors of different sizes and capacities to smooth the operating voltage;

● Choose a working current of about 22mA, not more than 100mA, and not less than 5mA.

As for the accuracy of the working voltage itself, it does not affect the measurement accuracy. Any voltage within the range of 4.5V~5.5V can get good measurement results as long as it remains constant.

▲ Schematic diagram of the circuit for measuring low resistance devices

● The +5V and GND of the power supply are connected to the double slots on the top and bottom of the panel package;

● Use C1 (0.1uF) and C2 (10uF) at the top and bottom to filter the working power supply;

● R1 (220 ohms) is a known resistor connected between +5V and R2;

● R2 is the low-resistance resistor to be measured, connecting R1 to GND;

In fact, the circuit above is a simple voltage divider circuit, and the current flowing through R1 and R2 is the same. After measuring the voltage across R1 and R2, we can know the ratio of R2 to R1, and then calculate the resistance value of R2.

The above is to measure the resistor R2 directly on the breadboard. You can use alligator clips to lead R2 out of the breadboard, and you can measure other devices (leads, surface mount devices, igniters, etc.).

02 Known resistance

In the previous measurement scheme, R1 is a known resistor. It is best to choose a resistor with high power and low temperature drift coefficient. Usually, a resistor with 5% accuracy can meet the requirements.

According to Ohm's law, when 5V is applied to a 220 ohm resistor, 0.114W (1/10W) of power will be generated, which will be consumed as heat on R1. When the temperature of R1 rises, the resistance change of a resistor with a low temperature coefficient (less than ±50ppm) is less than that of a common resistor (temperature coefficient greater than ±100ppm). Therefore, using a stable resistor R1 with high power (ensuring that its temperature change is small) and low temperature coefficient is the key to improving measurement accuracy.

A 1/2W metal film 1% precision 220 ohm resistor with a temperature coefficient of 50ppm or a 3W, 20ppm, 220 ohm wire wound resistor can meet the measurement requirements. In the following measurement experiment, a very common 5% 1/4W, 350ppm, 220Ω carbon film resistor was used.

Use a multimeter to measure the resistance of R1 before inserting it into the breadboard. Do not measure it after inserting R1 into the breadboard, as this will give you an inaccurate reading.

▲ Use a multimeter to read the resistance value of R1

In fact, as long as the reading is stable, the specific value of the resistance will not affect the final measurement result. Usually, the resistance value is between 200 and 240.

After recording the measurement results, connect R1 to the breadboard.

03 Measuring samples

In order to verify the principle of the measurement circuit and the correctness of the measurement process, you can start with a resistance test that can be accurately measured by a multimeter. For example, you can use a 10 ohm resistor as R2 for measurement.

Turn on the 5V power supply and measure the voltage across R1. Here, for example, it reads 4.7696V.

▲ Measure the voltage across R1: 4.7696V

Since the resistance of R1 is much larger than that of R2 (10 ohms), the voltage on R1 is relatively large and should be greater than 4.5V.

Next, measure the voltage across R2, which is usually less than 0.5V. Select the millivolt range of the DC voltage of the multimeter to measure. If you use the ordinary volt range to measure, the accuracy of the voltage value may be reduced. Most digital multimeters include both millivolt and volt ranges.

For example, in the measurement below, the voltage across R2 is 216.64mV.

▲ Use the millivolt range to measure the DC voltage on the unknown resistor R2

Based on the above measurement results, the resistance value of R2 can be calculated.

This result is consistent with the actual situation. The R2 just measured is a 5% precision resistor, which means its resistance should be between 9.5 and 10.5 ohms.

▲ Use a multimeter to measure the resistance of R2 directly

If you use the resistance range of the multimeter directly, you can measure the resistance of R2 to be 9.9Ω, which is consistent with the previous measurement results.

04 Measuring some common low resistance devices

The above method is used to measure the low-resistance devices in the figure at the beginning of this article. Some of these low-resistance devices are already milliohm-level resistors, and the effective resistance cannot be read using a multimeter.

When measuring, measure the R2 voltage as close to the root of the device pin as possible, otherwise the measured resistance will include the resistance of the device pin. Here are some measurement results:

It can be seen that all measurement results are within the actual expected resistance range.

05 Improvements

Replacing R1 with a more powerful and stable winding resistor can greatly increase the number of resistance values ​​that can be measured. It is also important to maintain the stability of the ambient temperature during the measurement process. For example, the resistance value of the device measured at high temperature during the day and at low temperature at night will differ by about 1%.

Carbon film resistors have a large temperature coefficient, and their resistance values ​​vary greatly at different temperatures.

To improve measurement accuracy, it is best to:

● Use a higher power, lower temperature coefficient resistor as R1;

● Before measuring, it is best to wait for two minutes to allow the measuring device to reach thermal equilibrium and measure after the tissue is stable.

▲ High power and low temperature coefficient wirewound resistor

For high-power, low-temperature-coefficient resistors, the resistance value of 220 ohms will usually not vary by more than one-tenth of an ohm within the normal room temperature range.

References

[1] Measuring Resistances Less Than 1Ohm: https://www.robotroom.com/Measuring-Low-Resistances.html